Phase relations in the pseudobinary systems RAO3-R2Ti2O7 (R: rare earth element and Y, A: Fe, Ga, Al, Cr and Mn) and syntheses of new compounds R(A1−xTix)O3+x/2 (2/3≤x≤3/4) at elevated temperatures in air

Phase relations in the pseudo-binary systems RFeO 3 -R 2 Ti 2 O 7 (R: Lu, Ho and Dy), RGaO 3 -R 2 Ti 2 O 7 (R: Lu and Er), LuAlO 3 -Lu 2 Ti 2 O 7 and RAO 3 -R 2 Ti 2 O 7 (R: Lu and Yb. A: Cr and Mn) at elevated temperatures in air were determined by means of a classic quenching method. There exist Lu(Fe 1−x Ti x )O 3+x/2 , R(Ga 1−x Ti x )O 3+x/2 (R: Lu and Er) and Lu(Al 1−x Ti x )O 3+x/2 (2/3≤ x≤3/4) having the Yb(Fe 1−x Ti x )O 3+x/2 -type of crystal structure (x=0.72, space group: R 3 m , a(A)=17.9773 and c(A)=16.978 as a hexagonal setting) in these pseudo binary systems. Eighteen compounds R(A 1−x Ti x )O 3+x/2 (R: Lu-Sm and Y, A: Fe, Ga and Al) were newly synthesized and their lattice constants as a hexagonal setting were measured by means of the X-ray powder diffraction method. The R occupies the octahedral site and both A and Ti does the trigonalbipyramidal one in these compounds. Relation between lattice constants for the rhombic R(A 1−x Ti x )O 3+x/2 and the monoclinic In(A 1−x Ti x )O 3+x/2 are as follows, a h ≈5 x b m , c h ≈3 x c m x sin β and a m =3 1/2 x b m , where a h and c h are the lattice constants as a hexagonal setting for R(A 1−x Ti x )O 3+x/2 and a m , b m , c m and β are those of the monoclinic In(A 1−x Ti x )O 3+x/2 . Crystal structural relationships among α-InGaO 3 (hexagonal, high pressure form, space group: P 63/ mmc ), InGaO 3 (rhombic, hypothetical), (RAO 3 ) n (BO) m and RAO 3 (ZnO) m (R: Lu-Ho, Y and In, A: Fe, Ga, and Al, B: divalent cation element, m, n: natural number), the orthorhombic-and monoclinic In(A 1−x Ti x )O 3+x/2 (A: Fe, Ga, Al, Cr and Mn) and the hexagonal-and rhombic R(A 1−x Ti x )O 3+x/2 (R: Lu-Sm and Y, A: Fe, Ga and Al) are schematically presented. We concluded that the crystal structures of both the α-InGaO 3 (high pressure form, hexagonal, space group: P 63/ mmc ) and the hypothetical InGaO 3 (rhombic) are the key structures for constructing the crystal structures of these compounds having the cations with CN=5.