1980 X-Ray Study of the Air-oxidised a-Ga~_Se8 and Ga._tea Powders

8/3/2019 1980 X-Ray Study of the Air-oxidised a-Ga~_Se8 and Ga._tea Powders

1/6

Bull. Mater. Sci., Vol. 2, Number 1, January 1980, pp. 55-60. (~) Printed in India.

X-ray s tudy o f the a ir -ox id i sed a -G a~_Se8 and G a ._Tea powders

A K MUK.HERJEE, U DHA W AN, K D KUN DRA and S Z A L INational Physical Laboratory, Hillside Road, New Delhi 1 10 012MS received 5 November 1979Abstract. X-ray studies of the stoichiometrically prepared a-Ga2Se3 and Ga~Te3are reported after various stages of air-oxidation in the temperature ranges 250 to825C and 250 to 650C respectively. Diffractometric powder data of Ga~Te8 arealso reported over the complete 20 range with remarkable difference in the relat iveintensities of the (444) and (642) reflections. In a-Ga2Se3 the oxidation proceedsby formation of the most stable phase, beta-gallium sesquioxide, complete oxidationoccurring at 650 C. For Ga2Te3 a mixture of Ga~TeO8 and TeO~ is obta ined asthe intermediate oxidation products in the range 500 to 600 C, while at 450 C someextra lines which could be indexed on the super-lattice cell of GaaTe3, along with Toand unchanged Ga~Te3 lines, are obs erved . Ox idatio nat the higher temperature of650 C led to the disappearance of TeO~ ines leaving Ga~TeO6 as the final well-crystal-lised phase.Keywords. X-ray st ud y; a-Ga2Ses, GaaTea ; air-oxidised powders.

1. Introductiona -GaaS% and GaaTea a r e t he As 'x 'Ba ' t y pe t e t r ahed ra l s em i -cond uc t ing com -p o u n d s h a v i n g t h e d e f e c t z in c b l o n d e st r u c t u r e , w i t h o n e - t h i r d o f t h e m e t a l s u b - l a t ti c ep o s i t i o n s u n o c c u p i e d . T h e l a t t ic e p a r a m e t e r s o f G a~ Sc a a n d G a ,, T% w e r e o r i g i n a l l yr e p o r t e d b y H a h n a n d K l i n g l e r (1 94 9) a s 5 . 4 1 8 k X a n d 5 . ~8 79 k X r e s p e c t iv e l y .I n t h e c a s e o f G a 2S % , W o o l l e y a n d K e a t i n g ( 1 96 1) f o u n d t h a t t h e o d d o r d e r l in e si n t h e p o w d e r p h o t o g r a p h w e r e b lu r r e d, w h i l e t h e e v e n o r d e r l i n es w e r e s h a r p .L a t e r , K h a n a n d A l l (1 97 8) r e p o r t e d t h e l a t t i c e p a r a m e t e r o f a -G a 2S e3 , b a s e d o nt h e s h a r p h i g h a n g l e l in e s, a s 5 .4 3 3 4 - 0 . 0 0 1 A . B u t n o w o r k h a s y e t b e e n r e p o r t e do n a i r - o x id a t io n a n d c o n c o m i t a n t p h a s e t r a n si t io n s , i f an y , o f t h e t w o c o m p o u n d s .W e h a v e c a r r i e d o u t x - r a y s t u d i e s o f t h e s t o i c h i o m o t r i c a l l y p r e p a r e d a - G a~ S ea a n dG ',h Tc s a lo n g w i t h t h e i r a i r - o x i d a t i o n p r o d u c t s i n t h e t e m p e r a t u r e r a n g e 2 5 0 C ~8 2 5 C a n d 2 5 0 C - 6 5 0 C r e s p e c t iv e l y . M a s s c h a n g e d a t a a r e a l s o r e p o r t e d .

