Gamma-radiation and isotopic effect on the critical behavior in triglycine selenate crystals

Rod&.

Pergamon

Phys. Chem. Vol. 44, No. 5, pp. 527-530, 1994 Copyright 0 1994 ElsevierScienceLtd

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GAMMA-RADIATION AND ISOTOPIC EFFECT ON THE CRITICAL BEHAVIOR IN TRIGLYCINE SELENATE CRYSTALS M. E. KASSEM,* A. E. HAMED, L. ABULNASR~ and S. ABBOUDY Physics Department, Faculty of Science, Alexandria University, Alexandria, Egypt (Received 25 March 1993; accepted 22 July 1993)

Abstract-Isotopic effects in pure and y-irradiated triglycine selenate crystals were investigated using the specific heat (C,) technique. The obtained results showed an interesting dependence of the critical behavior of C,, on the deuterium content. With increasing content of deuterium, the character of the phase transition changed from a second order (A-type) to a first order transition. After y-irradiation, the behavior of C, around the phase transition region was essentially affected. The transition temperature, T,, decreased and AC, depressed, and the transition hecame broad. It was noted that the effect of y-irradiation is opposite to the isotopic effect.

INTRODUCTION

portant information about the radiation damage in the crystal. The aim of the present work is to study the influence of the y-irradiation on the critical behavior of specific heat properties of DTGSe crystals deuterated by different amounts of deuterium (22, 50 and 83%). The isotopic effect is also considered.

It is well known (Strukov et al., 1978) that yirradiation resuits in essential changes of the physical properties of the hydrogen bonded ferroelectrics. Dielectric and thermal properties of deuterated triglycine selenate crystals (DTGSe) have been investigated by many authors (Gesi, 1976; Gridnev, 1987; Stankowska and Hamed, 1988a, b; Hamed et al., 1990, 1991). Their studies revealed that the phase transition changed to first-order after full deuteration of the crystals. Since a first-order phase transition in a defective crystal takes place by the movement of a phase boundary through a system of a randomly located obstacles (Gridnev, 1987); the manifestation of the specific heat critical behavior must depend on various external interactions, which alter the state of the lattice and the defect structure of the crystal. Strukov et al. (1989) have studied the influence of y-irradiation on the phase transition in triglycine selenate (TGSe) crystals. The results obtained demonstrated a change in the nature of the phase transition in crystals as a result of irradiation. Using dielectric measurements, the influence of y-irradiation in DTGSe crystals has been studied by Stankowska and Hamed (1988a, b). They found that the character of phase transition changed from first to second-order after irradiation. To the best of our knowledge, the specific heat properties of y-irradiated DTGSe crystals have not been investigated yet, although this would give im-

EXPERIMENTAL DTGSe was prepared by recrystallizing TGSe crystals from a 99.75% solution of heavy water (D20). The crystallization was repeated three times, each from a fresh portion of heavy water. Single crystals were obtained from each of these portions by a slow evaporation method (Hamed 1990). The amounts of deuterium in the crystals were estimated from the transition temperature, T,, of the samples as previously described (Hamed et al., 1990, 1991). The specific heat was measured in the temperature range from 285 to 3 13 K using a Perkin-Elmer model DSC-4 Differential Scanning Calorimeter. The melting of indium was used to calibrate the DSC-4 in terms of temperature and heat of fusion. Sample weights were chosen to be between 16 and 18 mg. All measurements were carried out at a constant heating rate of 3 K/min. The DTGSe crystals were irradiated by y-rays with a dose of 1 MR. The source was @‘Co and the dose rate was 300 R/min.

RESULTS AND

Specific heat (C,) of three different crystals of DTGSe each having a different content of deuterium (22, 50 and 83%) was measured as a function of

*Present address: Physics Department, Faculty of Science, Qatar University, P.O. Box 2713, Doha, Qatar. tAuthor to whom all correspondence should be addressed. RFC 4‘liSF

DISCUSSION

527

M. E. KASSEMet al.

528 temperature T, to clarify the critical around the transition point T,. Figure 1 presents the temperature excess specific heat (AC,) where:

behavior

of C,

dependence

of

and C, is the specific heat below zero polarization (in the paraelectric region T > T,). From Fig. l(a), it is clear that AC, decreases with an increase of deuterium content, while the transition temperature r, increases. This is in fair agreement with the reported data on dielectric and thermal properties (Gesi, 1976; Gridnev et al., 1987; Stankowska and Hamed, 1988a, b; Hamed et al., 1990, 1991). The behavior of C, at T, for DTGSe crystal deuterated by 22% is clearly of the L-type, or having a second order phase transition similar to the case of undeuterated TGSe crystal (Ema et al., 1977). By increasing the deuterium content, the broad peak of C, became narrower and there was a tendency towards a first order character of the phase transition. For a DTGSe crystal deuterated by 83% deuterium, the anomaly of C, was sharp and completely different from that obtained for DTGSe, and DTGSe, crystals. This indicates that the character of phase transition in such a case starts to be first order which confirms the previously reported data (Hamed et al., 1990, 1991). It is shown in Fig. l(b) that the behavior of C, is essentially affected after y-irradiation. The peak of C,, became broad and the phase transition became

