High-pressure studies of the absorption edges of three thallous halides

Visible and near-infrared optical-absorption edges of TlCl, TlBr, and TlI were measured as a function of pressure in a diamond anvil cell. The indirect band gap of the three thallous halides shifts red (to lower energy) with pressure and was followed into the near infrared. The direct band gaps of TlBr and TlCl also shift red at a slower rate. The calculated band structures [J. P. Van Dyke and G. A. Samara, Phys. Rev. B 11, 4935 (1975)] of the thallous halides as a function of pressure are quantitatively compared with the experimental results.