Tantalum clusters in oxidic matrices -: Synthesis, crystal structure and magnetic properties of Eu1.83Ta15O32

The mixed-valent oxotantalate Eu1.83Ta15O32 was prepared from a compressed mixture of Ta2O5 and the metals in a sealed Ta ampoule at 1400degreesC. The crystal structure was determined by means of single crystal X-ray diffraction: space group R, a = 777.2(6) put and c = 3523.5(3) pm, Z = 3, 984 symmetrically independent reflections, 83 variables, R-F = 0.027 for I > 2sigma (I). The structure is isotypic to Ba2Nb15O32. The salient feature is a [Ta( +8/3)(6)O-12 O-i(6)a] cluster consisting of an octahedral Ta-6 core bonded to 12 edge-bridging inner and six outer oxygen atoms. The clusters are arranged to slabs which are sandwiched by layers of [Ta(+5)(3)O-13] triple octahedra. Additional Ta(+5) and Eu(+2) atoms provide the cohesion of these structural units. Twelve-fold coordinated Eu(+2) atoms are situated on a triply degenerate position 33 pm displaced from the threefold axis of symmetry A depletion of the Eu(+2) site from 6 to 5.5 atoms per unit cell reduces the number of electrons available for Ta-Ta bonding from 15 to 14.67 electrons per cluster. Between 125 and 320 K Eu1.83Ta15O32 is semi-conducting with a band gap of 0.23 eV The course of the magnetization is consistently described with the Brillouin function in terms of a M-mol/(N(A)mu(B)) versus B/T plot in the temperature range 5K - 320 K and at magnetic flux densities 0.1 T - 5 T At moderate flux densities (< 1T) the magnetic moment agrees fairly well with the expected value of 7.94 mu(B) for free Eu (2+) ions with 4f(7) configuration in S-8(7/2) ground state. Below 5 K, anisotropic magnetization measurements at flux densities B < I T point to an onset of an antiferromagnetic ordering of Eu spins within the layers and an incipient ferromagnetic ordering perpendicular to the layers.