Ferroelectric-paraelectric transition in lithium trihydrogen selenite

Solid State Communications Vol. 4 pp.653-655, 1966.

Pergamon Press Ltd. Printed in Great Britain

F E R R O E L E C T R I C - P A R A E L E C T R I C TRANSITION IN LITHIUM TRIHYDROGEN SELENITE * G. A. S a m a r a and D.H. Anderson Sandia L a b o r a t o r y , Albuquerque, New Mexico 87115 (Received 28 October 1966 by H. Suhl)

A ferroelectric-to-paraelectric transition has been observed in LiH3 (SeO3)= under hydrostatic pressure. At 12.5 kbar the Curie point is 720 C, and the dielectricconstant in the paraelectric phase obeys a Curie-Weiss law with C = 4.42 x 1~4 0 -u KandTo = 6 9 . 5 0 C.

F E R R O E L E C T R I C I T Y in lithium trihydrogen selenite, LIH3 (SeO,)~ , was first reported by Pepinsky and V e d a m " in 1959. The crystal is monoclinic with the ferroelectric axis close to the [40~[] direction. R is ferroelectric at room temperature with a large spontaneous polarization of ~15 ~C / c m = a n d has the somewhat unusual property that it remains ferroelectric up to the reeking point, 110 o C. Both small-field dielectric constant and specific heat measurements over the temperature range - 180 to 110 ° C failed to reveal any transitions below the meRing point.~ W e recently examined the effect of hydrostatic pressure on the dielectric properties of the related (but not tsomorphous) ferroelectric NaDa (SeO,)= . The Curie point of the sample used was -14.50 C, = and R decreased with pressure at a rate of 3.3 o C/kbar." This result along with the fact that the melting point of most solids is raised with increasing pressure suggested that the Curie point of LiHs (SeOs)=, if it exists at all, m a y be able to observe a ferroelectric-to-paraelectric transition at high pressure.

0. 67 x 0. 42 x 0. 11 cm cut with the s h o r t e s t dimension along the polar axis. The capacitance and dielectric loss were m e a s u r e d at 10 k c / s using a t r a n s f o r m e r r a t i o a r m bridge with an a c c u r a c y of • 0. 01%. Figure 1 shows the changes of the dielectric constant, e, and dielectric loss, tan ~, with t e m p e r a t u r e dependence of c is in good a g r e e ment with e a r l i e r r e s u l t s . ~ Above 75 o C tan 6 i n c r e a s e s v e r y rapidly with i n c r e a s i n g t e m p e r ature and so does cabove 90°C. This behavior is believed to be caused by the large i n c r e a s e in sample conductivity as conditions approach the melting point. At 12.5 kbar the r e s u l t s show the behavior expected at a f e r r o e l e c t r i c - t o - p a r a e l e c t r i c transition. The dielectric constant r e a c h e s a peak value > 1 0 4 at a Curie point of 72°C above which it d e c r e a s e s monotonically with i n c r e a s i n g t e m p e r a t u r e . A s h a r p peak in tan 8 was also exhibited at the transition. No t h e r m a l h y s t e r e s i s was observed in the transition t e m p e r a t u r e on heating and cooling. In the p a r a e l e c t r i c phase ¢ obeys the usual C u r i e - W e i s s law

W e have measured the temperature dependence of the small field dielectric constant of LiHs (SeOs)=, under a hydrostatic pressure of 12.5 kbar. The measurements were made in a 30-kbar p r e s s u r e apparatus using a 50-50 mixture of n-pentane and isopentane as the p r e s s u r e fluid. The apparatus and techniques have been d e s c r i b e d e l s e w h e r e .4 The sample was a single c r y s t a l ~ This work was supported by the U.S. Atomic E n e r g y Commission.

¢= C / ( T - T o ) .

(1)

Figure 2 shows a Curie plot for the data in Fig. I. At 1 2 . 5 k b a r C = 4 . 4 2 x 1 0 4 °K and To = 69.5 ° C. In the f e r r o e l e c t r i c phase ¢ also obeys equation (I) quite well over a 20°C range below the Curie point with C = - 1.17 x 104 o K

653

654

F E R R O E L E C T R I C - P A R A E L E C T R I C TRANSITION

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Vol. 4, No. 12

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T e m p e r a t u r e dependences of the dielectric constant and loss tan 6 of lithium trihydrogen selenite at both I bar and 12.5 kbar. and T o = ?2.5° C.

