Brillouin scattering and crystal optical investigations of (C2H5NH3)2CuCl4

Physica 119B (1983) 269-278 North-Holland Publishing Company

BRILLOUIN SCATTERING O F (C2HsNHs)2CuCI4

AND CRYSTAL OPTICAL INVESTIGATIONS

W . K L E E M A N N a n d F.J. S C H A F E R Fachbereich 10, Universitiit G H Duisburg, Lotharstr. 65, 4100 Duisburg 1, Fed. Rep. Germany

E. K A . R , I i J J ~ M A K I , R. L A I H O a n d T. L E V O L A Wihuri Physical Laboratory, University of Turku, 20500 Turku 50, Finland Received 28 September 1982

The structural phase transitions (SPT) of the layered compound (C2HsNH3}2CuC14 are investigated by Brillouin scattering and by crystal optical methods, which allow to propose a revised phase sequence as follows: D~ (364 K) II (-347 K) D~ (--233 K) IV (39 K) V. It contains three phases (II, IV and V) with lower than orthorhombic symmetry. The newly detected low-temperature SPT are continuous, whereas those above RT are of first order. Anomalous softening and hysteresis of C~ and C22 is found in the vicinity of T~2~ 347 K. At T~3~ 233 K these coefficients show anomalies obeying (T~3- T) ~/2 laws. Contributions due to 2d and 3d magnetic order are observed in the refractive indices around TN = 10.2 K.

1. Introduction

T h e s e q u e n c e of s t r u c t u r a l p h a s e t r a n s i t i o n s observed in layer type compounds (CnH2n+INH3)2MCI4 (n = 1, 2 a n d M = M n 2+, F e 2+, C u 2+, C d 2+) has b e e n e x t e n s i v e l y inv e s t i g a t e d d u r i n g r e c e n t years. T h e s e crystals consist of n e a r l y i s o l a t e d l a y e r s of MCI6 octahedra sharing corners and being tilted backward a n d f o r w a r d to f o r m w h a t is k n o w n as a p u c k e r e d s t r u c t u r e . T h e alkyl a m m o n i u m m o l e cules a r e s i t u a t e d in cavities b e t w e e n t h e octah e d r a a n d a r e l i n k e d to CI ions b y h y d r o g e n b o n d s f r o m t h e i r NH3 g r o u p s [1]. T h e p h a s e t r a n s i t i o n s can b e d e s c r i b e d by an o r d e r - d i s o r d e r m o d e l of t h e o r i e n t a t i o n s a n d t h e r e o r i e n t a t i o n s of t h e (CnH2~+INH3) m o l e c u l e s [2]. D u e to h y d r o g e n b o n d NH3-C1 t i p p i n g of MCI6 octah e d r a will t a k e p l a c e u p o n t r a n s i t i o n t o t h e disordered structure. An interesting situation arises in (CnH2n+INH3)2CuCI4, w h e r e t h e t r a n s i t i o n f r o m t h e h i g h - t e m p e r a t u r e t e t r a g o n a l p h a s e to t h e r o o m - t e m p e r a t u r e o r t h o r h o m b i c s t r u c t u r e has b e e n s u g g e s t e d to p r o c e e d in t w o steps: 17 ~ D4h

18 15 D2h " ~ D2h

[3].

