Spectroscopy at very high pressures. XLIII. The Raman spectra of mercury (II) cyanide

Phase transitions have been observed in mercury (II) cyanide near 2.5, 89, 19 and 60 kbar. The I/II transition appears to be of second order with the structure of phase II closely related to that of the parent molecular type. Using Landau theory, three possible structures have been identified for the new phase II, of which S24 is marginally better in accord with the evidence. A major reconstruction of the lattice occurs at the II/III transition; the coordination number of mercury appears to increase to four, and the structure becomes covalently bonded, rather than molecular. Phase IV has a spectrum consistent with it having the cubic anti-cuprite structure adopted at STP by Zn(CN)2 and Cd(CN)2. Above 60 kbar, brown polymeric Hg(CN)2V is formed irreversibly. It has a single IR absorption