The growth of “thick” single crystals of Bi4Ti3O12 from bisnuth borate solutions

Journal of Crystal Growth 35 (1976) 36--40 © North-I lolland Publishing Company

THE GROWTH OF “THICK’ SINGLE CRYSTALS OF Bi

4Ti3O1 2FROM BISMUTH BORATE SOLUTIONS

T.M. BRUTON

*

Ii.Ju/lard Research Laboratories, Red/u/I. Surrey, England Received 11 April 1975 revised manuscript received 28 March 1976

Earlier work has shown that low growth temperatures arc favourable for the growth of Bi4Ti3O1 2 and that the addition of B203 allows growth at temperatures lower than the minimum for the Bi203 -—Ti02 system. This paper describes the growth of single crystals by slow cooling with spontaneous nucleation in an open crucible for a number of Bi203— B203 — Ti02 compositions. An optimum composition was found from which crystals measuring 35 X 35 x 2 mm were grown. The critical variable in tise growth of Bi4Ti3O1 2 was shown to be the width of the metastable region of supersaturation.

1. Introduction

which reproducibly yielded single crystals of dimenSiOilS up to 35 X 35 X 2 flint.

Bismuth titanate Bi4Ti3O17 is a layer-structure ferroelectric in which the c domains may be switched by low voltages giving good optical contrast when viewed in a suitable manner [1]. This effect may be utilised for erasable optical mass memories and in display devices [2,31.One of the major problems has beers the production of large crystals of good optical quality. The layer structure results in a strong platelike morph. ology and crystals a few hundred microns thick but having large surface areas have often been produced

2. Experimental A vertical tube furnace heated by silicon carbide rods was used and the temperature controlled by a West Viscount 3 term controller with a Pt/Pt I 3~7Rh thermocouple as the sensing element. The mdl was contained in a cylindrical platinum crucible (less than 40 ppm rhodium) ol dimensions 37 mm internal diam-

141. In earlier work a method for the growth of crystals 10 X 10 X I miii using the system Bi203—Ti02 was described and it was observed that the lowest temperatures were most favourable to the growth of Bi4Ti3O12 [5]. In this technique a seed crystal was introduced into a presaturated solution. Spurious nucleation also occurred on the seed supports and in general spontaneous nucleation yielded a thin crystal which was the source of secondary nucleation later in the growth. A number of additions to the binary system were tried. B203 was the best and a tentative phase diagram was constructed. This paper describes the optimnisation of crystal growth by spontaneous nucleation in the Bi703—Ti02— B703 system *

eter. 36 mm high and 1 mm wall thickness, supplied by Johnson Matthey Metals Ltd. The starting materials werc spectroscopically pure grade (Johnson Mattliey Chemicals Ltd, grade I) bismuth oxide, titanium oxide and boric acid (l13B03). The principal impurities were iron and chromium in amounts of the order of 10 ppm atomic in the bismuth oxide, but no impurities were detected above the 1 ppm level in the titanium oxide or the boric acid. Appropriate weights of the component oxides were loaded into the crucible arid incited in a separate muffle before the crucible was placed in the furnace. The crucible was supported in the furnace by an alumina tube inside which the thermocouple was positioned so that the junction touched the base of the crucible. The crucible was heated to a soak temperature -~ approximately 30°Cabove the anticipated crystallisation temperature at a rate of 50°C ii I arid held for 16 Ii. A monitor thermocouple was then placed in

. . Present address: Morgan Tlsermic Teehnolo°icalCentre, Bewdley Road, Stourport on Severn, Wores. DY13 80R,

