Sur le diborure et le tétraborure de magnésium. Considérations cristallochimiques sur les tétraborures

Abstract

MgB 2 and MgB 4 have been prepared at high temperature in sealed molybdenum vessels from mixtures of the elements. The utilization of an excess of metal in the vessel generates a pressure of magnesium vapor which inhibits thermal decomposition of the compounds during the synthesis. The structure of MgB 4 has been established from single crystal data collected on an automatic diffractometer. MgB 4 is orthorhombic, space group Pnam with a = 5.464; b = 7.472; c = 4.428Å and Z = 4. The structure of MgB 4, is based on chains of boron pentagonal pyramids in which the averaged BB bond is 1.787 Å. Interchain BB bonds of 1.730 Å are responsible for the three-dimensional boron framework. The Mg atoms, located in tunnels, form zigzag chains. The structure of MgB 4 is compared to those of ThB 4 and CrB 4; it is concluded that the size of the metal atom plays an important role in the nature of the boron framework which exists. In MgB 4, the fundamental unit of the boron skeleton is a pentagonal pyramid; a new feature, in boron-rich borides where this type of coordination polyhedron was previously found only in B 12 icosahedra.