60

c

t

19

21

23

25

r’. ?\

25

2-l

29

31

27

29

31

DTGSe3

33

35

37

39

T WI

Fig. 2. Excess specific heat, AC,, as a function of temperature, or DTGSe, (22%) and DTGSe, (83%) crystals before and after y-irradiation.

smeary. This could be attributed to the presence of the internal bias electric field formed during y-irradiation process (Stankowska et al., 1988b). To clarify the effect of y-irradiation on C, in each case, AC, was plotted as a function of temperature in Fig. 2 for DTGSe, and DTGSe,. It is clear that AC, after y-irradiation is depressed while T, is shifted towards lower temperature as shown in Table 1. The downward shift observed in T, is due to the reduction in the number of the ferroelectrically active dipoles occurring after the y-irradiation process (Pawlowski, 1978). The contribution of the non-interacting defects (small doses of irradiation used in the experiment) into free energy is additive (Strukov et al., 1980). The specific heat defects-stipulated part can be obtained by the usual expression,

40

AC,=

20 2 .$ s k

(1)

where

0

18

20

22

24

26

28

30

32

34

32

34

J (b)

.

60 I

1 MR

DTGSe,

o DTGSe2 1 MR A DTGSe3 1 MR

18

20

22

24

26

28

30

T (“C)

Fig. 1. (a) Excess specific heat, AC,, as a function of temperature, for DTGSe,, DTGSe, and DTGSe, crystals before y-irradiation. (b) Excess specific heat, AC,, as a function of temperature, for DTGSe,, DTGSe, and DTGSe, crystals after y-irradiation by dose of 1 MR.

r = (T - T,)/T,, N = defect concentration, value at the spherical nucleus with a radius d, and A,, = constant. To compare equation (1) with experimental results, the anomalous part of C,(AC,) at T > T, was plotted against r -I as illustrated in Figs 3(a), (b) and (c) for DTGSe, , DTGSe, and DTGSe,, respectively, before and after irradiation. The linear parts of the curves and the tilts increase when the defects concentration goes up (Strukov et al., 1980). As shown from Figs l(b) and 2; after P, = polarization

Table I. Transition temperature T, for DTGSe, and DTGSe, crystals before and after y-irradiatinn

Crystal

OMR

IMR

DTGSe, (22%) DTGSe, (50%) DTGSe, (83%)

23.7”C 26.2”C 3lS”C

22°C 24.7”C 29.75”C

Gamma-radiation

and isotopic effect in TGSe

529 AC, = Ar -‘,

where A is a constant, and the critical exponent (a) can be estimated from Fig. 4. The slope of the straight line obtained by plotting In AC, against In T in the lower temperature region T < T, [Fig. 4(a)] gave c( = 0.3, 0.4 and 0.54 for DTGSe, DTGSe, and DTGSe,, respectively. These values are larger than expected. The expected ones using the 3d Ising model, where u = 0.1 (Abello et al., 1985). After y-irradiation, the critical exponent a was found to be smaller as shown in Fig. 4(b). For DTGSe,; c( = 0.1 while for DTGSe,; c( = 0.3. The temperature dependence of C,, near a secondorder phase transition obeys an exponential law (Burtseva et al., 1988).

DTGSe, .OMR Y 1 MR

20

0

40

60

80

100

120

140

(b) 50 40

c

(2)

/

-

DTGSe,

AC, = Z (g)exp(z)

.OMR

(3)

x 1 MR

20

0

I

I

I

I

I

I

40

60

80

100

120

140

-

40

-

160

DTGSex

Cc) 50

1

l OMR x 1 MR _ ..-

30 20

-

I i ,$_x.

_/!I

__--a

*-

*.

,** a

__x_-_-x---x.

where N is the number of atoms displaced from the equilibrium position, U is the activation energy, R is the universal gas constant and Z is the coordination number (the number of neighbors of each atom). The dependence of ln(AC,. T*) on l/T is plotted for DTGSe, and DTGSe, crystals, (Fig. 5). The experimental data showed approximately straight lines, with the exception of temperature in the vicinity of T,. The slope clearly increases with deuterium content, while it decreases after y-irradiation in both cases.

10

0

I

I

I

I

I

l

20

40

60

80

100

120

1 140

l

(a)

160 4

t-1

Fig. 3. (a) Excess specific heat, ACP, as a function for DTGSe, T > T,. (b) for DTGSe, T > T,. (c) for DTGSe,

(22%) Excess (50%) Excess (83%)

crystal specific crystal specific

before and heat, AC,, before and heat, AC,,

after as a after as a

of T-‘, y-irradiation at function of 5 -‘, y-irradiation at function of c -I,

3

2

1

crystal before and after y-irradiation at T> T,.

