Synthesis, crystal structures, metal-atom distribution and magnetic properties of Cr5−xTixSe8 (x ≈ 2, 3, 4)

The non-stoichiometric chromium titanium selenides Cr5−xTixSe8 (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 MSe6 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.

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