Lithium iodate: Phase transitions revisited

Physica 144B (1987) 277-291 North-Holland, Amsterdam

LITHIUM IODATE: PHASE TRANSITIONS REVISITED Jean-Michel C R E T ' F E Z , E t i e n n e C O Q U E T

and B e r n a r d M I C H A U X

Laboratoire d'Optique du Rdseau Cristallin, Universitd de Bourgogne, 6 Bd. Gabriel, 21100 Dijon, France

Jean PANNETIER

and J a c q u e s B O U I L L O T

Institut Laue-Langevin, 156X, 38042 Grenoble, France

Patrick O R L A N S Laboratoire de Chimie Physique des Processus Industriels, E.N.S. des Mines, 158 Cours Fauriel, 42023 Saint-Etienne, France

A n d r 6 N O N A T and J e a n - C l a u d e M U T I N Laboratoire de R~activit~ des Solides, Universit~ de Bourgogne, B.P. 138, 21004 Dijon, France

Received 18 September 1986 Revised manuscript received 29 October 1986 The sequence of phase transitions of lithium iodate powders has been investigated by neutron thermodiffractometry. Upon heating, powders obtained by grinding hexagonal crystals exhibit the well known a---~y---~/3 sequence. The intermediate orthorhombic phase 3, is never observed as a single phase but always coexists either with a or with/3-LilO 3. The transition temperatures and the ct--~3" transformation rate are found to be strongly dependent on the crystallite dimension, especially for grain sizes smaller than 100/~m. The hysteresis of the reversible a ~ 3' transition, the kinetics of both a ~ ~ and 3,---,/3 transitions, and the influence of proton contamination are also investigated. Contrary to this, natural powders obtained by fast evaporation of an aqueous solution do not exhibit any intermediate phase and transform directly from a to the/3 tetragonal modification. Further DTA and X-ray investigations show that the occurrence of the 3" phase is related to the mechanical treatment of the sample (grinding, polishing...), i.e. to the presence of defects. Previous literature data are reviewed and the apparent discrepancies concerning the transition processes are elucidated on the basis of the present results.

1. Introduction L i t h i u m iodate has two stable crystalline m o d ifications at r o o m t e m p r e a t u r e and n o r m a l or low pressure: a (hexagonal, P63, Z = 2) and /3 (tetragonal, P42/n , Z = 8). E i t h e r t~ o r / 3 crystals can be o b t a i n e d d e p e n d i n g o n the g r o w t h conditions: t e m p e r a t u r e , p H and e v a p o r a t i o n rate o f the m o t h e r a q u e o u s solutions [1]. U p o n heating, /3-LilO 3 is stable up to the melting point (708 K), w h e r e a s a - L i l O 3 is k n o w n to u n d e r g o two p h a s e transitions according to the s e q u e n c e a ~ 7---~/3. T h e first transition is reversible and takes place at a b o u t 520 K with a large hysteresis; the s e c o n d o n e is irreversible a n d occurs at a b o u t 550 K. T h e structures o f all three m o d -

ifications have b e e n d e t e r m i n e d and refined; the m o r e recent papers o n the m a t t e r deal with the t h e r m a l d e p e n d e n c e o f structural p a r a m e t e r s [ 2 4], and the structure o f the i n t e r m e d i a t e o r t h o r h o m b i c phase y - L i I O 3 (Pna21, Z = 4 ) [5,6]. Since single crystals twin, crack and take a whitish colour at the transition, p o w d e r e d samples are generally used in the investigations o f transformations. A survey o f published d a t a o n the p h a s e transitions o f L i I O 3 reveals large discrepancies in transition t e m p e r a t u r e s as well as in the t e m p e r a t u r e range w h e r e the y phase exists [7-13]. Several p a r a m e t e r s , such as the c o n t a m i n a t i o n by H I O 3, related to the crystal g r o w t h conditions ( p H o f the m o t h e r solution) or the t h e r m a l history o f samples are usually

0378-4363/87/$03.50 (~) Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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considered to be responsible for these discrepancies. Both a---> 3' and 3,--->/3 transitions appear to be of first order-type and may therefore be influenced by the particle size of powdered sampies. The aim of our investigation was first to study the role of this parameter by means of neutron thermodiffractometry (NTD). This method [14] is indeed well suited to a material like lithium iodate: owing to the strong absorption of iodine, X-ray diffraction studies probe only a small layer at the surface of samples while neutron beams see the bulk; in addition, the main structural feature of the a ~ 7 transition is displacements of lithium and oxygen atoms [5] which barely contribute to the X-ray diffraction pattern but provide the largest contribution to the neutron intensities. In view of the interesting results obtained using NTD, this study was further complemented by X-ray diffraction, optical and DTA experiments.