2. Experimentala - G a lS e ~ a n d G a ~ Te a w e r e p r e p a r e d f r o m t h e s t o i c h i o m o t r i c m e l t s o f s p e c t r o -s c o p ic a ll y p u r e c o m p o n e n t s s e a le d i n e v a c u a t e d f u s e d q u a r t z c a p s u l e s a n d h e a t e di n a r o c k i n g f u r n a c e a b o v e t h e r e s p e c t i v e m e l t i n g p o i n t s ( 1 02 0 C ; 7 9 0 C ) f o r

55


2/6

56 A t~ Mukherjee et alabout 6 hr, followed by slow cooling to the solidus in 2 hr and final cooling withinth e furnace to room temperature. The ingots thus obtained were polycrystalline,colours being dark red for a-Ga=Se3 and black for Ga=Tea. Powder photographsof a-Ga~Sea showed that lines with even indices and h + k + l = 4n were sharpwhile those with all indices odd were diffuse, similar to the observation of Woolleyand Keating (1961). Apart from the limited 20 range of the powder data of Hahnand Klingler (1949) and absence of relative intensity information by Newman et al(1960, no further x-ray information is available on Ga~Tea. Complete powderdata for Ga~Te, have been obtained with a Philips 1310 diffraotometer and CuK=radiation with pulse height discrimination in conjunction with NaI (TI) scintillationcounter.Preliminary experiments on air-oxidation of a-GazSe~ and GazTe3 indicated nofurther change in the powder patterns of each compound after 8-10 hr of air-oxidation at the given temperature and conditions of experiment. Hard stickingof the oxidised materials with fused-quartz or platinum crucibles in GazTes restric-ted their use as suitable containers for the oxidation experiment. Finally a chargeof approximately 250 mgm of --200 B.S. mesh powder of each compound wasthinly spread in a gold crucible and placed in a muffle furnace previously heatedto a Fredetermined temperature and kept overnight. The sticking of the air-oxidised Ga2Te3 powders was somewhat less when the gold crucible was used.During the oxidation period, the muffle was kept slightly ajar to allow easy accessof air and it was on this account as well as due to mains fluctuations that furnacetemperature varied + 10 C from the set temperature. X-ray powder photo-graphs were taken in 11.46 cm diameter Philips camera using CuK= radiation.

3. Res ults and discussionThe lattice parameter of Ga~Tes extrapo!ated from the high angle reflections in thepowder film using Taylor and Sinclair (1945) function was found to be 5.898 4-0.001 A. Table 1 gives the powder data of Ga~Te8 obtained with a Philips 1310diffractometer and CuK= radiation along with the previously reported data of Hahnand Klingler (1949). To measure the peak intensities of the weak reflections inthe high angle region, the divergence slit and the counting rate were suitablyadjusted and the intensity range necessary to accommodate the full pattern wasthus found to be 1000 -- 1 as given m table 1. The peak intensities up to 20 =30.3 corresponding to the 200 reflections are for the unresolved a~a~components,while for the sue0eeding reflections the peak intensity of the az component hasbeen given in table 1, the background having been smoothly drawn and allowedfor.A comparison of the present observations with the published powder data ofOa2Tea (table 1)showsthat the (111)reflection is actually the strongest peak in thediffraotograms in contrast to the (311) peak. The d~ values computed for theextrapolated a, -~ 5.898 & naturally agree well with the experimental values sincethe diffractometer values are not affected by the absorption correction in an angular-d ~ n d o n t manner as in the cylindrical specimen case. However, the computedvalues of a0 for the Hahn and Klingler data show a systematic decrease withincrease in 0 in the high 0 region which is quite the reverse of the trend expected


3/6

X - r a y s t u d y o f a i r - o x id i s e d p o w d e r s 5 7Table 1. Co mp arison of the present observ at ions wi th JCPD$~" da ta o n Ga~Tes.

