Journal of Solid State Chemistry 198 (2013) 210–217

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Journal of Solid State Chemistry

0022-45

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Mixed metallic Ba(Co,Mn)X0.2�xO3�d (X¼F, Cl) hexagonal perovskites

Mihaela Iorgulescu a, Pascal Roussel a, Nathalie Tancret a, Nicolas Renaut a, Nicolas Tiercelin b,Olivier Mentre! a,n

a UMR CNRS 8181, Unite de Catalyse et de Chimie du Solide (UCCS), ENSCL, University Lille Nord de France, F-59000 Lille, Franceb International Associated Laboratory LEMAC: IEMN, UMR CNRS 8520, University Lille Nord de France, EC Lille, F-59655 Villeneuve d’Ascq, France

a r t i c l e i n f o

Article history:

Received 26 July 2012

Received in revised form

27 September 2012

Accepted 28 September 2012Available online 17 October 2012

Keywords:

Hexagonal perovskites

Face-sharing tetramers

Oxohalides

Manganese cobalt mixed oxides

Mixed valence

Shallow donors

96/$ - see front matter & 2012 Elsevier Inc. A

x.doi.org/10.1016/j.jssc.2012.09.040

esponding author. Fax: þ33 3 20 43 68 14.

ail address: olivier.mentre@ensc-lille.fr (O. M

a b s t r a c t

We show here that the incorporation of Mn in Ba–Co-oxohalide, BaCoX0.2�xO3�d, hexagonal perovskite

stabilizes the 6H-form (stacking sequence (chhhch0); c, h¼[BaO3] and h0 ¼[BaOX] layers), with

tetramers of face-sharing octahedra) rather than the trimeric 10H-form. On the contrary to previous

results on the Fe incorporation in similar system leading to more reduced 10H-compounds, the Mn

effect is to increase the mean (Co/Mn) valence better suited to the 6H form. We experienced a poor Mn/

Co miscibility during our syntheses leading to great difficulties to isolate mixed Co/Mn single phase

materials and/or weak reproducibility. Powder neutron diffraction data shows a mixed Mn/Co

octahedral occupancy, while the tetrahedra are filled by Co3þ cations. Anionic vacancies were refined

in the h0-[BaO1�zX1�x] layer and the next c-[BaO3�z] layers, while the h-[BaO3] layers are not oxygen

deficient. Magnetic properties suggest that a part of Mn cations remain paramagnetic until low

temperature, while isolated spin clusters (probably driven by AFM Co tetrahedral dimers) behave as

low-dimensional AFM systems. Transport measurements reveal a transition from high-temperature

metallic to low-temperature semi-conducting states that could occur from defect shallow donor upon

the Mn for Co substitution.

& 2012 Elsevier Inc. All rights reserved.

1. Introduction

The well-known perovskite structure, of ideal formula ABO3,can be built from two extreme close-packing types of AO3 layerswith the B cations in the octahedral holes: (i) the cubic stacking(ABC) sequence, with all BO6 octahedra sharing corners within theccc (¼3C) framework, where c refers to the cubic layers, e.g.,BaTiO3 [1]. (ii) the hexagonal (AB) sequence with octahedral Bcations sharing faces into infinite one-dimensional chains withinthe hh (¼2H) hexagonal polytype, e.g., BaCoO3 [2]. Between thesetwo extreme forms, a multitude of intermediate hexagonalstructures with sizeable proportions of c and h layers can beformed [3]. For a review, see [4]. In the past decade, we haveinvestigated a series of modular halogeno-cobaltites in which theslicing between individual hexagonal-perovskite blocks isachieved by anion deficient hexagonal-[BaOX1�x] (X¼F, Cl) layers[5–10]. At both sides of this central layers terminal, tetrahedra arecreated. In the Co-oxo-halides, the elementary blocks containeither trimeric (n¼3) or tetrameric (n¼4) linear strings of n face-sharing octahedra, similarly to what was reported for tetrameric12H and trimeric 5H–BaCoO3�d oxides [11,12], while all other n

ll rights reserved.

entre!).

values could be expected, taking into account the filiations of thesecompounds with the parent one-dimensional 2H–BaCoO3 compound(n¼p). We have recently reported that the stabilization of one or theother polytype is related to the mean metal oxidation number [13]:i.e., 6H–Ba6Co6X1�xO16�d (X¼F, Cl) shows a mean Co valence of�þ3.25 (X¼F) and �þ3.37 (X¼Cl) while the trimeric 10H–Ba5Co5X1�xO13�d are formed on heating in a reduction-process andcould be stabilized at room temperature by quenching, leading to aCo valence of �þ3.20 (X¼F) and �þ3.24 (X¼Cl). The here-reportedoxidation numbers were deduced from both neutron diffraction (ND)data and redox titration in good agreement. Similar trends wereconfirmed after incorporating Fe cations in the structure ofBa5Co5�yFeyX1�xO13�d.[13]. It was verified that the mixed (Co, Fe)distribution solely stabilizes the ‘‘most-reduced’’ trimeric phaseswhile clearly Fe plays as a reducing agent as the mean (Fe/Co)valence progressively lowers down to þ3.13 in 10H–Ba5Fe5

