Gel growth of single crystals of barium and strontium tungstates

Gel growth of single crystals of barium and strontium tungstates

I Journal of Crystal Growth 18 (1973) 199-201 © North-Holland Publishin~ Co. GEL G R O W T H TUNGSTATES OF SINGLE CRYSTALS OF BARIUM A N D STIq:O...

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Journal of Crystal Growth 18 (1973) 199-201 © North-Holland Publishin~ Co.

GEL G R O W T H TUNGSTATES

OF SINGLE CRYSTALS

OF BARIUM

A N D STIq:ONTIUM

A. R. PATEL and S. K. ARORA

Department of Physics, Sardar Patel University, Vallabh Vidyanayar, Gujarat State, India ,"eceived 29 September 1972; revised manuscript received 12 November 1972 An account of the growth of single crystals of the tungstates of barium and strontium in ge.ls has been presented. While "'impure" gel has been found to produce three-dimensional spherulites, good quality single crystals have been obtained by using "'pure" gel. Some aspects, such as (i) crystallization apparatus, fib effect of pH value, temperature, and concentration changes in reactants, have been studied and described. A simple procedure to control nucleation sRes in gels has been adopted, which has resulted into a dramatic increase in the size of the crystals.

A m o n g s t a w i c e variety o f techniques prevalent

ployed to take place in the gel medium:

today for the grow ih of single crystals, gel technique

BaCI2+Na2WO4 ~ BaWO4 $ + 2 Na + + 2 C 1 - ,

has gained considerable m o m e n t u m mainly due o the

SrCI2 + Na2WO4 --* SrWO4 ~ + 2 Na + + 2C1-.

work of Henisch t- s) The present report deal: w~th the growth oI stngle crystals of the tungslates of b m u m and strontium tn sthca gels

When N/10 BaCI2 and N/10 NazWO4 were used es the feed solutions, the nucleation centres were tremendously large. Thre>dimensmnal spheruhtes with 0 few

E ~pcrimcntal and oh ~ercatzon v

cm

The crystallmzalton apparatus used here are ~l) ,~tngle-tube-sy,,tem and Ilt} c~uble-lube-system, suggested by Patel and Bhat"), and are depicted dmgramatical~y in hgs. ia and lb. I1 was verified that smaller tubes have been Rmnd to impmr the size of the crystals. These two systems have an add~tmnal attractive advantage of prowding a reasonably good facdtty for inveshgat~ng the possibility of c~)'stal growth on seeds in gels The crystals g~own ~n the tube {~n either system) from one gel can be seeded as such ~nto another gel before setting by sm~ply transferringthe tube, conta|mng the ~.rvstals embedded ~n gel ~nto another beaker conl a ~ n g a fresh gel .,,olt,:am Further growth may be iheret~y al~m'ed to take -place on the ~hu,,-,;eed÷d

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F~g. I Crystull~zahon apparatus with d i m e n s i o n s (a) single-tube-system; (b) double-tube-system.

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crystals ,\,,oa~a~ab~c ~ommerv'~al gr:'dc sod~urr~ s~hcale was d~ssotved ~n d~st~lled water to gtve a resulttng spemfic F~av~tv of l 05 to which was then added the reqmred quanl~ty of 2N acetic ac~,d so as to result m gel solution of 7 '~ pH value l~n order to g~ow BaWOa artd SrWO,~ crys~a~so the lk~ttow~ng chcmtcal reactions w~ :e era-

2 cm

indicated:

200

A. R. P A T E L

A N D S. K. A R O R A

tree-leaf-hke crystals of size up to 3 mm were obtained. Attempts were also made to grow the crystals at different temperatures between 20 °C and 38 °C, but the re, ults obtained were Identical, proving thereby that the temperature did not produce any noticeable effect on either the size or the quality or the morphology of the c~3:stals. The gel pH also was not found to be a very s~gmficar.t factor in deciding the size or perfection of the cr..-stals. However, working with pH values between 7 and 8, the crystals grew fewer in number and slightly bigger in size than those obtained from gels of other pl-i values. Studies on the influence of the change ~n concentration of nutrients used showed that (i) clearer spherulites and leaf-shaped crystals were

