On the Polymorphism in Lanthanum Polysulfide (LaS2)

The polymorphism in lanthanum polysulfide (LaS{sub 2}) was investigated using X-ray diffraction and vibrational spectroscopy. Samples were prepared by traditional solid state techniques and by a method based on high pressure CS{sub 2} treatment, and then subjected to various heating and cooling schedules. In agreement with previous work, monoclinic ({alpha}) and orthorhombic ({beta}) forms were found, and a new tetragonal polymorph ({gamma}) with the anti-Fe{sub 2}-As structure was obtained in some synthesis runs. Unlike previous findings, no reversible transition between the {alpha} and {beta} phases was observed. The tetragonal {gamma} phase is most likely a spatially averaged disordered structure, with local symmetry lower then tetragonal. The CS{sub 2}, synthesis method gave rise to a disordered version of the monoclinic {alpha} phase, with broadened diffraction peaks. This material was slightly paramagnetic at room temperature and showed evidence for antiferromagnetic ordering below 50 K, unlike the ordered {alpha} and {beta} phases, which were diamagnetic at all temperatures. Low-temperature EPR and Raman spectroscopy indicates that the behavior of the disordered {alpha}-LaS{sub 2} is due to the presence of paramagnetic S{sup {minus}} defects and rapid S-S bond switching in the S-S basal planes.