England. 36

TM. Brriton / Growth of thick single crs’stals of bismuth titanate

the surface of the solution and the temperature was adjusted manually to within a few degrees of the crystallisation temperature. The monitor thermocouple was removed from the solution which was then cooled at 1°Ch~ through 200°C.The solution was exammed periodically and if nucleation did not occur in the centre of the surface the temperature was increased, the crucible position adjusted and the cooling programme restarted. At the end of the slow cooling cycle the furnace temperature was lowered to 800 C at 50°Ch~ and the furnace switched off. After a further 4 hours the crucible was removed from the fur. nace and the Bi4Ti3 012 crystals were removed from the solidified matrix by leaching in hot concentrated hydrochloric acid diluted with an equal amount of water. A large vertical temperature gradient in the solution is desirable for the growth of Bi4Ti3O12. in this series of experiments the vertical temperature gradient in the liquid was 2°Cmmt which is close to the 3°Cmm~. measured external to the crucible, preferred, by Morrison et al. [51.This large gradient served two purposes. One was to stir the solution by natural convection and the other to stabilise the crystal interface by decreasing the degree of supersaturation away from the crystal. Although in very viscous melts (~-‘40mole% B203) there was little convection and nucleation occurred throughout the solution, Crystals were grown from varying B203 compositions in the range between 86mole%Bi203—l4mole% Ti02—Omole%B203 to 45%Bi203—14%Ti02—41% B203. In some experiments 12 mole% Ti02 was used as the starting composition (see table 2).

3. Results and discussion The addition of B203 radically affected the mode of growth of Bi4Ti3O12. The results can be broadly classified into three regions according to B203 compositions and are summarised in table 1. The distribution of the crystals in the solidified matrix was a characteristic of each region and is illustrated in fig. 1. In region I a single crystal nucleated initially and covered the surface but further crystal plates nucleated below it and in region II a number of crystals nucleated at one point on the surface forming a thick plate with several grains. In general crystals in region II were less

37

Table 1 Three Bi

4Ti3O12 growth regions in the systems Bi203—Ti02-B203 for varying B203 contents Region Composition range Crystal product (mole% B203) ____________

-______

1

0—18

11

18—35

____________

Yellow twinned crystal plates up to 1 mm thick Single plate, pale yellow

up to 2 mm thick, contaming several grains III

35—45

Many thin poor quality plates grey-green

___________ ._

______________—________________

twinned than region 1. (Fig. 2 shows a typical region II crystal containing few twins.) An optimum composition was sought in which a single crystal could be nucleated and grown without any subsequent secondary nucleation. Fig. 3 shows the number of single crystals nucleated at different B203 levels in the solution. Each point on the plot was an average of several growth runs. A minimum number of crystals, usually one, was obtained at 20 mol% B203 and the preferred cornposition for growth was 68mole%Bi203—l2moIe% Ti02—2Omoie% B203. Using this composition a numher of large single crystals have been grown with dimensions of 35 X 35 X 2 mm and two such crystals are illustrated in fig. 4. Both crystals have inclusions at the centre and at the edges where the crystals touched the crucible wall. Fig. 2 shows the twin structure of crystal B. The size of the inclusions in crystals grown from the optimum composition varied from crystal to crystal, depending on the degree of thermal asymmetry in the liquid.

—~

~ mole ..

1835 ~

B2 03

mole

B2 03

3~-45 mole °/~B2 03

-

l~ mg. 1. The effect of the addition of B203 to the solution

showing the three types of crystal plate morphology which occurred and their approximate extent in B203 content.

TM. Brriton / Growth of thick single crystals of bisomut/i tita,ratc

38

~

a

B~03

1mg. 2. A single crystal grosvn from 68~B005, I 20 1 nO2. 200 B2 03 showing twin structure -

The imisprovememit in crystal qtnalihy amid size that was obtained by the addition of boric oxide warrammhs sonic dnscussiomm of the role of B203 iii this growths systemis. The following model was postulated as being consistent wills all the kmiown experimental facts. It is

0100

~ 10

/

/

~

1

/

/

/

/

/

/

/3

3

~L 0

10

20

Concentration

(mole

3/3

B203)

Fig. 3. The graph of number of crystals obtained in one growtlm run as a function of tIme B203 content of the solution.

