Solution Methods

The solution techniques utilized to grow bulk ln203 crystals include hydrothermal, flux, and salt electrolysis. Although the hydrothermal conditions were studied for ln203 with the use of water or alkaline solutions, the growth of bulk ln203 crystals by the hydrothermal method has not been developed. Salt electrolysis was discussed rarely. The most frequently used solution technique for bulk ln203 crystals is the flux method.

Hydrothermal

A few studies on the growth of bulk ln203 crystals by the hydrothermal method were reported. Roy and Shafer [143] investigated the In203-H20 system in hydrothermal conditions, at a temperature range of25-800°C and pressure of 3.45 x 106-1.39 x 10s Pa. It was concluded that at least three phases exist in the In203-H20 system in a pressure range of 1.38 x 107 Pa to 9.65 x 107 Pa: (a) In(OH)3, which is stable up to about 260°C; (b) InOOH, which seems to be substantially stable between about 260 and 440°C; and (c) ln203, which is stable above 440°C. The reactions and, therefore, the formation of ln203 proceed as follows:

Best results for ln203 crystals were obtained at a temperature range of432-449°C, pressure 6.89 x 107 Pa, and growth time of 48- 96 h. At a higher pressure, 8.27 x 107 Pa, the growth time could be shorter (14 h). However, no information on crystal size and quality was given.

Christensen et al. [144] continued research on the growth of ln203 of Roy and Shafer by the hydrothermal method in the ln203- H20-Na20 system. Cubic ln203 was obtained at temperatures between 307 and 500°C, pressure of 9.5 x 106-8.1 x 107 Pa atm, Na20 concentration of 0-3.3 M, and growth time of 17-173 h. At certain growth conditions, also rhombohedral ln203 was obtained, usually in combination with InOOH. Another investigated system was In203-D20-Na20. Here, both cubic and/or rhombohedral ln203

was obtained as well. Information on crystal size, perfection, and growth habit was, however, not provided.

Flux

For growing ln203 crystals from high-temperature solutions, only PbO + B203 flaxes were used (Remeika and Spencer [133], Chase and Wilcox [145], Chase and Tippins [146], Chase [147], Chase and Teviotdale [148], Wen et al. [149], and Hagleitner et al. [150]).

The starting material consisting of ln203, the PbO + B203 flux, and possibly a dopant was loaded after mechanical mixing into a Pt crucible with a Pt lid, which was next placed into a furnace heated to the maximum temperature Гмдх = 1200-1320°C and held at such maximum temperature for several hours. Next, the furnace was slowly cooled down to a final growth temperature Ту = 500-1100°C at the rate of 1-10 K/h, and during that cooling, ln203 crystals were formed. At the final growth temperature Ту, the crucible was removed from the furnace and it naturally cooled down to RT, or the flux was poured after removal. The crystals were released from the flux by immersing the solidified flux in a hot solution of HN03 +

H20, which dissolves the flux but does not attack the ln203 crystals.

The obtained undoped crystals were typically yellowish or greenish plates with the edge size approaching even 10 mm [133], wherein the thickness was typically 1 mm, with a maximum volume of about 0.1 cm3. The reported dopants were Mg [146,150] and Sn [149].

Examples

(i) Remeika and Spencer [133] used as a starting material powders of ln203 (4 g), B203 (4 g), and 50 g of PbO (50 g), which were loaded into a 100 cm3 Pt crucible covered with a Pt lid. The crucible with the powders was placed in a horizontal, resistive furnace, which was heated up to TMAX = 1200°C and held at that temperature for 4 h. Next, the furnace was cooled down at the rate of 10 K/h to Ту = 500°C at which the crucible was removed from the furnace and cooled down to RT. The crystals were removed from the flux with a hot solution of HN03 + H20. The obtained crystals were slightly yellow-green and transparent prismatic plates of 10 x 10 x 1 mm3 in size.

(и) Chase and Wilcox [145] grew ln203 crystals from a 200 g starting material containing 5 mol.% ln203, 20 mol.% B203, and 75 mol.% PbO. The starting material was loaded in a 100 ml Pt crucible with a Pt lid having an opening for a thermocouple. The crucible was held at ГМАХ = 1250°C for 4 h in a large muffle furnace with very low vertical temperature gradients, cooled down to TF = 1000°C at the rate of 4-6 K/h, and then the crucible was removed from the furnace. It was found that at constant furnace temperature (125СГС), there were temperature fluctuations of 0.3 К for about 20 min, while during cooling down, the fluctuations were periodic steps of about 0.14 К approximately every 5 min.

At TMAX = 1250°C, crystals did not grow, but they did during cooling down to Tp. In the crystals, striations were observed, the period of which corresponded to the period of temperature fluctuations.