Cl1�xO13�d. The Fe incorporation is also accompanied by drasticmagnetic and electric effects that emerge from the interplay betweenhigh-spin (HS) Fe species and low-spin (LS) Co species in the linearchains. In the present work, our goal is to probe similar effects afterincorporating mixed (Co/Mn) sites in the parent oxo-halide com-pounds. Indeed, we note from the literature the existence of thetrimeric 5H–BaMn0.2Co0.8O2.80 [12] (with (Mn,Co)þ3.6), while theincrease of the Mn/Co ratio leads to the preparation of the tetrameric12H–BaMn0.4Co0.6O2.83 (with (Mn,Co)þ3.66) [11]. Once more the

M. Iorgulescu et al. / Journal of Solid State Chemistry 198 (2013) 210–217 211

stabilization of more-reduced trimeric compounds versus more-oxidized tetrameric ones seems respected. The potentiality to estab-lish of a generalized model available for oxo-halides as well as foroxides or related materials deserves attention. In this work weprepared and analyzed the mixed metallic Co/Mn series for bothX¼F and Cl, and pay a special attention to the observed structuraltype in correlation with the metal valence. The full structural(including cationic distribution and oxygen vacancies localization),magnetic and electric characterization will be discussed.

Fig. 1. (a) Phase diagram for Mn-doped Co-oxo-halides. The 10H phase shown by

red dots, 6H by blue dots and 2H–BaMnO3 (noted 2H) by green dots. Single -

phases are denoted as ‘‘s.p.’’, biphasic mixtures are represented by multicolor dots.

For Co–10H and 6H compounds (y¼0), q¼quenched and s.c¼slowly cooled;

(b) 6H-structural type with atoms labeling; (c) 10H–structural type, (d) 2H

structural type.

2. Experimental procedures

As a general statement, the synthesis of mixed Ba6

Co6�yMnyX1�xO16�d compounds is rather difficult and not wellreproducible compared to Co-only and mixed Co/Fe oxo-halides.At best, it implies high temperature treatments and multiple re-grinding steps, starting from BaCO3 (Sigma-Aldrich, 99%), Co3O4

(Alfa Aesar, 99.7%), MnO (Sigma-Aldrich, 99%) and BaCl22H2O(Prolabo, 99%) or BaF2 (Aldrich, 99.99%). The mixtures are heatedat 700 1C for 12 h and quenched to room temperature, thenpressed into pellets, heated at 1000 1C for 72 h and finallyquenched again to room temperature. The powders are thenmixed with 1/10 M excess of BaCl2 �2H2O (or BaF2 for oxo-fluorides) and heated again at 1000 1C for 48 h, with anotherquenching to room temperature. The obtained black powders arewashed with boiling water and filtered to eliminate the BaCl2/BaF2 excess. The products were obtained as dark powders andcharacterized by powder X-ray diffraction (XRD) using a D8Bruker diffractometer, CuKa radiation. The mean oxidation statesof Co/Mn were determined by iodometric titration as detailed in[13]. For y¼2(X¼Cl) (the only sample obtained as a single-phasein sufficient quantity), we benefited of neutron diffraction data atroom temperature (high resolution 3T2 diffractometer,l¼1.225 A, Laboratoire Leon Brillouin, Saclay, France). The struc-tural refinement was performed using FullProf program [14]. Thehigh temperature XRD data were also performed using an AntonPaar XRK900 chamber. The magnetic susceptibility was measuredusing a quantum-Design SQUID magnetometer. The electricalproperties were measured by the four points method, from RTto 7 K using a Helium-free ARS cryostat.

3. Difficulties and limits

The phase diagram obtained for Co–Mn binary substitution isgiven in Fig. 1(a). According to our results taking single phasematerials as the ultimate criterion, it highlights a poor Co/Mnmiscibility for oxochlorides, even worst for oxofluorides. As aresult of importance for both X¼Cl and F, even if most of the timea mixed phase with secondary phases is obtained, the majorphases correspond mainly to the 6H-compounds (see Fig. 1(b))rather than the 10H (see Fig. 1(c)). According to the preceptsdeveloped above, it intuitively suggests an oxidizing effect of Mnions after incorporation in this structural type. This was com-forted by our redox titrations performed for single-materials y¼2and y¼4 which indicate a constant but high (Co/Mn) valence allalong the 6H–Ba6Co6�yMnyClO16�d line compatible with the 6Hpredominance. At this stage, the poor Mn/Co miscibility in the6H-crystal structure compared to the previously described Fe/Cobinary system is questionable. It is even worst in the X¼F case forwhich only one single-phase compound (y¼1.2) was obtained.The great stability of the 2H–BaMnIVO3 (Fig. 1(d)) often found as asecondary phase seems to represent an important obstacle. In factthe recent literature gives two examples of mixed Ba–Mn/Coperovskites for which high temperature treatments (T41100 1C)