suggested that time most important role played by the B 203 was to mmmci ease the widths of thc nict istablc mc from] in which the solution w is supcm slum tIed but iso mmdc thou occum med The mode of ~i ow hit in thc ss s tem Bu203 TrO-, us such that as the solution is cooled a crystal is isucleated on the surface amid rapidly cxpamids laterally to cover the melt surface. Growth is then only possible us the 1001] du ectiomi w Iimcli is t slow gi owthm direction A high commccmih m itiomi .,i adicmmt builds up below the ~ owing cm ystal iii on dci to m each the lar3e drivimig force needed for 0i owth us ths it du cc tion A point is mc melted whe n the supc m satu m i lion mc quircd to milammitammi growth is gi c itci than that n cqulic d (or spontaneous nude it ions amid addil mis ii em ~ stals thems grow below tlmc c xmslin0 p1 tIc It is kitown I hi it time mddmtioms of boric O\ioc mmici c iScS the svidt It of time imietastable region [81 so that in growth from 13i7 03 -~TiO a crystal again miucleates amid covers the liquid surl ace but the supersaturation rechuired or comitinued growths is now less than that required for miucleatiomi of a new crystal. however, the additiomi of time boric oxide also suppressed the initial msucieaiiomt of a crystal on the surface so that at the sight boric oxide comitemits a hugh supersaturation builds up before miucleatiomi and when the crystal does nucleate there is a rapid unstable imtial growth givimig a dendritic core amid a mnultigrain crystal plate. At very highs B203 coiltemit the supersaturatiomi for the initial nucleatioms is so high that omice nucleation occurs many crystals form throughout the melt. The optimimuism boric oxide addition is the omme which broadens the imielastable region sufTiciemitly to maintain stable growths after ntncleation but not so much as to cause mmmultigraimm formation after miucleatiou. In practice this was not achieved. An inclusion formed during the imiitial nucleation amid propagated through the crystal: some secondary nocleation occasiotnahly occurred when the crystal was more than 1 .5 mm thick. Experimental evideisce was found for the increase in width of the nmetastable region with B203 addition. The width of metastable region was measured by a thermobalamice technique [6] in which the weight of a small crystal was nsonitored in a slowly cooled unsaturated solution and time saturation temperature was taken as the temperature at which imo loss in weight occurred. The umsdercoohimig required for miucleatiomi was found by weighung a platimsum wire in the same solution amid fimsding (lie ternperature at which crystallisatiomi occurred on time wire.

TM. Bruton / Growth of thick single crystals of bismuth titanate

39

~i*P’hicrnLjb lig. 4. Two crystals grown from 680 Bi

203, 12% Ti02, 20’/~B203 in transmitted non-polarised light.

The width of the metastable region was found as 4°C for 86rnole%Bi203—l4mole%Ti02 and was approximately 10°C for 68mole%Bi203—l2mole%Ti02— 2Omole%B203. The addition of Si02 instead of B203 should have the same effect in broadening the metastable region since both are chain forming oxides. Some compositions containing Si02 were tried but at low Si02 contents there was no appreciable effect (as in the case of B203), and at higher Si02 contents Bi4Ti3012 was not formed. Ge02, which might be expected to behave in the same way, had been added and some thickening of the crystal noted [7]. As there is only a narrow range in which a single crystal is nucleated and as B203 compositions in region II can supportsupersaturation without nucleation a seeded solution growth technique should give large inclusion free crystals. When seeded growth was attempted a number of difficulties were encountered, The seed crystal must be supported with the plate parallel to the solution surface as any other alignment results in many fine plates randomly nucleated from the seed plate [5,7]. Since the seed plates were thin

when the nucleii merged, inclusions formed at their junction [5]. Because of the difficulties with seeded growth and because spontaneous nucleation gave thick good quality crystals with inclusion-free areas greater than the device requirements, spontaneous nucleation was used as the preferred growth technique. Luke has recently reported that impurities at the 100 ppm level have a very significant effect on the switching properties of the ferroelectric domains [9]. Mass spectroscopic analysis of crystals grown in the present work are summarised in table 2. The principal impurities are Pt and B, but these did not appear to have a significant effect on the ferroelectric properties of the crystals. The boron level in the crystal increased rapidly from 2 ppm concentration in crystals grown from Bi203—Ti02 alone to 200 ppm when 4 mole% of B203 was added. There was no apparent further increase in the boron content of the crystals with increasing B203 content in the solution. That the boron level has little effect on the ferroelectric properties was confirmed by coercive field measurements which gave the very low value of 190 V cm 1 for c domain switching in a crystal with 2 ppm boron and also in a