2. Experimental 2.1. Sample preparation

Large ot-LiIO 3 single crystals were grown by slow and controlled evaporation of slightly supersaturated aqueous solutions at about 315 K using c-cut plates as seed crystals. The p H of the mother solution was either 6 to obtain "neutral samples" or 2 for "acid samples". When single crystalline samples were needed, small plates were cut parallel or perpendicular to the c-axis, and polished on nylon cloth with diamond paste. Powdered samples were obtained by grinding large single crystals in a mortar. This powder was then passed through sieves to provide several samples of selected grain size, with average particle dimensions ranging from less than 20 tzm up to 500/xm. Particle sizes were controlled directly by optical microscopy for the largest grains (/>150 tzm) and by using an automatic particle sizer (MALVERN 2200/3300) for the smallest ones. Lithium iodate powder was also obtained directly by fast evaporation of an aqueous solution ( p H = 6) at a temperature between 313 and 318 K. The particle-size determination for this

sample gave a broad distribution spreading from 10 to 150/zm with a maximum at 55/xm. Examination under the microscope revealed that the grains of this powder consist of small hexagonal needles (10 to 20/xm in length) and larger conglomerates of several needles. We call this sample a "natural powder" as opposed to the "ground powders" obtained by grinding crystals. 2.2. Experiments

The neutron diffraction patterns of LilO 3 powders were recorded on the D1B two-axis diffractometer at the Institut Laue-Langevin. This powder diffractometer is equipped with a 400 cell 3He multidetector covering an angular range 20 =80°; a flux of about 1.5 × 106 neutrons cm -2 s -1 provided by a HOPG monochromator allows a complete powder diffraction pattern to be recorded simultaneously on a time scale of a few minutes. The selected grain-size samples (about 10 g in mass) were enclosed in cylindrical vanadium containers (10 mm in diameter, about 50 mm in height). The heating device was a conventional furnace with a vanadium heater working under a residual pressure lower than 10 -4 Torr. The temperature was measured with a Ni-Cr thermocouple in contact with the sample holder. For 4 samples (average grain size: 15,130,370, 480/xm), the temperature was first increased rapidly from 290 to 420K (60K/h) then more slowly at the constant rate of 10 K / h in the range of the phase transitions and up to about 600 K. The diffraction diagrams were recorded every 6 min during which time the temperature increment was 1 K. This variation is of the same order of magnitude as the temperature gradient inside the sample. Data were collected with a wavelength of 2.516(1)/~ in the angular range 20 = 25 ° to 105°. A section of a typical measured 3-dimensional thermodiffractogram is given in fig. 1. This figure clearly illustrates the thermal behaviour of the sample, namely the appearance and disappearance of Bragg peaks for every lithium iodate modification. The background level is low and not altered upon heating. At the end of the experiments the samples were as white as they were before the thermal treatment, which indicates that during the ~ 20 h of

J.M. Crettez et al. / Lithium iodate

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an experiment the samples grown from quasineutral solutions did not release iodine, i.e. contain no excess HIO 3 in agreement with the chemical analysis reported by Arend et al. [8]. DTA investigations were performed on a PERKIN-ELMER 7/4 DTA 1700 apparatus. The sample weights were about 60 mg and the heating rate 20 K/min for all experiments. X-ray diffraction diagrams were recorded on a horizontal-axis powder diffractometer equipped with a linear position-sensitive detector (LETI). Scanning electron microscopy photographs were obtained by using a CAMBRIDGE 250 Mk 2 apparatus. The macroscopic behaviour of oriented crystal plates during a first heating was observed through a magnifier; for the sake of convenience and to allow quantitative measurements, this experiment was recorded with a video-camera. The samples were placed on a copper heating plate, whose temperature was increased at a constant rate of about 24 K / h and measured with a thermocouple in contact with the metal in the near vicinity of the LiIO 3 plates. 2.3. Neutron data analysis

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In thermodiffractograms, every neutron diffraction pattern was analyzed by assuming Gaussian lineshapes in order to obtain position, halfwidth and integrated intensity of the reflections [15]. As in a preliminary analysis [13] with only the smallest particle-size sample (sieved through a 20/xm mesh and with a 10-20/xm distribution as found from the particle sizer), the relevant parameter to characterize the evolution of the amount of the different phases as a function of temperature is the ratio I ( T ) / I M : the symbol I ( T ) represents the intensity of a selected reflection of a given phase (a, 3' or/3) at the temperature T; for a and /3 phases, I M is the average intensity of this peak, respectively, before the appearance and after the vanishing of the 3' phase; for the 3' phase, I M is taken as the strongest intensity of this reflection observed during the heating process. This procedure actually neglects the thermal variation of the DebyeWaller factors; this is a reasonable approximation, however, for low-angle reflections and over

