Purification and growth of mercurous chloride single crystals

Abstract

Mercurous chloride ( Hg 2 Cl 2) exhibits an extraordinary combination of properties including transmittance from 0.35 to 20 μm, an anomalously slow sheer wave velocity (347 m/s), large birefringence and a high acousto-optic diffraction efficiency which make it an attractive candidate for a number of optical signal processing devices. For this reason we have developed improved techniques to purity and grow single crystals of this compound. While we observed that Hg 2 Cl 2 is insoluble in water, alcohol, and ether, and therefore purification by simple recrystallization is difficult, the material has sufficiently high vapor pressure that it can be purified by repeated sublimation. Purification is complicated by the oxidation of Hg 2 Cl 2 in presence of traces of moisture. We found that the orthorhombic compound Hg 5 Cl 2 O 4, analog to the mineral pinchite, was the main constituent of the black-brown residue formed under vapor transport conditions which produced colored Hg 2 Cl 2 single crystals. Moisture control coupled with multiple sublimations to minimize metallic contaminants has produced colorless oriented H 2 Cl 2 single crystals with reduced light scattering compared to crystals grown from unpurified feedstock.