Using the same starting composition and maximum temperature, Chase [147] and Chase and Teviotdale [148] additionally used smaller, 50 ml crucibles, held time at T’max of 4 to 10 h, TF = 900,1050, and 1100°C to which the crucible was cooled down at the rate of 1 to 10 K/h, and at which the crucibles were removed from the furnace. In some cases, the flux was poured from the crucibles after removal from the furnace, while in other cases, the crystals were separated from solidified fluxes by soaking in hot 20% HN03 for 4 to 24 h.

  • (hi) Using a similar flux composition, crucible volume, maximum temperature, and holding time at the maximum temperature, Chase and Tippins [146] performed growth experiments of Mg-doped ln203 crystals. MgO concentration in the starting composition was 0-2 mol.% in favor of the PbO content. The furnace was cooled down to TF = 900°C with the rate of 2.3 to 4 K/h. At 900°C, the crucibles were removed from the furnace and naturally cooled down to RT. The crystals were grown at the crucible bottom and wall as well as on the flux surface in the form of plates. The ln203 crystals obtained from an MgO- free flux were black and had a size of 5 x 5x3 mm3, while those from the flux containing MgO were green to yellow with a size of 2 x 2 x 1 mm3.
  • (iv) Mg-doped ln203 crystals were also grown from the flux by Hagleitner et al. [150]. A Pt crucible with the starting material was heated up in a furnace to ГМАХ = 1200°C and held at that temperature for 4-10 h. Next, the temperature of the furnace was slowly decreased to TF = 500°C at the rate of 3 K/h, and then the furnace was turned off. The crystals were extracted from the solidified flux with a water solution of HN03 (HN03:H20 = 1:4). The obtained crystals were yellow plates of edge size 1-2 mm, as shown in Fig. 5.9. The crystals had multiple domains and high level of stresses (stress birefringence shown in Fig. 5.9).
ln0 crystals obtained from the PbO + B0 flux containing MgO

Figure 5.9 ln203 crystals obtained from the PbO + B203 flux containing MgO.

Reprinted with permission from Ref. [150], Copyright 2012, American Physical Society.

(v) Wen et al. [149] conducted the flux growth of pure and Sn- doped ln203 crystals. The starting material containing powders of ln203, Sn02, as well as flux components B203 and PbO (total amount 43.5 g), was mixed and loaded into a Pt crucible covered with a lid to minimize evaporation of the flux. Sn02/In203 molar fraction in the starting material ranged from 0 (pure ln203) to 20% (Sn-doped ln203). The crucible with the starting material was heated in a furnace up to T’max - 1320°C in ambient atmosphere at the rate of 60 K/h and held for 6 or 12 h. Next, the furnace temperature was lowered down to TF = 850°C at the rate of 5 K/h and then the crucible was removed from the furnace. The crystals were removed from the solidified flux by soaking the Pt crucible in an aqueous HN03 solution. The obtained crystals were typically 2x2xl mm3 in size and contained Pb impurities. In the case of Sn doping, the ratio of Sn/In in ln203 crystals never exceeded 1% pointing to the solubility limit of Sn02 in ln203.

Growth kinetics and crystal quality

Chase [147] noticed that ln203 crystals grown from the flux have {100} habit at high supersaturation modified by small {211} faces at low supersaturations, because {100} faces are more stable at high supersaturations, while {211} at low supersaturations. The growth proceeds by two-dimensional corner and edge nucleation followed by growth at screw dislocations. Two-dimensional nucleation requires much higher supersaturation than the growth at a screw dislocation. This is manifested by hillocks, the apex of which revealed after etching at least one pit that is associated with the screw dislocation. Growth on such screw dislocations requires lower supersaturations as compared with the nucleation on edges or corners.

The major defects are trapped flux elements and striations shown in Fig. 5.10. The striations have a thickness from <10 pm to 1 mm. The striations occur radially and are related to growth mechanism and growth rate. The reason for the striations could be temperature oscillations: one of about 0.07 К magnitude with a period of 5 min, and the other of random oscillations with a magnitude of 0.5 K. During the initial nucleation from edges and corners under high supersaturation, rapping of flux elements occurs. As the growth proceeds, the supersaturation decreases and the growth at screw dislocations dominates. Due to low growth rate at low supersaturation, there are lower local changes in the supersaturation and less flux impurities are trapped with a more uniform manner. Such growth mechanisms promoting faster initial growth at the edges and corners followed by a slower growth at screw dislocations generate different structure of striations and different concentration of flux elements from the edges and corners toward the center of the face, which in fact is concave at the central part with formed steps.

