Metal-metal bonding and mixed-valent tantalum in La₂Ta₃S₂O₈

The compounds La₂Ta₃S₂O₈ and La₂Nb₃S₂O₈ have been prepared and characterized. From singlecrystal X-ray diffraction measurements, La₂Ta₃S₂O₈ crystallizes in space group D¹²_2h-Pnnm of the orthorhombic system with four formula units in a cell of dimensions a = 9.876(7), b = 11.768(7), and c = 7.658(5)Å (T = 111 K). The compound is isostructural to a series of oxyselenides Ln₂Ta₃Se₂O₈ (Ln = La, Ce, Pr, and Nd) previously reported. The three-dimensional structure is built up from layers of tricapped trigonal-prismatically coordinated La atoms alternating with layers of octahedrally coordinated Ta atoms. One Ta atom is in a distorted octahedral site surrounded by six O atoms and forms edge-sharing dimers with itself. The other Ta atom is in a distorted octahedral environment with two O and four S atoms in its coordination sphere and forms edge-sharing chains with bridging S atoms. The one-electron band structure of La₂Ta₃S₂O₈ was calculated from the tight-binding method with an extended Hückel-type Hamiltonian. The band structure calculations support the hypothesis of separate Ta(+4) and Ta(+5) sites in the Ln₂Ta₃Q₂O₈ (Q = S and Se) compounds and point to the driving force behind the TaTa pairing that occurs along the octahedral chains. La₂Nb₃S₂O₈ has the same structure, as deduced from a comparison of X-ray powder patterns.