On the energy band structure and the phase transitions in a cuprous chloride crystal

To explain the break‐down of the Lyddane‐Sachs‐Teller (LST) relation and the compressibility anomaly for CuCl a new energy band model is proposed to include a small indirect gap comparable to the binding energy of direct excitons. It is found for such a model that the condensation of excitons can result in the break‐down of the LST relation as well as a number of other anomalies of properties, i.e. CuCl is a high‐temperature excitonic dielectric with an effective mass ratio mh/mc ≈ 102 of electrons and holes. It is also shown that high pressure can induce a possible excitonic dielectric‐superconductor phase transition.