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J.M. Crettez et al. I Lithium iodate

a small t e m p e r a t u r e range. T h e Bragg reflections used to obtain the curves of the transformed fraction vs. t e m p e r a t u r e were chosen so that they are single, strong enough to allow accurate intensity determination and not overlapping other peaks of a parent phase: 002(a), 002(3,) and 201(/3), respectively, at 0 = 2 8 . 8 °, 28.4 °, 19.2 °. We verified that the ratio I ( T ) / I M (i.e. the

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transformed fraction) for a given phase does not depend on the choice of the selected reflection. The evolution of I ( T ) / I M vs. t e m p e r a t u r e is reported in figs. 2a and 2b for two samples of different grain size. The general features of the curves are the same for all samples we investigated; but for the larger particle sizes (~>300/~m) the intensity determination is less accurate owing probably to some preferred orientation in the sample. In the thermodiffractograms, a few reflections seem not be altered during the a ~ ~/transformation. The Bragg p e a k at 0 = 27.5 ° in fig. 1 is such an example. It consists in two reflections ( l l 0 ( a ) and 301(3')) which are very close in angular position and have nearly equal calculated intensity (Illo(~)/I31o(~)= 1.1)*. At higher temperatures this p e a k is succeeded by the 311 (/3) reflection which is stronger.

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3. Results of NTD experiments (powders) 3.1. Kinetics o f transitions ~• 5O

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The experiments described in this p a p e r have been p e r f o r m e d under non-isothermal conditions. H o w e v e r , in order to assess the influence of the t e m p e r a t u r e increase and of the true kinetics of the transformation, two limited isothermal investigations of the transformations a--> y and y-->/3 have also been performed. In these experiments, the t e m p e r a t u r e of the sample was raised to 543 and 553 K respectively (this was actually part of hysteresis experiment similar to that described in section 3.3) and the thermal evolution of the transformed fractions was measured over a period of about one hour. The results are given in fig. 3. They show that the a---> y transition reaches equilibrium, but not completion, after about 40min. This means firstly that, in the range where the 3' phase coexists with a, the equilib* Structure factors have been calculated with the LAZY PULVERIX program [16] and were based on published a and y structure parameters at 575 and 488 K, respectively [2, 5].

J . M . Crettez et al.

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rium ratio a/3' depends only on temperature and secondly that all the curves of transformed fraction vs. temperature given in the following do not represent the true equilibrium fractions. The true equilibrium fractions would look like the dotted curves in fig. 2a if the kinetics effects were taken into account. Because all measurements have been performed at the same heating rate, a direct comparison between the different curves is possible. The isothermal transformation y--->/3 measured at 553 K exhibits a rather different behaviour: at this temperature, the transformed fraction varies linearly with time at a rate of 0.7% per minute up to completion. This implies that once nuclei of/3 are formed in a powder of lithium iodate, this sample will transform completely into /3 after some time (about 2.4 h at 553 K). A constant 3'//3 ratio may be obtained only if the temperature is suddenly decreased when a given amount of/3 phase is formed.

Lithium iodate

281

3.2. Influence of particle size Although the different transformation curves (figs. 2a, 2b) look rather similar, they display significant differences. The most prominent features are the following: (i) Within the limits of the intensity determination (about a few percent), the y phase always coexists either with a- or/3-LilO3; this implies that the a and/3 phases never coexist when the a--->/3 transformation proceeds through the intermediate y phase. (ii) The temperature (T~v) of onset of the a to 3' transformation is strongly grain-size dependent, at least below 100/xm. This is presented in fig. 4 which also shows the temperature ( T ~ ) of onset of /3-LilO3, and the temperature T~ at which the sample, is complete transformed into /3. These curves delimit the range of existence of the three phases during the first heating (at 10 K/h) of LilO 3 powders prepared from crystals grown at p H = 6 . In this respect it is worth mentioning that the upper curve of fig. 4 would not exist in a true equilibrium diagram because, at any temperature above the line Tve, a sample of y-LilO 3 would finally transform completely into /3 after some time (see section 3.1). The range of existence of the y phase (T~ - T,~ 45 K) is nearly independent of the grain size while the domain in which a and y phases coexist in the absence of the /3 phase (i.e. Tr~-T~) decreases regularly from 35 K for a particle size smaller than 10/xm to 20 K at 500/zm. (iii) The rate of transformation of o~to y (i.e. the slope of I/I M = f(T)) is observed to be constant up to about 70% of transformation, but this rate also is grain-size dependent. Although the limited number of experiments does not allow to find out a relationship between these two parameters, the trend is clear: average grain size (/xm)

