On the kinetics of polymorphic transitions in highly deuterated NH4HSeO4 crystals

Solid State Communications, Vol. 64, No. 6, pp. 957-960, 1987. Printed in Great Britain.

0038-1098/87 $3.00 + .00 © 1987 Pergamon Journals Ltd.

ON T H E KINETICS O F P O L Y M O R P H I C T R A N S I T I O N S IN H I G H L Y D E U T E R A T E D NHaHSeO 4 CRYSTALS H. Pykacz Institute of Physics, Technical University of Wroclaw, 50-370 Wroclaw, Poland and Z. Czapla Institute of Experimental Physics, Wroclaw University, 50-205 Wroclaw, Poland

(Received 24 May 1987 by G.S. Zhdanov) The temporal and temperature variation of the dielectric constant has been measured in ND4DSeO4 with over 90 mole percent of deuterium content. The temperatue of thermal equilibrium of P212121 and /2 phases is 322 K and the transition between phases/2 ~ I1 occurs at 274.4 K. Below 322K P212121 is a stable phase while the /2 and I1 phases are metastable. INTRODUCTION NH4HSeO 4 CRYSTALS W E A K L Y D E U T E R A T E D (molar fraction of deuterium below 40%), just as non-deuterated, belong to monoclinic system o f / 2 symmetry and on cooling transform to ferroelectric I1 phase at T,.. Tc increases with deuterium content and is 250 K for 0% D. Highly deuterated crystals (above 50% D) have the P212~ 21 symmetry which on cooling does not change [1]. In these crystals on heating an irreversible phase transition to I2 phase was observed at about 340 K [2-4]. Recently, it was shown [5] that the/2 phase of the crystals with over 90% D is metastable at room temperature and the crystals return to P212121 phase after about 60h. Thus, two polymorphic/2 and P212121 phases coexist temporarily. Longtime instability of N M R spectrum [6, 7] and dielectric constant [8] below 271 K observed for non-deuterated crystals recently [9] has been attributed to transformation to P212121 phase. Also it was shown that the thermal equilibrium temperature of P212~21-/2 phases increases with deuterium content. Aim of the Communication is the study of kinetics of phase transition in highly deuterated, D ~> 90%, crystals on the basis of temporal and temperature variation of the dielectric constant.

perature are shown in Fig. 1. At room temperature, RT, sb for a virgin sample is 13 and is nearly constant on cooling to 270 K. Low value of Sb denotes P21212~ symmetry of crystal. On heating slight increase of Sb begins at 330 K (as it is seen from the segment of the curve with magnified scale) and high increase occurs in the range of 340-343 K. High value Of Sb(200) at 345 K means that the crystals has /2 symmetry [2, 3]. On cooling Sb diminishes to the temperature of 277 K and then increases abruptly at the transition to I1 phase, being maximal at Tc = 274.4 K. Above T, dielectric constant on heating, s~, is significantly larger than on cooling, ~ , and the difference ebh--S~decreases with temperature. At RT eb amounts to 13 or 76 or about 130 as it is seen in Fig. 1. To explain the differences in value of eb additional measurements were performed. After heating (to 350 K) - - cooling cycle sb was measured as a function of time, t, at the constant tem-

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RESULTS A N D DISCUSSION

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Dielectric constant along b axis, Sb, was measured at 1 kHz with 5 kV m 1field applied. Temperature was changed with the rate of 4.35 10-3 K s-i in the vicinity of the phase transition and 2 or 3 times larger far from the transition. Results of measurements of sb vs tem-

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Fig. 1. Temperature dependence of dielectric constant along the b axis. Arrows indicate temperature run.

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T R A N S I T I O N S IN H I G H L Y D E U T E R A T E D NH4HSeO4 CRYSTALS

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Vol. 64, No. 6

immediately repeated heating process (e~). The value of (e~b--e~)/S~increases from zero at RT to 4% at 310 K. 60 This result confirms the existence o f / 2 phase on cool295 K 8b o ing from 340 K. After heating-cooling cycle the crys40 o tal was kept at RT for 4 days in one case, 12 h in the o other one and heating-cooling cycles were repeated. o 20 o o o .~o \~ The results shows (Fig. 3) that P2~ 2~2~ and 12 phases occur in the crystal on heating while only 12 phase I I I I ; 2 6 6 7 8 occurs on cooling. It is also seen that transformation (x24h) process P212121 ~ /2 on heating proceeds at lower Fig. 2. Temporal changes of dielectric constant. temperature when the fraction o f / 2 phase in crystal is Dashed line represents dielectric constant of P2: 2] 2t larger. Domains with /2 phase become nucleation phase. centres for the new 12 phase. To find the temperature of thermal equilibrium perature of 295 K. The results are shown in Fig. 2. eb between P2t 212t a n d / 2 phases relaxation processes of diminishes from 76 to 13 during over 8 days (shape of eb were measured at several temperatures. After heatcurve eb(t) and transition time depend on the quality ing to 350K and then cooling to 340.5 K temporal of the crystal). It means that the transformation from changes of eb were measured, eb becomes constant with /2 to P2t 2t 2: at RT proceeds very slowly. The trans- time and equal to 165 (Fig. 4). It means that the 12 formation process 12 ~ P2] 212] at RT proceeds dis- phase is stable at 340.5 K. For the virgin crystal with tinctly faster after several transformations of the crys- P2~212j phase, after heating to 337K, time depental at 345 K. It can last 3 days as is marked by a cross dence of eb was measured (Fig. 5). Continuous in Fig. 2, or even less. Spontaneous deformation and increase of eb up to 153 indicates that at 337K the strain microcracks accompany the phase transforma- transformation from P2: 2~21 t o / 2 phase occurs. At tion P2: 2~2~ --*/2 at 340 K. After several transforma- 337 K /2 phase is stable and eb (153) is significantly tions the crystal becomes opaque. These microcracks lower than at 345 K. A virgin crystal with P2~2~2~ accelerate the / 2 - + P2~212~ process and affect the phase was heated to 320 K, its temperature was stabivalue of eb most evidently around To. lized and two measurements of eb were performed at Time of cooling from 345 K to RT is about 1 h. In At = 0.5h interval. The result of temporal changes this time decrease of eb is about 2% at RT in a nearly re(At) - e(O)]/e(O)At was zero. Measurements were perfect crystal (Fig. 2). We conclude, that eb(T ) curve repeated increasing temperature every 2 K and the on cooling in the range 345-295 K describes 12 phase, following results were obtained: 0 (322 K), and the fraction of P212:2] phase is not larger than 1.3.10-2h 1(324K),4.10 2 h - 1 ( 3 2 6 K ) , 1 0 . 1 0 - 2 h -l 2%, otherwise than in [5]. The solid line in Fig: 3 (328 K), 32" 10-2h -l (330 K). Results of the measureshows temperature dependence of eb for the virgin ments at 330 K are shown in Fig. 5. They indicate that sample on heating to 345 K, cooling to RT (at) and not only at 330 but also at 324 K the P2m2~2 t phase is unstable and the transformation to the 12 phase proF., b /. ceeds slowly. When the measurements presented in 200 Fig. 5 at 330 K were completed the crystal was imme80