1 I -7

-8

y-irradiation, the effect of these defects was apparent in all crystals (DTGSe, , DTGSe, and DTGSe,) as a broadening in the transition, a decrease in AC, and a downward shift in T,. However, it is also noted from Figs 3(a), (b) and (c) that the difference in the slopes of the linear parts of the curves, before and after y-irradiation, decreases with an increase of deuterium content and becomes nearly the same in the case of DTGSe,. This means that the critical behavior of C, in the case of DTGSe, and DTGSe, is quite different from the case of DTGSe,, which confirms the conclusion that the phase transition character in the DTGSe, crystal is changed to be a first order transition. Assuming that the excess specific heat AC, can be expressed by (Abello et al., 1985):

I

I

I

-6

-5

-4

DTGSel

0

(b)

x OMR:a=0.3

5

0

t

lMR:a=O.l

4

3

2

1

A

OMR:a=0.4

0

lMR:a=0.3

t

-8

-7

-6

-5

-4

In t Fig. 4. (a) Variation of (In AC,) against In T for DTGSe,, DTGSe, and DTGSe, crystals before y-irradiation. (b) Variation of (In AC,) against In T for DTGSe, and DTGSe, crystals before and after y-irradiation.

530

M. E.

KASSEMer al.

Professor E. F. El-Wahidy the head of Physics Department, Faculty of Science, Alexandria University, Egypt, for their fruitful

discussion

and support.

REFERENCES

DTGSy

Abello L.. Chhor K. and Pommier C. (1985) Thermodynamic studies on successive phase transition in LiKSo, crystals at low temperature. J. Chem. Thermodyn. 17, 1023-1034. Burtseva V. P., Vasilev V. E. and Varikash V. M. (1988) Phase transition in mixed [(NH,), _rHX]2 SO, crystals. Sov. Phys. Solid State 30, 8777878. Ema K., Hamono K., Kurihara K. and Hatta I., (1977) A. C. calorometric investigations of specific heat anamoly in ferroelectric TGSe. J. ?hys. Sot. Jpn. 43, 19541961: Gesi K. (1976) Dielectric study on the phase transition in triglycine selenate-deutrated- triglycine selenate system.

’

J. Phys. Sot. Jpn. 41, 565-569. (104/T)

K-’

Fig. 5. Dependence of In(AC,T2) against on 104/T for DTGSe, and DTGSe, crystals before and after y-irradiation.

This could be an indication that the activation energy increases as deuterium content increases and decreases with ~-irradiation. From the above analysis it is worth noting that:

(1) With an increase

(2)

of deuterium content. , T. increased, AC, decreased, activation energy increased, c( increased and the phase transition character changed to be first-order. After y-irradiation, Z’, decreased, AC, depressed, the activation energy decreased, c( decreased and the phase transition became smeary. L

It can be concluded that the effect of y-irradiation is in the opposite direction from that of the isotopic effect (deutetium substitution). Acknowledgements-The authors would like to thank Professor J. Stankowski, IFM, PAN, Poznan, Poland and

Gridnev S. A., Popov V. M. and Shuvalov L. A. (1987) Manifestation features of the low frequency mechanism of dielectrics loss in DTGSe crystals. Sou. Phys. Solid Stale 29, 1147-t 150. Hamed A. E., Kassem M. E., Kandil S. H. and El-Samahy A. E. (1990) On the phase transition of DTGSe single crystals. Phase Transition 110, 119-127. Hamed A. E., Kassem M. E., El-Osairy M. and Okas A. M. (1991) Specific heat and electrical conductivity of deutrated triglycine selenate single crystals. Phase Transition. 29. 219-225.

Pawlowski ‘A. (1978) Critical phenomena in y-irradiated TGS. Acta. Univ. Wratislav. 338, 119-123. Stankowska J. and Hamed A. E. (1988a) On the character of the phase transition of DTGSe crystals. Acta Phys. Pal. A74, 1, 3743. Stankowska J. and Hamed A. E. (1988b) The influence of y-irradiation on the character of the phase transition of DTGSe crystals. Ferroelecrrics 81, 103-106. Strukov B. A., Taraskin S. A., Meleshima V. A., Belgina N. V. and Yurin V. A. (1978) Thermal properties, y radiation defects and inhomogeneity of TGS crystals. Ferroelectrics 22, 727-728.

Strukov B. A., Taraskin S. A., Minaeva K. and Fedorikhim V. (1980) Critical phenomena in perfect and imperfect TGS crystals. Ferroelectrics 25, 399402. Strukov B. A., Taraskin S. A., Fedorikhim V. and Minaeva K. (1989) Effect of defects and impurities upon the phase transition in TGS crystals. J. Phys. Sot. Jpn. 49, 7-9.