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transformation from 3, to/3 is not influenced by the grain size of the starting a-LilO 3 sample. For a heating rate of 1 0 K / h this rate is about 8 % K - 1 ; this corresponds to a transformation rate of 1.1% per minute, i.e. about twice the value measured under isothermal conditions (Section 3.1).

3.3. Hysteresis o f the a ~ 3, transformation The reversibility and the hysteresis of the a ~---3, transition have been mentioned already by many authors [7, 8, 11]. A diffraction experiment aiming at illustrating these phenomena has been performed by using the following temperature program: fast temperature increase from 295 to 473 K in two hours, then from 473 to 531 K at a rate of 5 K/h and finally a temperature decrease at a rate of 10K/h. The sample used for this experiment was a neutral powder with a grain size in the range 45-65/xm. The thermal evolution of the intensity of the 002(a) and 002(3,)

reflections is plotted in fig. 5. The y phase appears at 522K, then grows up in the a + 3' mixture. At the upper temperature of the experiment (531 K), 73% of the sample was transformed into 3,-LilO 3. During cooling the ratio 3,/a remains constant down to 468 K, below this temperature 3' transforms back into a and vanishes at 395 K. Therefore, the hysteresis range calculated as the difference between the temperature of onset of y upon heating and that of the beginning of the retransformation into a upon cooling is about 54 K; this is to be compared with the value of 47 K obtained by Matsumura [7] and Arend et al. [8] from DTA measurements. In a second hysteresis experiment with a different sample and heating rate, but including two heating and cooling cycles, we observed qualitatively the same behaviour; in addition, we measured exactly the same temperature of formation of 3,-LiIO 3 during the two successive heating sequences. This would suggest that the size of the crystallites has not been modified by the partial transformation t~ --~ 3' ~ a.

J.M. Crettez et al. / Lithium iodate

to about 70% of conversion, but the rate of transformation (9% K -1) is slightly larger than the one observed for a neutral powder of the same grain size. Besides, the rate of transformation 3" to/3 is not modified.

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3.4. Influence of proton contamination The properties of lithium iodate are reported to be strongly influenced by the crystal growth conditions, in particular the pH of the mother solutions [8, 9]. Acid crystals have a higher content of protons than neutral crystals. A neutron thermodiffractometry experiment has been carded out under the conditions described in section 2.2 but with a powdered sample (10-20/zm) obtained by grinding a large single crystal grown from a solution at pH = 2. The general features of the transformations are similar to that observed for a neutral sample (fig. 2a), but the transition temperatures T~v and Tva are depressed to 522 and 548 K, respectively, instead of 529 and 561 K. Once again the transformed fraction from ot to 3" is a linear function of the temperature up

3.5. Phase transformation of natural powders During X-ray diffraction experiments aiming at the study of the thermal evolution of the a-phase structural parameters, our attention has been focused on the behaviour of samples prepared under slightly different conditions. It was found that cut crystal pieces exhibit the expected a ~ 3'-'~/3 sequence of transformations whereas hexagonal-shaped needles (~0.2 x 0.2 x 0.5 mm 3) taken from the mother solution break up at about 533 K without showing any reflection of the intermediate 3" phase [2]. In order to elucidate this question we have studied by means of NTD the behaviour of a natural powder obtained as described in section 2.1. Part of the thermodiffractogram of a natural powder collected under the conditions (10 K/h) described in section 2.2 is shown in fig. 6a. Compared to fig. 6b which is the corresponding part extracted from fig. 1, this thermodiffractogram shows that a-LiIO 3 natural powder transforms directly into /3-LiIO 3 without the occurrence of the intermediate 3" phase. The plot of the ratio I(T)/I M derived from the intensity of the {002(a), ll0(a)} and {220(/3), 201(/3)} reflections is reported in fig. 2c and leads to the following observations: (i) The temperature of onset of the /3 phase for the direct t~-->/3 transformation ( T ~ = 506 K) is significantly lower than T~v (520 K) and Tv~ (545 K) measured on a ground powder of the same average particle size (55/~m) as shown in fig. 4. (ii) The direct a--*/3 transformation spreads over the same temperature range (A T ~ 43 K) as that of ground powders, but, obviously, implies the coexistence of a and/3 phases. (iii) The rate of transformation from a to /3-LiIO 3 (2.9% K -1) is lower than that of both a--* 3" and 3'-*/3 transitions of ground powders,

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even for the smallest grain-size sample studied. Here again, it must be noticed that once the seeds of /3 have appeared in the sample, the transformation would probably continue under isothermal conditions up to completion.

