JOURNAL OF SOLID STATE CHEMISTRY 138, 272 — 277 (1998) ARTICLE NO. SC987803 A Structural Study of the Perovskite Series CaTi 12x Fe x Nb x O 3 Anton R. Chakhmouradian and Roger H. Mitchell Department of Geology, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario, Canada P7B 5E1 Received November 12, 1997; in revised form February 9, 1998; accepted February 17, 1998 An X-ray powder diffraction study of the series CaTi 12x Fe x Nb x O 3 is presented. The series comprises orthorhombic perov- skites (Pbnm, a+b+2a p , c+2a p , Z  4) in the range 04x40.3, and monoclinic perovskites (P 2 1 /n, a+b+2a p , c+2a p , O90°, Z  4) in the range 0.44x40.5. The struc- ture of the orthorhombic members is derived from the cubic aristotype by octahedral rotation a  a  c  . The structural distor- tion in the monoclinic members involves octahedral rotation and short-range cation ordering at the B-site (4c and 4d ). In the series CaTi 12x Fe x Nb x O 3 , the unit-cell parameters and degree of oc- tahedral rotation increase with x. The [111] p tilt angle increases from 16.1° in CaTiO 3 to 17.6–18.9° in CaFe 1/2 Nb 1/2 O 3 (for the NbO 6 and FeO 6 octahedra, respectively). In contrast to previous studies, here the diffraction pattern of the end-member CaFe 1/2 Nb 1/2 O 3 is interpreted to exhibit splitting of the hkl and h0l lines indicative of a monoclinic derivative of the CaTiO 3 -type structure.  1998 Academic Press INTRODUCTION Among naturally occurring perovskite-type compounds, the mineral perovskite (ideally CaTiO  ) is the most abun- dant in the Earth’s crust. Compositional variations of nat- urally occurring perovskite-group minerals are important indicators of geochemical evolution in geological systems. Perovskite commonly contains significant amounts of Fe and Nb, thus forming a solid solution series with CaFe  Nb  O  (1, 2). The maximum content of the CaFe  Nb  O  end-member (up to 23.5 mol.%) calculated from the electron-microprobe analyses, is found in the min- eral latrappite (2). The crystal structure of latrappite has been refined using the Rietveld method and shown to be orthorhombic and similar to that of CaTiO  perovskite (2). The structure of CaTiO  perovskite has been studied extensively over the past five decades (3—9). This structure is derived from the aristotype by octahedral tilting about three four fold axes of the cubic subcell, and represents one of the fundamental perovskite hettotypes. The tilting results in orthorhombic symmetry of CaTiO  (space group Pnma or Pbnm). Most authors use the unconventional setting Pbnm with a+b+2a  , c+2a  (5—8). In Glazer’s (10) notation, the octahedral tilting exhibited by CaTiO  is written as aac. The compound CaFe  Nb  O  has not been encoun- tered in nature, but has been synthesized and shown to have the perovskite-type structure (11). Filip’ev and Fesenko (11) claim that CaFe  Nb  O  has a monoclinically deformed perovskite cell, and exhibited no cation ordering at the B-site. Therefore, the actual symmetry of this perovskite was suggested to be orthorhombic with a+b+2a  , c+a  . The proposed monoclinic deformation of the perovskite cell implies a shear distortion such that the angle  between a  and c  is no longer 90°. The aluminous analogue of CaFe  Nb  O  with the formula CaAl  Nb  O  was suggested to have 1 : 1 cation ordering at the B-site, resulting in a doubling of the periodicity along c (11). It is noteworthy that among the complex oxides AB  B  O  (A "Ca, Sr, Ba, Pb; B"Al, In, Ga, Mn, Cr, Fe, Y, Bi, ¸n; B"Nb, Ta, Sb), the majority exhibit partial or com- plete ordering of cations at the B-site (11—15). According to Fesenko et al. (12), the ordering is controlled by the differ- ence in charge (q) and radius (R  "R  !R  / R  ) of the cations. For the complex perovskites AB  B  O  , R  should exceed 0.09 for ordering to occur (12). High R  (when B approaches or exceeds A in radius) may result in the ‘‘inverse’’ perovskite structure (by analogy with the inverse spinel structure (11, 13)). For Fe- bearing perovskites, R  and thus the probability of cation ordering at the B-site, would depend on the spin state of Fe (16). R  calculated for CaFe  Nb  O  using Shan- non’s ionic radii (16), is 0.14 for the low-spin Fe and a mere 0.01 for the high-spin Fe. This suggests that the ordering is more likely to occur when the Fe cations are in the low-spin state. In the absence of long-range cation ordering, the perovskite structure may still exhibit reduction in symmetry due to short-range ordering, as has been dem- onstrated by Woodward (15) for CaFe  Ta  O  (15). The present work is a part of the comprehensive study of naturally occurring perovskite-type compounds and their synthetic analogues. The objective of the present work was to determine the structural characteristics of the series CaTi  Fe  Nb  O  , with particular emphasis on the crys- tal chemistry of the end-member CaFe  Nb  O  (x"0.5). 272 0022-4596/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.