Bismutocolumbite, Bi(Nb0.79Ta0.21)O4, stibiocolumbite, Sb(Nb0.67Ta0.33)O4, and their structural relation to the other ABO4 minerals with stibiotantalite (SbTaO4) structure
Zubkova, N.V.; Pushcharovsky, D.Y.u.; Giester, G.; Smolin, A.S.; Tillmanns, E.; Brandstätter, F.; Hammer, V.; Peretyazhko, I.S.; Sapozhnikov, A.N.; Kashaev, A.A.
published: Apr 19, 2002
The crystal structures of bismutocolumbite, (Bi0.99Sb0.01)(Nb0.79Ta0.21)O4 (I) and stibiocolumbite, Sb(Nb0.67Ta0.33)O4 (II) were established from X-ray single crystal diffraction; I, orthorhombic, space group Pnna, a = 5.668(1)Å, b = 11.725(2)Å, c = 4.971(1)Å, V = 330.4Å3, Z = 4, dcalc. = 7.726, dobs. = 7.17-7.56g/cm3, F(000) = 614, R = 0.025 for 820 unique reflections with I > 2σ(I); II, orthorhombic, space group Pna21a = 5.557(1)Å, b = 4.932(1)Å, c = 11.808(2)Å, V = 323.6Å3, Z = 4, dcalc. = 6.316, F(000) = 538, R = 0.023 for 1437 reflections with I > 2σ(I). In both structures (Nb,Ta) atoms have octahedral coordination. In I Bi-O distances within distorted octahedron range from 2.126 to 2.738Å; (Nb,Ta)-O distances are between 1.852-2.148 Å, in II Sb-O distances range from 2.008 to 2.619Å within distorted pseudotrigonal bypiramid; (Nb,Ta)-O distances are between 1.848 - 2.085Å. Both structures contain sheets of corner-linked (Nb,Ta)O6-octahedra separated by significantly distorted BiO6(SbO5)-polyhedra with four shorter distances opposite to the lone-pair electrons of Bi3+(Sb3+). The repulsive forces between lone-pair electrons and the O atoms lead to the distortion of (Nb,Ta)O6 octahedra in the stibiotantalite structure type. Two shortest bonds (Nb,Ta)-O(1) = 1.852Å in I as well as (Nb,Ta)-O(4) and -O(3) = 1.848 and 1.891Å, respectively, in II occur with ligands which are oriented in the direction of the lone-pair electrons of Bi3+(Sb3+) and which form the elongated bonds Bi-O(1) = 2.738Å in I and Sb-O(3) = 2.619Å and Sb-O(4) = 3.012Å in II. The structures of I and II with a close packing indices (CPI) of 54 and 55, respectively, can not be considered as atomic ones in contrast to some other ABO4 oxides with 6-fold coordination of cations, namely wolframite (Fe,Mn)WO4 (CPI = 68) and rutile-like CrVO4 (CPI = 74). The structural transitions (Pna21 to Pnna) in (Bi,Sb)(Nb,Ta)O4 series should be considered as a function of chemical composition, temperature and pressure.