appear necessary, i.e., 12H–BaMn0.4Co0.6O3�d [11] and 5H–BaMn0.2Co0.8O2.83 [12]. However, due to the presence of halides,the temperature was here limited to 1000 1C while severalheating/grinding steps assorted with various cooling types.Quenching to room temperature was finally preferred. TheFig. 2(a) shows the RT-XRD patterns of most representativeprepared compounds. We note the difficulty to distinguish theparasitic 2H–BaMnO3 XRD lines since they are all included in thetypical XRD-pattern of the 6H-phase. Here we often used the ratiobetween intensities and/or quantitative Rietveld analyses toconclude. As no continuity on the solid-solution lines seemsachieved after our syntheses, the results of the XRD will bedetailed term by term.

4. Results and discussion:

4.1. Ba6Co6�yMnyClO16�d series

a)

Term y¼1 (X¼Cl): Ba6Co5Mn1ClO16�d displays a XRD patternwith a splitting of all diffraction lines with half–half intensities(see zoom on two diffraction lines in Fig. 2(b). The distortion ofthe cell from hexagonal to orthorhombic is rather commonand consists of an elongation of one parameter within the (a,b) plane such that the trigonality is lost, (see 10H–BaFeO2.80

[15] with the orthorhombic Cmcm space group). We have longhesitated about this possibility due to the correct agreementobtained after pattern-matching of the XRD pattern(RBragg¼12.5%, RF¼8.32%, space group Ammm, a¼14.55(8) A,b¼5.69(5) A, c¼9.88(5) A, a¼b¼g¼901). However, latter high

y = 3

y = 0

y = 2

y = 1

y = 4

Inte

nsity

(arb

. uni

ts)

10 20 30 40 50 60

y = 0

y = 3

y = 2y = 1

y = 4

(104)

(110)

2θ(°)

2θ(°)

29 30 31 32 33

6H

6H + 6H’6H SP

6H + 2H

6H SP

Fig. 2. (a) XRD patterns of Ba6Co6�yMnyClO16�d. The 2H–BaMnO3 as impurity for

y¼3 can be observed by the difference in intensity of the (1 0 4) line compared to

the others; (b) Zoom between 291 and 331 2y (on (1 0 4) and (1 1 0) characteristic

diffractions peaks).

M. Iorgulescu et al. / Journal of Solid State Chemistry 198 (2013) 210–217212

resolution XRD experiments at low angles have also revealedsplit (0 0 1) lines. This fact refutes the orthorhombic deforma-tion or any other lower symmetry. We thus retained a‘‘mixture model’’ as the most plausible, in good agreementwith the poor Co/Mn miscibility met in the full Co/Mncomposition range. The refined lattice parameters for thetwo concomitant hexagonal are:a1¼5.69(4) A, c1¼14.53(1) A and a2¼5.67(6) A andc2¼14.54(3) A. It involves a different Co/Mn repartition intwo related phases. The high temperature behaviour of thissample is of interest and also gives clues for the veracity of the‘‘two-phases mixture’’ model. It is shown on Fig. 3 and clearlypictures several distinct interesting phenomena:

i) From RT to 700 1C, the splitting seems to be accentuatedleading to a broadest multiplet structure at 700 1C(Fig. 3(a) and (b)).

ii) Above 700 1C, we observe a coalescence of the two com-ponents leading to a single phase materials at hightemperature whose lattice parameters have been success-fully refined through a pattern matching procedure at900 1C leading to parameters a¼5.791(3) A andc¼14.749(4) A (Fig. 3(c)).

iii) A reversible phase separation is observed on cooling(Fig. 3(d) and (e)). To explain this behaviour we suggestthat thermodynamics plays against the stabilization ofsingle phase materials at this particular Co/Mn ratio, suchthat the relative ratio of each metal should be consideredas a crucial parameter to stabilize a single 6H-material.

b)

Terms y41: The term y¼2(Mn) of the solid solution wasobtained only once as a single phase, but in large amount�6 g—sufficient for ND experiment. Its structural study willbe described later. The term y¼3 was always obtained in a

biphasic mixture with 2H–BaMnO3 [16–18] (�5–10%) whiley¼4 (Mn) was systematically prepared as a single phase insmall amounts. Larger Mn ratio does not stabilize the 6H-formbut long strings of face-sharing octahedral predominate (e.g.,2H–BaMnO3). We recall that in this area we recently isolatedthe compound Ba8Mn6Co2ClO22�d with oligomers consistingof six faces sharing octahedral [10].

c)

Lattice parameters evolution: The cell parameters of the 6H–Mn/Co oxo-chloride phase (present solely or within a mixture)were refined from XRD data. It shows slight variation of thecell volume along with the Co/Mn substitution (DaE0.1 A andDcE0.15 A) (Fig. 4) reflecting the small difference betweenthe ionic radii of Co3/4þ and Mn3/4þ in octahedral coordination[19].

