J. Japan. Assoc. Min. Petr. Econ. Geol. 76, 253-261, 1981 CRYSTALLIZATION AND PHASE TRANSFORMATION OF ANATASE UNDER THE HYDROTHERMAL CONDITIONS OF CONTROLLED Po2 YOSHIRO OHTSUKA Institute of Geoscience, The University of Tsukuba, Ibaraki YOSHINORI FUJIKI National Institute for Researches in Inorganic Materials, Ibaraki YOSHIO SUZIKI Institute of Geoscience, The University of Tsukuba, Ibaraki The control effect of Po2 by oxygen buffers was hydrothermally examined, and it was observed that the oxygen buffers of HM and NNO react effectively for longer time 300 hours using an H20 solvent at 530°C under 1000 atm. The hydrothermal reactions in the system TiO2O•EnH2O-KF-K2HP04-H20 produced anatase, potassium hexatitanate and rutile with increasing temperature, and it was not found the effects of Po2 on the temperature range of their formation. Anatase grown by controlled Po2 is characterized by the increase of c parameter with decreasing Po2. Anatase-rutile transformation was examined under the hydrothermal conditions of controlled Po2. The rate equation of this transition abides by the Avrami model equation. It was found that the kinetic data of pure and stabilized anatase are expressed by the first order equation in an H20 buffer. However, the stabilized anatase abides by the higher order equation than n=1 in the buffers of HM and NNO. by hydrothermal solutions. Occasionally, INTRODUCTION this mineral occurs in pegmatites, in druse Natural TiO2 minerals occur in three in granite and in veinlets in diabase, diorite polymorphs; rutile (tetragonal), anatase and other igneous rocks (Palache et al., (tetragonal), and brookite (orthorhombic). 1963). Artificial anatase is prepared by In these phases, only anatase (or perhaps various method; the treatment of titanium brookite) is known to be metastable with salts in aqueous solutions, and the oxidation respect to rutile over the wide range of or the hydrolysis of organic compounds of temperature and pressure (Navrotsky and titanium, although the growth of single Kleppa, 1967). crystals is difficult (in Gmelins Handbuch, Natural anatase occurs in an association 1951; Czanderna et al., 1957). Anatase with quartz, rutile, brookite, hematite and single crystals are hydrothermally grown other minerals in vein or crevice deposits of at relatively high temperature by using the so-called Alpine type formed from suitable stabilizer and solvent (Izumi and alteration and leaching of gneisses or schists Fujiki, 1976). (Manuscript received November 29, 1980) 254 Yoshiro Ohtsuka, Yoshinori Fujiki and Yoshio Suzuki It has been well known that the transi (Pt-(Au)) ), which placed in a cold seal tion rate in anatase-rutile transformation type pressure vessel. The starting material is retarded by the presence of anions such as was sealed in the small Au capsule (4mm in P043-, SO42-, No3-, Cl- and F- (Knoll and I. D. and 15mm long) having a small hole; Kuhnhold, 1957; Rao et al., 1959), and of this capsule and the solvent were placed in some metal oxides such as ZnO, A1203, Na20, the dissolution zone in the middle Pt W03, Nb205 and Si02 (Rao et al., 1959; lida capsule (5mm in I. D. and 70mm long), and Ozaki, 1961; Hishita et al., 1978; which was sealed; furthermore, this double Suyama and Kato, 1978). This retardation capsule and oxygen buffer containing water effect is explained in terms of the stabilized were placed in the large Au capsule (6mm in structure of anatase. Besides, the rate I. D. and 90mm long), which was sealed and the temperature of the transition are (Fig. 1). known to be greatly affected by reducing conditions and total pressure (Shannon, 1964; Vahldiek, 1966). Osborn (1953) has reported that the reaction boundary between anatase and rutile is dependent upon total pressure under hydrothermal conditions. In the present work, the effect of Po, on the formation of anatase and phase trans formation from anatase to rutile are in vestigated under the hydrothermal condi Fig. 1. The cutaway sketch of a run assembly. tions of controlled Po2. (1) thermocouple (2) pressure vessel (3) large Au capsule (4) Pt capsule EXPERIMENTAL (5) solvent (10% KF+2.5%K2HPO4) (6) amorphous TiO2 (7) small Au capsule Crystallization of anatase (8) supporter (9) oxygen buffer Amorphous Ti02•EnH2O obtained by The reaction temperature was measured hydrolyzing a titanium alcoxide, Ti [OCH on the outside of the vessel, then it was (CH3)2]4, was employed as a starting materi corrected to the inside temperature from al. The growth of single crystals was calibration curve (Fig. 2). This calibration carried out by a mixed solution of 10% KF curve was made by measuring simultaneously solvent and 2.5% K2HPO4 stabilizer. on the outside and the inside of the vessel Oxygen partial pressure in hydrothermal by two thermocouples. In the case of the reactions was controlled with the oxygen growth reaction, the temperature between buffer method developed by Eugster (1957). the dissolution zone and crystallization Oxygen buffers employed in this study and zone is different, and the temperature their dissociation equations are as follows: gradient is also exhibited in Fig. 2. 