4. Results of DTA experiments (powders) As expected from the previously mentioned NTD results, the DT A curves of a natural and of

a ground powder are different (figs. 7a and 7c). The curve of a natural powder exhibits only one endothermic peak at 511 K, whereas that of a ground powder (70/xm in average grain size) shows two peaks (endothermic at 521 K and exothermic at 583 K) in agreement with previous investigations. Fig. 7b shows the D T A curve of a ground natural powder (grinding time ~-= 5 min in a D A N G O U M A U system): the two thermal events (endothermic at 519 K and exothermic at 569 K) are found again, like in the case of pow-

J.M. Crettez et al. / Lithium iodate

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(ii) If the grinding time z of a natural powder is increased, the temperature T~v of the beginning of the endothermic peak increases; this can be attributed to the decrease of the crystallite dimensions ( T ~ = 519 K for ~- = 5 min, 523 K for 10 min, 529 K for 15 min). The temperature of the beginning T 1 and of the maximum T 2 of the exothermic peak also shift to higher temperatures ( T 1 = 569 K and T 2 = 581 K for ~"= 5 min, 573 and 5 8 6 K for 10min, 577 and 5 9 0 K for 15 min). (iii) When the heating cycle of ground powders (natural or others) is stopped during the first half of the rising of the endothermic peak, an exothermic event always appears during the cooling cycle (at 466 K for a ground powder and at 439 K for a ground natural powder with r = 15 min); this indicates the occurrence of the 3' phase in the sequence of phase transitions. If the heating cycle is stopped during the second half of the rising of the endothermic peak, no thermal event appears during the cooling cycle; this indicates that the sample is then totally transformed into t h e / 3 phase in agreement with the value of T~ obtained by neutron diffractometry (see fig.

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Fig. 7. DTA curves of LilO 3 powders (heating rate 20 K/ min): (a) a natural powder (pH=6, average grain size 55/zm); (b) a ground natural powder ( p H = 6, grinding time 5min); (c) a ground powder (pH=6, average grain size 70 #m). Arrows indicates the transition temperatures observed in neutron thermodiffractometry (see fig. 4). T~ is the temperature at which the sample is totally transformed into the/3 phase when the heating rate is 10 K/h. ders obtained by grinding a single crystal. Additional D T A experiments show that: (i) When the heating cycle of a natural powder is stopped at 534 K, 550 K or at a higher temperature, i.e. after the maximum of the endothermic effect, no thermal event appears in the cooling cycle; this confirms that the sample did not transform into the 3" modification (it transformed partly or totally t o / 3 ) .

Our N T D results indicate that the transition temperature Tvt~ is spread over the range 530565 K, that is at temperatures much lower than the onset of the exothermic peak in all reported D T A experiments (573 K for small grain-size powders [8], and 603-613 K [11]). Therefore this definitively rules out the assignment of the exothermic peak to the 3'---,/3 transformation. All these results lead to the conclusion that both a ~ 3' and 3,--->/3 transitions occur in sequence in a rather narrow temperature range and cannot be separated in most D T A or DSC experiments. In other words, the endothermic peak observed in D T A is the superposition of two distinct but not resolved processes a ~ y and y---~/3. Let us mention, however, that the results published by Matsumura (see fig. 1 in ref. 7) strongly suggest the existence of two partly resolved peaks. The enthalpy of the direct ot ~ / 3 endothermic transformation of a natural powder is A H 1.1kcal/mol while the total enthalpy of the a - * 3' and y--* fl transformations of ground pow-

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J . M . Crettez et al.

Lithium iodate

ders is slightly higher: A H -~ 1.3 kcal/mol. This suggests that the 3'--*/3 transformation is endothermic like the a ~ 3' transition, but with a smaller enthalpy. This can actually be related to the variation of the volume per formula unit: the volume discontinuity is A(V/Z)= 4.2 A3 at the a --->3' transformation but only Zi(V/Z) ~ 0.7/~3 for the 3'--->/3 transformation [5].

5. Transformation of crystal plates Previous results obtained on powders have shown that the thermal behaviour of lithium iodate is strongly influenced by the size of the crystallites a n d / o r mechanical treatment of sampies. A natural extension of this work was then to investigate the transformation of large crystals. This study has considered both the macroscopic and microscopic effects of the transitions.