4.2. Ba6Co6�yMnyFO16�d case

The oxo-fluoride Co/Mn solid solution is even worst regardingthe Co/Mn miscibility. Only a 6H-single-phase was successfullyobtained for Ba6Co5Mn1FO16�d (y¼1) with parametersa¼5.680(2) A, c¼14.300(1) A, Fig. 5. For y¼3, a triphasic mixtureof two 6H phases (mixed Co/Mn) together with the 2H–BaMnO3.For the y¼4, a biphasic mixture of a 6H phase together with 2H–BaMnO3 was also observed while for y¼5 term, a mixture of 2H–BaMnO3 and Ba6Mn5O16 [20] was observed. Then, we concludethat the Mn/Co segregation in the coexisting phases is predomi-nant. Here, one could propose that according to its large ionicradius, Cl� is more easily ‘‘templating’’ the 6H-form that F� . Thesecompounds have not been considered for further investigations.

4.3. Structure of y¼2 and y¼4 (X¼Cl case)

a)

Cationic distribution: In order to investigate the Co/Mn rela-tive distribution in the three crystallographic positions offeredby the 6H-structural type, we studied the compoundBa6Mn2Co4ClO16�d (only one obtained as single phase in largequantity) by ND. The Fig. 6 shows the Rietveld refinement fit.The refinement was conducted with mixed refined Co/Mnsites, but keeping constant the total occupancy. The atomiccoordinates and anisotropic displacement parameters aregiven in Table 1.It results that the Mn cations preferentially occupy the face-sharing octahedra while tetrahedra are solely occupied by Cocations. Co-filled tetrahedra are consistent with the preferredtetrahedral environment found for MnV ions in perovskitesderivatives, e.g., Ba7Mn5Ca2O19 [21], Ba8Mn6Ca2O22 [22] orBa6Ru2Mn2Na2O17 [23] (Fig. 7(a)–(c). In the case of mixedM3þ /4þ valence, the tetrahedral preference for Co3þ wasvalidated also by DFT calculations performed on a similarcompound, Ba8Mn6Co2ClO22�d [10].Inside the tetramers we note that the particular occupancies ofthe marginal M1 site together with the relative Co and MnFermi lengths (b) (63% Co, bCo¼�0.373/ 37% Mn, bMn¼0.25)lead to difficulties in the refinement of the z coordinate due tothe almost null contribution of this site in the structure factors,i.e., ai[Co]xbCoþai[Mn]xbMnE0 (ai¼site occupation). Then,the value z(M1)¼0.7531 was fixed to the value found in theparent Ba6Co6ClO16�d compound [6]. The refinement leads tothe conclusion that Mn prefers the central M2 positions(65%Mn/35%Co). Similar behaviour has been reported in thecase of the oxide compound 12H–Ba6Co3.6Mn2.4O16.98 [11](71%Mn in this site comparatively to 33%Mn in the marginaloctahedra). A contrasted behaviour was reported for the 5H-crystal structure (trimeric) of 5H–Ba5Co4Mn1O14 [12] whereall trimeric sites roughly host a statistic Co/Mn mixture as

32

100

T (C)

900

31

31 32

900

100

T (C)

31 32

31 32

900

100

T (C)

Heating

2θ (°)

2θ (°)

2θ (°)

2θ (°)

100T (C

)

900

Cooling

900C

Fig. 3. HT-XRD of Ba6Mn1Co5ClO16�d upon heating (a) evidence of the materials at high temperature. (b) same picture with DT¼100 1C. (c) Pattern matching at 900 1C

using a single 6H-phase. ((d), and (e)): reversible cooling back to a biphasic mixture.

0 1 2 3 4 5 65.0

5.5

6.0

y

a (Å

)

14.5

15.0

c (Å)

x=Cl

2H –B

aMnO

3 (major phase)

+2H-BaMnO36H + 6H’

Fig. 4. lattice parameters versus x for the Ba6Co6�yMnyClO16�d solid solution.