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A d i r e c t s e c o n d - o r d e r t r a n s i t i o n f r o m D4~7 to D~, is n o t a l l o w e d b y t h e L a n d a u t h e o r y b u t it can b e r e a l i z e d , if t h e s t e p D ~ D ~ results f r o m dist o r t i o n s of CUE16 o c t a h e d r a by t h e J a h n - T e l l e r effect. From linear birefringence (LB) m e a s u r e m e n t s a f i r s t - o r d e r t r a n s i t i o n was o b s e r v e d at T e l = 3 6 4 K in E A C u C [4] a n d subs e q u e n t l y a s c r i b e d to t h e ~4hr~17~-r~18,..,2h t r a n s i t i o n . L a t e r on, by using t h e s a m e e x p e r i m e n t a l m e t h o d , T e l l o et al. [5] h a v e o b s e r v e d this t r a n sition at 361 K a n d a n o t h e r a n o m a l y at To2 = 349 K, which t h e y c o n n e c t e d with D~ 8 ~ D~,. Since p h a s e t r a n s i t i o n s a r e k n o w n to b e easily o b s c u r e d by f e r r o e l a s t i c d o m a i n effects w h e n using o p t i c a l t r a n s m i s s i o n m e t h o d s [4], it s e e m e d to b e w o r t h w h i l e to r e p e a t t h e L B m e a s u r e m e n t s on c a r e f u l l y s e l e c t e d s t r a i n - f r e e s a m p l e s a n d to m o n i t o r t h e d e v e l o p m e n t of d o m a i n s u n d e r a m i c r o s c o p e . H e n c e , o n e a i m of t h e p r e s e n t p a p e r was to e l i m i n a t e c o n t r a d i c t i o n s c o n c e r n i n g t h e o r d e r of t h e S P T at Tel [4, 5] a n d to confirm t h e v e r y e x i s t e n c e of t h e S P T at To2 [5]. A p a r t f r o m t h e L B m e t h o d direct m e a s u r e m e n t s of t h e r e f r a c t i v e indices a n d of t h e o r i e n t a t i o n of t h e i n d i c a t r i x s e e m e d to b e useful in o r d e r to o b t a i n m o r e i n f o r m a t i o n on t h e p h a s e II in t h e t e m p e r a t u r e r a n g e T~2 < T < Tc~.

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A n o t h e r t e m p e r a t u r e range of interest lies between 210 and 270 K, where a large LB variation [6] and a diffuse D T A anomaly [7, 5] have been observed. The nature of this so-called thermochroic transition [8, 7] is still poorly understood, thus demanding for alternative investigation methods. A p a r t from refractive index and indicatrix orientation m e a s u r e m e n t s the determination of the elastic properties seemed to be very promising. It has been shown both by ultrasonic m e a s u r e m e n t s in M A M C [9] and by Brillouin scattering investigations in M A M C and E A M C [10] that certain elastic modes soften or change discontinuously upon phase transitions of these crystals. W e report here Brillouin scattering results for longitudinal phonons over a wide t e m p e r a t u r e range in E A C u C . Finally, for the sake of completeness, new results on LB and refractive index anomalies at the three-dimensional (3d) spin-ordering temperature (TN = 10.2 K) which were unresolved in our former experiments [6], will be presented.

2. Experimental procedure Single crystals of E A C u C were prepared from aqueous solutions. These compounds crystallize in the form of platelets with the long edges along [110] and [110]. Prior to m e a s u r e m e n t s the samples (typically 4 × 4 × 0.5 m m 3) were inspected under a polarizing microscope and only twinand strain-free crystals were accepted. E A C u C has a transparent region centering around 600 nm at r o o m t e m p e r a t u r e [6]. Therefore the Brillouin scattering m e a s u r e m e n t s were made by using the 568 nm line of a Kr laser. T o avoid heating the crystal, the laser power was limited to 15 mW. U n d e r these circumstances, only scattering from the longitudinal phonons was strong enough to be detected with sufficient accuracy. The spectrum of the scattered light was analyzed by a three-pass F a b r y - P 6 r o t interferometer, the free spectral range of which was calibrated with an accuracy of 0.1%. The hight e m p e r a t u r e m e a s u r e m e n t s were made with the sample on a hot stage in a sealed oven, which

was designed for light scattering studies at elevated temperatures. The low-temperature investigations were made with the sample fixed on a cold finger in a dewar. The t e m p e r a t u r e of the sample could be stabilized to better than 0.1 K, but it is likely that the t e m p e r a t u r e inside the scattering volume was 1 - 2 K above the temperature of the rest of the crystal. The LB was measured with sodium vapor light at A = 589.3nm with a computer-controlled modulation technique as described previously [4]. The resolution limit of the LB is about l(I 8. T e m p e r a t u r e scans between 5 and 4 0 0 K are possible with a stability of better than 0.05 K at an absolute accuracy of about 0.1 K. Measurements are p e r f o r m e d through a polarizing microscope allowing for a proper selection of single domains and careful in situ orientation. Rotations of the neutral directions due to changes of the index ellipsoid orientation (e.g. in a monoclinic or triclinic phase) are measured by simply placing the sample between crossed polarizers being parallel to the initial neutral axes. In these experiments I measures 02, where 0 is the rotation angle, provided that 0 < 7r/2. Refractive indices are measured at room temperature with an A b b e type refractometer. The t e m p e r a t u r e variation of the in-plane indices (nj and n2) is determined using a Lebedeff type interferometer consisting of two beam-splitting parallel calcite plates in a configuration as described by D o m a n n and Kasten [11]. Using H e N e laser light (632.8 nm) index changes of 6 n ~ 10 ~' were resolvable. However, due to minute changes of the sample position within the interf e r o m e t e r during t e m p e r a t u r e scans, the absolute accuracy is reduced to only about 10 5. Better results should be possible with a microscopically monitored position control, which is planned for future investigations.

