A New Metallic Layer Structure

Abstract

DURING the course of an investigation into the system manganese/mercury, I confirmed the existence of the intermediate phase Mn₂Hg₅ reported by Prelinger¹ and described its crystal habit and determined the dimensions of its unit cell². Since then, satisfactory Weissenberg photographs have been obtained from very small crystals (0.01–0.02 mm. in diameter). The only systematic absences observed were 0kl for k odd, which lead to three possible space groups, namely, P4/mbm (D ⁵ _4h), P&4bar;b2 (D ⁷ _2d) and P4bm (C ² _4v). Furthermore, the hk0 and hk2 intensities display the same trend, which is different from that of the hkl intensities. If the ten mercury atoms in the cell are all placed at z = 0 and the four manganese atoms at z = ½, it is apparent that the geometrical structure factor for the first of the three space groups is identical for all reflexions with l = 2n, but the manganese contributions reverse their signs for l = 2n + 1. This arrangement of atoms is only possible in the space group P4/mbm. From packing considerations a structure could be derived which gave good agreement between the observed and calculated intensities. The geometrically derived parameters are u = 0.075 and v = 0.206 for the eight mercury atoms at positions ± (uv0 ; vu0; u + ½,½ − v,0; v + ½, n + ½,0) and u = 0.178 for the manganese atoms at ± (u,u + ½, ½;, u + ½, u,½), the two remaining mercury atoms being situated at special positions (½,0,0) and (0,½,0). Further refinement of these parameters by Fourier methods is under way; the results will be published more fully elsewhere.