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Crystal structure and physical properties of newboride Ca1+Co4B4

To cite this article Yukari Katsura et al 2009 J Phys Conf Ser 176 012016

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Crystal structure and physical properties of new boride Ca1+εCo4B4

Yukari Katsura Hiraku Ogino Yutaka Matsumura Shigeru Horii Jun-ichi Shimoyama and Kohji Kishio Department of Applied Chemistry School of Engineering the University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-8656 Japan

tt67114maileccu-tokyoacjp

Abstract We report the crystal structures and physical properties of a novel boride Ca1+εCo4B4 This is a Gd1+εFe4B4-type compound composed of tetrahedral chains of CoB and infinite atomic chains of Ca with independent translation periods cCa and cCo This is the first boride of this structural type that has a divalent metal at the rare earth site The very low electrical resistivity showing linear decrease with decreasing temperature indicated a highly metallic nature of Ca1+εCo4B4 The very small magnetization suggested that Ca1+εCo4B4 is paramagnetic First-principles calculations supported this paramagnetism and revealed the nature of chemical bonding and electric conduction in this compound

1 Introduction Ternary metal borides RMxBy where R is an electropositive metal (usually rare earth metals) and M is a transition metal have a wide variety of compositions and crystal structures Alkaline earth elements are known to have similar electronegativity and ionic radii with rare earth elements However ternary borides containing alkaline earth elements have much extent for exploration compared to the well-studied rare earth elements

There exist three structural prototypes for ternary borides that has the composition RM4B4 CeCo4B4 [1] LuRh4B4 [2] and NdCo4B4 [3] structures These structures are all composed of M4 tetrahedra and B2 dimers in different configurations The structure of the newly synthesized Ca1+εCo4B4 is presented in figure 1 This structure belongs to Gd1+εFe4B4 structure[4-9] which is an incommensurate variation of NdCo4B4 structure

NdCo4B4 structure belongs to a tetragonal space group P42n Among above three structures NdCo4B4 is the only structure with edge-sharing Co4 tetrahedra forming one-dimensional tetrahedral chains of Co These chains are bonded to adjacent chains by B2 dimers forming channels of CoB A single atomic chain of R atoms is formed inside each channel R atoms form a body-centered tetragonal lattice with neighboring R chains The c-axis length of the R lattice (cR) is equal to the c-axis length of the MB lattice (cM)

In Gd1+εFe4B4 structure the spacing of R atomic chain is 5-15 shorter than the period of M4 tetrahedral chains so that cR and cM are independent Therefore stoichiometry of these compounds can not be described with simple integers and the value of ε varies from 006 to 018 These structures can be expressed as pseudo-commensurate supercells Rp(M4B4)q where p and q are integers

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

ccopy 2009 IOP Publishing Ltd 1

It is difficult to clarify why some of the borides prefer commensurate structures and why others prefer incommensurate ones The commensurate NdCo4B4 structure are observed in RCo4B4 (R = La Pr Nd) [3] LaRu4B4 [11] ROs4B4 (R = La-Nd Sm Eu Th) [12-14] and RIr4B4 (R = Y La-Nd Sm-Tb Th) [12-14] On the other hand Pr1+εMn4B4 [15] R1+εFe4B4 (R = Sc Y La-Nd Sm Gd-Tm Lu) [6810] and Sm1+εCo4B4 [16] are known to have the incommensurate Gd1+εFe4B4 structure No RM4B4 or R1+εM4B4 phases with alkaline earths for R have been reported so far

The c-axis lengths of R lattices (cR) and a-axis lengths show strong dependence on the ionic radii of R3+ The c-axis lengths of MB lattices (cM) remain almost fixed regardless of R The relatively short a-axis length and cR in Ce1+εM4B4 (M = Fe [8] Os [12]) suggest that the high valence state above +3 is favored for R when M has 8 valence electrons

Physical properties of NdCo4B4-type and Gd1+εFe4B4-type borides have not been studied except for magnetic properties Lack of magnetism in Fe atoms in R1+εFe4B4 is observed from 57Fe-Moumlssbauer studies of R1+εFe4B4 (R = Nd Gd) Ferromagnetism in R1+εFe4B4 is observed only when R ions have magnetic moments due to f-electrons (R = Pr Nd Gd-Ho) [817] Few studies were carried out on the electrical resistivity of R1+εM4B4 Electronic structure of these compounds has been completely unknown

In this study we discovered Ca1+εCo4B4 as the first R1+εM4B4 phase with alkaline earth element at the R site We investigated its crystal structure electrical and magnetic properties and its electronic structure thus far

2 Experimental A mixture of Co and B powder with a molar ratio of 111 was pressed into pellet and heated in a quartz ampoule at 900-1100degC for 48-72 h to obtain CoB precursor Boron was placed in excess to prevent formation of ferromagnetic impurities CoB was crushed into powder and filled in a closed Nb tube with Ca granules The tube was pressed and then heated in a quartz ampoule at 1100degC for 48-120 h so that liquid Ca could penetrate and diffuse into CoB

Distribution of impurities and composition were analyzed by EDX spectra of polished surface through SEM (KEYENCE VE7800) equipped with EDX spectrometer (EDAX PV775075ME) Powder XRD patterns of crushed sample washed by water were obtained by using Rigaku Ultima-IV with Cu-Kα radiation RIETAN-2000 code [19] was used for Rietveld simulation

Bright-field TEM images and electron diffraction patterns were obtained using JEOL JEM-2010F (200 kV) from a thin specimen prepared by JEOL Ion Slicer The script on EMS on Line [20] was used for simulation of TEM images Electrical resistivity of polycrystalline bulk was measured by the four-point-probe method under magnetic field of 0-5 T Magnetization of the bulk was measured by using SQUID magnetometer (Quantum design MPMS-XL5s)

