Perovskite, LiNbO_3, Corundum, and Hexagonal Polymorphs of (In_(1-x)M_x)MO_3

摘要

LiNbO_3 (LN), corundum (cor), and hexagonal (hex) phases of (In_(1-x)M_x)MO_3 (x = 0.143; M = Fe_0.5Mn_0.5) were prepared. Their crystal structures were investigated with synchrotron X-ray powder diffraction, and their properties were studied by differential thermal analysis, magnetic measurements, and Mossbauer spectroscopy. The LN-phase was prepared at high pressure of 6 GPa and 1770 K; it crystallizes in space group R3c with a = 5.25054(7) A, c = 13.96084( 17) A, and has a long-range antiferromagnetic ordering near T_N = 270 K. The cor- and hex-phases were obtained at ambient pressure by heating the LN-phase in air up to 870 and 1220 K, respectively. The cor-phase crystallizes in space group R-3c with a = 5.25047(10) Å, c = 14.0750(2) Å, and the hex-phase in space group P6_3/mmc with a = 3.34340(18) Å, c = 11.8734(5) Å. T_N of the cor-phase is about 200 K, and T_N of the hex-phase is about 140 K. During irreversible transformations of LN-(In(1+x)M_x)MO_3 with the (partial) cation ordering, the In~(3+), Mn~(3+), and Fe~(3+) cations become completely disordered in one crystallographic site of the corundum structure, and then they are (partially) ordered again in the hex-phase. LN-(In(1+x)M_x)MO_3 exhibits a reversible transformation to a perovskite GdFeO_3-type structure (space group Pnma; a = 5.2946(3) Å, b = 7.5339(4) Å, c = 5.0739(2) A at 10.3 GPa) at room temperature and pressure of about 5 GPa.