Alkaline-earth metal mercury intermetallics A11-xHg54+x (A = Ca, Sr).

Re-examination of the mercury-rich regions of the Ca-Hg and Sr-Hg phase diagrams has shown that the phases previously identified as "AHg 3.6" should be reformulated as A(11-x) Hg(54+x) (A = Ca, Sr). The crystal structures for representative members of these A 11- x Hg 54+ x phases were determined from single-crystal X-ray diffraction data (Pearson symbol hP65, space group P6; a = 13.389(1) A, c = 9.615(1) A for Ca(10.92(2))Hg(54.08) (x = 0.08(2)); a = 13.602(2) A, c = 9.818(1) A for Sr(10.48(4))Hg(54.52) ( x = 0.52(4))) and confirmed by powder Rietveld refinements ( R B = 0.020 for Ca(10.7(2))Hg(54.3) and 0.014 for Sr(10.7(3))Hg(54.3)). Diverse coordination polyhedra surround the A (CN14-16, multiply capped pentagonal or hexagonal prisms as well as Friauf polyhedra) and Hg atoms (CN11-13, pentacapped trigonal prisms and icosahedra). Partial disorder of Hg into one of the A sites accounts for the nonstoichiometry in the A(11-x)Hg(54+ x) phases. If this disordered A site is completely occupied by Hg atoms, the composition is constrained to a maximum of x = 2 in A(11-x)Hg(54+ x), corresponding to a small homogeneity range of "A(0.14-0.17)Hg(0.86-0.83)"; the true homogeneity range is likely narrower. The structure can be regarded as being built up from a stacking of triangular nets with hexagonal voids that are filled with single atoms or various clusters. In particular, the presence of triangular Hg 3 clusters in ordered orientations distinguishes this structure from that of the related Gd 14Ag 51-type structure, in which triangular Ag 3 clusters are in disordered orientations. Band structure calculations reveal a small degree of electron transfer from the A to Hg atoms, supporting the presence of a partially anionic mercuride substructure.