A new approach to electronic transitions in the samarium monochalcogenides

Restricted Access.The samarium monochalcogenides are treated here as ionic crystals with the samarium ion having anomalous compression characteristics arising from the easy delocalisation of a 4f electron. The compression curve of the samarium ion has been determined from experimental PV data in SmS while the compression curves of the chalcogen ions are already available. It is postulated that the compression properties of samarium are the same in all crystals. The theory successfully predicts the following: (i) the transition pressures in SmSe and SmTe; (ii) the occurrence of continuous and discontinuous transitions; (iii) the slope of the phase boundary in SmS; (iv) the properties of Sm1-xGdxS mixed crystals; (v) the optical band gaps