3. Brillouin scattering results Starting from the principle of conservation of energy and m o m e n t u m in the inelastic scattering process of light from phonons, the Brillouin frequency shift p can be represented in the form

W. Kleemann et al.

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(1)

w h e r e v is t h e v e l o c i t y of s o u n d a n d A is t h e w a v e l e n g t h of t h e i n c i d e n t light. T h e r e f r a c t i v e indices ni a n d ns a r e d e f i n e d in t h e d i r e c t i o n s of t h e i n c i d e n t a n d s c a t t e r e d lights, which p r o p a g a t e with t h e m u t u a l a n g l e 4'. In this w o r k t h e p h o n o n m o d e s a s s o c i a t e d with t h e C u , C22 a n d C 3 3 elastic coefficients w e r e i n v e s t i g a t e d . B e c a u s e t h e velocity of s o u n d a n d a c o m p o n e n t C , of t h e elastic stiffness t e n s o r a r e r e l a t e d as C, = p v 2 w e o b t a i n in t h e b a c k - s c a t t e r i n g g e o m e t r y , which was e m p l o y e d t h r o u g h o u t t h e p r e s e n t e x p e r i ments, (2)

cii = A 2pt, Z/4n2 .

W h e n c a l c u l a t i n g C , f r o m eq. (2) t h e f o l l o w i n g R T v a l u e s of t h e r e f r a c t i v e indices w e r e u s e d : nl = 1.667 a n d n2 = 1.671 ( i n - p l a n e ) a n d nc = 1.655 ( o u t - o f - p l a n e ) . T h e d e n s i t y of E A C u C is p = 1.70 I71. 3.1. H i g h - t e m p e r a t u r e p h a s e transitions

Fig. 1 s h o w s t h e t e m p e r a t u r e d e p e n d e n c e of

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t h e B r i l l o u i n shifts a s s o c i a t e d with t h e longit u d i n a l p h o n o n s p r o p a g a t i n g in t h e m o s t p r o m i n e n t p l a n e of an E A C u C crystal. In t h e hight e m p e r a t u r e t e t r a g o n a l p h a s e t h e o p t i c a l axis is n o r m a l to this p l a n e . R e f e r r i n g to t h e s y s t e m of r e f e r e n c e axes u s e d by T e l l o et al. [5] we r e l a t e C33, C n a n d Cz2 to t h e m o d e s p r o p a g a t i n g along, t h e r e s p e c t i v e x', y ' a n d z ' axes of t h e s u g g e s t e d o r t h o r h o m b i c r o o m t e m p e r a t u r e p h a s e of t h e crystal. It is f o u n d t h a t t h e B r i l l o u i n shifts corr e s p o n d i n g to C n a n d Cz2 coincide a b o v e 364 K. This a g r e e s with t h e t e t r a g o n a l s t r u c t u r e of the h i g h - t e m p e r a t u r e p h a s e . T h e v a l u e of C~ll = C~2 t u r n s o u t to b e 1.4 × 10 l° N / m 2. A s is e v i d e n t f r o m figs 2, 3 a n d 4 t h e influence of t h e t r a n s i t i o n at Tcj = 364 K is not d i s c e r n i b l e in o u r Brillouin data. I n s t e a d , a c l e a r s o f t e n i n g of C n a n d C2e is o b s e r v e d a r o u n d T e e - 3 4 9 K. T h i s is t h e s a m e t e m p e r a t u r e which T e l l o et al. [5] h a v e c o n n e c t e d with t h e t r a n s i t i o n D2h18-->D2h15 on t h e basis of t h e i r b i r e f r i n g e n c e m e a s u r e m e n t s . A b o v e 349 K t h e t e m p e r a t u r e d e p e n d e n c e of t h e elastic coefficients is g o v e r n e d by a t e r m p r o p o r t i o n a l to ( T - Tc2)-1/2. B r i l l o u i n shifts of s t r a i n - f r e e crystals of E A C u C e x h i b i t h y s t e r e s i s b e l o w 349 K. T h i s is s h o w n in fig. 5 for a d i r e c t i o n of p r o p a g a t i o n of p h o n o n s which c o r r e s p o n d s a p p r o x i m a t e l y to