2Ni+02=2NiO (NNO).........(1) On the other hand, effectiveness of 4Fe304+02=6Fe203 (HM)............(2) oxygen buffer method was checked by double 2H2+02=2H20 (H20)............(3) capsule, (Au-(Pt)) or (Au-(Ag•EPd) ). The The hydrothermal growth reactions reactant and solvent were sealed in the were conducted in sealed triple capsule, (Au- small Pt or Ag•EPd capsule (5mm l. D. and Crystallization and phase transformation of anatase 255 the amorphous Ti02•EnH2O was employed as a starting material, because this transition follows a reaction path of amorphous reactant•¨anatase•¨rutile. However, the amorphous reactant crystallizes easily into anatase by dehydration at 120•Ž at 1 atm, and is recognized to crystallize into anatase during heating up to a given temperature above 300•Ž under the hydrothermal con ditions. So that, this amorphous reactant was considered to be anatase (or pseudo anatase) in a reaction process. RESULTS AND DISCUSSION Fig. 2. Calibration curves between the inside and the outside temperatures of a Effectiveness of oxygen buffer method vessel. The recrystallization zone has a distance of 5 cm from the dissolution Effectiveness of oxygen buffer method zone. was checked by using various oxygen buffers 30mm long), then, this capsule and oxygen (Table 1). In the dissociation equilibrium of H20, a starting Fe304 sample must be buffer were placed in the large Au capsule oxidized to Fe203, but still remains for 312 (6mm I. D. and 50mm long), which was hours at 590•Ž under 1000 atm. It means sealed. In this case, the reaction tem that the oxidation reaction of Fe304 could perature has very small gradient or no not be reached into an equilibrium. This gradient because of short capsule. is due to lower Po, than theoretical Po2 Phase transformation from anatase to rutile owing to metallic wall in the vessel (Laudies In the experiments on phase transition, and Nielsen, 1961). In the system of an Table 1. The control effect of Po, by Eugster method ★He: Hematite (Fe2O3), Mq: Magnetite (Fe3O4) △= Equilibrium PO2 is not controlled 256 Yoshiro Ohtsuka, Yoshinori Fujiki and Yoshio Suzuki HM or an NNO buffer using an H20 solvent, K2HPO4 solution instead of the H20 solvent a starting Fe203 or NiO sample was always was used, the dissociation equilibrium of reduced into Fe304 or Ni phase, respectively. HM or NNO needed a long time, and that of Namely, the Po, is well controlled in these an IM buffer could not be obtained; under reaction system. When a 10% KF+2.5% the conditions of the present experiments. Crystallization under controlled Po2 The results of hydrothermal reactions in the system TiO2• nH2O-KF-K2HPO4-H20 under controlled Po2 are shown in Fig. 3. Anatase, potassium hexatitanate (K2Ti6O13) and rutile were synthesized with increasing temperature, and the formation field of these phases was markedly depended on reaction temperature rather than Po2. This order of synthetic phases toward high tem perature side is concordant with that in which the zigzag chain sharing the edges of TiO6 octahedral becomes more straight (Fig. 4). Only anatase crystals were produced at temperatures below 460•Ž and most of them exhibited bipyramidal shape surround- ed by {101}and {103}faces (Fig. 5 (a)). The Fig. 3. The phase relation grown in the sys well known habit consisting of {101} and tem Tio2•E nH2O-KF-K2HPO4-H2O {111} faces of natural anatase never occured under various Po2 and temperature under the present conditions. These conditions. Fig. 4. Structural change with the increase of growth temperature among three phases grown in Fig. 3. Crystallization and phase transformation of anatase 257 Fig. 5. Anatase, potassium hexatitanate (K2Ti6O13) and rutile crystals grown under hydrothermal conditions of controlled Po2. (a) Anatase crystal and potassium hexatitanate crystals (needle crystals) (b) Rutile crystals grown under higher Po2 conditions (c) Rutile crystals grown under lower Po2 conditions crystals are of approximately 0.1mm in constant and Po, change is exhibited in Fig. size. The color of anatase crystals was 6. It is found that although the c affected by the reaction temperature rather parameters increase with decreasing Po,, than Po2. Namely, the anatases grown the change of the a parameters is very little. below 460•Ž tinged with light yellow, and Consequently, it roughly shows a tendency those coexisting with K2Ti6O13 at higher that the unit cell volumes increase, and that temperatures than 460•Ž are blueish. K2 the densities decrease with decreasing Po2. Ti6013 crystals are colorless, and have needle Although this tendency of the cell parameter form (about 2mm length) elongated along change for rutile is known as an evidence of the b-axis. Rutile crystals were synthesized oxygen vacancy or titanium interstitial, above 520•Ž.