(a) (b) F~g 2 ta) Single crystals o f BaV/O,, grown ~! "'pure" gel ~,cate ~n ram) tb) Single cystals o f SrWO4 grown b - " p u r e " gel ISca]e

III, r l I l l ,

grown using higher concentration (N/10) of Na2WO4 and relatively lower concentration IN/20) of BaCIz, and ~ lower concentration (N/20 and less )of Na2WO4 and higher concentration (N/10 and more) of BaCI2 produced poorer and opaque crystals. It may be conjectured at present that the spheruhtes are not due to faster precipitation because they have been formed even with very dilute nutrient solutions. It was thought, therefore, :.o ascertain whether their formation arises due to some impurities present in the gel. To do that, further investigations on the growth of BaWO~ and SrWO4 crystals, were conducted with "pure" get which was prepared b~ filtering out the suspended ~mpurities from sodi,~m silicate and treating it as usual to give the resultant sp.gr. 1.05 and pH 7.5. With this "pure'solution, single cry: "als up to 4 mm across were

obtained. Furthermore, relatively higher strength, e.g.: N/2 or N/5, of Na2WO4 and lower strength, e.g. N/5 or N/10 of BaCl2 (and SrCl2 in the case of SrWO4) in the case of BaWO4 crystals gave quite regularly faceted and transparent crystals up to 5 mm across. Some of these are shown in fig. 2. It may conceivably be said, therefore, that abnormal growth, such as spherulites, can be prevented by removing the susp-.nded impurities (which may probably be Na, Si, Ca) in sodium silicate. Nucleation control

Crystal growth using gels is very simple. Equally difficult is it to control the nucleation sites in gels. We have tried the process of seeding of the crystals from one gel system to another, but this did not produc: an appreciable change in size or perfection. Instead, this resulted into a large number of unavoidable secondary nucleation centres. We have, however, been successful in limiting the excessive nucleation in the following way. For growing BaWO4 crystals, 20 ml of 0.02 N BaClz was poured in tube 2 and 20 ml of 0.03 N Na2WO4 in tube 1 after the gel was properly set. Then the strength of both the feed solutions was increased gradually by changang the concentration of the freshly added reactants at the rate of 0 025 N per day. This was done every day by removmng 15 ml of both the nutrients from theIr respective tubes and replacing them by an equal amomlt of concentrated solution This process was continued up to seven weeks and the crystals were harvested in the eighth week. It is considered that beginning with very dilute reactants, ionic diffusion through gel may be slow and sup:rsaturation small. Under these circumstances, comparatively few nuclei are formed. On increasing the concentrations, I further growth of the existing nuclei would be preleren= tml to the for matlon ofaddition ones. A typical BaWO~ crystal thus grown is shown (upper one) in fig. 2a, whil~ fig. 2b shows (upper one) a crystal of SrWO4. Thl¢ "'~ particular procedure, has given a dramatic increase ia the size of crystals~ which grew up t.o .abou! 7 m ~ across, as is evident from fig. 2. The crystals were identified by chemical avaly~is X-ray diffraction techniques. The crystals were fc to have a tabular habit with {001}, 1tl0} ant" { habit laces. Both BaWO~ and SrWO~ exhibited feet {001} cleavages.

GEL GROWTH OF SINGLE CRYSTALS OF BARIUM AND ~ STRONTIUM TuNGSTflT'ES,' ~" ,. !~ ~ . ]~i'~

References 1) H. K. Henisch, J. Dennis and J. I. Hanoka, J. Phys. Chem. Solids 26 (1965) 493. 2) H. K. Henisch, J. I. Hanoka and J. Dennis, J. Electrochem. Soc. 112 (1965) 627.

3) J. Dennis, H. K: Herdsch 'and P. Chcrin, J. Electrocheat. Soc. 112 (1,65) 1240. 4) J. Dennis anG H. K. Henisch, J. EIectrochem. Soc. 114 (1967) 263. 5) H. K. Henisch, Crystal Growth in Gels (Pennsylvania Univ. Press, 1970). 6) ~ R. Patei and H: L. Bhat, J'. Crystal growth~12 (1972) 28&"