High-Pressure Synthesis and Physical Properties of a Spinel Compound FeAl 2 S 4 .

A spinel compound FeAl 2 S 4 was successfully synthesized under high-pressure and high-temperature conditions and was systematically characterized via the structural, magnetic, and specific heat measurements. It crystallizes into a cubic structure with the space group Fd 3̅ m (no. 227) and the lattice constant a = 10.0207(2) Å. A Fe/Al site inversion is found; that is, the molecular formula can be rewritten as (Fe 1- x Al x )(Al 2- x Fe x )S 4 , and the inversion parameter x is about 0.22. Magnetic susceptibility measurements indicate that FeAl 2 S 4 undergoes a spin glass behavior, which is confirmed by ac susceptibility and specific heat measurements. The freezing temperature T f ∼ 10.5 K and Weiss temperature T θ ∼ -107.4 K lead to a high frustration parameter f = | T θ / T f | of about 10, which suggests that FeAl 2 S 4 is a high-frustration magnet. Our results indicate that high pressure can help stabilize the spinel structure with small R̅ σ and the cation inversion plays an important role in the formation of the spin glass state.