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et al. / 13rillouin scattering a n d crystal optics of E A C u C

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with C ~ = 1.5 x 10 m N / m 2 and a 0 . 0 5 x 1 0 1 ° N / m 2 K 1/2. This phase transition appears in the s a m e t e m p e r a t u r e region, w h e r e S t e a d m a n and Willett [7] and T e l l o et al. [5] have found a diffuse a n o m a l y of the enthalpy. T h e y interpreted this a n o m a l y as a t h e r m o c h r o i c phase transition due to a coordination change of the c o p p e r ions, as originally p r o p o s e d by WiUett [8]. In addition it can be noted that the t e m perature d e p e n d e n c e of C . and C22 r e s e m b l e s the 225 K transition in E A M C [10], w h e r e it is caused by a reorientation of the (C2HsNH3)groups.

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Fig. 6 s h o w s the t e m p e r a t u r e d e p e n d e n c e of the LB in the t e m p e r a t u r e range b e t w e e n 5 and 370 K as m e a s u r e d in the (bc) c l e a v a g e plane. T h e neutral axes w e r e d e t e r m i n e d at R T and refer to the axes Y'oand z' [5] or to the lattice constants b = 7 . 4 7 A and c = 7 . 3 5 A of the

W. Kleemann et al. / Brillouin scattering and crystal optics of E A C u C

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orthorhombic room t e m p e r a t u r e ( O R T ) phase Pbca (D~) [7]. The curve agrees with previous results [6, 4] in its global shape, which comprises a first-order j u m p at TeE = 364.0 K, a steep, but continuous, decrease around 230 K, a flat minim u m near 100 K and another flat m a x i m u m near 25 K. The first-order character of the I - I I SPT at T~ is more clearly shown in fig. 7 (dotted curves) being in contrast with the result of Tello et al. [5]. Their continuously changing LB curve results, in our opinion, from an undetected

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273

superposition of twin domains [6, 4], which can perfectly be avoided by the use of our microscopical selection method. On the other hand, in agreement with Teilo et al. [5], our novel experiments exhibit also another small first-order anomaly in the t e m p e r a t u r e range between 340 and 355 K, which was undetected in our former investigations [6, 4]. As indicated in fig. 6 the LB j u m p is of the order 10-4. In contrast with the I - I I transition at Tc~ and in agreement with Brillouin (fig. 5) and thermal expansion [5] results a considerable hysteresis is observed. T h e LB scans presented in fig. 7 reveal ATe2 = 7.5 K centered around the approximate value of the equilibrium transition temperature, To2 = 346.5 K. It has been observed that ATe2 increases with increasing internal strains contained in the sample. E.g. the O R T phase III may be preserved on heating in stressstabilized single domains up to temperatures very near to Tc~. Accordingly, the I I I - I I and the I I - I transitions can lie very close to one another and frequently a complicated mixture of twin domains of phases III and II is observed. On cooling, on the other hand, phase III domains may immediately occur just below Tc~, which are continuously growing at the expense of phase II domains until the latter disappear at temperatures well below 340 K. Our former experiments on the critical behavior of the LB around Tel [4] were carried out erroneously on these phase III domains. Hence, the I I - I I I transition remained undetected at this occasion. It is important to remark that each I I - I I l transition is accompanied by a domain switching (interchange of b and c axes within a given domain) as can be derived from the change of the apparent LB sign at To2 (fig. 7). This must be taken into account in the interpretation of the ferroelastic I I - I I I transition. New ideas on this p h e n o m e n o n are indeed necessary, since we can show that the intermediate phase II is not orthorhombic as predicted by Petzelt [3] and assumed up to date [4-6]. As can be seen in fig. 7 the optically neutral directions do not coincide in phase II with those of the O R T phase III. T h e rotation angle of the indicatrix is very small, 0 < 1°, thus being negligible in the LB