5.1. Macroscopic investigations Two small plates (about 2 mm in thickness) were cut respectively perpendicular (on the left in figs. 8) and parallel to the c axis (on the right) in a large hexagonal neutral single crystal. For such a neutral crystal, the + c direction is that of the highest growth rate [17]. During the film from which three "vpical photographs have been extracted (fig. 8) it c~,J be seen that: (i) At temperatures below 494 K, no visible change occurs in both samples: they remain transparent as at room temperature. (ii) The transition begins at 494.3 K: a white zone appears on the left side (i.e. on the + c side) of the plate cut parallel to the c axis and on the upper part of the other sample. This means that the transformation starts from an edge of the crystal. Let us denote by t o the corresponding time. (iii) Shortly after the onset of the transition, crystals start to crack; cleavage lines start from the transformed area on the edge and run through the crystals, parallel to natural faces (fig. 8a: T = 495.3 K, t = t o + 135 s). (iv) As the cleavage lines progress through the sample cut perpendicular to c, they become

Fig. 8. Photographs showing the macroscopic evolution of small LilO 3 plates during the first heating: (a) at 495.3 K; (b) at 496.2 K; (c) at 497.4 K.

nucleation zones for the transformation (fig. 8b: T = 4 9 6 . 2 K , t = t 0 + 2 9 0 s ; and fig. 8c: T = 497.4K, t = t o + 465 s) which, in turn, induce new cleavage lines, still parallel to the natural faces of the plate, i.e. at 60° from the first lines. As a result, the starting single crystal is rapidly

J . M . Crettez et al.

Fig. 9. Scanning electron microscope photograph showing the column-like structure of blocks after transformation of a t~-LiIO 3 plate into/3-LilO 3.

split into a mosaic of smaller crystals. The velocity of the a-3, boundary plane, parallel to a natural face, is estimated to be 0.05 mm/s. Examination of the second crystal, cut parallel to a natural face, shows a progressive transformation from one edge in the - c direction; the boundary velocity is measured as 0.006 mm/s. (v) The transparent zones disappear at 500 K (respectively 508 K) for the sample cut perpendicular (respectively parallel) to the c axis. Both samples have been heated up to 533 K without showing any further macroscopic change. At the end of the experiment, the samples are puffed and crumbly in consistency. Indeed scanning electron microscopy of a piece taken inside a transformed plate reveals that the samples are no longer homogeneous, but appear as separate blocks, the ordering of which gives a column-like structure directed along the c axis of the original hexagonal crystal (fig. 9). The X-ray diffraction patterns of the transformed samples indicate the tetragonal/3 modification.

5.2. Microscopic study (X-ray) Powder experiments (see section 3.5.) have shown that as-grown crystallites of t~-LilO 3 transform directly into / 3 - L i I O 3. Additional Xray experiments were performed with large single crystals on a two-axis diffractometer: the

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crystal was oriented to be in Bragg reflection and the diffraction pattern was collected at different constant temperatures in the range of phase transitions. In a first experiment, we investigated a natural face (i.e. neither sawed nor polished) from a large t~-LilO 3 crystal. We observed a direct transformation from a-(single crystal) to /3(microcrystalline) LilO 3. A second experiment was then performed on a plate cut from a large single crystal, perpendicular to the c axis. The measurement was performed with the 002(a) reflection. At the transition, one observed the onset of the 002(3,) Bragg peak; both 002(a) and 002(3,) lines coexist as the transition sweeps through the crystal. Incidently, this provides a direct evidence of the relationship [5] between the cells of a- and 3,-LilO 3. At higher temperature, the complete diffraction pattern of/3-LilO 3 was obtained in addition to the reflections of o~- and 3,-LilO 3. Comparison of the diffracted intensities of this/3-phase with that of a powdered /3-LilO 3 sample at the same temperature, however, indicates strong texture. From an analysis of this texture one could derive a relationship between the orientation of the unit cells of 3, and/3-LilO3:a ~ is parallel to a~, be to cv, and c~ to b~. This is actually the orientation of the stereoviews given in the paper of Liminga et al. [6] in order to compare the atomic arrangement of the three polymorphs of lithium iodate. The simultaneous observation of lines from the three phases of LilO 3 in the X-ray diffraction patterns of small plates seems to contradict the NTD results (fig. 2 and section 3.2); this is however an artefact arising from the slowness of the transformation and the measurement procedure: the X-ray beam is diffracted from a crystal surface of about 5 x 0.2 mm 2 and may, at some stage of the transformation, probe at the same time transformed and untransformed zones of the surface (see fig. 8).