Figure 1 Crystal structure of Ca1+εCo4B4 (a) View from [001] direction (b) Illustration of two c- axis parameters cCa and cCo (c) Connection between two tetrahedral chains The bonds between atoms are presented for visual aid and they do not necessarily mean presence of chemical bonds

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

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Figure 2 (colour online) Experimental and simulated powder XRD patterns of Ca1+εCo4B4

Figure 3 (a) Electron diffraction pattern from [110] direction The diffraction spots marked with 0 1 2 and 3 correspond to 000 001 and 002 reflections from CoB sublattice and 002 reflection from Ca sublattice respectively Other satellite spots are produced by multiple scattering over the two sublattices (b)(c) Bright field TEM images from [001] and [110] directions with simulated images for commensurate CaCo4B4

Spin-polarized first-principles calculation was carried out by FLAPW method using WIEN2k code

[21] with GGA (PBE96) electron correlation potential 8times8times14 k-point mesh was employed Energy and charge were converged within 00001 Ry and 0002e Muffin-tin radii for Ca Co and B were 250 200 and 163 Bohr respectively Electron isosurface and Fermi surface were visualized by XCrySDen [22]

3 Results and discussion Dark gray polycrystalline bulk was obtained by removal of Nb tube From powder XRD pattern shown in figure 2 the phase was characterized by a tetragonal lattice with a = 6992 Aring and c = 3861 Aring These values were similar to the lattice parameters for NdCo4B4 (a = 7070 Aring c = 3822 Aring) [3] and the CoB sublattice in Sm1+εCo4B4 (a = 7051 Aring cCo ~ 3871 Aring) [16] implying that these peaks originate from CoB lattice in CaCo4B4 or Ca1+εCo4B4 The strong 310 reflection is a typical feature for powder diffraction patterns of NdCo4B4-type and Gd1+εFe4B4-type compounds Most of the peaks were successfully reproduced by Rietveld simulation of both commensurate cell CaCo4B4 and incommensurate cell Ca12(Co4B4)11The presence of Ca sublattice (body-centered tetragonal cCa ~ 350-353 Aring) was implied from the diffraction peaks such as 2θ = 283deg(101) and 385deg(211) Although the peaks from Ca sublattice were much broader than the ones from CoB sublattice this could be explained by low periodicity of Ca atoms in the chain

A more convincing evidence for the presence of incommensurate sublattice was obtained from selected area electron diffraction pattern from Ca1+εCo4B4 Diffraction spots from two sublattices with

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

3

different c-axis parameters cCo and cCa are clearly observed in figure 3(a) Satellite spots due to multiple scattering by the two sublattices were observed along c-axis Such pattern was consistent with TEM studies of R1+εFe4B4 [710] Bright-field TEM images in figures 3(b) and 3(c) from [001] and [110] directions of Ca1+εCo4B4 show a good agreement with simulated patterns

From SEM observations of fractured surface in figure 4 grain sizes were 5-10 microm Standardless EDX analysis on the grains showed the composition of Ca to Co ~ 10-11 40 supporting that the resultant phase is CaCo4B4 or Ca1+εCo4B4 The grains are well connected and the volume fractions of unreacted Ca metal and impurities were small enough for resistivity measurements No contamination due to Nb tube was observed in the samples The density of the bulk was 436 gcm-3 which was 77 of the calculated density for Ca1+εCo4B4 (568 gcm-3)

As shown in figure 5 the Ca1+εCo4B4 bulk exhibited highly metallic resistivity with a slight increase in resistivity under magnetic field Magnetic susceptibility of the best Ca1+εCo4B4 bulk was 30times10-4 (2 K 1 T) and 17times10-4 (340 K 1 T) suggesting that Ca1+εCo4B4 is paramagnetic However the gradual decrease in susceptibility at higher temperatures implied the presence of small amount of ferromagnetic impurities with Curie temperatures higher than 340 K

First-principles calculations were carried out on a commensurate cell CaCo4B4 with addition of 02 electrons per formula unit to reproduce the Fermi level in the incommensurate cell Electron isosurface of CaCo4B4 shown in figure 6(a) reveals the presence of rod-like Co-B-B-Co covalent bonds which connect the tetrahedral chains of Co CoB channel and Ca appear to be ionically bonded Density of states (DOS) in figure 6(b) shows that CaCo4B4 is almost paramagnetic Pauli paramagnetic susceptibility is estimated to be 20times10-4 at 0 K however this value should be lower in the sample because the DOS in the actual cell would be decreased due to lowered symmetry The Fermi level lying in Co d-band indicates that the metallic bonds in tetrahedral chains of Co are responsible for electric conduction Figure 6(c) shows four sets of Fermi sheets stacked along c-axis suggesting anisotropy in conductivity with high conductivity along c-axis

4 Conclusion We investigated the crystal structure and physical properties of a novel ternary boride Ca1+εCo4B4 characterized by an incommensurate Gd1+εFe4B4-type structure The presence of incommensurate substructures along c-axis was confirmed by the powder XRD pattern and electron diffraction patterns of Ca1+εCo4B4 Among all NdCo4B4-based borides this is the first compound that has a divalent metal at the rare earth site Such displacement of the rare earth sites in ternary borides will be one of the powerful methods for finding novel ternary borides From resistivity and magnetization measurements Ca1+εCo4B4 was implied to be a paramagnetic metal A preliminary first-principles calculation on the commensurate cell showed that CaCo4B4 is paramagnetic It was also found that the tetrahedral chains of Co atoms are responsible for electric conduction and Ca atoms are ionically bonded to the CoB channel structure Single crystal growth of Ca1+εCo4B4 is needed for refinement of atomic positions and further studies on physical properties especially the magnetization and anisotropic physical properties

0 100 200 3000

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T K

ρ micro

Ωcm

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Figure 4 Secondary electron image of a fractured surface of Ca1+εCo4B4 bulk