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Vol. 64, No. 6 160 eb

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TRANSITIONS IN H I G H L Y D E U T E R A T E D NH4HSeO4 CRYSTALS

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Fig. 5. Changes o f e b with time performed at 330 K are marked by open circles and that at 337K by full circles.

Fig. 7. The crystals were in I1 phase for 1 h (full circles) and for 16 h (open circles), next were heated to 283 K and temporal changes of eb were measured.

diately cooled to 320 K its temperature was stabilized and temporal changes of eb were recorded. The results are shown in Fig. 4. Diminishing ofeb with time means that both phases P212] 2, and 12 exist in the crystal and the latter is unstable. Thermal equilibrium of P2~ 2] 2] and 12 phases in the investigated crystal occurs at t e m p e r a t u r e Ttr = 322 ___ 2 K. Below Tt, /2 phase is metastable, however, relaxation time at 295 and at 283 K (measured but not shown) lasts several days. The phase transition f r o m / 2 to 11 occurs at temperature T, = 274.4K (Fig. 1). Value of/~max at Tc depends on the quality of crystals and usually it is in the interval 140-216. Below 274 K crystal is in P2] 212, or in I1 phase (Fig. 1). To check which phase is stable temporal measurement of eb was performed at 272 K for two samples with different degree of defects (different Cmax) after previous transformation at 345K. Results show (Fig. 6) that I1 phase is metastable, however, transition time to P212121 phase is long enough. On heating ~b is considerable higher than on cooling e~ at T > T~. Inequality e~ > ~ in the range 280-295 K suggest the coexistence of/2 and I1 phases. Temporal measurements of/~b at 283 K were performed for the crystal which previously had been in I1 phase at 270 K for 1 and 16 h (Fig. 7). eb diminishes

with time much faster than for the crystal with /2 phase at the same temperature 283 K. Therefore, we infer that on heating at temperatures above 280 K three phases I1, 12 and P212]2] temporarily coexist and the transformation I1 ~ / 2 is faster than between /2 ~ P2] 212] phases. It is known for non-and weakly deuterated crystals that spontaneous polarization, Ps, appears along b axis below T,. To prove the existence of P~ the hysteresis loop by Sawayer-Tower technique was performed on cooling and on heating in the temperature range 290-270 K. Ps is too low to be measured, however, from the shape of the loop we conclude that Ps is different from zero above Tc on heating. These results confirmed the existence of I1 phase above 280 K on heating.

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In ND4DSeO 4 crystals even three polymorphic phases can coexist temporarily. The temperature of thermal equilibrium of P2t212i a n d / 2 phases, I]r, is 322 K. Kinetics of phase transformation P2] 2t 2] --*/2 increase with T-Ttr. Strain microcracks occur during the phase transformation P2] 212] --* /2 at 340K. Kinetics of the phase transformation /2 -* P2] 212~ increases with the number of defects. For a nearly perfect crystal the transformation occurs for several days at room temperature.

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CONCLUSIONS

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Acknowledgement-- The work was supported by INT i BS PAN from the central programme "Structure, phase transition and properties of molecular system and condensed phases".

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REFERENCES 0

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Fig. 6. Temporal changes of eb performed for two different samples.

1. 2.

Z. Czapla, O. Czupifiski & L. Sobczyk, Solid State Commun. 40, 929 (1981). Z. Czapla, O. Czupifiski & L. Sobczyk, Solid State Commun. 51, 309 (1984).

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TRANSITIONS IN HIGHLY DEUTERATED NH4HSeO4 CRYSTALS H. Pykacz, J. Mr6z, Z. Czapla & O. Czupiriski, Acta Phys. Polonica A67, 727 (1985). A.A., Sukhovsky, Yu. N. Moskvich, O.V. Rozanov & I.P. Aleksandrova, Ferroelectrics Left. 3, 45 (1984). H. Pykacz & Z. Czapla, Ferroelectrics Lett. (in press). I.P. Aleksandrova, Yu. N. Moskvich, O.V. Rozanov, A.F. Sadreev & A.A. Sukhovsky,

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