11 20 30 40 50 60

y = 0

y = 1y = 3

y = 4

y = 5

2H -BaMnO3

Ba6Mn5O16

24 26 28 30 32

y = 0y = 1y = 3y = 4y = 5

2� (°)

2� (°)

Fig. 5. (a) Ba6Co6�yMnyFO16�d XRD patterns; in black-terms for y¼1 to 5; in blue

Ba6Co6ClO16�d calculated XRD pattern; (b) zoom between 24 and 321 2y showing

the difficulty to separate 6H-phases and 2H–BaMnO3.

M. Iorgulescu et al. / Journal of Solid State Chemistry 198 (2013) 210–217 213

detailled on the Fig. 8(a)–(c). By comparison, one should recallthat in the case of the 10H–Ba5(Co,Fe)5XO13�d compounds, thecentral trimeric octahedral site is preferentially occupied byCo, while for the marginal octahedral and tetrahedral sites, amixed Co/Fe distribution is observed [13].Finally, regarding the M–M distances and keeping in mind thelow evolution of the c lattice parameters along the Co–Mn line,the central M2–M2 distance increases with the Mn substitu-tion while M2–M1 decreases (Fig. 8(d)). In the tetrahedraldimers, the Co3–Co3 distances remain unchanged in good

agreement with the conservation of its unique Co cationicnature.The crystal structure for y¼4 (X¼Cl) was refined by theRietveld method using XRD data. At this stage, we do not have

FigRie

TabAto

Ba6

A

B

B

B

B

C

C

C

O

O

O

O

C

wer

M. Iorgulescu et al. / Journal of Solid State Chemistry 198 (2013) 210–217214

access to a clear Mn/Co contrast, nor reliable information onthe oxygen vacancies. But for reasons given above, the tetra-hedra can be assumed as fully occupied by Co3þ and a mixed

. 6. (a) XRD Rietveld refinement for y¼4 (RBragg¼7.71%, RF¼5.76%); (b) ND

tveld refinement for y¼2 (RBragg¼3.91%, RF¼2.82%).

le 1mic positions and isotropic displacement parameters for y¼2 a term (from ND Rietveld

Co6�yMnyClO16�d series.

tom Wyck Occ x

a1 1c 1 1/3

a2 2 g 1 0

a3 2i 1 2/3

a4 2 h 1 0

o1/Mn1 2 h 0.635(3)/0.365(3) 1/3

o2/Mn2 2 h 0.346(2)/0.654(2) 1/3

o3/Mn3 2 h 1/0c 2/3

1 6n 0.941(6) 0.012(2)

0.029(8)

2 6n 1 0.1872(5)

0.1717(5)

3 3j 1/3 0.644(1)

0.644

4 3k 1 0.031(1)

0.046(7)

l1 1a 0.942(1) 0

0.850(4)

a Refined formula: Ba6(Mn2.03Co3.96)þ3.37Cl0.942O15.646.b For XRD refinement, some constraints were applied to the thermal parameters (i.e., all

e kept also identical for same chemical natures.c The refinement of Co3/Mn3 leads to nearly full Co occupancy and was then blocked

Figdra

Mn3þ/Mn4þ valence would fill the tetrameric oligomers. Thepertinent distances are listed in Fig. 8(d), and one shouldnotice the excellent matching of intermetallic distances alongthe face-sharing tetramers, in good agreement with similardistances in Ba8Mn6Co2ClO22�d, for which an octahedral Mnagainst tetrahedral Co was validated by GGAþU calculations[10]. Only the Co–Co distances between two corner-sharingtetrahedra are slightly shortened from 3.52(2) A for y¼2(powder-ND results) to 3.42(2) A for y¼4 (Powder-XRDresults).

b)

Oxygen vacancies and charge distribution: In addition to thepartial deficiency refined for the Cl position in the h0-layers,the c–[BaO3] layer also shows extra vacancies (occ. O1¼94%),similarly to what was observed in the 10H–Ba5(Co,Fe)5X1�xO13�d. The refinement of oxygen occupancy in h–[BaO3] layers leads to fully occupied sites (Table 1). Thereforethis behaviour contradicts the preferred vacancies in face-sharing h-layers deduced from the accurate ND analysis of the

refinement) and for y¼4 b term (from XRD Rietveld refinement, in italic) of the

y z U(A2)

2/3 0 0.0144(7)

0.0135(2)

0 0.7963(2) 0.0038(6)

0.8003(1) 0.0135(2)

1/3 0.3532(2) 0.0087(9)

0.3521(5) 0.0135(2)

0 0.5 0.0111(9)

0.0135(2)

2/3 0.75318 0.0228(7)

0.7581(5) 0.0099(1)

2/3 0.5848(8) 0.0030(7)

0.5844(1) 0.0099(1)

1/3 0.8792(5) 0.0010(8)

0.8819(5) 0.0213(4)

�0.494(1) 0.8307(1) 0.0083(1)

�0.485(5) 0.8341(1) 0.0098(6)

�0.6255(5) 2/3 0.0083(1)

�0.6564(4) 0.0098(6)

0.356(1) 0 0.0083(1)

0.356 0.0098(6)

�0.484(1) 1/2 0.013(2)

�0.477(7) 0.0098(6)

0 0 0.0216(9)

0.0098(6)

U(Ba) were kept identical, all U (anions) were kept identical and all U(M sites)

to 100% Co.

