Huntite-borate crystallization in stoichiometric melts

Journal of Crystal Growth 133 (1993) 181—184 North-Holland

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CRYSTAL GROWTH

Letter to the Editors

Huntite-borate crystallization in stoichiometric melts Valery I. Chani

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Kiyoshi Shimamura, Keiji Inoue and Tsuguo Fukuda

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai-shi 980, Japan Received 19 April 1993; manuscript received in final form 6 July 1993

3~,Li~, The crystal formation of huntite borates LnAl Ti4~, v5~,Mo6~ and Ge4~, was observed in 3(B03)4, stoichiometric where Ln systems is Yb after or a combination heating to 1340—1370°C of (Yb,Tm,Er)and doped cooling by Bito room temperature. All crystals grown by spontaneous crystallization were studied by X-ray diffraction powder analysis and only the huntite-borate phase was detected. The dependence of stability of the huntite-borate structure on crystal composition is discussed.

Single crystals of the huntite borates LnA1 3 (B03)4 have been the subject of increasing interest during recent years as promising materials for lasers and nonlinear optics [1,21.It is known [1—3] 3+ that in ± these crystals rare-earth elements Ln and Bi3 occupy trigonal prismatic (p) positions, Al3 + occupy octahedral (o) positions and can be completely substituted by Ga3 + and Fe3 and B3 + occupy oxygen triangles. Usually the huntite borates melt incongruently; that is why crystallization from high-temperature solutions is widely used for the preparation of these crystals. For practical applications it is preferable to grow these crystals from stoichiometric melts, but unfortunately there are few reports about congruently melting compositions for the huntite borates. Our previous papers [1,2]reported that within the investigated systems, the probability of congruent melting is higher for the (Yb,Er)Al3(B03)4 3 ± cations composition compared with and Bi3 ± forasp-positions and larger Ga3 + Ln or Fe3 + for 0-positions. The present paper reports on huntite-borate crystallization in stoichiometric melts based on this crystal composition. Thus it 15 PO5SF ble to consider the reported results as a first step

1

Present address: General Physics Institute, Vavilov Street 38, Moscow 117942, Russian Federation,

0022-0248/93/$06.O0 © 1993

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for growth of huntite borates by the Czochralski method. Thewith huntite-borate crystals grown from melts the composition givenwere in table 1. Starting materials were (4-9’s) powders. The charges were weighed out and fractional amounts were placed in covered platinum crucibles of about 50 ml volume, and the crucible was placed in a furnace. The furnace temperature was increased to T(1) during 10 h, kept during t(1) hours and cooled to T(2) during 0.5 h. For the crystal growth, the melt was cooled slowly from T(2) to T(3) during t(2) hours; all parameters of the temperature program are presented in table 1. After crystallization, the melts were cooled to room temperature during 10 h and the crystals were removed from the melt by hot HNO 3 and water using it was ultrasonic necessary vibration, to removeand the dried. crystalsSometimes mechanically because the mass of the crystallized huntite borate was more than 80 wt% of the initial melt and it was impossible to extract the crystallized matter by chemical methods. The obtained crystals were mostly transparent hexagonal rods, colored by Er3~,Ti4~and Mo6~ the colors of the grown crystals are presented in table 2. It is necessary to note that crystals 11-3,

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182

/ Huntite-borate crystallization in stoichiometric melts

V.1. Chani et al.

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Fig. 1. Fragment of the phase diagram of the Ln 203—Al203— B203 system for the 12
11-5, 15-1, 15-2 and 15-3 were slightly green, because the concentration of MoO 3 in The thesesize melts was higher as compared with others. of the crystals was about 1 X 1 X 1 mm. All grown crystals were studied by X-ray diffraction powder analysis (Cu Ka, 1.5418 and only the huntiteborate phase was detected. It should be noted that the uniformity of all prepared crystals excluding the 10-3 composition was relatively high as compared with our previous results [1,2]. The full width at half maximum (FWHM) for all peaks on X-ray pictures was about 20 = 0.1°—0.2°. All studied melt compositions were collected on the fragment of the phase diagram in fig. 1.

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The concentration of Ln203 in all studied melts was slightly higher than for theoretical stoichiometric composition (12.5 mol%), because as it

183

was shown in ref. [2] for Ho3

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Moreover, in our latest investigations we have found cationsthat substitute the concentrations AI3~ cations of Ybinand 0-positions. Er in the o-sublattice achieve 0.07 atoms per formula unit.

Thus for absolute of melt and crystal compositions it wascoincidence necessary to use systems enriched by Ln 203. It is important to note that todiagram, investigate the studied compositions on phase 2TiO2,

V205, 2MoO3 and 2GeO2 were added with the

Al 203 concentration, because the4~, cation sizes for V5~,Mo~ coordination number n 6 Ge =

and Ti4~are close to the size of the i:~cation in accordance with ref. [4].Moreover, the sizes of

these cations are related by 44’)
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Table 2 Composition, color and hexagonal lattice parameters a and c of the grown crystals Melt No. 10-3 11-1 11-2 11-3 11-4 11-5 13-3 13-5 15-1 15-2 15-3

