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JOURNAL OF SOLID S TAT E CHEMISTRY 39, 259-261 (1981)

X-Ray Diffraction Study on (Y c, /3 Phase Transition of Cu,SeThe phase transition of a superionic Cu,Se conductor was investigated by X-ray di ffraction methods.In the experiment the Debye line intensity variation with temperature showed a behavior usuallyexnected for the second-order transition. However, the transition was found to be of the first order.The anomalous behavior is explained.

A fair amount of theoretical and experi-mental work has been done to study thedynamics of the diffusion process in variousinorganic solids possessing the commonfeature of at least one high-temperaturephase with exceptionally high ionic conduc-tivity. These materials are called superionicconductors (e.g., AgI, Ag,Se, Ag,S, CuBr,Cu,S, Ag,$I, RbAg&, Cu,Se). The transi-tions to the superionic phase are of thetypical first order with a considerablechange of the lattice structure (e.g., AgI,Ag,Se, Ag&S, CuBr, Cu2S; (I)). Howeverthe transition may also be of the secondorder (e.g., RbAgJ, (I)) or of mixed behav-ior (e.g., AgBSI (2)). Cu,Se has not yet beendefinitely classified (I), but the transitionfrom a low-temperature (Yphase to a high-temperature superionic p phase is known tobe complex (3). The fact that, on heating,the equilibrium state at each temperaturecan be easily established (4, 5) makes thissystem very interesting for investigation.

According to Murray and Heyding (3) (Yphase has a large monoclinic unit cell,which transforms to the cubic p phase atabout 136C. They also performed DTAmeasurements. From their results it can beconcluded that the CY ) fi phase transitionof Cu,Se should be of the first order.The polycrystalline samples of nominalCu,Se used in our experiments were pre-pared as described previously (5). The in-tensities of Debye reflections of Cu,Se wererecorded using powder samples in an Anton

Paar high-temperature vacuum furnacemounted on a Siemens X-ray diffractome-ter. Ni-filtered CuKa radiation was used.In addition, a Nonius Guinier -de Wolf qua-druple-focusing camera was used for thestructure analysis at room temperature.As can be seen from the diffraction pat-terns shown in Fig. 1, only one Braggreflection (030), occurs clearly resolved inthe low-temperature phase (Fig. lb), andthis peak was used for the examination ofthe temperature dependence of the peakintensity. However, in the 28 range shownin Fig. lb, there exist about 40 additionalpeaks of (Yphase of very low intensity, notrecorded by the diffractometer, but clearlydetected on the film of Guinier-de Wolffocusing camera.

,: E> s-cu, Ser8=

k.i_._l

w-2 a 50 LO 30 20 10

- ZB PI

b 50 LO 30 20- 28i'l

FIG. 1. X-Ray powder ditlkaction patterns of CuzSeat (a) 146C and (b) room temperature.259 0022-45%/81/110259-03$02.00/O

CopyrigJtt I@ 1981 by Academic Press, Inc.All rights of reproduction in any for m reserved.

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260 NOTES

TEMPERATURE iC1FIG. 2. Temperature dependence of intensity (nor-malized to room temperature intensity) for the 28 =13 peak of cY-Cu$e in Fig. lb. 0, values for heating;0, values for cooling.The temperature in the temperature cam-era could be varied very quickly, but the

transformation of LY o p and vice versa wasfaster than the fastest temperature changeattainable in the camera. Figure 2 showsthe temperature dependence of the peakintensity of the (030) Bragg reflection. Theintensity decreases as the temperature wasraised, and vice versa. The peak disappearscompletely at about 142C.It may be mentioned that neither theform of the (030) line profile nor the temper-ature of the disappearance or appearance ofthe (030) line were altered by repeatedheating and cooling runs. The measure-ments were repeatable and showed no hys-teresis effects. The (030) peak appeared tochange intensity to a new equilibrium valueas soon as the temperature was changed.Annealing for up to 2 hr did not affect thebehavior. When the temperature waschanged, peak intensity developed in theusual way. Hence the phenomenon is nottime dependent, but only temperature de-pendent. This means that, with changingtemperature, atoms take up almost instan-taneously the position appropriate to thenew structure.In view of the continual variation ofdiffraction intensity (without any jump atthe transition point) and the absence ofhysteresis about the transition point, thetransition (Y * /3 should be of the secondorder. But if the structure changes aretaken into account, the transition should be

