CRYSTAL STRUCTURE AND OPTICAL OBSERVATIONS OF BiV04

A single-crystal X-ray diffraction analysis has been performed on fergusonite BiVO4 synthesized by pulling method. At room temperature the crystal belongs to monoclinic system, with space group B2/b, z = 4, and cell parameters a = 7.247?, b=5.096?, c = 11.702? and r =134.18o. The structure has been analysed by two and three-dimensional Patterson procedure, and calculated through three-dimensional electron density. A full-matrix, least square refinement gave R=0.067, Rw =0.058 for 335 independent reflections. An absorption and secondary extinction correction gave final R = 0.058. The Bi and V atoms occupy a respective set of 4e special positions and the O atoms two sets of general positions. The oxygen atoms take part in four-fold coordinations around vanadium in the sense that they form regular VO4 tetrahedra. The structure also shows eight-fold oxygen coordinations around bismuth and form distorted Bi-O dodecahedron. The space groups for temperatures above and below Tc are found to be I41/a and 12/b (or B2/b) respectivelv. The phase transition from tetragonal to monoclinic does skip over a possible intermediate orthorhombic modification. The phase transition is apparently driven by a lone-pair distortion of the Bi-O polyhedron. Bi3+ and V5+ displace alternatively up and down along the c axis, resulting in a layer-like Character for monoclinic BiVO4 The character has been proved by optical observations. The BiVO4 crystal cleaves easily to give smooth (cleavage) faces parallel to plane perpendicular to the c axis.Three kinds of samples, i.e crystals with cell-like structure and disc shape, crystal ribbon with natural cleavage and powder crystal grain, were used for optical observation, and the relationship between the transmittance of BiVO4 and optical wave-length was studied.