Figure 5 Temperature dependence of resistivity of Ca1+εCo4B4 bulk

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

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Figure 6 (colour online) (a) Electron isosurface of CaCo4B4 (b) Spin-polarized density of states of CaCo4B4 (c) Fermi surface of CaCo4B4 calculated by FLAPW method

Acknowledgments The authors thank T Itoh Y Kakegawa K Ibe and H Tsunakawa (the University of Tokyo) for their assistance on TEM observations This work was performed under the inter-university cooperation research program of the Institute for Material Research Tohoku University This study was partially supported by National Institute for Materials Science (NIMS) and by the Japan Society for the Promotion of Science under the Grant-in-Aid for JSPS Fellows and the Grant-in-Aid for Scientific Research of Priority Areas (MEXT) No16080103

References [1] Kuzma Yu B and Bilonizhko N S 1971 Kristallografiya 16 1030 [2] Yvon K and Johnston D C 1982 Acta Crystallogr B 38 247 [3] Kuzma Yu B and Bilonizhko N S 1978 Dopov Akad Nauk Ukr RSR Ser A 275 [4] Chaban N F Kuzma Yu B Bilonizhko N S Kachmar O O Petriv N V 1979 Dopov Akad

Nauk Ukr RSR Ser A 10 873 [5] M Sagawa M Fujimura S Yamamoto H Matsuura Y and Hiraga K 1984 IEEE Trans Magn

20 1584 [6] Givord D Moreau J M and Tenaud P 1985 Solid State Commun 55 303 [7] Bezinge A Braun H F and J Muller J 1987 Phys Rev B 36 1406 [8] Niarchos D Zouganelis G Kostikas A and Simopoulos A 1986 Solid State Commun 59 389 [9] Zhao Z B and Hong S Y 1992 J Phys Condens Matter 4 9221 [10] Oumlsterreicher K and Oumlsterreicher H 1987 J Solid State Chem 70 313 [11] Gruumlttner A and Yvon K 1979 Acta Cryst B 35 451 [12] Rogl P 1979 Monatsh Chem 110 235 [13] Hiebl K Rogl P and Sienko M J 1982 Inorg Chem 21 1128 [14] Rogl P 1980 Monatsh Chem 111 517 [15] Zavalij P Yu Mykhalenko S I and Kuzma Yu B 1994 J Alloys Compd 203 55 [16] Zavalij P Yu Kuzma Yu B and Zavalii L V 1990 Kristallografiya 35 638 [17] Givord D Li H S Moreau J M and Tenaud P J Magn Magn Mat 1986 54-57 445 [18] Buschow K H J de Mooij D B Daams J L C and van Noort H M 1986 J Less-Common Metals

115 357 [19] Izumi F and Ikeda T 2000 Mater Sci Forum 321-324 198 [20] Jouneau P -H and Stadelmann P 1985 Centre Interdeacutepartemental de Microscopie Electronique

EPFL Lausanne [21] Blaha P Schwarz K Madsen G Kvasnicka D and Luitz J 2001 WIEN2k An Augmented Plane

Wave+ Local Orbitals Program for Calculating Crystal Properties (Vienna University of Technology)

[22] Kokalj A 2003 Comp Mater Sci 28 155

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

5

It is difficult to clarify why some of the borides prefer commensurate structures and why others prefer incommensurate ones The commensurate NdCo4B4 structure are observed in RCo4B4 (R = La Pr Nd) [3] LaRu4B4 [11] ROs4B4 (R = La-Nd Sm Eu Th) [12-14] and RIr4B4 (R = Y La-Nd Sm-Tb Th) [12-14] On the other hand Pr1+εMn4B4 [15] R1+εFe4B4 (R = Sc Y La-Nd Sm Gd-Tm Lu) [6810] and Sm1+εCo4B4 [16] are known to have the incommensurate Gd1+εFe4B4 structure No RM4B4 or R1+εM4B4 phases with alkaline earths for R have been reported so far

The c-axis lengths of R lattices (cR) and a-axis lengths show strong dependence on the ionic radii of R3+ The c-axis lengths of MB lattices (cM) remain almost fixed regardless of R The relatively short a-axis length and cR in Ce1+εM4B4 (M = Fe [8] Os [12]) suggest that the high valence state above +3 is favored for R when M has 8 valence electrons

Physical properties of NdCo4B4-type and Gd1+εFe4B4-type borides have not been studied except for magnetic properties Lack of magnetism in Fe atoms in R1+εFe4B4 is observed from 57Fe-Moumlssbauer studies of R1+εFe4B4 (R = Nd Gd) Ferromagnetism in R1+εFe4B4 is observed only when R ions have magnetic moments due to f-electrons (R = Pr Nd Gd-Ho) [817] Few studies were carried out on the electrical resistivity of R1+εM4B4 Electronic structure of these compounds has been completely unknown

In this study we discovered Ca1+εCo4B4 as the first R1+εM4B4 phase with alkaline earth element at the R site We investigated its crystal structure electrical and magnetic properties and its electronic structure thus far

2 Experimental A mixture of Co and B powder with a molar ratio of 111 was pressed into pellet and heated in a quartz ampoule at 900-1100degC for 48-72 h to obtain CoB precursor Boron was placed in excess to prevent formation of ferromagnetic impurities CoB was crushed into powder and filled in a closed Nb tube with Ca granules The tube was pressed and then heated in a quartz ampoule at 1100degC for 48-120 h so that liquid Ca could penetrate and diffuse into CoB

Distribution of impurities and composition were analyzed by EDX spectra of polished surface through SEM (KEYENCE VE7800) equipped with EDX spectrometer (EDAX PV775075ME) Powder XRD patterns of crushed sample washed by water were obtained by using Rigaku Ultima-IV with Cu-Kα radiation RIETAN-2000 code [19] was used for Rietveld simulation

