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Investigations of γ-irradiationeffect on specific heat of(CH3)2NH2Ga(SO4)2·6H2O AND(CH3)2NH2A1(SO4)2·6H2Oferroelectric crystals in the lowtemperature rangeA. U. Sheleg a , T. I. Dekola a & N. P. Tekhanovich aa Institute of Solid State and Semiconductor Physics ,National Academy of Sciences of Belarus , Minsk,BelarusPublished online: 15 Mar 2011.

To cite this article: A. U. Sheleg , T. I. Dekola & N. P. Tekhanovich (2001)Investigations of γ-irradiation effect on specific heat of (CH3)2NH2Ga(SO4)2·6H2OAND (CH3)2NH2A1(SO4)2·6H2O ferroelectric crystals in the low temperature range,Ferroelectrics, 249:1, 207-217, DOI: 10.1080/00150190108225997

To link to this article: http://dx.doi.org/10.1080/00150190108225997

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Ferroelectrics, 2001, Val. 249(3-4). pp. 207-217 Reprints available directly from the publisher Photocopying permitted by license only

0 2001 OPA (Overseas Publishers Association) N.V. Published by license under the

Gordon and Breach Science Publishers imprint. Printed in Malaysia

INVESTIGATIONS OF ?-IRRADIATION EFFECT

AND (CH3)2NH2Al(S04)2*6H20 FERROELECTRIC ON SPECIFIC HEAT OF (CH3)2NH2Ga(S04)2*6H20

CRYSTALS IN THE LOW TEMPERATURE RANGE

A.U. SHELEG*, T.I. DEKOLA and N.P. TEKHANOVICH

Institute of Solid State and Semiconductor Physics of National Academy of Sciences of Belarus, F? Brovki 17, 220072 Minsk, Belarus

(Received April 17,2000; In final form May 29,2000)

The specific heat measurements were made on dimethylammonium aluminium sulphate (DMAAS) and dimethylammonium gallium sulphate (DMAGS) crystals pure in the temperature region from 80 to 300 K and y-irradiated in the region of their phase transitions (PT) by the adiabatic method. It was established, that under y-irradiation the ferroelectric nearly tricritical PT at Tc=152 K is shifted to the second-order PT ever more in DMAAS. It was shown that temperature region of the ferroelectric phase existence between Tc1=135 and Tc2=119 K is narrowed as a result of irradiation in DMAGS. The analysis of the experimental results is discussed in the framework of the Landau theory.

Keywords: Specific heat, phase transition, y-irradiation.

I. INTRODUCTION

Crystals of dimethylammonium gallium sulphate (CH3)2NH2Ga(S04).6H20 (DMAGS) and dimethylammonium aluminium sulphate (CH3)2NH2Al(S04).6H20 (DMAAS), the ones of a new family of ferroelectric-ferroelastic compounds, have been much investigated by various methods during the last ten years. It is known [1,2] that DMAAS crystal at Tc=152 K undergoes the second-order proper ferroelectric phase transition (PT) from the high-temperature ferroelastic phase 2/m to the ferroelectric one m. Below Tc= 152 K the crystal exhibits ferro- electric properties and above this temperature it is ferroelastic up to the decom-

* Corresponding Author: e-mail: sheleg@ifttp.bas-net.by

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208 A.U. SHELEG et al.

position temperature -358 K [2]. Besides, as shown in [3], the second low-temperature Pr, whose nature is still to be explained and the symmetry of the corresponding low-temperature phase has not yet been established, is observed in the vicinity of 75 K. The additional anomalies in these crystals were observed while studying dielectric and pyroelectric properties at 11 1 K [4].

DMAGS crystal is the isomorphous analogue of DMAAS and is ferroelastic at room temperature. With lowering the temperature two Pr’s were detected at T,1=133 K and T,2=113 K while studying dielectric and pyroelectric properties [5]. DMAGS crystal is in the ferroelectric state between the temperatures T,, and T,2, below the Tc2 - in the low-temperature paraelectric phase, and above the T,, - in the paraelectric-ferroelastic phase [6] . The temperature dependent meas- urements of birefringence revealing the PT in DMAGS at T,,= 136 K and Tc2= 123 K were camed out on heating in [7]. In [6,8] the X-ray studies of the DMAGS crystallographic parameters were performed in the region of the low-temperature IT and it was established that with lowering temperature there was an sharp decrease of the lattice cell volume and the hysteresis of the transi- tion temperature was observed (on cooling T,2=104 K and on heating TC2=122 K). It should be noted that PT at TC1=133 K is the second-order transition and at Tc2=113 K - the first-order one as shown in [5], while the authors of [7] believed that both the PT’s belonged to the first-order transitions.

