A New Polymorph of FeAlO 3 at High Pressure Takaya Nagai,* ,† Daisuke Hamane, † P. Sujatha Devi, ‡ Nobuyoshi Miyajima, § Takehiko Yagi, § Takamitsu Yamanaka, | and Kiyoshi Fujino † DiVision of Earth and Planetary Sciences, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan, Electroceramics DiVision, Central Glass and Ceramic Research Institute, Kolkata 700-032, India, Institute for Solid State Physics, UniVersity of Tokyo, Kashiwa 277-8581, Japan, and Department of Earth and Space Science, Graduate School of Science, Osaka UniVersity, Toyonaka 560-0043, Japan ReceiVed: August 8, 2005; In Final Form: August 29, 2005 Synchrotron X-ray diffraction measurements confirmed that a new polymorph of FeAlO 3 could be synthesized at about 1800 K and 72 GPa. This phase can be indexed on an orthorhombic cell and transforms into the trigonal form on release of pressure. The c/a ratio of about 2.71 of the trigonal phase suggests corundum structure of FeAlO 3 rather than LiNbO 3 or ilmenite structure. This conclusion also suggests that the high- pressure orthorhombic phase could be the Rh 2 O 3 (II) structure rather than the GdFeO 3 -type perovskite structure. 1. Introduction The Al 2 O 3 -Fe 2 O 3 system is important not only in nature but also in industry. It is well-known that the solid solution of (Al,Fe) 2 O 3 is limited at ambient pressure, although R-Al 2 O 3 (corundum) and R-Fe 2 O 3 (hematite) are isostructural, and the ionic radii of Al 3+ and Fe 3+ are quite similar in an octahedral site. FeAlO 3 , an intermediate composition of Al 2 O 3 /Fe 2 O 3 ) 1:1, appears to be isomorphous with the FeGaO 3 form at high temperature. 1 Preliminary reports have said that FeAlO 3 with the FeGaO 3 form transforms to the tetragonal garnet form and then the GdFeO 3 -type perovskite form at high pressures. 2,3 However, further details have been unclear so far, and our preliminary experiments using a Kawai-type multianvil apparatus showed that the FeGaO 3 -type of FeAlO 3 decomposes into Fe-bearing corundum and Al-bearing hematite up to 25 GPa at about 1300- 1500 K (See Figure 1). Recently, the solubility of Al 3+ into MgSiO 3 perovskite has been focused on. The bulk modulus of Al-bearing MgSiO 3 perovskite is significantly smaller than that for Al-free MgSiO 3 perovskite at lower mantle conditions. 4,5 It was also pointed out that the electrical conductivity of Al-bearing silicate perovskite is much greater than that of Al-free silicate perovskite. 6 The substitution mechanism of Al 3+ into Fe-free MgSiO 3 perovskite could be understood in the MgSiO 3 -MgAlO 2.5 system. 7 On the other hand, some studies pointed out that the MgSiO 3 -FeAlO 3 system could be important in understanding the substitution mechanism of Al 3+ into (Mg, Fe)SiO 3 perovskite with higher Al content. 8-10 Thus, clarification of the existence of FeAlO 3 perovskite at high pressure and high temperature should give important information to understand the correlation of Al 3+ and Fe 3+ solubility in silicate perovskite. FeAlO 3 is reported to be ferromagnetic, magnetoelectric, and piezoelectric. Existence of corundum and hematite in the orthorhombic FeAlO 3 phase makes it difficult to synthesize FeAlO 3 in the pure form by conventional methods. Devaux et al. 11 reported the synthesis of impurity-free FeAlO 3 by oxalate precipitation. We have carried out high-pressure and high- temperature experiments with the pure FeGaO 3 type of FeAlO 3 powder prepared by a citrate-nitrate gel combustion process using a double-sided laser-heated diamond anvil cell technique. 12 We report here a new high-pressure polymorph of FeAlO 3 that has been identified by synchrotron X-ray diffraction measure- ments and a structural transition exhibited by the same during decompression. 2. Experimental Section The FeGaO 3 type of FeAlO 3 powder was used as a starting material and was synthesized as follows. An amorphous citrate- nitrate gel combustion process was followed to prepare FeAlO 3 precursor powder. For a typical batch preparation, 0.5 M solutions each of Al(NO 3 ) 3 ‚8H 2 O and Fe(NO 3 ) 3 ‚8H 2 O were * Corresponding author. E-mail: nagai@ep.sci.hokudai.ac.jp. † Hokkaido University. ‡ Central Glass and Ceramic Research Institute. § University of Tokyo. | Osaka University. Figure 1. A backscattered image of a recovered sample from 25 GPa at about 1300-1500 K. Bright gray colored grains are Al-bearing hematite and dark gray colored grains are Fe-bearing corundum. 18226 2005, 109, 18226-18229 Published on Web 09/13/2005 10.1021/jp054409s CCC: $30.25 © 2005 American Chemical Society