the platinum wires which supported them inevitably

crystal with 200 ppm boron. Full details of the ferro-

came into contact with the solution and formed sites for spurious nucleation. It was also found that unless

electric properties will be reported elsewhere [10]. The other impurities are well below the levels indicated

the seeding was precisely controlled so that some dis-

by Luke to be significant. The grey-green colour ob-

solution of the seed occurred prior to growth, nucleation occurred at several points on the seed plate and

served in crystals grown from high borate content fluxes correspond to a high chromium impurity con-

151. Bruton / Grout/i of thick sing/c crystals of bismuth tita,ratc

40 Table 2

Impurities determined by mass spectrometry in crystals of Bi4TisOma grown in the ternary system Bi0

203 — B2O~--Ti02 ( impu-

rities in ppm alomuic) (‘onsposition of solution

Colour

Boron

Platinuns

Iron

Cbmromium

Strontium

Calcium

Magnesium

Yellow Yellow Yellow Pale yellow Pale yellow Grey greens Grey green

0.3 2 200 40

70 70 150 100

4 21.5 20 10

3000

150

20

150 400

30 60

10 50

0.1 0.3 2 3 7 15 40

0.4 2 0.6 0.7 0.2 0.1 0.1

30 20 20 6 8 4 7

<6 <6 <8 <8 <7 <20
Bi203/B203/ Ti02 ( nriole’t ) 86 86

82 70 56 48 45

0 0 4 18 32 38 41

14 14 14 12 12 14 14

centration (~“~40 ppm). Results on crystals grown in earlier studies showed that a marked greyish colour was obtained in crystals with sigrsificammt. rhodium contemnts (~l0 ppm) amid for this reason platinum crucibles with how rhmodiommi content (‘-~40ppm) were used amid thsermiiocoople junctions were placed in the solution only tor long enough to take a single teniperature miieasuremiiemit . . . . . The difficulties of contaitmmmsg bismuthn oxide melts in platinum have beets reviewed by Robertson [11]. It has been found in this study that the use of mixed hismutli oxide-- boric oxide fluxes prolomiged the crucible lute compared with their use with bisniothi oxide melts -

alone as less platmnonn was dissolved from the platinum wail amid grain boundary failure occurred less readily.

I gratefully acknowledge the close co-operation of -

.

Mr. A.J. Fox and Dr. E.A. Fletcher for their measure.

References Ill S.F. Cumnnmns and EL. Cross, J. Appi. Plsys. 39(1968) 2268. 121 S.A. Kenneman, G.W. Taylor and A. Miller, lerroelectries 1 (1970) 227. 131 W.J. Takem, NP. lormugoni and M.H. Francoinbe, AppI. PIss’s. Lett. 22(1974) 26. . 141 AD. Morrison, l.A. Lewis and A. Miller, lerroelectrics 1 (1970) 75. [51 TM. Bruton, Ferroelectrics 7 (1974) 259. 161 TM. Bruton, J. Solid State Chemis. 9 (1974) 173. 171 D.J. Epstein, US Air Force Avionics Lab. Tech. Report

181

Acknowledgements

-

I am grateful to Messrs. J .A. Roberts, J .B. Ciegg amid 1.M. Gale for time crystal analyses, and to my colleagues Mssrs. 0.F. Hill, P.A.C. Whiffin amid Dr. J.C. Brice for much encouragement amid good advice.

.

.

.

mnents of the electrical properties of the crystals and in

defining die objective for crystal growth development.

AFAL-TR-TO-172 August (1970). D. LOwell and H.J. Scheel, Crystal Growth from High Tensperature Solution (Academic Press, London, 1975).

191 T.E. Luke, J. Appl. Phys. 45 (1974) 1605. 1101 A.J. lox Philips Res. Rapt. to be published. -,

[lt~ J.M. Robertson, J.P.M. Damen, H.D. Jonker, M.J.G. van Hout, W. Kamnminga and A.B. Voernians, Platinum Metals Rev. 18 (1974) 15.