Bright-field TEM images and electron diffraction patterns were obtained using JEOL JEM-2010F (200 kV) from a thin specimen prepared by JEOL Ion Slicer The script on EMS on Line [20] was used for simulation of TEM images Electrical resistivity of polycrystalline bulk was measured by the four-point-probe method under magnetic field of 0-5 T Magnetization of the bulk was measured by using SQUID magnetometer (Quantum design MPMS-XL5s)

Figure 1 Crystal structure of Ca1+εCo4B4 (a) View from [001] direction (b) Illustration of two c- axis parameters cCa and cCo (c) Connection between two tetrahedral chains The bonds between atoms are presented for visual aid and they do not necessarily mean presence of chemical bonds

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

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Experimental Rietveld simulation with an incommensurate cell (Ca12(Co4B4)11 Pccn)

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unts

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331

511

542

702

800

101(

Ca)

Figure 2 (colour online) Experimental and simulated powder XRD patterns of Ca1+εCo4B4

Figure 3 (a) Electron diffraction pattern from [110] direction The diffraction spots marked with 0 1 2 and 3 correspond to 000 001 and 002 reflections from CoB sublattice and 002 reflection from Ca sublattice respectively Other satellite spots are produced by multiple scattering over the two sublattices (b)(c) Bright field TEM images from [001] and [110] directions with simulated images for commensurate CaCo4B4

Spin-polarized first-principles calculation was carried out by FLAPW method using WIEN2k code

[21] with GGA (PBE96) electron correlation potential 8times8times14 k-point mesh was employed Energy and charge were converged within 00001 Ry and 0002e Muffin-tin radii for Ca Co and B were 250 200 and 163 Bohr respectively Electron isosurface and Fermi surface were visualized by XCrySDen [22]

3 Results and discussion Dark gray polycrystalline bulk was obtained by removal of Nb tube From powder XRD pattern shown in figure 2 the phase was characterized by a tetragonal lattice with a = 6992 Aring and c = 3861 Aring These values were similar to the lattice parameters for NdCo4B4 (a = 7070 Aring c = 3822 Aring) [3] and the CoB sublattice in Sm1+εCo4B4 (a = 7051 Aring cCo ~ 3871 Aring) [16] implying that these peaks originate from CoB lattice in CaCo4B4 or Ca1+εCo4B4 The strong 310 reflection is a typical feature for powder diffraction patterns of NdCo4B4-type and Gd1+εFe4B4-type compounds Most of the peaks were successfully reproduced by Rietveld simulation of both commensurate cell CaCo4B4 and incommensurate cell Ca12(Co4B4)11The presence of Ca sublattice (body-centered tetragonal cCa ~ 350-353 Aring) was implied from the diffraction peaks such as 2θ = 283deg(101) and 385deg(211) Although the peaks from Ca sublattice were much broader than the ones from CoB sublattice this could be explained by low periodicity of Ca atoms in the chain

A more convincing evidence for the presence of incommensurate sublattice was obtained from selected area electron diffraction pattern from Ca1+εCo4B4 Diffraction spots from two sublattices with

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

3

different c-axis parameters cCo and cCa are clearly observed in figure 3(a) Satellite spots due to multiple scattering by the two sublattices were observed along c-axis Such pattern was consistent with TEM studies of R1+εFe4B4 [710] Bright-field TEM images in figures 3(b) and 3(c) from [001] and [110] directions of Ca1+εCo4B4 show a good agreement with simulated patterns

From SEM observations of fractured surface in figure 4 grain sizes were 5-10 microm Standardless EDX analysis on the grains showed the composition of Ca to Co ~ 10-11 40 supporting that the resultant phase is CaCo4B4 or Ca1+εCo4B4 The grains are well connected and the volume fractions of unreacted Ca metal and impurities were small enough for resistivity measurements No contamination due to Nb tube was observed in the samples The density of the bulk was 436 gcm-3 which was 77 of the calculated density for Ca1+εCo4B4 (568 gcm-3)

As shown in figure 5 the Ca1+εCo4B4 bulk exhibited highly metallic resistivity with a slight increase in resistivity under magnetic field Magnetic susceptibility of the best Ca1+εCo4B4 bulk was 30times10-4 (2 K 1 T) and 17times10-4 (340 K 1 T) suggesting that Ca1+εCo4B4 is paramagnetic However the gradual decrease in susceptibility at higher temperatures implied the presence of small amount of ferromagnetic impurities with Curie temperatures higher than 340 K

First-principles calculations were carried out on a commensurate cell CaCo4B4 with addition of 02 electrons per formula unit to reproduce the Fermi level in the incommensurate cell Electron isosurface of CaCo4B4 shown in figure 6(a) reveals the presence of rod-like Co-B-B-Co covalent bonds which connect the tetrahedral chains of Co CoB channel and Ca appear to be ionically bonded Density of states (DOS) in figure 6(b) shows that CaCo4B4 is almost paramagnetic Pauli paramagnetic susceptibility is estimated to be 20times10-4 at 0 K however this value should be lower in the sample because the DOS in the actual cell would be decreased due to lowered symmetry The Fermi level lying in Co d-band indicates that the metallic bonds in tetrahedral chains of Co are responsible for electric conduction Figure 6(c) shows four sets of Fermi sheets stacked along c-axis suggesting anisotropy in conductivity with high conductivity along c-axis

4 Conclusion We investigated the crystal structure and physical properties of a novel ternary boride Ca1+εCo4B4 characterized by an incommensurate Gd1+εFe4B4-type structure The presence of incommensurate substructures along c-axis was confirmed by the powder XRD pattern and electron diffraction patterns of Ca1+εCo4B4 Among all NdCo4B4-based borides this is the first compound that has a divalent metal at the rare earth site Such displacement of the rare earth sites in ternary borides will be one of the powerful methods for finding novel ternary borides From resistivity and magnetization measurements Ca1+εCo4B4 was implied to be a paramagnetic metal A preliminary first-principles calculation on the commensurate cell showed that CaCo4B4 is paramagnetic It was also found that the tetrahedral chains of Co atoms are responsible for electric conduction and Ca atoms are ionically bonded to the CoB channel structure Single crystal growth of Ca1+εCo4B4 is needed for refinement of atomic positions and further studies on physical properties especially the magnetization and anisotropic physical properties