In this paper we present the results of the specific heat investigation of DMAGS and DMAAS crystals in the temperature range from 80 to 300 K and also the influ- ence of y-irradiation on the specific heat in the PT regions is discussed.

11. EXPERIMENTAL RESULTS

The specific heat of both the crystals was measured by using an adiabatic vac- uum calorimeter. The heating rate for the DMAGS samples (m=2.94 g) varied 0.03-0.10 Wmin, and for the DMAAS (m=7.67 g) - 0.04-0.09 Wmin. The tem- perature of the samples was controlled by a platinum resistance thermometer. The specific heat measurements were taken at intervals of 0.6-2.5 K for DMAGS and 0.8-1.8 K for DMAAS with an accuracy of 0.3 %. The samples were y-irradiated at room temperature using y-installation from a Co6’source. The dose rate in the irradiation zone was -300 RJs. The irradiation dose accumu- lated by consecutive expositions in the same sample was 10 and 100 MR for DMAGS and 10,50 and 100 MR for DMAAS.

The temperature dependence of the specific heat for DMAAS crystal in the tem- perature region from 80 to 300 K is illustrated in Figure 1. As it is seen, the h y p e anomaly appears at T,=152 K indicating the second-order PT for a pure crystal.

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y-IRRADIATION EFFECT ON SPECIFIC HEAT OF DMAAS AND DMAGS 209

400

c’

E

oh

Y, 300 :

200

I I I

100 200 300

T, K FIGURE 1 Temperature dependence of specific heat of DMAAS

The results of the specific heat measurements of the unirradiated sample of DMAGS crystal are given in Figure 2. Two clearly defined anomalies are observed with maxima at temperatures TCl=135 K and Tc2=119 K in the temper- ature dependence curve Cp(T). As indicated in the figure, the anomaly at Tcl has the A-shape expected for the second-order PT. The specific heat anomaly at Tc2=119 K represents a sharp symmetric peak. This indicates that PT at T,, is of the fist-order in accordance with papers [5-81.

The specific heat background interpolated with polynomial

Glatt = Ai . Ti is shown by the dashed lines in Figures 1 and 2. The values

of the enthalpy and entropy changes of transition at Tc=152 K in DMAAS, which i=O

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210 A.U. SHELEG et al.

I I I

100 200 300

T, K

FIGURE 2 Temperature dependence of specific heat of DMAGS

are equal to 702 J/mol and 4.8 J/(K-mol), respectively, are determined by inte- grating. The latent heat and the entropy jump at TC2=119 K in DMAGS equal to 615 J/mol and 5.1 J/(K-mol), respectively, and the values of the enthalpy and entropy changes of transition at Tc1=135 K - 341 J/mol and 2.5 J/(K.mol), respectively.

The dependence CJT) for DMAAS irradiated by various doses of y-quanta is shown in Figure 3. As it is seen, the shift of the FT point to the low-temperature region is observed with the increasing dose of irradiation and in this case the shift of the transition is not accompanied by any observable anomaly smearing. Besides the maximum specific heat value decreases distinctly.

The temperature dependence of the specific heat for y-irradiated DMAGS crys- tal is shown in Figure 4. It is seen that the peak value of the specific heat in the

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.).-IRRADIATION EFFECT ON SPECIFIC HEAT OF DMAAS AND DMAGS 211

0 - 1 0 - 2 A - 3 x - 4

300

G

I I

140 160

T, K FIGURE 3 Temperature dependence of specific heat of DMAAS for D=O - 1 , 10 - 2,50 - 3, 100 MR - 4

vicinity of Tc2 decreases and shifts to higher temperatures with the increasing dose of irradiation. The successive shift of the PT point at Tcloccurs to lower temperatures and is followed by decreasing and smearing of the anomaly. Thus, the area of the ferroelectric phase existence is narrowed in DMAGS under y-irra- diation.