Present invest iga t ionJCPDS data (card No. 5-734)

a0 = 5- 886 Ade,~. (A) us ing hklao = 5.8 98 A dobs. (3,) I / Io 1/Io dobs. (A ) ao (cal.) A

3.405 3.403 1000 111 60 3.393 5.8772" 949 2" 949 138 200 20 2" 938 5.8762" 085 2" 085 766 220 90 2.081 5.886! .77 8 ! .77 7 237 311 100 ! .771 5.8741" 703 !" 702 37 222 20 1.69 9 5.8861-475 1-474 43 400 40 1-472 5.88 81.353 1.353 102 331 69 1-35 t 5.8891.319 !.31.8 33 420 40 !.3 17 5.8901" 204 1.204 89 422 80 1.202 5.8891.135 1.135 40 333, 15 60 1.131 5.877I" 043 1.042 20 440 45 1.040 5.8830.9969 0.9968 23 531 80 0.9939 5.8800.9830 0.9827 8 600,442 20 0.9801 5-8800.9326 0.9326 !4 620 80 0.9296 5.8790.8994 0.8995 11 533 . . . . . .0.8891 0.8890 5 622 . . . . . .0.8513 0.8512 6 444 . . . . . .0.8259 0.8259 12 551, 117 . . . . . .0.8179 0.8180 3 640 . . . . . .0.7882 0.7882 2 2 642 . . . . . .

* Peak intensi t ies of the unresolved a~a2 doubl e t on t he d i f frac tome t e r cha r t s fo r ( I l l ) and(200) , and for the ax component of the subsequent re f lec t ions , above background.t Join t Co mm it tee on Pow der Diffrac t ion Standards .

f o r c y l in d r i c al s p e c im e n s if a b s o r p t i o n is t h e m a i n f a c t o r a f f e c t i n g 0 . O u r p o w d e rf i lm s al s o i n d i c a t e t h e s a m e i n t e n s i t y r e l a t i o n b e t w e e n th e 1 11 a n d 3 11 r e f l e c t io n sa s th e d i ff r a c t o m e t e r d a t a w h i c h a r e in c o n t r a s t t o th e H a h n a n d K l i n g l e r f i n d i n g sg i v e n i n th e c a r d . T h e o b s e r v e d u n u s u a l l y l o w i n t e n s i t y o f (4 4 4) re f le c t i o n c o m -p a r e d t o t h a t o f (6 4 2) c o u l d n o t b e e x p l ai n ed i n t e r m s o f t h e r a n d o m d i s t ri b u t io no f v a c a n t m e t a l s u b - l a t t ic e s i te s i n t h e d e f e c t z i n c - b l e n d e s t r u c t u r e w h e r e a llt h e r e f l ~ t i o n s h a v i n g h q - k q- l = 4 n s h o u l d b e s t r o n g , s i n c e f o r th e s e ] F ts =( f r , q - 2~3fan)~ a n d th e a t o m i c s t r u c t u r e f a c t o r s o f T e a n d G a d e cr e as e a l m o s t i nth e s a m e r a t i o i n t h e 0 r e g i o n c o r r e s p o n d i n g t o 4A.A. a n d 6 4 2 . T h e r e a s o n f o rt h i s d i s c r e p a n c y i s u n d e r i n v e s t i g a t i o n u s i n g s i n g l e c r y s t a l m e t h o d s .

T h e m a s s c h a n g e s r e c o r d e d o n a i r - o x i d a t i o n o f a- G a~ S e8 a n d G a 2 T e3 a t d i ff e r e n tt e m p e r a t u r e s a r e g i v e n i n t a b l e 2 . F o r a -G a ~ S es a s te a d y l o ss i n m a s s w a s r e c o r d e du n t i l i t b e c a m e p r a c t i c a l ly c o n s t a n t i n t h e t e m p e r a t u r e r a n g e 6 5 0 ( 2 -8 2 5 C , w h e r ex - r a y p o w d e r p a t t e r n s s h o w e d o n l y t h e f l - G a z O~ p h a s e . W i t h G a z T e a g a i n in m a s sw a s o b s e r v e d a t 5 5 0 C a n d a b o v e , a t e m p e r a t u r e w h e r e G a ~ T e O 6 a n d T e O z w e r ei d en t if ie d . A s t u d y o f th e x - ra y p a t t e r n s o f a - G u iS e 8 a l o n g w i t h t h e o x i d is e d p h a s e sa t d i f fe r e n t t e m p e r a t u r e s r e v e a l e d t h e p r e s e n c e o f a w e a k a n d b r o a d l i n e a t t h e


4/6

58 A K Mukherjee et alTable 2. Per cent changes in mass recorded in tl~,e air-cxidation of a-GuiSes andGa2Tes+, -- indicate gain/loss in mass.

