The structure of Ca3(VO4)2

Abstract

Abstract The crystal structure of Ca3(VO4)2 is rhombohedral with space group R3c and unit-cell parameters a = 14.124(1) Å and α = 44.96(2)°, with equivalent hexagonal axis lengths of a = 10.809(1) and c = 38.028(9) Å. The rhombohedral cell contains seven formula units. The structure was refined by full-matrix least-squares utilizing 1252 symmetry-independent reflections to a final R value of 0.049. One formula unit lies on the threefold axis with one of the calcium-ion sites on this axis half-occupied and another site octahedrally coordinated to oxygen atoms. The remaining calcium ions are coordinated to 8, 7 and 8 oxygen atoms with Ca–O bond lengths of 2.525, 2.442 and 2.570 Å respectively. The mean V–O bond length for the three orystallographically independent VO4 groups are within 0.01 Å of 1.69 Å. The structure is related to that of Ba3(VO4)2 but with a formula-unit density 7/8 as large, due to the loss of one formula unit per cell from chains along the threefold axis. This change in structure follows from the smaller average cation-oxygen bond length in the calcium salt. R values as low as 0.041 can be obtained with models that allow for differences in the environment of the half-occupied cation site, presumably dependent upon whether it is filled or empty.