Crystal composition

4+ (Yb,Bi)A13(B03)4 : Li + : Ge (Yb,Tm,Er)Al 6~ YbAl 5’~:Mo6~:Ti4~:Bi3~ 3(BO3)4:Li~:Mo YbAl3(BO3)4:Li~:V 6~:Bi31’ YbAl3(BO3)4:Li~:Mo 4~ YbAl3(BO3)4:Li~:Ti 6~ YbAl3(BO3)4:Li~:Mo6’~:Bi3~ YbAl3(BO3)4:Li~:Mo6” 3(B03)4:Li~:Mo (Yb,LiXAI,Mo) 3~ (Yb,LiXAI,Mo) 3(B03)4 : Bi 3(B03)4 (Yb,LiXAI,Mo)3(B03)4

Color White Pink Lilac White Lilac White White White White

a9.282 (A) 9.335 9.317 9.324 9.315 9.328 9.311 9.341 9.350

c7.203 (A) 7.262 7.264 7.246 7.247 7.251 7.247 7.258 7.266

c/a 0.776 0.778 0.780 0.777 0.778 0.777 0.778 0.777 0.777

White White

9.299 9.314

7.224 7.263

0.777 0.780

184

VI. Chani et al.

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Huntite-borate crystallization in stoichiometric melts

(Ln,Li,BiXA1,Me)3(B03)4 where Me is Ge, V, Mo and Ti. In this way the investigated system Ln2 03—A1203—B203 can be rewritten as (Ln203 + Li20 + Bi203)—(Al203 + Me20~)—B203. Using charge compensation principles for Li— Me pairs for the solubility limits, all studied compositions can be rewritten as: LiAlGe2(B03)4, LiA12V(B03)4, LiAl233Mo067(B03)4 and LiA1 Ti 2(B03)4. Our experiments are the first reported study of these compositions. We had three purposes for studying the crystal growth conditions for Ge, V, Mo and Ti doped huntite borates: (i) to decrease the melting point of these stoichiometric mixtures, (ii) to decrease the viscosity of the melts and (iii) to study spectral properties (color) of doped crystals. Moreover Li 20 is much cheaper than Ln 2°3• For practical applications the Li(Al,Mo)3(B03)4 crystals are more suitable as compared with other Li(Al, Me)3(B03)4 compositions because the bond energy for this composition is relatively low [1,2]. At the same timeLithese are almost colorless. That is why andcrystals Mo-containing huntite borates were selected for a more detailed study (compositions 15-1, 15-2 and 15-3). The best productivity for the crystal growth process (crystal/melt weight ratio) was achieved from the 15-3 melt (84 wt%). Using the melt 15-3 (table 1) composition, it is possible to represent it as a mixture of YbAl3(B03)4 and LiAl233Mo067 (BO3)4 (about 52:48 mol% or 59:41 wt%). Thus using the productivity of this melt (84 wt%), at least 25 wt% of the crystallized material was LiAl233Mo067(B03)4. By using the molecular weights of both materials, it is possible to recalculate that the prepared (Yb,LiXA1,Mo)3(BO3)4 crystals consist of at least 0.38 atoms of Li and 0.25 atoms of Mo per formula unit of huntite. We could not control melting of this mixture because all crucibles were covered during the heating—

cooling process. However, we did not use special equipment for preparation of homogeneous mixtures before melting. Therefore, we firmly believe that it was impossible to obtain 84 wt% productivity of crystal growth by solid state reaction. Moreover, should be noted that substitution 3’~and itAl3~cations by Li’~and Mo6~1S of Yb accompanied by an increase in the hexagonal

lattice parameters a and c as seen in table 2 (compositions 15-2 and 15-3). In general, the 15-3 melt composition differed from the 15-2 composition only in the concentration of Li2O and MoO3 (table 1). We tried to grow crystals of the huntite borates from a stoichiometric melt with composition similar to 11-5 by the Czochralski method and utilization of an open crucible and a RF furnace in normal atmosphere conditions. Unfortunately we had no success because it was observed visually that the evaporation of the melt at a temperature of about 1400°Cwas very high. The main cause of this phenomenon is thought to be the loss of B203. As a result, the composition of the melt was changed and we could not completely melt this mixture using a Pt crucible. We have reported the crystallization parameters of huntite borates LnAI3(B03)4 where Ln is Yb3~,Ge4~,V5~,Mo6~and or an (Yb,Tm,Er) combination doped Li’~, Ti4~ from by almost Bi stoichiometric melts. The crystals of (Yb,LiXA1,Mo) 3(BO3)4 have a concentration of Li and Mo not less than 0.38 and 0.25 atoms per formula unit respectively. For these crystals it was shown that substitution of a combination of Yb—Al by a combination of Li—Mo was accompanied by an increase in the lattice parameters of huntite. Our experiments indicate that formation of the studied huntite borates in stoichiometric melts is possible only in covered crucibles, because the evaporation of the melt was very high. Optimization of crystal composition is necessary for further preparation of huntite-borate crystals from the melt. The results of these experiments will be reported when they are available.

References [1] V.1. Chani, in: Melt Growth, Vol. 9, Future Crystal Growth Technology (IMR Tohoku University, Sendai, 1993) p. 304. [2] V.1. Chani, K. Shimamura, K. Inoue, T. Fukuda and K. Sugiyama, J. Crystal Growth 132 (1993) 173. [31Tech. L.I. Maltseva, 15 (1980)N.I. 35. Leonyuk and T.I. Timchenko, Kristall [4] RD. Shannon. Acta Cryst. A 32 (1976) 751.