6. Discussion

The results gathered from the various techniques mentioned in the above sections allow to

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get a detailed picture of the sequence of transformations of lithium iodate. However, before we proceed further on this subject, it seems worthwhile to review previous literature data.

6.1. Interpretation of previously published results The first point to consider regards the transition temperatures and domain of existence of a, 7 and/3 phases. As we mentioned earlier, there are rather large discrepancies in the published literature concerning these two parameters [7, 8, 11]. A common feature of all up to now reported DTA experiments performed on lithium iodate samples in the range 423 K to about 600 K is the occurrence of two thermal events: (i) An endothermic peak observed in the range 513-533 K depending on the author but also, within an investigation, on the experimental conditions (sample preparation, first or second heating). This peak is ascribed to the a --~ 3' transition; Matsumura [7] observed, however, that this peak is split into two components. (ii) An exothermic peak observed in the range 557-613K. Most workers ascribed this to the 3'---~/3 transformation. Liminga et el. [6], however, suggest that this peak is probably caused by contaminants. This peak progressively vanishes [11] while cycling the transformation between 423 and 573 K. All these puzzling results can now be interpreted by considering the influence of the grain size on the transformations. As a matter of fact, the experiments quoted in the literature have been performed on specimens of unspecified grain size and/or purity but, obviously, all results must fit within the limits of stability given on diagrams like fig. 4. For instance, Matsumura [7] and Arend et el. [8] give Tar = 520 and 522 K (neutral powder), respectively, which would correspond to a particle size of about 50/zm. Arend et el. [8] observed a gradual disappearance of the a ~ 3' transition while cycling an acid powder between 423 and 543 K. In fact, at the upper limit of the first cycle, a fraction of the sample had already transformed into the/3 phase. This, of course, went unnoticed as long as the exother-

mal effect was assigned to the 3'--~/3 transition. During the following cycles, the amount of /3LiIO 3 kept on growing at the expense of the 7 phase, so that after a few cycles the transformation into /3-LiIO 3 had reached completion. Czank et el. [11] observed during a first heating cycle a transition at 533 K: this is the transition temperature expected for finely divided powders (see fig. 4).

6.2. Phenomenological description of the transitions Our results clearly demonstrate that many features of the transitions of lithium iodate are not intrinsic effects (i.e. effects that result from the chemical composition and structural arrangement within the unit cell) but depend strongly on the size of the crystal and on previous mechanical treatment (the history of the sample). As a consequence, it is convenient to distinguish not between bulk and powder samples as was usually done before, but between as-grown (or natural) samples (large single crystals or small crystallites) which are studied directly after being drawn from the growing solutions and other samples which have undergone any kind of stress or mechanical treatment such as grinding for powders or sawing and polishing for large crystals. The first kind o f samples transforms directly from a- to /3-LilO 3 structure through a firstorder destructive phase transition, like the second kind of samples first transforms reversibly (from a structural point of view) to 7-LiIO 3, and then, at higher temperature, irreversibly to /3LiIO3; the parameters of these transformations (temperature, transformation rate) may vary, depending on the morphology and history of the sample.

6.3. Mechanism and kinetics of the ~ ~ 7 transformation Besides the characteristics mentioned above, the a --~ 3, phase transition of lithium iodate displays a set of typical features:

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is structurally reversible, with a large hysteresis, but morphologically irreversible; - if the temperature is stabilized, the transformation is stopped: for a given size of crystallites, the "equilibrium" transformed fraction fv depends only on temperature (fig. 3); - y - L i I O 3 exists as a stable phase only in a narrow temperature range a n d in the presence o f o t - L i I O 3 (this isothermal stability does not exist for the y +/3 mixture). In this section, we will show how some of the features of the a ~ y transformation can be explained, at least qualitatively, by simple geometrical considerations. The macroscopic behaviour of small plates of a - L i I O 3 during thermal transformation points to a nucleation-growth process for the a --~ 3' transition. Such processes are usually analyzed by an A v r a m i - J o h n s o n - M e h l (AJM) equation; most of the experiments presented in this work, however, have been performed under non-isothermal conditions. This precludes [18] the possibility to extract true kinetics parameters (e.g. the dimensionality of the transformation) from the experimental data. In addition, the A J M model usually assumes a random nucleation; this is clearly not the case for lithium iodate: Fig. 8 shows that, at the beginning of the transformation, the nucleation centers are found exclusively on the surface of the crystals, more precisely on the + c top edges of the crystals. The transformation was also found to progress with different velocities perpendicular and along the - c axis, with o I >> vii for a given constant heating rate. This leads us to set up a macroscopic geometrical model to interpret the a--~ 3' transformation of LiIO 3 powders. The grains of powders are considered as hexagonal prisms. In fig. 10, the hatched zone represents the fraction of a grain transformed into 3' at a given temperature. So, in the first step of the transformation, both velocities v± and oil contribute to the transformed fraction until a whole section perpendicular to c is formed. Then, in a second step, the velocity vii will mainly contribute to the transformed fraction. A detailed calculation would need the knowledge of the shape of the c t - y boundary surface, but the curves f~ (T) exhibit the expected behaviour,

