JOURNAL OF SOLID STATE CHEMISTRY 137, 143 — 147 (1998) ARTICLE NO. SC977741 Synthesis and Structure of BiCa 2 VO 6 I. Radosavljevic, J. S. O. Evans, and A. W. Sleight 1 Department of Chemistry and Center for Advanced Materials Research, Oregon State University, Corvallis, Oregon 97331-4003 Received June 17, 1997; in revised form November 21, 1997; accepted December 18, 1997 A new compound, BiCa 2 VO 6 , has been synthesized and its structure determined. This compound crystallizes in space group Cmc2 1 (No. 36) with a  8.892 A  , b  11.961 A  , c  5.546 A  , and Z  4. Both high-resolution X-ray and neutron powder diffraction data were used in the refinement of the BiCa 2 VO 6 structure. This structure can be described as composed of (BiO 2 )  chains, VO 3 4 tetrahedra, and Ca 2 cations leading to a BiO 2 Ca 2 VO 4 struc- tural formula. This compound is potentially ferroelectric where switching polarity would mainly involve rotations of VO 4 tetra- hedra.  1998 Academic Press INTRODUCTION The bright yellow color of bismuth vanadate (BiVO  ) gives it considerable potential as a pigment, and this com- pound is also known to be ferroelastic (1). When BiVO  is suitably doped, it becomes a highly selective catalyst for the partial oxidation of propylene and butene (2). We have been investigating phases in Bi /V /A/O systems where A is Mg, Ca, Sr, and Ba. Known and structurally characterized quaternary phases in the Bi/Ca/V/O system are Bi  CaV  O  (3), BiCa  V  O  (4), BiCa  V  O  (5), BiCaVO  (6), and Bi  Ca  V  O  (7). Among these phases, second harmonic generation was observed in BiCa  V  O  , indicating pos- sible potential as a nonlinear optical material (8). In this paper we report the synthesis and crystal structure of a new quaternary phase in this system, bismuth calcium vanadate, BiCa  VO  . EXPERIMENTAL A polycrystalline sample of BiCa  VO  was synthesized using a stoichiometric mixture of Bi  O  (Cerac, 99.999%), CaCO  (Mallinckrodt), and NH  VO  (Johnson Matthey, 99%). The mixture was thoroughly ground, heated at 950°C for 30 hr, and then cooled to room temperature at a rate of 5°C/min. The pale yellow product was analyzed using an To whom correspondence should be addressed. electron microprobe, which confirmed a single-phase prod- uct with a Bi : Ca : V ratio of 1 : 2 : 1. Powder X-ray diffraction data for BiCa  VO  were col- lected using CuK radiation on a Siemens D5000 diffrac- tometer with vertical Soller slits and an energy-dispersive Kevex detector. Initial X-ray diffraction data were collected using a Si internal standard for the purpose of indexing and refinement of cell parameters. A second data set was then collected for Rietveld analysis without an internal standard. Details of data collection are given in Table 1. Neutron diffraction data were collected on a 15-g sample of BiCa  VO  at Brookhaven National Laboratory using the high-resolution powder diffractometer at beam line H1A of the high-flux beam reactor with a wavelength of 1.8857 A  . Further details of neutron diffraction data collection are given in Table 1. Second harmonic generation was sought using a Nd :YAG laser with a wavelength of 1064 nm. STRUCTURE SOLUTION Accurate peak positions for the first 30 reflections in the X-ray diffraction pattern were obtained using a pseudo- Voigt peak shape in the program Profile within the Siemens Diffract/AT suite (9). Diffractometer zero point and sample height errors were corrected using a Si internal standard. Autoindexing using the program Visser (10) suggested an orthorhombic cell with a figure of merit of M  "227. Cell dimensions were further refined using Refcel software (11), giving a"8.891(1) A  , b"11.950(2) A  , and c"5.542(1) A  . Systematic absences suggested two possible space groups: Cmc2  (No. 36) and Cmcm (No. 63) (12). The second data set, collected without an internal stan- dard, was used for application of direct methods and sub- sequent structure refinement. Initial attempts at structure solution were performed in space group Cmc2  . Integrated intensities for the first 144 reflections were extracted using a Pearson profile function in the program Extra (13). Direct methods were applied to structure solution using the pro- gram Sirpow (14). This procedure suggested positions for Bi, Ca, and V. Least squares refinement and sub- sequent difference Fourier synthesis within the GSAS suite 143 0022-4596/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.