111. DISCUSSION

The obtained experimental C,(T) dependences in the ferroelectric phase before and after irradiation can be analyzed by the use of phenomenological theory of

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- 1 0 - 2 x - 3

I I

120 140

T, K

FIGURE 4 Temperature dependence of specific heat of DMAGS for D=O - 1, 10 - 2, 100 MR - 3

ferroelectricity. As known [9], according to the Landau theory the anomalous part of the specific heat in the ferroelectric phase can be given as

It is possible to determine the coefficients of thermodynamic potential for the pure and irradiated samples of DMAAS and DMAGS, using the quantity (AC‘J’ as a function of T,-T for T<Tc (Figures 5, 6). It is apparent that the respective dependences closely resembling the linear ones are consistent with Eq. (1). The values of the Landau expansion coefficients A, B, C in SI of both the crystals for different doses of irradiation are presented in the Table. The val- ues for AT were determined from the dielectric constant measurements made in

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y-IRRADIATION EFFECT ON SPECIFIC HEAT OF DMAAS AND DMAGS 213

I I I

0 2 4 6

Tc-T, K

FIGURE 5 Temperature dependence of (ACJ2 values for doses 1 4 in Figure 3

[ 1, 51, and were considered constant under irradiation. According to the Landau theory for the tricritical point, as is the case in DMAAS, the distinct characteris- tics of the specific heat behaviour in the ferroelectric phase in the vicinity of the transition point obeys the power law with a parameter 0.5 [9]. The analysis of the

experimental data for DMAAS crystal has shown, that the AC,

dependence is performed with a=0.47*0.06. This fact as well as the positive sign of the coefficient B confirms the existence of the second-order phase transition close to tricritical point in DMAAS. The positive magnitude of B for DMAGS also specifies the second-order ferroelectric PT at Tcl =135 K in this crystal. The coefficients of B determining the specific heat value in the IT points of the crys-

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214 A.U. SHELEG et al.

0 2

T,-T, K FIGURE 6 Temperature dependence of (ACC,)-’ values for doses 1-3 in Figure 4

tals increase during irradiation. The increase of the coefficient B indicates the gradual removal of the FT from the tricritical point along the PT curve in DMAAS. The B and C coefficients change in such a way that the value

N = / B2 describing the degree of proximity of the transition to the SATCT, tricritical point increases [9]. This points to the fact that in DMAAS under y-irra- diation the PT character can change from close to the tricritical point to the sec- ond-order FT. A similar effect was found in TGSe and DTGSe crystals previously [ 10,111.

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1-IRRADIATION EFFECT ON SPECIFIC HEAT OF DMAAS AND DMAGS 215

TABLE Thermodynamic parameters of the PT in DMAAS and DMAGS

Crystal D, MR B, J-'mol

DMAAS 0

10

50

100

DMAGS 0

10

100

152.16

151.25

149.85

148.68

135.35

135.21

131.77

0.3 1 x 1 0-8

0.33x10-*

0.38x10-'

0.46x10-'

0 . 7 3 ~

1 . 4 8 ~

3 . 0 7 ~ 1 0 - ~

It is known [12], that the concentration change of the ferroelectrically-active dipoles under y-irradiations could be inferred from the specific heat measurement. The relative change of the excess enthalpy of the PT is connected to that of the number of ferroelectrically-active dipoles NO /NO during the irradiation by a ratio:

1/2 ND AQDTQ nr,=(zzz) '

where Qo and To - the excess enthalpy and the temperature of the IT for the pure crystal, respectively; Q D and TO - the excess enthalpy and the temperature of the PT for the irradiated crystal. The excess enthalpy was calculated as

* = 1 (cexp - clatt)dT' TI and T2 are temperatures beyond which the anom- Ti

alous part of the specific heat can be neglected. The dependences of the relative values of the excess enthalpy and the number of the ferroelectrically-active dipoles on the dose of irradiation are plotted in Figure 7. As indicated in the fig- ure, the sharp decrease in the number of the ferroelectrically-active dipoles in the crystal lattice occurs already at an irradiation dose of 10 MR for both the crys- tals.

The irradiation damages the crystal lattice structure that can cause in a ferroe- lectric crystal a change of structure symmetry which destroys the ferroelectric properties. In our study this manifests itself by the displacement of the PT point to the lower temperatures for DMAAS and by a narrowing of the ferroelectric phase for DMAGS. The suppression of the ferroelecric properties in the consid- ered crystals under y-irradiation (the effect is more intensive for DMAGS as compared to DMAAS (Figure 7)) is brought about by the increase in number of

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0

m - 2 0 - 3 a - 4

50

D, MR 100

FIGURE 7 Dose dependence of the relative excess enthalpy of the FT (1 - DMAAS, 2 - DMAGS) and concentration of the ferroelectrically-active dipoles (3 - DMAAS, 4 - DMAGS)

the destroyed ferroelectrically-active dipoles at the expense of the damage of the chemical bonds responsible for the appearance of these dipoles.

Hence y-irradiation results not only in the decrease of the anomalous part of the specific heat in the PT range, but also in the reduction of the concentration offer- roelectrically-active dipoles, that entails changes of the FT temperatures.

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y-IRRADIATION EFFECT ON SPECIFIC HEAT OF DMAAS AND DMAGS 217

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