A structural study of the perovskite series CaTi1-2xFexNbxO3

An X-ray powder diffraction study off the series CaTi1-2xFex Nb-x O-3 is presented. The series comprises orthorhombic perovskites (Pbnm, a approximate to b approximate to root 2a(p), c approximate to 2a(p), Z = 4) in the range 0 less than or equal to x less than or equal to 0.3, and monoclinic perovskites (P2(1)/n, a approximate to b approximate to root 2a(p), c approximate to 2a(p), beta not equal 90 degrees, Z = 4) in the range 0.4 less than or equal to x less than or equal to 0.5. The structure of the orthorhombic members is derived from the cubic aristotype by octahedral rotation a(-)a(-)c(+). The structural distortion in the monoclinic members involves octahedral rotation and short-range caticon ordering at the B-site (4c and 4d), In the series CaTi1-2xFexNbxO3, the unit-cell parameters and degree of octahedral rotation increase with x. The [111](p) tilt angle increases from 16.1 degrees in CaTiO3 to 17.6-18.9 degrees in CaFe1/2Nb1/2O3 (for the NbO6 and FeO6 octahedra, respectively). In contrast to previous studies, here the diffraction pattern of the end-member CaFe1/2Nb1/2O3 is interpreted to exhibit splitting of the hkl and h0l lines indicative of a monoclinic derivative of the CaTiO3-type structure. (C) 1998 Academic Press.