TANTALUM CLUSTER IN AN OXIDIC MATRIX - SYNTHESIS AND STRUCTURES OF MIXED-VALENCE OXOTANTALATES M2-DELTA-TA15O32 (M = K, RB (DELTA = 0) - M = SR (DELTA = 0.15), BA (DELTA = 0.12))

The mixed-valent oxides Sr1.85Ta15O32 (1), Ba1.88Ta15O32 (2), K2Ta15O3 2 (3), Rb2Ta15O32 (4) were prepared from appropriate mixtures of Ta2O5 , tantalum and the corresponding carbonate at 1520-1670 K in sealed ta ntalum tubes. According to X-ray single crystal structure analyses the oxides crystallize in the space group R (3) over bar, Z = 1. The latt ice parameters in the hexagonal setting are a = 777.36(11), c = 3516.2 (7) pm for 1, a = 778.87(11), c = 3548.1(7) pm for 2, a=780.7(2), c=35 73.1(11)pm for 3, and a=781.90(11), c = 3593.0(7) pm for 4. The oxide ions form a defect dense packing with the layer sequence chhhh. Anti-c uboctahedral sites are completely occupied by the alkali metal cations . The alkaline earth cations occupy 92 to 94% of such sites; they are displaced from the centres. Smaller voids are located in the centres o f the cuboctahedral Ta6O12 clusters forming the characteristic structu ral unit of these low-valent oxotantalates. In case of 3 and 4 the clu sters have 13 electrons, in case of 1 and 2 they have close to 15 elec trons available for Ta-Ta-bonding. Moreover, the structures of the alk ali and alkaline earth metal compounds differ notably with respect to the spectrum of Ta-O and Ta-Ta distances in the Ta3O13 octahedra tripl es forming another characteristic structural unit for these oxides. Su ch differences are traced back to distinct local charge balances for t he uni- and divalent cations. The oxides 2, 3 are semiconductors with band gaps ranging from 130 to 360 meV.