Structure cristalline d'un conducteur métallique bidimensionnel: Le bronze violet de potassium et molybdene K 0.9Mo 6O 17

Abstract

Single crystals of the purple bronze K 0.9Mo 6O 17 have been grown by a electrolytic reduction technique. Measurements of the electrical resistivity show that this compound is a quasi two-dimensional metal. A crystal structure determination is used to explain the strongly anisotropic resistivity. K 0.9Mo 6O 17 is found to be trigonal with lattice parameters: a = 5.538 Å and c = 13.656 Å; Z = 1. The space group is P 3 . The K cations are surrounded by 12 oxygen anions which form icosahedral sites. One third of the Mo cations (Mo(1)) occupy tetrahedral sites, while the Mo(2) and Mo(3) cations have octahedral surroundings. The structure can be described as slabs of ReO 3-type connected by KO 12 icosahedra. Each slab is built up of 4 layers, parallel to the (001) plane, of Mo(2) and Mo(3) octahedra sharing corners. Two slabs are connected by K icosahedra which share 2 faces with the upper and lower neighboring Mo(3) octahedra. Each K icosahedron is edge linked to 3 upper and 3 lower Mo(1) tetrahedra. Each Mo(1) tetrahedron is itself corner linked to 3 Mo(3) octahedra of the neighboring layer. The deformations of the occupied polyhedra are a good illustration of the Pauling's third rule. There is no Mo(1)OMo(1) bonding, so that the MoOMo bonding, infinite in the a and b directions, is disrupted in the c direction. The Zachariasen's bond length-bond strength relation has been applied to the MoO bonds. The computed effective mean Mo valences are +6 for Mo(1) on tetrahedral sites and about 5.1 and 5.8, respectively, for Mo(2) and Mo(3) on octahedral sites. The 4 d electrons of Mo atoms are so located in the two-dimensional infinite slabs of octahedra. The conduction band is expected to be an antibonding π∗ band resulting from the hybridization of the Mo 4 d t 2 g and oxygen p π states. Thus, the structural properties should lead to a very anisotropic Fermi surface and thus, to a quasi two-dimensional electrical conductivity.