Crystal Structure and Characterisation of Mercury(II) Dichromate(VI)

Abstract

Dark-red single crystals of HgCr2O7 were grown by reacting HgO and CrO3 in excess at 200°C for four days. The crystal structure (space group P32, Z = 3, a = 7.2389(10), c = 9.461(2) A, 1363 structure factors, 57 parameters, R[F2>2σ(F2)] = 0.0369, wR(F2 all) = 0.0693) was determined from a crystal twinned by merohedry according to (110). It consists of nearly linear HgO2 units (\({\bar {d}}\)(Hg–O) = 2.02 A) and dichromate units that are linked into infinite chains ‘O3Cr–O–CrO3–Hg–O3Cr–O–CrO3’ running parallel to the c-axis. Six additional Hg–O contacts between 2.73 and 2.96 A stabilise the structural arrangement. The dichromate anion exhibits a staggered conformation with a bent Cr–O–Cr bridging angle of 140.7(6)°. Upon heating above 300°C, HgCr2O7 decomposes in a two-step mechanism to Cr2O3. The title compound was additionally characterised by vibrational spectroscopy.