0 100 200 3000

20

40

60

80

5 T 0 T

T K

ρ micro

Ωcm

Ca1+εCo4B4

Figure 4 Secondary electron image of a fractured surface of Ca1+εCo4B4 bulk

Figure 5 Temperature dependence of resistivity of Ca1+εCo4B4 bulk

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

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Figure 6 (colour online) (a) Electron isosurface of CaCo4B4 (b) Spin-polarized density of states of CaCo4B4 (c) Fermi surface of CaCo4B4 calculated by FLAPW method

Acknowledgments The authors thank T Itoh Y Kakegawa K Ibe and H Tsunakawa (the University of Tokyo) for their assistance on TEM observations This work was performed under the inter-university cooperation research program of the Institute for Material Research Tohoku University This study was partially supported by National Institute for Materials Science (NIMS) and by the Japan Society for the Promotion of Science under the Grant-in-Aid for JSPS Fellows and the Grant-in-Aid for Scientific Research of Priority Areas (MEXT) No16080103

References [1] Kuzma Yu B and Bilonizhko N S 1971 Kristallografiya 16 1030 [2] Yvon K and Johnston D C 1982 Acta Crystallogr B 38 247 [3] Kuzma Yu B and Bilonizhko N S 1978 Dopov Akad Nauk Ukr RSR Ser A 275 [4] Chaban N F Kuzma Yu B Bilonizhko N S Kachmar O O Petriv N V 1979 Dopov Akad

Nauk Ukr RSR Ser A 10 873 [5] M Sagawa M Fujimura S Yamamoto H Matsuura Y and Hiraga K 1984 IEEE Trans Magn

20 1584 [6] Givord D Moreau J M and Tenaud P 1985 Solid State Commun 55 303 [7] Bezinge A Braun H F and J Muller J 1987 Phys Rev B 36 1406 [8] Niarchos D Zouganelis G Kostikas A and Simopoulos A 1986 Solid State Commun 59 389 [9] Zhao Z B and Hong S Y 1992 J Phys Condens Matter 4 9221 [10] Oumlsterreicher K and Oumlsterreicher H 1987 J Solid State Chem 70 313 [11] Gruumlttner A and Yvon K 1979 Acta Cryst B 35 451 [12] Rogl P 1979 Monatsh Chem 110 235 [13] Hiebl K Rogl P and Sienko M J 1982 Inorg Chem 21 1128 [14] Rogl P 1980 Monatsh Chem 111 517 [15] Zavalij P Yu Mykhalenko S I and Kuzma Yu B 1994 J Alloys Compd 203 55 [16] Zavalij P Yu Kuzma Yu B and Zavalii L V 1990 Kristallografiya 35 638 [17] Givord D Li H S Moreau J M and Tenaud P J Magn Magn Mat 1986 54-57 445 [18] Buschow K H J de Mooij D B Daams J L C and van Noort H M 1986 J Less-Common Metals

115 357 [19] Izumi F and Ikeda T 2000 Mater Sci Forum 321-324 198 [20] Jouneau P -H and Stadelmann P 1985 Centre Interdeacutepartemental de Microscopie Electronique

EPFL Lausanne [21] Blaha P Schwarz K Madsen G Kvasnicka D and Luitz J 2001 WIEN2k An Augmented Plane

Wave+ Local Orbitals Program for Calculating Crystal Properties (Vienna University of Technology)

[22] Kokalj A 2003 Comp Mater Sci 28 155

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

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10 20 30 40 50 60 70 80 90 100 110 120 130 140

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551

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114

721

730

314

533

651

613

414

623

Experimental Rietveld simulation with an incommensurate cell (Ca12(Co4B4)11 Pccn)

Inte

nsity

co

unts

211(

Ca)

331

511

542

702

800

101(

Ca)

Figure 2 (colour online) Experimental and simulated powder XRD patterns of Ca1+εCo4B4

Figure 3 (a) Electron diffraction pattern from [110] direction The diffraction spots marked with 0 1 2 and 3 correspond to 000 001 and 002 reflections from CoB sublattice and 002 reflection from Ca sublattice respectively Other satellite spots are produced by multiple scattering over the two sublattices (b)(c) Bright field TEM images from [001] and [110] directions with simulated images for commensurate CaCo4B4

Spin-polarized first-principles calculation was carried out by FLAPW method using WIEN2k code

[21] with GGA (PBE96) electron correlation potential 8times8times14 k-point mesh was employed Energy and charge were converged within 00001 Ry and 0002e Muffin-tin radii for Ca Co and B were 250 200 and 163 Bohr respectively Electron isosurface and Fermi surface were visualized by XCrySDen [22]

3 Results and discussion Dark gray polycrystalline bulk was obtained by removal of Nb tube From powder XRD pattern shown in figure 2 the phase was characterized by a tetragonal lattice with a = 6992 Aring and c = 3861 Aring These values were similar to the lattice parameters for NdCo4B4 (a = 7070 Aring c = 3822 Aring) [3] and the CoB sublattice in Sm1+εCo4B4 (a = 7051 Aring cCo ~ 3871 Aring) [16] implying that these peaks originate from CoB lattice in CaCo4B4 or Ca1+εCo4B4 The strong 310 reflection is a typical feature for powder diffraction patterns of NdCo4B4-type and Gd1+εFe4B4-type compounds Most of the peaks were successfully reproduced by Rietveld simulation of both commensurate cell CaCo4B4 and incommensurate cell Ca12(Co4B4)11The presence of Ca sublattice (body-centered tetragonal cCa ~ 350-353 Aring) was implied from the diffraction peaks such as 2θ = 283deg(101) and 385deg(211) Although the peaks from Ca sublattice were much broader than the ones from CoB sublattice this could be explained by low periodicity of Ca atoms in the chain

