Crystal structure of double vanadates Ca9R(VO4)7. II. R = Tb, Dy, Ho, and Y

Crystal structures of the compounds Ca 9 R (VO 4 ) 7 ( R = Tb ( I ), Dy ( II ), Ho ( III ), and Y ( IV ) have been studied by the method of the full-profile analysis. All the compounds are crystallized in the trigonal system (sp. gr. R 3 c , Z = 6) with the unit-cell parameters ( I ) a = 10.8592(1), c = 38.035(1), V = 3884.2(2) Å 3 ; ( II ) a = 10.8564(1), c = 38.009(1) Å, V = 3879.6(2) Å 3 , ( III ) a = 10.8565(1) and c = 37.995(1) Å, V = 3878.3(2) Å 3 , and ( IV ) a = 10.8588(1), c = 37.995(1) Å, V = 3879.9(2) Å 3 . In structures I–IV , rare earth and calcium cations occupy three positions— M (1), M (2), and M (5). Rare earth cations occupy the R 3+ positions almost in the same way: 2.7–2.6(2) cations in the M (1) position; 2.7–2.3(2) cations in the M (2) position, and 0.6–1.0(1) cation in the M (5) position. At the same time, the occupancy of the M (5) position regularly increases with a decrease of the R 3+ radius.