W. Kleemann et al. I Brillouin scattering and crystal optics of E A C u C

274

measurements. Again no hysteresis is found for the 0 j u m p at T~, whereas for the example shown in fig. 7 ATe2 amounts to 14K centered a r o u n d Tc2 ~ 347.5 K. Differently orienting stress seems to be responsible for slightly differing 0 values in up and down scans of T. Both SPT, at T~ and at Tc2, are also reflected by the t e m p e r a t u r e dependence of the principal refractive indices n~ and n2 measured in the (bc) plane (fig. 8). As for the LB the most prominent changes occur at Tel, I(~r/1,2[ ~ 1 0 3 whereas the effects are one order of magnitude smaller at T~2. Hysteresis at To2 is also clearly observed in these experiments. A r o u n d T~3 ~ 233 K as determined by D T A [5] and from C11 (fig. l) both curves, nl(T) and n2(T), exhibit points of inflexion, which m a r k the smooth change between different slopes dnj/dT (j = 1, 2) being characteristic for the O R T and the low-temperature phase, respectively (fig. 8). It is seen that the indices are differently sensitive to temperature. Similar effects in high-temperature parent phases have recently been observed on the cubic refractive indices of ABF3-type antiferromagnets like KNiF3 at the magnetic PT [12] and on BaTiO3 at the ferroelectric PT [13], respectively. In both types of materials the anomalies above Tc are due to fluctuations (6rl 2) of the order parameter, ~7,

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being the sublattice magnetization m for KNiF3 and the polarization P for BaTiO3, respectively• The I I I - I V transition of E A C u C is probably purely structural and thus constitutes an example for a third class of transitions, where fluctuations of ~7 are undoubtedly observed in the refractive indices and their differences. The onset of L R O below T~3 is more clearly marked in the LB measured in a (111) plane, i.e. on a sample, which is hit by the light b e a m under an angle of 45 ° with respect to the normal direction a. Fig. 9 shows that 2~nHE very abruptly changes at T~3 = 222K. It has been found that the actual value of T~3 may vary between 220 and 240 K depending on the sample. More clearly the phase change at T~3 is evidenced by measurements of the indicatrix orientation. As shown in fig. 9 a large deviation from orthorhombicity ( 1 = 0 ) arises in the (111) plane below T~3, whereas only spurious effects are visible at inspection under normal incidence in the (100) plane. This seems to indicate that the index ellipsoid is only allowed to rotate around one well-defined axis lying within the (bc) plane. Hence, we assume the low-temperature phase to be monoclinic (MLT phase) with its monoclinic axis lying along b or c of the O R T phase. This is consistent with the observation that C~ and C22,

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Fig. 9. T e m p e r a t u r e dependence of A n m and of the rotation of the indicatrix (intensity) with respect to its orientation in the O R T phase around To3.

W. Kleemann et al. / BriUouin scattering and crystal optics of E A C u C

on the one hand, and nl and n2, on the other hand, are differently sensitive to T around Tea (cf. figs. 1 and 8). The monoclinic axis being least affected by the SPT may hence tentatively be correlated with our quantities (?22 and n2. The intensity I as shown in fig. 9, depending on 0 and An simultaneously, seems to saturate at low temperatures after passing large variations just below Tea. However, according to our data of the in-plane refractive indices another anomaly seems to occur at To4 = 39 K. This is shown in detail in fig. 10. Relatively sharply defined bending points seem to mark the onset of another second-order SPT into another monoclinic (or triclinic) L T phase. Brillouin scattering measurements have unfortunately not been carried out in this temperature range. Hence, the very existence of this SPT still remains questionable and necessitates further investigations. Here we present only a preliminary analysis of our data in fig. 10 under the assumption that the LB of the parent M L T phase IV asymptotically varies like the expected [14] Ta-law of the lattice energy (full lines in fig. 10). The additional anomalies of nl and n2 (dashed lines in fig. 10) asymptotically (t ~ 0) vary like

Snj oc

(j = 1, 2),

t2B

where t

= (Tc4-T/Tc4

(4) and / 3 - 0.5 is the critical

-'x

-

~" ~,1~"

L

,o

i

2o

3'o

i

~o

s'o

60

temperature (K)

Fig. 10. T e m p e r a t u r e d e p e n d e n c e of nz and n2 in the lowtemperature region. T h e anomalies 18nil, j = 1, 2, below T¢4 = 39 K (dashed lines) are obtained after subtracting T 3background contributions (full lines).

