Chalcogen substitution in the Ta 4P 4S 29 tunnel structure: Synthesis and structure of TaPS 6Se

Abstract

A compound with a composition close to TaPS 6Se was prepared either from the elements or from Ta PS 6 and Se heated respectively at 630 and 460°C in evacuated silica tubes for 20 days. The crystal symmetry is monoclinic, space group P2, with the following cell parameters: a = 15.657(2) Å, b = 6.8714(8) Å, c = 22.273(3) Å, β = 135.09(1)°, V = 1691.7(5) Å 3, and Z = 8. The structure determination made from 3238 reflections and 244 parameters yielded a reliability factor of R = 0.050. TaPS 6Se atomic arrangement is very similar to that of Ta 4P 4S 29, with the same basic tunnel framework in which, instead of ordered sulfur helices, selenium chains, partially substituted by sulfur, are inserted. The chains of composition (Se 8S 2) present a periodicity of 2 b, but owing to the existence of four different statistically distributed chains, no superstructure is observed. The origin of the network and chain commensurability is related to the chain composition and SeSeS angle distortion, along with some small substitution by selenium within the TaPS 6 framework itself. This substitution induces some distortion in the [Ta 2S 12] bicapped biprisms and in the [PS 4] tetrahedra that constitute the basic structure of the compound. Some elongated interatomic distances were found in the substituted S 2 pairs (mean d ss = 2.123 Å) and in the mean TaTa distances (mean d TaTa = 3.426 Å). TaPS 6Se is semiconducting and diamagnetic in agreement with d 0 tantalum cations.