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SOME THERMODYNAMICAL PROPERTIES OF KDP-DKDP MIXED CRYSTALS
B. Strukov, A. Baddur, V. Koptsik
To cite this version:
B. Strukov, A. Baddur, V. Koptsik. SOME THERMODYNAMICAL PROPERTIES OF KDP- DKDP MIXED CRYSTALS. Journal de Physique Colloques, 1972, 33 (C2), pp.C2-155-C2-157.
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JOURNAL DE PHYSIQUE Colloque C2, supple'ment au no 4, Tome 33, Avril 1972, page C2-155
SOME THERMODYNAMICAL PROPERTIES OF KDP-DKDP MIXED CRYSTALS
B. A. STRUKOV, A. BADDUR and V. A. KOPTSIK Department of Physics, Moscow State University, Moscow, USSR
Rksumk. - Nous Btudions la chaleur specifique, la permittivitk et la polarisation spontank de milieux mixtes KDP-DKDP (KH2(1-x)D2zP04). NOUS trouvons que la tempQature de transi- tion depend d'une maniere non linkire de la concentration de deutkrium. La transition de phase du premier ordre des cristaux DKDP se deplace en fonction de la concentration en deutkrium vers la transition pratiquement continue du KDP. La constante de Curie et la variation d'anisotropie a la transition de phase sont d'une maniere appreciable dans les cristaux haute concentration en deuterium.
Abstract. - The specific heat, dielectric permeability and spontaneous polarization of the mixed KDP-DKDP (KH2( I - ~ ) D ~ ~ P O ~ ) crystals have been studied. The deuterium concentration dependence of the transition temperature is found to be nonlinear. The first order phase transition in DKDP crystals shifts to the nearly continuous one in KDP while deuterium concentration changes. The Curie constant and surplace entropy of the phase transition increase appreciably in crystals with high deuterium concentration.
The shift of the Curie temperature and some thermodynamical properties as a function of deute- rium content in KDP-DKDP mixed crystals were considered in [I], [2] ; the isotope effect was regarded as a result of changing of the tunneling motion in partially deuterated crystals. In the number of expe- rimental works the thermodynamical properties of pure KDP and DKDP with very high deuterium concentration were investigated. There is only one paper where high frequency dielectric properties of the KDP-DKDP mixed crystals were considered [3].
In this paper the result of experimental investigation of the electrical and thermal properties of KDP- DKDP mixed crystals are presented. The special notice was given to the nearest vicinity of the ferro- electric Curie points.
The real deuterium concentration in crystals (grown from starting solution with x = 0.18, 0.30, 0.40, 0.70, 0,85 and 0,96) was found out by determination of the density of water obtained by burning of the crystals in an oxygen stream (cr drop )) method) [4]. Accuracy of this method was estimated as
+
1.5 %. It was discovered that the deuterium concentration in crystals was appreciably smaller than in solutions : 0.09, 0.17, 0.29, 0.45, 0.70, 0.73, correspondingly.We took transition temperatures from dielectric constant and specific heat measurements and obtained T,(x) dependence which is shown in figure 1. I t is clear that the dependence is nonlinear ; the experi- mental points deviate from the stright line to higher values of T,. Kaminov's data [4] are plotted in figure too. Author of [4] considered his data to fit linear dependence better (stright line at Fig. 1).
FIG. 1. - The dependence of the transition temperature on the deuterium concentration in KDP-DKDP mixed crystals.
0 : our data, : data taken from [3].
The dielectric permeability as a function of tempe- rature for all crystals is shown in figure 2. The curves were obtained under condition of accurate temperature stabilization in critical regions ; changing of the deute- rium content leads to some variation of E,(T) depen- dence. There is decreasing of the contribution to the dielectric constant connected usually with dynamic properties of a domain structure of the crystals while
G x x) increase. At the same time the Curie constant
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972254
C2-156 B. A. STRUKOV, A. BADDUR AND V. A. KOPTSIK
FIG. 2. - The temperature dependence of dielectric constant ec in KDP-DKDP mixed crystals. 1 - x = 0, 2 - x = 0.09, 3 - x = 0 . 1 7 , 4 - x = 0 . 2 9 , 5 - x = 0 . 4 5 , 6 - ~ = 0 . 7 3 .