275

E

r~ i

8

l

tO

J

J

L

12 temperature (K)

1,;

IG

Fig. 11. T e m p e r a t u r e d e p e n d e n c e of IdAnl2/dT I around the N6el t e m p e r a t u r e TN = 10.2 K.

exponent of the order parameter of phase V. It should be mentioned that recent low-temperature N M R measurements [14] have also shown distinct deviations from the O R T phase, but only limited information on the crystal structure can be drawn from these results. At the lowest temperatures ( T < 2 0 K) finally magnetic anomalies due to 2d spin-ordering are observed in the indices and in the LB as shown very clearly by means of the temperature derivative of An in fig. 11. As discussed previously [6] the relatively broad peak around 13 K coincides with the peak of the magnetic specific heat, Cm, of the 2d Heisenberg ferromagnet with S = 1/2 [15]. Moreover, owing to the better resolution compared with our former experiment [6] we also observe the LB peak due to the onset of 3d spin-order at the N6el temperature, TN---10.20 K. Very typically for the magnetic LB [16] the different contributions to Cm (here the 2d and the 3d peaks, respectively) are represented at different weights by the LB. In the present case the 3d peak is clearly overemphasized when compared with the Cm data [15].

5. Discussion

5.1. High-temperature phase transitions Tello et al. [5] have used the phenomenological Landau theory in order to predict ther-

276

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modynamic anomalies at Tel and To2. Their calculations, however, are based on the assumption ~ l T ~ r~2h ~ l s ~ r,--2h, ~ 5 the I - I I of the phase sequence r,--4h transition being continuous and the I I - I I I transition being discontinuous, respectively. Since the symmetry of phase II is actually lower than orthorhombic (presumably monoclinic) and the I - I I transition is discontinuous, the results can only be approximately valid. Lacking more exact structural data at present, they may nevertheless be useful for a qualitative discussion of our Brillouin scattering data. These are in line with their conclusion that the elastic coefficients are not influenced by the I - I I transition. However, the possibility of a small smooth or even stepwise anomaly cannot be definitely excluded because they would be m a s k e d by the softening of phonons due to the transition at T~2. Regarding this transition, stepwise anomalies were predicted for Cll, C22 and C33 [3]. It is evident that at the phonon frequencies dealt with in the present experiments (733 shows no anomaly at any of the phase transitions. From the expression of Tello et al. [5] the t e m p e r a t u r e d e p e n d e n c e of CI~ is obtained in the form

~2~2 CH I I=I CllI1 go + g2~ b2 + g4~b4,

l = 1, 2

(5)

for a 1st order orthorhombic-to-orthorhombic phase transition. The t e m p e r a t u r e dependence of the anomalous part of the elastic coefficient depends on that of the order p a r a m e t e r and of the relative importance of the factors in the denominator. By assuming go'~g2ff) 2, g 4 ~ 4 the eq. (5) can be reduced to the form derived by Hirotsu et al. [17] for a 1st order cubic-to-tetragonal phase transition. The stepwise behavior predicted by this calculation is qualitatively seen in the t e m p e r a t u r e dependence of C22 in EACuC. In the vicinity of To2, both Clt and C22 have a contribution proportional to I T - T~]-j/2. According to the phenomenological theory developed by Levanyuk [18] for second-order phase transitions an anomaly with this kind of t e m p e r a t u r e dependence results from interactions between a