Temperature of oxidation250C 350 C 450C 55 0 C 65 0 C 825C Calculated massgain/loss due tooxidation

a-Ga~Se~ --6% --16~ - - 2 5 % - - 3 9 % - - 5 0 % --49% ~ 50% lossGa,Tea Nil Nil Nil +14~ +25~ .. ~ 30~ gain

position of the strongest line of fl-Ga~Oa in the powder pattern of the oxidiseda-GaeSes material at 350 C, thus indicating the presence o f large enough crystallitesof the oxide even tho ugh the loss in mass had started at 250 C. The crysta llinephase analysis of the oxidised samples in the range 350 C-600 C showed a mix-ture of fl-Ga~O3 and the star ting material, a-GazSes. The diffuse character of thea-Ga~Sea lines with h. k. /, all odd, appeared to be unaffected by heating in air a tthese temperatures. Finally, at 650C the oxida tion of a-Ga~Sea was found tobe complete with the formation of fl--GazO3 as the x-ray powder patterns of thesample oxidised at higher temperatures of 750 C, 825 C did not change exceptthat the high angle lines became progressively sharper indicating larger and well-crystallised grains.For Ga~Tes, powder photographs o f air-oxidised materials in the range 250 C-450 C showed Te lines along with the start ing phase. It was interesting to no tethat the x-ray pattern of the mater ial oxidised at 450C exhibited several weakextra lines which could be indexed on the super-lattice cell of GaaTes with latticeparameters: a-----4.17, b = 23.60, c = 12.52A, as reported by Newman andCundaU (1963), in add ition to the Te and GazT8 phases. Table 3 gives the x-raypowder data of Ga~Tes air-oxidlsed at 450 C conta ining these lines. Subseqttentlya mixture of Ga~TeO6 and TeOz, former being the predominant phase, was observedin the range 500 C-600 C. With increase of temperature o f oxidation at 650 C,TeO~ (m.p. 733 C) lines disappeared f rom the x-ray powder pattern leaving GazToO~as the final weU-crystallised stable phase. The attainment of the cons tan t massin the case of a-GaeSes and of near theoretical gain in the case of GazTea can betaken as indicating the completion of oxidation according to the equations:

2Ga~Se~ + 30,. --, 2Gazes + 3Sea : mass loss ,-~ 50~ with evapora tion of Se.GazTes + 503 --* Ga~TeOe + 2TeO~ : mass gain ,~ 30. 5~ , with TeOz retained

in sample.The x-ray patt erns of the final oxidised products in both the eases, at 825 C fora.GazSe3 and at 650C for G-azTeg, were found to be in good agreement withthe published da ta for fl-Ga~O3 and GazTeOe respectively.

The process of air-oxidation of solid materials is principally controlled by thediffusion of oxygen atoms in the lattice. In a-GazS% the Se atoms are eliminatedfrom the lattice creating vacant sites for relatively easy diffusion of oxygen atoms.


5/6

X-ray study of air-oxidised powders 59This resul ts in the format ion of / / -GazO3, the most s table form of gal l ium sesqui-oxide.

For GazTe3 air-oxidised at 450 C, the presence of super-lat t ice l ines can beexpla ined as due to a volume fract ion of the ini t ia l mater ia l which t ransforms tothe larger cel l s t ructure und er the ac t ion of heat and rest r ic ted sugp ly of oxygen,while the outer layers of the grains suffer Te dissociation which is present on thegrains in the elemental form. However, the width of the GazTe3 cubi c (111) reflec-t io n at d = 3.405 A is such as to rule out th e presence of anoth er l ine close by atd = 3.37 A, indexible as (070) of th e super-lat t ice cell. This absence of (070)

Tab l e 3. Powder data for air-oxidised Ga2Te3 at 450C (upto 20,-~ 82).

