M2X INTERMETALLICS - NONMETAL INSERTION IN A H.C.-LIKE METALLIC DISTRIBUTION

A simple structural model is proposed on the basis of an ideal hexagon al close-packing (A)(infinity) of the metal atoms M. The metalloid ato ms X, located in an ordered manner in the metallic planes with the sta cking sequence ..(A)(infinity)or..(B)(infinity).., generate two types of elementary units called units U-I and U-II. These units are hexagon al prisms with the fundamental vectors a(0), b(0), c(0) and the elemen tary volume V-0. When the exclusive occurrence of U-I (or U-II) only f ields 6-prismatic and triangular sites of metalloid atoms X, additiona l tetrahedral and pyramidal sites of X atoms are present when units U- I and U-II exist simultaneously. The structures of compounds with the general formula M2X (M=3d, 4d, 5d and (or) 4f, 5f elements, X = B, P, As, Sb, Si, Ge, ...) are described in terms of intergrowth mechanisms, Binary, ternary, or even quarternary structures of compounds, with M/ X ratios equal or close to two, appear as superstructures of the eleme ntary units, Insofar as no ordering is considered along the stacking d irection, the vectorial combinations of the fundamental vectors in the form A = h.a(0) + s.b(0), B = k.a(0) + t.b(0), C = c(0) result in sup ercells with the volume V = (h.t - k.s).V-0. The attainable symmetry ( hexagonal, tetragonal, orthorhombic, monoclinic,...) strongly depends on the particular values of the integers h, s, k, t. The criteria of o ccurrence of various series of compounds are presented together with t heir crystallographic features and structural relationships are emphas ized, Moreover, the model demonstrates that numerous compounds with th e predicted unit cell parameters should be obtained in each crystal st em. (C) 1998 Academic Press.