sound wave and thermal fluctuations of the order parameter. In E A C u C fluctuations of the order p a r a m e t e r may arise from dynamic J a h n - T e l l e r distortions of the CuC16 octahedra. It is well known that the strength of the coupling of an order p a r a m e t e r and an acoustic m o d e depends on their frequencies. Consequently, the elastic coefficients may have different values depending on the frequency of the measurement. This effect was shown to be quite large in M A M C [10]. Unfortunately, neither ultrasonic nor neutron scattering results are available for discussion of the acoustic dispersion in E A C u C . It would be particularly interesting to investigate the frequency and t e m p e r a t u r e dependence of ultrasonic velocity and absorption near the phase transition observed at 349 K. Since both SPT at Tel and To2 are discontinuous, the appearance of the monoclinic intermediate phase II is not forbidden by symmetry. The large D T A anomaly at Tel [5] even seems to exclude a close relation between the phases I and II. Lacking structural data at present we can only guess what really happens. It is well known [3] that two different driving forces are responsible for the phase change from T H T (D4~7) to O R T (Dl~): (i) the J a h n - T e l l e r (JT) effect, which tries to lift the degeneracy of the 2Eg ground state of the Cu 2. ion in order to diminish the total energy of the system, (ii) N H . . . CI hydrogen bonds, which cause different tilts of the CuC16 octahedra depending on the orientation of the NH3 group with respect to its eight neighboring chlorine atoms. Tentatively we propose the J T distortion to be the leading force becoming static at Tcl, whereas the NH3 group is still allowed to rotate freely around the tetragonal ct axis. This motion, on the other hand, may be the reason for a subsequent rearrangement of the neighboring equatorial CI atoms to form a (nearly) square arrangement as in phase I. This can be achieved by alternate rotations of the CuC[6 octahedra around ct. If we start from the tilt system a°a°c ~ of the parent phase I (THT) in the notation of D e p m e i e r et al. [19], we then arrive at "a°a°c + JT", where JT means the JT distortion resulting in elongated Cuel6 octahedra with the long axes alternating between [110[, and

W. Kleemann et al. / Brillouin scattering and crystal optics of EAC2uC

[ll0]t from site to site. The resulting structure is monoclinic with its z axis along ct and with the possibility of forming twin domains in agreement with the experimental observations. Within this picture at T~2 the lock-in of the NH3 groups into an orthorhombic configuration [19] takes place and the rotation of the CuC16 octahedra around ct is no longer necessary. Hence, phase III may be characterized by the tilt system " a - a - a ° + JT", which leads to the familiar O R T structure [7]. It remains to be shown that the II-III transition implies the interchange of short and long intraplanar axes as evidenced by spontaneous domain switching (fig. 7).

5.2. Low-temperature phase transitions Following the ideas of Section 5.1 the III-IV transition at about 230 K may be induced by a monoclinic NH3 configuration, which causes similar tilts as in the 6-phase of M A M n C [19]. Hence, a monoclinic phase emerges, which may tentatively be described by the tilt system " a - a - c + J T " with an in-plane monoclinic z axis. The proposed I V - V transition at 3 9 K finally seems to involve another monoclinic or a triclinic low-temperature phase, the stabilizing force of which is not known at present. Specific heat measurements carried out over a larger temperature range than in previous experiments [15] would be welcome to clarify the situation around To4. The similarity between the I l l - I V transition in E A C u C and t h e / 3 - 6 transition in E A M n C [10] has already been mentioned in view of the Brillouin scattering results (fig. 1). This is less obvious from the LB data, since the I l l - I V anomaly is much larger in E A C u C and its sign is different from that of the /3-6 anomaly in E A M n C . Most striking, however, is the large contribution of fluctuations to the total LB change in the copper salt (fig. 6), whereas only small effects are observed in E A M n C (fig. 4 in ref. [10]). Presumably these differences arise from the fact that despite the similarity of the tilt systems a large JT distortion is involved in EACuC. This causes a descent in symmetry (EAMnC: L,'2hl~'1,l"~lS~ 8" L"2h, E A C u C : D15<--->CSh, ten-