of the first order. In order to clear up thispoint, further information was needed. Itwas found that line (030) disappeared with-out notable broadening, which could indi-cate that the transformation is probably notmonophase. Careful X-ray examinationconfirmed this point. There exists a two-phase (C\I+ /3) field, the lower end of whichcoincides with the rapid drop of intensity(about 13oC, Fig. 2). The upper end isdetermined by the disappearance of line(030), which vanishes at about 142C.Is the two-phase field an intrinsic or onlyan apparent phenomenon for the phasetransition of Cu,Se? At least two possibili-ties must be taken into consideration:(a) If the diffractometer samples wereslightly richer in Se than Cu,Se, then thegradual disappearance of the (030) re-flection for cY-Cu,Se with inreasing tem-perature would be expected because of theshift of the Cu,-,Se boundary to composi-tions richer in copper.We examined several Cu2-sSe samples ofdifferent copper content in the range 2.0 1 22 - x 2 1.99. The samples with stoichio-metric index 2.00 > 2 - x 2 1.99 were two-phase alloys with (Yand /3 as constituents, (Ywas naturally the main phase. In thesesamples the disappearance of the (030) areflection proceeds throughout the temper-ature interval from room temperature up to((Y + p)-/3 phase boundary.Samples with index 2.01 2 2 - x > 2.00also proved to be two-phase alloys ((Y +Cu), but with (Yphase as the main constitu-ent and with minor quantities of pure Cu.For these samples the disappearance of the(030) line of a-phase was always between130 and 142C.(b) The inhomogeneity of a samplewith various nonstoichiometric composi-tions could also have caused that the transi-tion temperature could be altered from partto part in the sample, and consequently themixed-phase region would be observed.Therefore the syntheses were prepared very

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NOTES 261carefully. Some parts of each bulk samplewere examined by means of X rays and nodifference in the phase composition of anyof the parts of one particular sample wasfound on X-ray powder photographs.Moreover, samples obtained in differentsyntheses but of equal compositionsshowed the same behavior when heated inthe X-ray camera and when the disappear-ance of the (030) reflection was examined.We may conclude that the intermediatetwo-phase (Q + p) field is an intrinsicphenomenon for the phase transition ofCu,Se.Why is there no hysteresis? It is obviousthat in this system diffusion is fast enoughat these temperatures to bring the specimeninto equilibrium immediately after the tem-perature is changed. In view of this and ofthe existence of the two-phase field near thetransition point, the temperature variationof the (030) line can be understood even forthe first-order transition. All these factsallow us to conclude that the (Y t) p transi-tion on Cu,Se is of the first order.

ReferencesI. N. G. PARSONAGE AND L. A. K. STA VELE Y, Dis-

order in Crystals, Oxford Univ. Pres s (Claren-don), London (1978).2. S. HOSHINO, T. SAKUMA, AND Y. FUJI, .I. Phys.sm. Japan 47, 1252 (1979).

3. R. M. MURRAY AND R. D. HEYDING, Can. J.Chem. 53, 878 (1975).

4. Z. OGORELEC AND B. ~ELUST KA, J. Phys. Chem.Solids 30, 149 (1 9).

5. A. TONEJC, Z. OGORELEC AND B. MESTNIK, J.App l. Cry stallogr. 8, 375 (1975).

A. TONEJCA. M. TONEJC

Institute of PhysicsUniversity of ZagrebP.O.B.-304Zagreb, Yugoslavia

Received January 27, 1981; in revisedform April 22, 1981