By etching the crystals in hot [90°C] 50% HN03 or in 20% HCl/80% HN03 for 0.5 min to 12 h, Chase and Teviotdale [148] revealed etch pits that are parallel to the intersection of the {111} and {100} planes. Extended etching period in HN03 (12 h or more] produced etch tubes at the bottom of the surface pits, which intersect the {100} faces parallel to the {111} planes. These etch tubes are dislocation related, caused likely by screw dislocations. The etch pits are always elongated parallel to the <100> crystallographic direction. About 5% of the crystals showed twin boundaries, which were 90° rotated about the <100> axis. The twins were generated during nucleation or shortly thereafter. The distribution of the pits was found uniform, and its density was at the level of 103 cm'2. An extended etching caused disappearing of most of the pits with remaining etch tubes. The source of the etch tubes extending from the etch pits is screw dislocations located either at the center of a dendrite (i.e., they were formed during crystal nucleation) or from inclusions trapped from the flux. Some of the etch tubes propagated not perpendicularly to a growing face, but also laterally and parallel to the growth direction, which can be associated with dislocation climb during crystal growth. There are also etch tubes, which changed propagation direction by about 90°. This may happen if a dislocation propagates parallel to the growth direction near the crystal edge, and it is captured by a slower growing face. Such dislocation trapping may cause changes in the relative growth rate of adjacent faces. The dislocations are growth centers and can be trapped by adjacent faces, which explain the observation that in solution growth, the majority of dislocations are located on the slowest growing faces. If the flux was poured after removal from the furnaces, extra etch pits were observed, which were formed likely by defects due to rapid growth since they were present mainly at the surface.

Striations in flux-grown ln0 crystals. Reprinted with permission from Ref. [147], Copyright 2006, John Wiley and Sons

Figure 5.10 Striations in flux-grown ln203 crystals. Reprinted with permission from Ref. [147], Copyright 2006, John Wiley and Sons.

According to growth experiments with Mg-doped ln203 crystals by Chase and Tippins [146], the crystals were grown at the crucible bottom and wall as well as on the flux surface. Those grown on the flux surface were plates with all {100} faces present, but in the case of the crystals at the crucible wall or bottom, one {100} face was missing. In203 crystals grown from an MgO-free flux had striations and flux elements trapped therein due to supersaturation. The crystals grown from the flux containing MgO had no striations and trapped flux elements. These differences in growth habit can be explained by MgO acting as nucleation centers, therefore avoiding high supersaturation necessary to grow nominally pure ln203 crystals. Lower supersaturation in the case of the MgO-containing flux allows for a slow and more uniform growth, minimizing at the same time the incorporation of flux elements and creation of striations, which are related to temperature fluctuations and associated incorporation of impurities (flux elements} following those temperature fluctuations. The concentration of Mg in the obtained crystals was 0.16 and 0.23 wt.% for Mg concentration of 1.75 and 3.5 wt.% in the flux, respectively. This leads to the segregation coefficient below 0.1.

The undoped ln203 crystals showed in absorption spectra free carrier absorption indicating semiconducting behavior, while those doped with Mg showed no free carrier absorption. This means that Mg acts as a compensator for electrical conductivity.

Hagleitner et al. [150] noticed that the flux-grown ln203 crystals had multiple domains and high level of stresses. The crystals also had a very high concentration of impurities (in wt. ppm): Pb = 4307 ± 74, Mg = 1388 ± 42, Pt = 155 ± 28, and other impurities (Zr, Sn, Sb, Nd, and Bi] < 50.

Salt electrolysis

Application of molten salt electrolysis to grow ln203 crystals was demonstrated by Teweldemedhin et al. [151]. The salt contained fine powders of ln203, Li2Mo04, and Mo03 in the molar ratio of 0.4:1:1 and total weight of about 39 g. The oxides were loaded into a porcelain or alumina crucible with an immersed Pt cathode of 0.5 * 2 cm size and Pt anode of 1 x l cm size. Electrolysis was carried out at 840°C for 6 days by passing a constant current of 35 mA.

The crystals obtained on the cathode from alumina crucibles were green-yellow, while those obtained from porcelain crucibles were dark green. If the amount of ln203 in the starting material was less than 0.4 mol, at a higher temperature (900°C) only very small crystals grew; however, at lower temperatures, Mo02 crystals were formed. Generally, larger ln203 crystals and larger quantity thereof were obtained with higher ln203 molar ratio, and at lower operating temperature. Usually ln203 crystals grew on the upper part of the cathode, which was not entirely immersed in the salt. The ln203 crystals were of size ~1.6 x 1.1 x 1 mm3. The crystals obtained in porcelain crucibles contained (in wt.%): Mo < 0.09, Li < 0.04, A1 ~ 0.13, Si ~ 0.2, and Sn < 0.06. The crystals obtained in alumina crucibles had similar impurities, but at twice lower concentration, except Sn. The color of the crystals did not change after annealing in non-reducing conditions (air, 02, inert gas) at 950°C for several hours.

 
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