A more convincing evidence for the presence of incommensurate sublattice was obtained from selected area electron diffraction pattern from Ca1+εCo4B4 Diffraction spots from two sublattices with

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

3

different c-axis parameters cCo and cCa are clearly observed in figure 3(a) Satellite spots due to multiple scattering by the two sublattices were observed along c-axis Such pattern was consistent with TEM studies of R1+εFe4B4 [710] Bright-field TEM images in figures 3(b) and 3(c) from [001] and [110] directions of Ca1+εCo4B4 show a good agreement with simulated patterns

From SEM observations of fractured surface in figure 4 grain sizes were 5-10 microm Standardless EDX analysis on the grains showed the composition of Ca to Co ~ 10-11 40 supporting that the resultant phase is CaCo4B4 or Ca1+εCo4B4 The grains are well connected and the volume fractions of unreacted Ca metal and impurities were small enough for resistivity measurements No contamination due to Nb tube was observed in the samples The density of the bulk was 436 gcm-3 which was 77 of the calculated density for Ca1+εCo4B4 (568 gcm-3)

As shown in figure 5 the Ca1+εCo4B4 bulk exhibited highly metallic resistivity with a slight increase in resistivity under magnetic field Magnetic susceptibility of the best Ca1+εCo4B4 bulk was 30times10-4 (2 K 1 T) and 17times10-4 (340 K 1 T) suggesting that Ca1+εCo4B4 is paramagnetic However the gradual decrease in susceptibility at higher temperatures implied the presence of small amount of ferromagnetic impurities with Curie temperatures higher than 340 K

First-principles calculations were carried out on a commensurate cell CaCo4B4 with addition of 02 electrons per formula unit to reproduce the Fermi level in the incommensurate cell Electron isosurface of CaCo4B4 shown in figure 6(a) reveals the presence of rod-like Co-B-B-Co covalent bonds which connect the tetrahedral chains of Co CoB channel and Ca appear to be ionically bonded Density of states (DOS) in figure 6(b) shows that CaCo4B4 is almost paramagnetic Pauli paramagnetic susceptibility is estimated to be 20times10-4 at 0 K however this value should be lower in the sample because the DOS in the actual cell would be decreased due to lowered symmetry The Fermi level lying in Co d-band indicates that the metallic bonds in tetrahedral chains of Co are responsible for electric conduction Figure 6(c) shows four sets of Fermi sheets stacked along c-axis suggesting anisotropy in conductivity with high conductivity along c-axis

4 Conclusion We investigated the crystal structure and physical properties of a novel ternary boride Ca1+εCo4B4 characterized by an incommensurate Gd1+εFe4B4-type structure The presence of incommensurate substructures along c-axis was confirmed by the powder XRD pattern and electron diffraction patterns of Ca1+εCo4B4 Among all NdCo4B4-based borides this is the first compound that has a divalent metal at the rare earth site Such displacement of the rare earth sites in ternary borides will be one of the powerful methods for finding novel ternary borides From resistivity and magnetization measurements Ca1+εCo4B4 was implied to be a paramagnetic metal A preliminary first-principles calculation on the commensurate cell showed that CaCo4B4 is paramagnetic It was also found that the tetrahedral chains of Co atoms are responsible for electric conduction and Ca atoms are ionically bonded to the CoB channel structure Single crystal growth of Ca1+εCo4B4 is needed for refinement of atomic positions and further studies on physical properties especially the magnetization and anisotropic physical properties

0 100 200 3000

20

40

60

80

5 T 0 T

T K

ρ micro

Ωcm

Ca1+εCo4B4

Figure 4 Secondary electron image of a fractured surface of Ca1+εCo4B4 bulk

Figure 5 Temperature dependence of resistivity of Ca1+εCo4B4 bulk

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

4

(a)

(b)

-10 -5 0 5 10

-10

0

10

E-EF eV

CaCo4B4

DO

S s

tate

s eV

-1 c

ell-1

Up spin

Down spin Total Co B Ca

(c)

Figure 6 (colour online) (a) Electron isosurface of CaCo4B4 (b) Spin-polarized density of states of CaCo4B4 (c) Fermi surface of CaCo4B4 calculated by FLAPW method

Acknowledgments The authors thank T Itoh Y Kakegawa K Ibe and H Tsunakawa (the University of Tokyo) for their assistance on TEM observations This work was performed under the inter-university cooperation research program of the Institute for Material Research Tohoku University This study was partially supported by National Institute for Materials Science (NIMS) and by the Japan Society for the Promotion of Science under the Grant-in-Aid for JSPS Fellows and the Grant-in-Aid for Scientific Research of Priority Areas (MEXT) No16080103

References [1] Kuzma Yu B and Bilonizhko N S 1971 Kristallografiya 16 1030 [2] Yvon K and Johnston D C 1982 Acta Crystallogr B 38 247 [3] Kuzma Yu B and Bilonizhko N S 1978 Dopov Akad Nauk Ukr RSR Ser A 275 [4] Chaban N F Kuzma Yu B Bilonizhko N S Kachmar O O Petriv N V 1979 Dopov Akad

Nauk Ukr RSR Ser A 10 873 [5] M Sagawa M Fujimura S Yamamoto H Matsuura Y and Hiraga K 1984 IEEE Trans Magn

