Abstract
The crystal structures of bismutocolumbite, (Bi 0 . 9 9 Sb 0 . 0 1 )(Nb 0 . 7 9 Ta 0 . 2 1 )O 4 (I) and stibiocolumbite, Sb(Nb 0 . 6 7 Ta 0 . 3 3 )O 4 (II) were established from X-ray single crystal diffraction; I, orthorhombic, space group Pnna, a = 5.668(1) A, b = 11.725(2) A, c = 4.971(1) A, V = 330.4 A 3 , Z = 4, d c a l c . = 7.726, d o b s . = 7.17-7.56g/cm 3 , F(000) = 614, R = 0.025 for 820 unique reflections with I>2σ(I); II, orthorhombic, space group Pna2 1 a = 5.557(1) A, b = 4.932(1) A, c = 11.808(2) A, V = 323.6 A 3 , Z = 4, dean = 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 2.738 A; (Nb,Ta)-O distances are between 1.852-2.148 A, in II Sb-O distances range from 2.008 to 2.619 A within distorted pseudotrigonal bypiramid; (Nb,Ta)-O distances are between 1.848 - 2.085 A. Both structures contain sheets of corner-linked (Nb,Ta)O 6 -octahedra separated by significantly distorted BiO 6 (SbO 5 )-polyhedra with four shorter distances opposite to the lone-pair electrons of Bi 3 + (Sb 3 + ). The repulsive forces between lone-pair electrons and the O atoms lead to the distortion of (Nb,Ta)O 6 octahedra in the stibiotantalite structure type. Two shortest bonds (Nb,Ta)-O(1) = 1.852 A in I as well as (Nb,Ta)-O(4) and -O(3) = 1.848 and 1.891 A, respectively, in II occur with ligands which are oriented in the direction of the lone-pair electrons of Bi 3 + (Sb 3 + ) and which form the elongated bonds Bi-O(1) = 2.738 A in I and Sb-O(3) = 2.619 A and Sb-O(4) = 3.012 A 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 ABO 4 oxides with 6-fold coordination of cations, namely wolframite (Fe,Mn) [ 6 ] W [ 6 ] O 4 (CPI = 68) and rutile-like CrVO 4 (CPI = 74). The structural transitions (Pna2 1 to Pnna) in (Bi,Sb)(Nb,Ta)O 4 series should be considered as a function of chemical composition, temperature and pressure.
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