277

tatively) and, obviously, an increased sensitivity to fluctuations. This view is supported by the observation that Tc3 s e e m s to be very sensitive to internal strain. As mentioned by Fousek and Petzelt [20] a direct proof of the very existence of the fluctuation part in morphic birefringence still lacks in the literature. Its absence is indeed expected in many cases for symmetry reasons. For example, above To, in all cubic systems the fluctuating parts of the refractive index, if any, should cancel in LB experiments. Only stress-induced anisotropic fluctuations may be responsible for small LB anomalies as observed, e.g., on the SPT of cubic ABFa-type crystals [4]. In an orthorhombic system, however, fluctuation contributions to all refractive indices nj (] = 1, 2, 3) and, hence, to the principal birefringences are expected quite generally, since the relevant symmetry adapted optical susceptibilities Xj (J = 1, 2, 3) and their variations 6Xj transform like the identity representation I = A in the space group G [20]. Fluctuation effects on the LB as at the III-IV SPT of E A C u C are likely to occur also in M A C u C [6] and in orthorhombic BaMnF4 at the commensurate-incommensurate SPT [21]. The rotation of the indicatrix axes as measured in the (111) plane of E A C u C (fig. 9) is proportional to off-diagonal elements 6Xjk of the polarizability tensor. They are expected to be non-zero in monoclinic and triclinic systems [20] in agreement with our tentative assignments of the transitions III-IV and IV-V.

6. Conclusion

Based on Brillouin scattering and crystal optical measurements the transition pattern of E A C u C has been shown to be more complex than it was believed up to date. The following series of phases is found: THT(364.0 K ) M H T ( ? ) ( - 3 4 7 K) ORT(220-240 K)MLT(?)(39 K) MLT(?)(T~ = 10.2 K)afm MLT(?)

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W. Kleemann et al. / Brillouin scattering and crystal optics of E A C u C

where the structures of the monoclinic (M) phases are still questionable. The onsets of the J a h n - T e l l e r distortion of the CUE16 octahedra and of the orientational lock-in of the NH3 groups are tentatively correlated with the SPT at Tel and To2, respectively, whereas different NH3 orientations seem to cause the strongly fluctuating SPT at T¢3. Detailed structural investigations are needed to test our conjectures. In particular, more data are required for the phase V below 39 K.

Acknowledgement Thanks are due to U. Spengler for constructing the Lebedeff interferometer and help with the refractive index measurements. Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

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[3] J. Petzelt, J. Phys. Chem. Solids 36 (1975) 1005. [4] W. Kleemann, F.J. Sch/ifer and J. Nouet, Physica 97B (1979) 145, and references therein. [5] M.J. Tello, J.L. Mafies, J. Fernandez, M.A. Arriandiaga and J.M. Perez-Mato, J. Phys. C: Solid State Phys. 14 (1981) 805. [6] G. Heygster and W. Kleemann, Physica 89B (1977) 165. [7] J.P. Steadman and R.D. Willett, Inorg. Chim. Acta 4 (1970) 367. [8] R.D. Willett, J. Chem. Phys. 41 (1964) 2243. [91 T. Goto, B. L/ithi, R. Geick and H. Strobel, J. Phys. C: Solid State Phys. 12 (1979) L303. [10] E. Kar~ij~im~iki, R. Laiho, T. Levola, W. Kleemann and F.J. Schiller, Physica l l l B (1981) 24. [11] G. Domann and A. Kasten, J. Magn. Magn. Mat. 13 (1979) 167. [12] P.A. Markovin, R.V. Pisarev, G.A. Smolensky and P.P. Syrnikov, Solid State Commun. 19 (1976) 185. [13] G. Burns and F.H. Dacol, Ferroelectrics 37 (1981) 661. [14] H. Kubo, N. Kaneshima, Y. Hashimoto, K. Tsuru and K. Hirakawa, J. Phys. Soc. Jap. 42 (1977) 484. [15] P. Bloembergen, K.G. Tan, F.H.J. Lefevre and A.H.M. Bleyendaal, J. Physique 32, Suppl. C-1 (1972) 879. [16] W. Kleemann, J. Ferr6 and F.J. Sch~ifer, J. Phys. C: Solid State Phys. 14 (1981) 4463. [17] S. Hirotsu, T. Suzuki and S. Sawada, J. Phys. Soc. Japan 43 (1977) 575. [18] A.P. Levanyuk, Soviet Phys. JETP 22 (1966) 901. [19] W. Depmeier, J. Felsche and G. Wilderrnuth, J. Solid State Chem. 21 (1977) 57. [20] J. Fousek and J. Petzelt, phys. stat. sol. (a) 55 (1979) 1l. [21] F.J. Schiller, W. Kleemann and T. Tsuboi, J. Phys. C: Solid State Phys. 16 (1983) in press.