-it

,

)

T Fig. 10. Schematicrepresentation of the propagation of the y phase (hatched zone) within a crystalliteinitiallya-LiIO3 and the connectionswith the different parts of the idealizedcurve transformed fraction vs. temperature. i.e. they can be considered, to a first approximation, as constituted of two linear parts with different slopes. Fig. 10 shows the connection between the morphology of the transformed zone and the rates of transformation of an idealized curve. During the transformation of L i I O 3 small plates, we also observed the occurrence of cleavage lines shortly after the onset of the a--* y transition (section 5.1); these lines became nucleation zones from which the transition started again. The onset of cleavage lines and their orientation can be explained by the significant differences between the a and y cell parameters. At 530 K, the 3 , - L i I O 3 orthorhombic cell parameters: ay -- 9.4166 ~ , by = 5.9593/~, cy = 5.3004 ~ have to be compared to the a - L i I O 3 cell parameters in an orthohexagonal description [5]: a~V~ = 9.5602/~, b e = 5.5196 A and c~ = 5.2369/~, indicating, in the plane perpendicular to the common polar e axis, a contraction along the a direction and an expansion along the perpendicular direction through the ot--~ y transformation. The onset of smaller crystallites due to the breaking of the grains is more important for large grain-size powders: this explains why the transformation rate for these powders is higher than that of small-particle powders (see section 3.2).

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In order to explain the variations of the temperature of onset of the 3' phase as a function of the grain size, one m a y consider a thermodynamical model in which the free enthalpy of the system contains a t e r m involving an energy corresponding to the boundary surface between the a and 3" phases [19]. Such a model leads to a linear variation of T~v vs. the inverse of the grain size (at least for small particle sizes) and to a t e m p e r a t u r e T,v of about 510 K which would correspond to an infinite particle dimension, i.e. to a large sample mechanically treated so that the 3" phase can appear.

of X-ray Bragg reflections o f / 3 - L i l O 3 just before and after the exothermic peak. All investigations reported in this p a p e r are limited to the phase transformations of hexagonal lithium iodate samples obtained f r o m aqueous solutions and existing at normal or low pressure. The sequence of transitions is investigated during the first heating and cooling cycle, the highest t e m p e r a t u r e reached by the samples (about 660 K in D T A experiments) being well below the melting point (about 708 K). It is worth mentioning that lithium iodate, cooled from the melt and submitted to well defined thermal treatments, exhibits a quite different and complex behaviour [21].

6.4. F u r t h e r q u e s t i o n s

The comparison between D T A and N D T results indicates that the thermal effect associated with the y--->/3 transformation is endothermic and usually hidden under the declining of the a --->y peak, therefore we are left with the question of the origin of the exothermic p e a k occurring at higher t e m p e r a t u r e . As this effect is not observed in natural powders, a first possible explanation would be to assign this effect to the release of an internal energy stored in the crystallites Of the a modification during the mechanical t r e a t m e n t of grinding. Such a p h e n o m e n o n has already b e e n observed for instance [20] in the case of PbF 2. It seems rather unlikely that this energy is not released until after the sequence of two phase transitions which completely transform the atomic structure of the compound. Let us mention in this respect that Liminga et al. [6] have observed this exothermic event below the ~ ~ 3' transition t e m p e r a t u r e in the case of one sample grown at p H = 7. A second alternative is to assume that this energy has not been provided by the mechanical t r e a t m e n t of the a sample, but by the lattice strains resulting f r o m the a---> 3' transformation itself. This would then imply that /3-LilO 3 form e d from 3'-LilO 3 is highly defective or distorted: the exothermic event could then correspond to a kind of "recrystallization" o f / 3 - L i l O 3. This assumption would have to be checked by further experiments, for instance by analyzing the profile

Acknowledgements The authors wish to thank P. Bert (Dijon) for the S E M photographs.

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