Crystal structure of InTi_0.75Fe_0.25O_3.375 and phase relations in the pseudobinary system InFeO₃-In₂Ti₂O₇ at 1300 °C

An average structure of incommensurately modulated monoclinic InTi_0.75Fe_0.25O_3.375 was determined using single-crystal X-ray diffraction. Unit cell parameters are a = 5.9207(8), b = 3.4249(5), c = 6.3836(9), and β = 107.96(1) with Z = 2, and the space group is C2/m. The structure with pseudorhombohedral symmetry involves two metal positions: M1, practically occupied by In, and M2, by Ti and Fe. Two layers are alternately stacked along the c axis; one is an edge-shared M1O₆ octahedral sheet and another consists of bipyramidal coordination at M2 sites. In the bipyramidal layer, a honeycomb lattice 68% occupied by oxygen ions surrounds the M2 site. These oxygen ions are subsequently displaced from their original positions to reduce mutual repulsion. This explains large displacement parameters U₁₁ and U₂₂ despite the normal U₃₃ at the M2 site as well as at the oxygen site. Solid solution for the present phase is limited to InFeO₃:In₂Ti₂O₇ = 2:3 at 1300 °C according to a phase relations study in the pseudobinary system InFeO₃-In₂Ti₂O₇. We propose a hypothetical compound with an unusual ABO₃ structure based on a comparison of this compound and related structures.