. 7. Crystal structure of perovskite-based-compounds with Mn5þ in tetrahe-

l environment from Ref. [21–23].

Fig. 8. (a)–(c) Cationic distribution for some Co–Mn mixed compounds: (a) 12H–BaCo0.6Mn0.4O2.83 from Ref. [11](b) 5H–BaCo0.8Mn0.2O2.8 from Ref.

[12](c) Ba6Co4Mn2Cl0.942O15.646 (i.e., BaCo0.66Mn0.33Cl0.157O2.607) from this work. Metal–metal and metal–oxygen distances for y¼0 from Ref. ([6],y¼2 and y¼4 are

given in (d).

M. Iorgulescu et al. / Journal of Solid State Chemistry 198 (2013) 210–217 215

several BaMnO3�d polytypes. In our case, we must thenconclude that �6% of the Co3þO4 tetrahedra are oxygendeficient in their basal planes. It leads to the formulaBa6Co3.96Mn2.03Cl0.942O15.646 associated to the mean (Co/Mn)þ3.37 oxidation degree, in perfect agreement with ouriodometric titration (þ3.37(1)). An ideal partially orderedmodel Ba6(Co3þ

1.26/Mn4þ0.74(Oh marginal)) (Co3þ

0.7 /Mn4þ1.3(Oh central))

Co3þ2(Td)Cl0.942O15.646 could account from the several features

listed below:i) The tetrahedral Co3þ state in its high-spin (HS) state was

unambiguously assigned from first principle GGAþU cal-culations in this structural type [24].

ii) A preference for tetravalent Mn species in competitionwith Co can be proposed (from the presence of 2H–BaMnIVO3 at the Mn-rich side).

iii) The increase of M2–M2 distance after Mn incorporationdistances would likely result from a greatest Mn4þ–Mn4þ

(65%Mn occupied) electrostatic repulsion in the centraloctahedral site

iv) The shortening of the Co–Co distances compared to theparent Ba6Co6ClO16�d would involve smaller LS Co3þ

species rather than Co4þ .

v) A similar charge distribution was proposed on pure oxide12H–BaMn0.4Co0.6O2.83 [11].

Then difficulties of preparation could occur from a Co3þ/Mn4þ

charge segregation. Clearly this obstacle vanishes forBa6Mn4Co2ClO16�d which systematically led to single-phasematerials (when prepared in small amount, �0.5 g). Here, takinginto account the fully occupied tetrahedral sites by HS Co3þ andthe titrated (Co/Mn)þ3.38 mean valence, one should deduce aMnþ3.57 mixed valence in the face-sharing Mn4O15 tetramers.This Mn4þ/Mn3þ

�1 ratio could improve the stabilization of thisparticular composition.

4.4. Transport properties:

The resistivity evolution with the temperature forBa6Mn2Co4ClO16�d (y¼2) and Ba6Mn4Co2ClO16�d (y¼4) areshown in Fig. 9(a). It is rather different than for the semi-conducting behaviour systematically found on parent trimericand tetrameric oxo-halides [9] and on trimeric Fe- doped com-pounds, Ba5(Co, Fe)5XO13�d.[13] Both compounds exhibit a tran-sition from high temperature metallic to low temperature

50 100 150 200 250 300-2

0

2

4

6lo

g ρ

(Ω*c

m)

T (K)

0.08 0.12 0.16 0.20

0

2

4

6

log

ρ (Ω

*cm

)

T-1/2 (K-1/2)

y = 0

y = 4y = 2

Fig. 9. Resistivity evolution (a) with T, (b) with T�1/2 for y¼0, 2and 4 terms of the

Ba6Co6�yMnyCl1�xO16�d series.

Fig. 10. (a) Ba6Co2Mn4ClO16�d magnetic susceptibility and inverse of magnetic

susceptibility at 0.2 T. In insert, magnetization at 2 K; (b) wm, wpara and wm�wpara

versus the temperature. In insert, zoom between 0 and 100 K.

