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.