We have repeated the m e a s u r e m e n t s at other p r e s s u r e s ( > 9 kbar ), and the r e s u l t s show that the Curie point of LiHs (SeO3) d e c r e a s e s linearly with i n c r e a s i n g p r e s s u r e at a rate of ~ 6 o C / k b a r . Extrapolating the r e s u l t s back to lower p r e s s u r e s indicates that the c r y s t a l would have a Curie point of ~ 145°C at a t m o s p h e r i c p r e s s u r e if it were not for melting before r e a c h trig this t e m p e r a t u r e . It is interesting to note here that the shift of the Curie point of this compound with p r e s s u r e is quite comparable to (_~ - 5.5 ° C/kbar).

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It is not possible, on the basis of the present evidence, to state unequivocally whether the transition is f i r s t or second order. The shape of the dielectric constant v s . t e m p e r a t u r e curve and the lack of a t h e r m a l h y s t e r e s i s at the t r a n s ition suggest that the transition in LiHa (SeO3)2 might be second o r d e r . The r a t i o of the slopes of the inverse susceptibility vs. t e m p e r a t u r e plots

T e m p e r a t u r e dependence of the r e c i p r o c a l of the dielectric constant of lithium trihydrogen selenite at 12.5 kbar. below and above the Curie point is - 3.8:1. This is much l a r g e r than the - 2:1 r a t i o predicted (to a f i r s t o r d e r approximation) for a second order transition by the phenomenological theory of f e r r o e l e c t r i c i t y . 6 If the transition is indeed second o r d e r , then the d i s c r e p a n c y can be a s s o c i a t e d with a large adiabatic c o r r e c t ion t e r m for this material. 6 The C u r i e - W e i s s t e m p e r a t u r e , T o , is about 2.5 ° C lower than the transition t e m p e r a t u r e . This is usually c h a r a c t e r i s t i c of a f i r s t o r d e r transition. For a second o r d e r transition the two t e m p e r a t u r e s coincide. In the p a r a e l e c t r i c phase the C u r i e - W e i s s constant of LiH3 (SeO3)~ ( C = 4.42 x 104 o K) is intermediate between those of the o r d e r - d i s order type f e r r o e l e c t r i c s such as Rochelle salt and KH= PO4 (C _~ 10 a o K) and the displacive type f e r r o e l e c t r i c s such as BaTiOs (C _~ 106 o K). .Acknowledgment - The authors acknowledge the technical a s s i s t a n c e of W.L. Chrisman.

Vol. 4, No. 12

FERROELECTRIC

- PARAELECTRIC

TRANSITION

655

References I. P E P I N S K Y R. and V E D A M

K., Phys. Rev. 114, 1217 (1959).

2.

B L I N C R. and V O V K D. Imve recently reported (Phys. Letters 19, 117 (1965)) on the variation of the Curie point of partially deuterated NaI-I~ (SeO3)2. m

3.

SAMARA G.A. (to be published)

4.

SAMARA G . A . J .

5.

SAMARA G.A. Bull. Am. Cer. Soc. 44, 638 (1965); Phys. Rev. (in press).

6.

JONA E and SHIRANE G., F e r r o e l e c t r i c Crystals, pp. 15 and 38 Macmillan, New York. (1962).

Phys. Chem. Solids 26_, 121 (1965).

Ein f e r r o e l e k t r t s c h - p a r a e l e k t r i s c h e r Ubergang ist unter hydrostatischem Druck in Li H3 (Se O~ ) ~ beobachtet worden. Bei 12,5 l~bar betr~gt der Curie°Punkt 72CC, und die dielektrtsche Konstant in der p a r a e l e k t r i s c h e n Phase gehorcht einem C u r i e - W e i s s - G e s e t z , wobei C = 4,42 x 104 °K, und To = 69, 5 o C.