M. Iorgulescu et al. / Journal of Solid State Chemistry 198 (2013) 210–217216

semiconducting behaviours. This aspect is not well understood sofar but is strongly reminiscent of the creation of shallow donorsdue to doping-defects or non-stoichiometry. For instance in TiO2,the oxygen non-stoechiometry is responsible for such behaviourleading to a donor band overlapped with the conduction band.This thermally populated band is responsible for the two distinctbehaviours versus temperature [25]. The BaMnO3�d polytypeswith mixed Mn3þ/Mn4þ states in variously sized face sharingoligomers being insulator, one should assign this behaviour to theeffect of Co-dopants. At least, about the Mn and Co chargedistribution discussed above, it is clear that the Co3þ/Mn4þ

distribution in the face-sharing tetramers for y¼2 plays in favourof the greater resistivity compared to the y¼4 term where Co3/4þ

would play the role of defects in a mixed valent Mn3/4þ matrix. Atlow temperature, the systems turn back to the standard insulat-ing state. The 1D-VRH (Variable Range Hopping) model driven bylinear subunits already proposed for similar compounds [8,9]seems predominant (Fig. 9(b)), as shown by the good linearity oflog(r) vs. T�1/2.

4.5. Magnetic properties

The magnetic susceptibility wm(T) estimated from M/H(T) tomagnetization M(H) at 2 K plots are shown in Fig. 10(a), forBa6Co2Mn4ClO16�d. They are typical of an AFM system with no longrange ordering. We recall the Mn preference for octahedral sites suchthat for this precise composition, Mn4O15 tetramers are expected,separated by Co2O7 dimers. The magnetic susceptibility (Fig. 10(a))curves show a typical paramagnetic behaviour, with a broad anomalyaround 10 K. In the high temperature domain, the Curie–Weiss lawgives meff¼12.21 mB/f.u. (-4.58 mB/metal) and yCW¼�556 K. Thevalue of the effective moment is in good agreement with theproposed ideal Co3þ(S¼3/2) (Td), Mn4þ(S¼3/2) (Oh), Mn3þ(S¼2)(Oh) distribution, with the expected meff¼11.22 mB/f.u., in a spin-onlyapproximation. We recall that the Co3þ valence of tetrahedral

positions in this structural type is supported by previous DFTcalculations [24]. The deviation from the Curie–Weiss law occursaround 150 K which could correspond to the AFM setting inside thecorner-sharing Co3þ tetrahedral dimers. At low temperature, asecond paramagnetic regime seems to dominate w(T) plots. Between2 and 8 K, it was fitted from the corresponding data-points leading tomeff¼2.66 mB/f.u. which corresponds to less than one Mn3þ(S¼3/2)(-3.87 mB) remaining ‘‘free’’ at low temperature. It is too large to beassigned to ‘‘residual’’ intrinsic paramagnetism (edges of the crystals,defects, etcy). The wm�wpara plot is shown in Fig. 10(b) (zoom in theinsert). Its shape is reminiscent of low dimensional AFM systems,possibly hold by Co2O7 tetrahedral dimers. The bordering-Mn wouldalso participate leaving the central Mn ‘‘uncoupled’’ and responsiblefor the important low-T paramagnetic contribution. This hypothesisremains an assumption and others scenario may happen as suggestedby related compounds presents in the literature. For example, for 5H–BaMn0.2Co0.8O2.8 (trimer) [12], a model of random anisotropic non-interacting monodomain ferromagnetic clusters was proposed. Therandomness in anisotropy is introduced by the Mn ions and preventsthe 3D long-range ferromagnetic order, achieved in the parentcompound BaCoO2.8 at �47 K [25]. The magnetic behaviour wasconsidered to be intermediate between the spin glass systemobserved for 12H–BaMn0.4Co0.6O2.83 [11] and the ferromagneticTo47 K ordered BaCoO2.8 [26]. Clearly, whatever the mechanismsare in our compounds, the Mn/Co co-presence seems to break the 3Dmagnetic ordering.

5. Conclusion

Here, we present new results regarding the the Ba–M–O–X

(M¼Co/Mn) chemical systems, leading to the stabilization of only6H tetrameric structural type. The general conclusions are listed

M. Iorgulescu et al. / Journal of Solid State Chemistry 198 (2013) 210–217 217

hereafter. We note the difficulty and the poor reproducibility ofthe syntheses of the mixed Co/Mn substituted compounds invol-ving low thermodynamic stabilities and bad Co/Mn miscibilitywithin the 6H-phase. The structural investigations by neutrondiffraction refinement for Ba6Mn2Co4ClO16�d displays the pre-ference of Mn for central faces sharing octahedra (0.35Co/0.65Mn), while in the marginal octahedra, a 0.63Co/0.37Mncationic distribution was observed. The tetrahedra are occupiedonly by Co. Then difficulties to prepare these compounds as asingle phase could occur from the Co3þ/Mn4þ charge segregation.The electrical investigations of two terms show a transition fromhigh temperature metallic to low temperature semiconductingbehaviour, strongly reminiscent of the creation of shallow donorsdue to doping-defects or non-stoichiometry. The magnetic sus-ceptibility of Ba6Mn4Co2ClO16�d shows a low dimensional AFMsystem, possibly driven by Co–Co dimers. The bordering-Mn alsoparticipates, but would leave the central Mn ‘‘uncoupled’’ andresponsible for an important low-temperature paramagneticcontribution. The main picture behind the mixed Co/Mn oxoha-lide perovskite series is that the competition between trimericand tetrameric face-sharing octahedral oligomers is dominated by6H-tetrameric compounds in good agreement with the Mn‘‘oxidizing’’ effect in these compounds. This result is to oppose towhat was previously observed on ‘‘reduced’’ Co/Fe compoundsthat adopt the trimeric 10H-form and show a lower Co/Fe meanoxidation state.

