Transition Metal Borides

Abstract

The crystal structure of CrB4 (ANDERSSON and LUNDSTRÖM, 1968) and MnB4 (ANDERSSON and CARLSSON, 1970) are closely related, although the former is orthorhombic with a = 4.744 Å, b = 5.477 Å and c = 2.866Å and the latter monoclinic with a = 5.503 Å, b = 5.367 Å, c = 2.949 A and β= 122.71. The relationship is, however, easily revealed if the MnB4 structure is described using a non-conventional I-centered cell with a = 4.630 Å, b = 5.367 A, c = 2.949 Å and β = 90.31 Å. From the projection of the CrB4 structure along its short axis (see Fig. 1) it is seen that the boron atoms form squares in the ab plane. These boron squares are connected in the c direction as well, thus generating a three-dimensional network, intersected by channels in the c direction. The chromium atoms are situated in these channels, forming chains with a fairly long Cr-Cr distance of 2.87 Å. It is a characteristic feature of the structure that it contains voids (at 0,1/2,1/2 and 1/2,0,0), which are large enough to accommodate atoms with radii less than 1.0 Å without any appreciable distortion of the lattice. The MnB4 structure may be regarded as a monoclinic distortion of the CrB4 structure involving minor changes in the interatomic distances but no changes in coordination. No further representatives of these structure types are known at present. Attempts to prepare iso-structural phases in the V-B system were unsuccessful, while in the Mo-B and W-B systems a phase of a different structure occurs at the corresponding composition.