Crystal structures, chemical reactivity, magnetic properties, and Mössbauer spectroscopy of the quasi-ternary channel compounds TlV 5− yFe yS 8 ( y = 0.5 – 1.5)

Abstract

Single crystal X-ray investigations reveal that the unit cell volume of TlV 5− y Fe y S 8 increases with increasing Fe content. This unusual and unexpected behavior is explained with the increasing metal-to-metal distances as a function of Fe abundance. Tl-poorer samples can be obtained with a topotactical redox reaction using iodine in donor solvents. During this reaction the host material is strongly attacked by the redox agent. The magnetic properties can be explained on the basis of the coexistence of a temperature-independent Pauli paramagnetism (TIP) and a Curie—Weiss-like contribution from localized magnetic moments. With raising Fe abundance the effective magnetic moment per Fe is reduced and an increase of the value for the TIP is observed. This is indicative of strong delocalization of metal 3 d electron density as a function of Fe abundance. The effective magnetic moment per Fe points to Fe 3+ in the low spin state ( S = 1 2 ). This is further supported by Mössbauer experiments. The Fe atoms are distributed on the three possible crystallographic sites with a preference for the M2 site. The isomer shifts as well as the quadrupole splittings are indicative of Fe 3+ in the low spin state. A local effective Debye temperature of 260(3) K was evaluated for the Fe 3+ ions, suggesting no significant differences of the stiffness of the compounds. For TlV 4FeS 8 and TlV 4.5Fe 0.5S 8 below about 20 and 10 K, respectively, an ordering of the localized electronic spins is observed, leading to a broadening of the spectra. The change of the internal magnetic fields with temperature does not follow the Brillouin function for S = 1 2 , suggesting a spin-glass behavior.