LASE2-X COMPOUNDS - VIBRATIONAL AND ELECTRICAL-PROPERTIES

The phase diagram in the system La-Se was investigated around the LaSe 2 composition. X-ray and microprobe analyses reveal the existence of o nly two phases, LaSe2 and LaSe1.9, with no solid solution behavior bet ween them. Infrared and Raman spectra of the non-stoichiometric phase suggest that the vacancies in the basal plane of the low-dimensional l attice are disordered. Polarized single crystal Raman spectra and a mo dified valence force field calculation for LaSe2 indicate that the ass umption of isolated (Se-Se)(2-) pairs in the anti-Fe2As structure is n ot justified. In accord with this, the observed resistivity for LaSe2 as a function of temperature is typical of a semimetallic compound. A transition to a semiconductor may occur below 110 K. The measured resi stivity as a function of temperature for the non-stoichiometric LaSe1. 9 phase is indicative of a semiconductor. The infrared data together w ith the electrical measurements suggest that this phase can be conside red as n-type semiconductor with the holes in the anionic sp valence b ands.