Mixed valency and phase transitions at the (001) SmS surface

A model for mixed valency and phase transitions at (001) SmS is suggested. Focussing on the electron-lattice coupling as a mechanism potentially responsible for both phenomena, the problem of the semi-infinite crystal is factorized and reduced to the investigation of a low-dimensional effective Hamiltonian matrix. Surface geometry and electronic structure are obtained simultaneously by numerical minimization of the free energy. The possible appearance of mixed valency and of phase transitions confined to the surface region and involving relaxed or reconstructed geometry is demonstrated.