Synthesis, crystal structures, metal-atom distribution and magnetic properties of Cr 5− xTi xSe 8 ( x ≈ 2, 3, 4)

Abstract

The non-stoichiometric chromium titanium selenides Cr 5− x Ti x Se 8 ( x ≈ 2, 3, 4) were prepared applying high-temperature solid-state reactions. The crystal structures have been refined with X-ray and neutron powder diffraction data applying the Rietveld method. The compounds crystallize in the space group F2/ m (monoclinic, non-standard setting of C2/ m) with four formula units per unit cell. The structures of all phases are related to the NiAs-type structure with ordered metal vacancies in every second metal-atom layer. Among the four different metal-atom sites two are fully occupied and the remaining two are only partially occupied. The MSe 6 octahedra share edges within layers and faces between layers. Consequently, long M–M separations occur in the layers whereas short M–M contacts result between M atoms of face-sharing octahedra. The lattice parameters of the samples and the M–M/M–Se bonds exhibit a remarkable deviation from Végard's rule which is caused by the distribution of Ti and Cr atoms over the independent crystallographic sites. The refinement of neutron diffraction data reveals that for x ≈ 2, 3, Ti atoms are located on three metal sites and only for x ≈ 4 the fourth position is also occupied by Ti. For all compositions a pronounced preference of Ti for sites within the full metal-atom layers is observed. Furthermore, a significant depletion of one partially occupied metal-atom site is accompanied by an increase of occupancy of the other partially occupied metal-atom site. The magnetic properties in the high-temperature range show Curie–Weiss behavior with negative paramagnetic Curie temperatures θ indicating strong antiferromagnetic exchange interactions which get weaker with increasing Ti content. The zero-field-cooled and field-cooled susceptibility curves diverge at low temperatures suggesting spin-glass behavior. The freezing temperature is lowered with increasing Ti content.