VARIANTS OF CATION ARRANGEMENT IN THE 3(6)-NET OF TANTALATES AND NIOBATES WITH 1/1-STOICHIOMETRY

We have analyzed and tabulated the variants of mutual ordered arrangements of cations of two types with 1:1 stoichiometry in cation matrices having planar trigon 3(6) nets. A net with two nodes in its unit cell has a sole variant, which occurs in the wolframite type structure. With four nodes in the cell, there are two variants: the scheelite type and the variant found in alpha- and beta-LaTaO4. With 6 nodes, there are six, and with 8 nodes, more than 20 variants. In the ABO(4) (A = REE, B = Ta, Nb) structural type (M'-fergusonite, fluorite, alpha-LaTaO4, beta-LaTaO4, and fergusonite) having a more or less distorted face-centered cubic cation subcell in common, we have the wolframite type of arrangement for M'-fergusonite and fluorite, the scheelite type for fergusonite, and the partly scheelite type for alpha- and beta-LaTaO4. We offer a system for representing the atomic arrangement by recording the alternation of atoms in linear rows of three directions.