20 1584 [6] Givord D Moreau J M and Tenaud P 1985 Solid State Commun 55 303 [7] Bezinge A Braun H F and J Muller J 1987 Phys Rev B 36 1406 [8] Niarchos D Zouganelis G Kostikas A and Simopoulos A 1986 Solid State Commun 59 389 [9] Zhao Z B and Hong S Y 1992 J Phys Condens Matter 4 9221 [10] Oumlsterreicher K and Oumlsterreicher H 1987 J Solid State Chem 70 313 [11] Gruumlttner A and Yvon K 1979 Acta Cryst B 35 451 [12] Rogl P 1979 Monatsh Chem 110 235 [13] Hiebl K Rogl P and Sienko M J 1982 Inorg Chem 21 1128 [14] Rogl P 1980 Monatsh Chem 111 517 [15] Zavalij P Yu Mykhalenko S I and Kuzma Yu B 1994 J Alloys Compd 203 55 [16] Zavalij P Yu Kuzma Yu B and Zavalii L V 1990 Kristallografiya 35 638 [17] Givord D Li H S Moreau J M and Tenaud P J Magn Magn Mat 1986 54-57 445 [18] Buschow K H J de Mooij D B Daams J L C and van Noort H M 1986 J Less-Common Metals

115 357 [19] Izumi F and Ikeda T 2000 Mater Sci Forum 321-324 198 [20] Jouneau P -H and Stadelmann P 1985 Centre Interdeacutepartemental de Microscopie Electronique

EPFL Lausanne [21] Blaha P Schwarz K Madsen G Kvasnicka D and Luitz J 2001 WIEN2k An Augmented Plane

Wave+ Local Orbitals Program for Calculating Crystal Properties (Vienna University of Technology)

[22] Kokalj A 2003 Comp Mater Sci 28 155

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

5

different c-axis parameters cCo and cCa are clearly observed in figure 3(a) Satellite spots due to multiple scattering by the two sublattices were observed along c-axis Such pattern was consistent with TEM studies of R1+εFe4B4 [710] Bright-field TEM images in figures 3(b) and 3(c) from [001] and [110] directions of Ca1+εCo4B4 show a good agreement with simulated patterns

From SEM observations of fractured surface in figure 4 grain sizes were 5-10 microm Standardless EDX analysis on the grains showed the composition of Ca to Co ~ 10-11 40 supporting that the resultant phase is CaCo4B4 or Ca1+εCo4B4 The grains are well connected and the volume fractions of unreacted Ca metal and impurities were small enough for resistivity measurements No contamination due to Nb tube was observed in the samples The density of the bulk was 436 gcm-3 which was 77 of the calculated density for Ca1+εCo4B4 (568 gcm-3)

As shown in figure 5 the Ca1+εCo4B4 bulk exhibited highly metallic resistivity with a slight increase in resistivity under magnetic field Magnetic susceptibility of the best Ca1+εCo4B4 bulk was 30times10-4 (2 K 1 T) and 17times10-4 (340 K 1 T) suggesting that Ca1+εCo4B4 is paramagnetic However the gradual decrease in susceptibility at higher temperatures implied the presence of small amount of ferromagnetic impurities with Curie temperatures higher than 340 K

First-principles calculations were carried out on a commensurate cell CaCo4B4 with addition of 02 electrons per formula unit to reproduce the Fermi level in the incommensurate cell Electron isosurface of CaCo4B4 shown in figure 6(a) reveals the presence of rod-like Co-B-B-Co covalent bonds which connect the tetrahedral chains of Co CoB channel and Ca appear to be ionically bonded Density of states (DOS) in figure 6(b) shows that CaCo4B4 is almost paramagnetic Pauli paramagnetic susceptibility is estimated to be 20times10-4 at 0 K however this value should be lower in the sample because the DOS in the actual cell would be decreased due to lowered symmetry The Fermi level lying in Co d-band indicates that the metallic bonds in tetrahedral chains of Co are responsible for electric conduction Figure 6(c) shows four sets of Fermi sheets stacked along c-axis suggesting anisotropy in conductivity with high conductivity along c-axis

4 Conclusion We investigated the crystal structure and physical properties of a novel ternary boride Ca1+εCo4B4 characterized by an incommensurate Gd1+εFe4B4-type structure The presence of incommensurate substructures along c-axis was confirmed by the powder XRD pattern and electron diffraction patterns of Ca1+εCo4B4 Among all NdCo4B4-based borides this is the first compound that has a divalent metal at the rare earth site Such displacement of the rare earth sites in ternary borides will be one of the powerful methods for finding novel ternary borides From resistivity and magnetization measurements Ca1+εCo4B4 was implied to be a paramagnetic metal A preliminary first-principles calculation on the commensurate cell showed that CaCo4B4 is paramagnetic It was also found that the tetrahedral chains of Co atoms are responsible for electric conduction and Ca atoms are ionically bonded to the CoB channel structure Single crystal growth of Ca1+εCo4B4 is needed for refinement of atomic positions and further studies on physical properties especially the magnetization and anisotropic physical properties

0 100 200 3000

20

40

60

80

5 T 0 T

T K

ρ micro

Ωcm

Ca1+εCo4B4

Figure 4 Secondary electron image of a fractured surface of Ca1+εCo4B4 bulk

Figure 5 Temperature dependence of resistivity of Ca1+εCo4B4 bulk

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

4

(a)

(b)

-10 -5 0 5 10

-10

0

10

E-EF eV

CaCo4B4

DO

S s

tate

s eV

-1 c

ell-1

Up spin

Down spin Total Co B Ca

(c)

Figure 6 (colour online) (a) Electron isosurface of CaCo4B4 (b) Spin-polarized density of states of CaCo4B4 (c) Fermi surface of CaCo4B4 calculated by FLAPW method

Acknowledgments The authors thank T Itoh Y Kakegawa K Ibe and H Tsunakawa (the University of Tokyo) for their assistance on TEM observations This work was performed under the inter-university cooperation research program of the Institute for Material Research Tohoku University This study was partially supported by National Institute for Materials Science (NIMS) and by the Japan Society for the Promotion of Science under the Grant-in-Aid for JSPS Fellows and the Grant-in-Aid for Scientific Research of Priority Areas (MEXT) No16080103