Air-oxidisedGa~Tea at450 C Te (JCPDS No. 4-554) GaaTe~ (Present film data)GazTea super cella= 4"17Ab = 2 3 " 6 0 Ac = 1 2 " 5 2 A

dobs. (A) Relative dobs. (A) I/Io hklintensity d0bs. (A) Relative hkl d~al. A)Intensity hkl

4"26 vw3"85 w 3"86 20 1003"405 m3"243 s 3"23 100 Ol3-077 vw2.943 m2.757 vw2.557 w2"355 m 2.351 37 1022"232 m 2"228 31 1102.092 m 2.087 11 1111"974 w 1.980 8 0031-929 vw 1.930 4 2001"901 vw1"840 w 1.835 20 2011"784 m 1.781 7 1121"702 vw1"623 w 1.616 12 2021"478 m 1.479 13 1131.456 w 1.459 8 2101"422 w 1.417 8 2111-382 w 1.383 7 104, 2031"357 w1"314 w 1.309 6 2121"262 vw 1.257 4 3011.207 w1.178 vw 1.180 3 302

4"28 0423.408 s 111 3.404 043

3.09 0142.949 w 200 2.947 1032.761 O442.564 1712.357 1632.085 m 220 2.084 183

1.905 056, 2501.777 m 311 1-777 1461.702 vw 222 1.701 0861.474 w 400 1.474 206

1.353 w 331 1.352 1, 12, 61.318 vw 420 1.318 2861"204 w 422 1.203 189

vw -~- very weak, w ~---weak, m = medium, s = strong.


6/6

60 A K M u k h e r j e e e t a li s a t v a r ia n c e w i t h t h a t o f N e w m a n a n d C u n d a l l ( 19 63 ), w h o d i d n o t r e p o r t th espac ings and r e l a t i ve i n t ens i t i e s o f GazTea supe r - l a t t i ce l i nes bu t on ly t he l a t t i cep a r a m e t e r s o f t h e s u p e r -c e l l. T h e a g g l o m e r a t i o n o f t h e o x i d i s e d m a s s a n d i t ss t ic k i n g t o t h e c o n t a i n e r i n t h e c a s e o f G a z T ea a t 5 50 C , a n d a t 6 5 0 C , t e m p e r a -t u r e s w e l l b e l o w t h e m e l t i n g p o i n t o f T eO ~ i s m o s t p r o b a b l y d u e t o t h e f o r m a t i o no f i t s g l a ss y p h a s e , D u t t o n a n d C o o p e r (1 9 6 6 ), w h i c h a l so e x p la i n s t h e d i s a p p e a r a n c eo f t h e T e O , l i n es i n t h e p o w d e r p a t t e rn s .

Reference sD u t t o n W A a n d C o o p e r W C 1 9 66 Chem. Rev. 6 6 6 5 7H a h n H a n d K l i n g le r W 1 94 9 Z. Anorg. Chem. 2 5 9 1 3 5K h a n M Y a n d A l i S Z 19 78 P a p e r s u b m i t t ed to t h e I n d i a n N a t i o n a l C o n f e r e n c e o n C r y s t a l l o -

g r a p h y , V a U a b h V i d y a n a g a rN e w m a n P C et al 1961 Philips Res. Rep. 1 6 4 1N e w m a n P C a n d C u n d a l l J A 1 96 3 Nature (London) 2011 876T a y l o r A a n d S i n c l a ir H 1 9 45 Prec. Phys. Soc. (London) 5 7 1 2 6W o o l l e y J C a n d K e a t i n g P N 1 96 1 J. Less-Common Metals 3 1 9

1980 X-Ray Study of the Air-oxidised a-Ga~_Se8 and Ga._tea Powders

Documents

Transcript of 1980 X-Ray Study of the Air-oxidised a-Ga~_Se8 and Ga._tea Powders