It was found that P,(T) dependence is essentially different for the crystals with high and low deuterium concentration. A polarization jump in the pure KDP and in the partially deuterated crystals is order of 1 pC/cm2 and in DKDP - near 4.3 pC/cm2, so one could expect the latent heat in DKDP nearly 20 times more than in KDP.
The results of the specific heat measurements are presented in figure 4. We have found that the latent heat is nearly 50 cal/mole deg for DKDP (x = 0.73) and decreases quickly while tr x H becomes smaller.
For ct x >) more than 0.5 one can see the surplace
specific heat in the wide temperature region above T,.
The surplace entropy of the transitions increases when
tt x )> becames larger (Fig. 5). It is interesting to note
that the surplace entropy of the composition x = 0.45
changes nearly linear from 3 200 OK (KDP) to 4 200 OK (DKDP, x = 0.73). The most interesting features can be observed in a critical region (Fig. 3) where data were obtained under both heating and cooling equili- brium conditions. At the KDP side there is small hysteresis of E, at the transition points (near 0.05 OK) and lack of nonequilibrium state when cooling. Never- theless this manifest hysteresis certainly gives evidence of the first order phase transition in crystals under
FIG. 4. -The temperature dependence of the specific heat in the mixed KDP-DKDP crystals. 1 - x = 0 (data taken from [ 5 ] ) , 2 - x = 0.09, 3 - x = 0.29, 4 - x = 0.45,
5 - x = 0.73.
FIG. 3. -The temperature dependence of I/&, in the vicinity of the phase transition in the mixed KDP-DKDP crystals.
1 - X = 0 , 2 - - x = 0 . 0 9 , 3 - x = 0 . 1 7 , 4 - x = 0.29, 5 - x = 0.45 , 6 - x = 0.73.
consideration. Increasing of the deuterium content (more than 50 %) leads to appearance of the nonequi- librium state under cooling too and the phase transi- tions assumes the features of the ordinary first order phase transition.
Spontaneous polarization of the crystals was mea- sured by the electrocaloric method near Curie points.
Extremely accurate measurements were carried out for x = 0 and x = 0.73 deuterium content crystals.
FIG. 5. - The dependence of surplace entropy on the deuterium concentration in the mixed KDP-DKDP crystals. 0 : our data,
@ : data taken from [5], : data taken from [6].
SOME THERMODYNAMICAL PROPERTIES OF KDP-DKDP MIXED CRYSTALS C2-157
is found to be the largest (AS = 1.15 cal/mole deg).
It is hard to know right now if it is accidental.
The increasing of AS while (( x )) increases seems to be understandable as one takes proton tunneling to be a main reason of an isotopic effect in the crystals under consideration. The cr delution )) of the proton sub- system by deuterons leads to decreasing of the degree of disorder in the polar phase and therefore to increas- ing of the surplace entropy of the phase transition [7].
We tried to find some explanation of Tc(x) depen-
dence using results of [I] and [2]. It was found that the molecular field approximation gives the results which are far from experiment ; the appropriate des- cription of the deviation from linearity Tc(x) depen- dence could be obtained in cluster approximatin [I], if one accept 52,/52, near 0.15.
The authors would like to thank Dr. I. A. Velichko for close cooperation and Prof. V. N. Setkina for the help in analysis of deuterium concentration in the crystals.
References
[I] TOKUNAGA (M.), MATSUBARA (T.), Progr. Theor. [5] STEPHENSON (C. C.), HOOLEY (J. G.), J. Amer. Chem.
Phys., 1966, 35, 581. SOC., 1944, 66, 1397.
121 KOPSKY (V.), prepint be published) in Czech. J . [6] REES (W.), phys. Rev., 1969, 181, 905.
Phys., 1970, B 21.
[3] KAMINOV (I. P.), Phys. Rev., 1965, 138, A 1539. [7] SVETINA (S.), BLINC (R.), Proceeding of the Internatio- [4] SHATENSHTEIN (A. I.), Isotopnii analis vodi, Izdatelstvo nal Meeting on Ferroelectricity (Prague, 1966),
Akademii Nauk SSSR, Moscva, 1957. vol. 1, p. 119.