References [1] Kuzma Yu B and Bilonizhko N S 1971 Kristallografiya 16 1030 [2] Yvon K and Johnston D C 1982 Acta Crystallogr B 38 247 [3] Kuzma Yu B and Bilonizhko N S 1978 Dopov Akad Nauk Ukr RSR Ser A 275 [4] Chaban N F Kuzma Yu B Bilonizhko N S Kachmar O O Petriv N V 1979 Dopov Akad

Nauk Ukr RSR Ser A 10 873 [5] M Sagawa M Fujimura S Yamamoto H Matsuura Y and Hiraga K 1984 IEEE Trans Magn

20 1584 [6] Givord D Moreau J M and Tenaud P 1985 Solid State Commun 55 303 [7] Bezinge A Braun H F and J Muller J 1987 Phys Rev B 36 1406 [8] Niarchos D Zouganelis G Kostikas A and Simopoulos A 1986 Solid State Commun 59 389 [9] Zhao Z B and Hong S Y 1992 J Phys Condens Matter 4 9221 [10] Oumlsterreicher K and Oumlsterreicher H 1987 J Solid State Chem 70 313 [11] Gruumlttner A and Yvon K 1979 Acta Cryst B 35 451 [12] Rogl P 1979 Monatsh Chem 110 235 [13] Hiebl K Rogl P and Sienko M J 1982 Inorg Chem 21 1128 [14] Rogl P 1980 Monatsh Chem 111 517 [15] Zavalij P Yu Mykhalenko S I and Kuzma Yu B 1994 J Alloys Compd 203 55 [16] Zavalij P Yu Kuzma Yu B and Zavalii L V 1990 Kristallografiya 35 638 [17] Givord D Li H S Moreau J M and Tenaud P J Magn Magn Mat 1986 54-57 445 [18] Buschow K H J de Mooij D B Daams J L C and van Noort H M 1986 J Less-Common Metals

115 357 [19] Izumi F and Ikeda T 2000 Mater Sci Forum 321-324 198 [20] Jouneau P -H and Stadelmann P 1985 Centre Interdeacutepartemental de Microscopie Electronique

EPFL Lausanne [21] Blaha P Schwarz K Madsen G Kvasnicka D and Luitz J 2001 WIEN2k An Augmented Plane

Wave+ Local Orbitals Program for Calculating Crystal Properties (Vienna University of Technology)

[22] Kokalj A 2003 Comp Mater Sci 28 155

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

5

(a)

(b)

-10 -5 0 5 10

-10

0

10

E-EF eV

CaCo4B4

DO

S s

tate

s eV

-1 c

ell-1

Up spin

Down spin Total Co B Ca

(c)

Figure 6 (colour online) (a) Electron isosurface of CaCo4B4 (b) Spin-polarized density of states of CaCo4B4 (c) Fermi surface of CaCo4B4 calculated by FLAPW method

Acknowledgments The authors thank T Itoh Y Kakegawa K Ibe and H Tsunakawa (the University of Tokyo) for their assistance on TEM observations This work was performed under the inter-university cooperation research program of the Institute for Material Research Tohoku University This study was partially supported by National Institute for Materials Science (NIMS) and by the Japan Society for the Promotion of Science under the Grant-in-Aid for JSPS Fellows and the Grant-in-Aid for Scientific Research of Priority Areas (MEXT) No16080103

References [1] Kuzma Yu B and Bilonizhko N S 1971 Kristallografiya 16 1030 [2] Yvon K and Johnston D C 1982 Acta Crystallogr B 38 247 [3] Kuzma Yu B and Bilonizhko N S 1978 Dopov Akad Nauk Ukr RSR Ser A 275 [4] Chaban N F Kuzma Yu B Bilonizhko N S Kachmar O O Petriv N V 1979 Dopov Akad

Nauk Ukr RSR Ser A 10 873 [5] M Sagawa M Fujimura S Yamamoto H Matsuura Y and Hiraga K 1984 IEEE Trans Magn

20 1584 [6] Givord D Moreau J M and Tenaud P 1985 Solid State Commun 55 303 [7] Bezinge A Braun H F and J Muller J 1987 Phys Rev B 36 1406 [8] Niarchos D Zouganelis G Kostikas A and Simopoulos A 1986 Solid State Commun 59 389 [9] Zhao Z B and Hong S Y 1992 J Phys Condens Matter 4 9221 [10] Oumlsterreicher K and Oumlsterreicher H 1987 J Solid State Chem 70 313 [11] Gruumlttner A and Yvon K 1979 Acta Cryst B 35 451 [12] Rogl P 1979 Monatsh Chem 110 235 [13] Hiebl K Rogl P and Sienko M J 1982 Inorg Chem 21 1128 [14] Rogl P 1980 Monatsh Chem 111 517 [15] Zavalij P Yu Mykhalenko S I and Kuzma Yu B 1994 J Alloys Compd 203 55 [16] Zavalij P Yu Kuzma Yu B and Zavalii L V 1990 Kristallografiya 35 638 [17] Givord D Li H S Moreau J M and Tenaud P J Magn Magn Mat 1986 54-57 445 [18] Buschow K H J de Mooij D B Daams J L C and van Noort H M 1986 J Less-Common Metals

115 357 [19] Izumi F and Ikeda T 2000 Mater Sci Forum 321-324 198 [20] Jouneau P -H and Stadelmann P 1985 Centre Interdeacutepartemental de Microscopie Electronique

EPFL Lausanne [21] Blaha P Schwarz K Madsen G Kvasnicka D and Luitz J 2001 WIEN2k An Augmented Plane

Wave+ Local Orbitals Program for Calculating Crystal Properties (Vienna University of Technology)

[22] Kokalj A 2003 Comp Mater Sci 28 155

16th International Symposium on Boron Borides and Related Materials IOP PublishingJournal of Physics Conference Series 176 (2009) 012016 doi1010881742-65961761012016

5