LaSe_2-x compounds: Vibrational and electrical properties

The phase diagram in the system La-Se was investigated around the LaSe₂ composition. X-ray and microprobe analyses reveal the existence of only two phases, LaSe₂ and LaSe_1.9, with no solid solution behavior between them. Infrared and Raman spectra of the non-stoichiometric phase suggest that the vacancies in the basal plane of the low-dimensional lattice are disordered. Polarized single crystal Raman spectra and a modified valence force field calculation for LaSe₂ indicate that the assumption of isolated (Se-Se)^2- pairs in the anti-Fe₂As structure is not 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 110K. The measured resistivity as a function of temperature for the non-stoichiometric LaSe_1.9 phase is indicative of a semiconductor. The infrared data together with the electrical measurements suggest that this phase can be considered as n-type semiconductor with the holes in the anionic sp valence bands.