Acknowledgments

The 00Fonds Europe!en de De!veloppement Re!gional (FEDER)-00,00CNRS00, 00Re!gion Nord Pas-de-Calais00, and 00Ministe!re de l’En-seignement Supe!rieur et de la Recherche00 are acknowledged forfundings for the X-ray diffractometers. The authors particularlythank the ANR (Agence Nationale de la Recherche) for financialsupport of the ‘‘projet-blanc’’ MAD-BLAST at the origin of this

work. The LLB (Florence Porcher) is thanked for her contributionin PND data collection.

References

[1] R.H. Buttner, E.N. Maslen, Acta Crystallogr. B 48 (1992) 764.[2] H. Taguchi, Y. Takeda, F. Kanamaru, M. Shimida, M. Koizume, Acta Crystallogr.

B. 33 (1977) 1299.[3] J. Darriet, A.M. Subramanyam, Chem. Mater. 5 (1995) 543.[4] R.H. Mitchell, Perovskites Modern and Ancient, Almaz Press, Ontario, Thunder

Bay, 2002 318.[5] K. Yamaura, R.J. Cava, SSC 115 (2000) 301.[6] N. Tancret, P. Roussel, F. Abraham, Solid State Chem. 177 (2004) 1023.[7] G. Ehora, C. Renard, S. Daviero-Minaud, O. Mentre, Chem. Mater. 19 (2007)

2924.[8] M. Kauffmann, O. Mentre, A. Legris, S. Hebert, A. Pautrat, P. Roussel, Chem.

Mater. 20 (2008) 1741.[9] O. Mentre, M. Kauffmann, G. Ehora, S. Daviero-Minaud, F. Abraham,

P. Roussel, SSS 10 (2008) 471.[10] M. Iorgulescu, H. Kabbour, N. Tancret, O. Mentre, P. Roussel, Chem. Commun.

46 (2010) 5271.[11] L. Miranda, A. Feteira, D.C. Sinclair, M.G. Hernandez, K. Boulahya,

M. Hernando, A. Varela, J.M. Gonzalez-Calbet, M. Parras, Chem. Mater. 20(2008) 2818.

[12] L. Miranda, J. Ramirez-Castellanos, A. Varela, J. Gonzalez- Calbet, M. Parras,M. Hernando, M.T. Fernandez-Diaz, M.Garcia Hernandez, Chem. Mater. 19(2007) 1503.

[13] M. Iorgulescu, P. Roussel, N. Tancret, N. Renault, F. Porcher, G. Andre,H. Kabbour, O. Mentre, Inorg. Chem. 51 (2012) 7433.

[14] J. Rodriguez-Carvajal, J. Phys. B 192 (1993) 55.[15] J.L. Delattre, A.M. Stacy, T. Siegrist, J. Solid State Chem. 177 (2004) 928.[16] A. Hardy, Acta Crystallogr. 15 (0) (1962) 179.[17] B.L. Chamberland, A.W. Sleight, J.F. Weiher, J. Solid State Chem. 1 (1970) 506.[18] T. Negas, R.S. Roth, J. Solid State Chem. 3 (1971) 323.[19] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751.[20] K. Boulahya, M. Parras, J.M. Gonzalez Calbet, J.L. Martinez, Chem. Mater. 14

(2002) 4006.[21] N. Floros, C. Michel, M. Hervieu, B. Raveau, J. Solid State Chem. 168 (2002) 11.[22] N. Floros, C. Michel, M. Hervieu, B. Raveau, Chem. Mater. 12 (2000) 3197.[23] E. Quarez, F. Abraham, O. Mentre, J. Solid State Chem. 176 (2003) 137.[24] O. Mentre, H. Kabbour, G. Ehora, S. Daviero-Minaud, G. Tricot, M. Whangbo,

JACS 132 (13) (2010) 4865.[25] K. Bapna, R.J. Choudhary, D.M. Phase, Mat. Res. Bull. 47 (2012) 2001.[26] K. Boulahya, M. Parras, J.M. Gonzalez-Calbet, U. Amador, J.L. Martinez,

V. Tissen, M.T. Fernandez-Diaz, Phys. Rev. B 71 (2005) 144402.