Convection and Diffusion Effects During Physical Vapor Transport of Hg2Cl2

We have carried out extensive experimentation on the physical vapor transport growth of mercurous chloride, which is an important material for opto-electronic devices. Because of the extraordinary combination of properties found in Hg2Cl2, including transmittance from 0.36 to 20 μm, anomalously slow soung velocity, high birefringence, and large acousto-optic diffraction efficiency, we are exploring the conditions for growth of large crystals of device quality. Our experiments have led so far to measured Hg2C12 growth rates that were orders of magnitude smaller than those expected by theories based on laminar, unidirectional flow between source and sink. Slight disturbances of the solid-vapor interface give rise to instabilities which lead to interfacial convection. This kind of convection should enhance the growth rate due to the acceleration of the materials exchange by hydrodynamic coupling, which is contrary to our findings. Interfacial convection is often connected with buoyancy-induced instabilities which dominate other instabilities under 1-g conditions. Despite attempts to reduce the Rayleigh numbers in our system, buoyancy-induced instabilities might be the cause of discrepancy between the measured and calculated growth rates. Observations of these phenomena under a microgravity environment might permit a better assessment of the transport mechanisms in physical vapor phase crystal growth of Hg2C12.