화학공학소재연구정보센터
Inorganic Chemistry, Vol.33, No.14, 3032-3037, 1994
11-Fold Superstructure of Tage4/11Te2 - A Novel Response to Charge-Transfer in the Maxte2 (M=nb, Ta A=si, Ge 1/3-Less-Than-or-Equal-to-X-Less-Than-or-Equal-to-1/2) Series
The commensurately modulated structure of TaGe4/11Te2 was determined from single-crystal X-ray diffraction data. TaGe4/11Te2 is orthorhombic; the basic unit cell dimensions are a = 6.4396(8) angstrom, b = 14.044(2) angstrom, and c = 3.8522(5) angstrom, with Z = 4. The (3 + 1)D superspace group is B2mm(00gamma), where gamma = 4/11. The structure consists of [Te/M, A/Te] sandwiches stacked in the AA/BB mode like other MAxTe2 phases (M = Nb,Ta; A - Si, Ge; 1/3 less-than-or-equal-to x less-than-or-equal-to 1/2). Also, Ta and Ge are in Te trigonal-prismatic and square coordinations, respectively. The intrasandwich cationic arrangement is, however, quite distinct from the pattern found in the parent compounds. Instead of a single zigzag strip of lone Ta atoms per sandwich per unit cell, there are two such strips plus two half-strips. It is shown that the extra number of strips is needed to obtain the proper number of short intersandwich Te-Te contacts required for the electronic stabilization of the compound. In this way an incommensurate modulated structure, as occurs for TaSixTe2 compounds, can be avoided. It is proposed that, although the average oxidation state of Te changes from 2- in MA1/2Te2 to 5/3- in MA1/3Te2, equivalent Te-Te distances need not change owing to the setting up of commensurately or incommensurately ordering motifs.