Structure and Magnetic Properties of KRuO₄

The crystal structure of KRuO₄ is refined at both 280 and 3.5 K from neutron powder data, and magnetic properties are reported for the first time. The scheelite structure, I4₁/a, is confirmed at both temperatures. Atomic positions of greater accuracy than the original 1954 X-ray study are reported. The rare Ru⁷⁺ ion resides in a site of distorted tetrahedral symmetry with nominal electronic configuration 4d¹(e¹). Curie-Weiss parameters are near free ion values for the effective moment and θ = -77 K, indicating dominant antiferromagnetic (AF) correlations. A broad susceptibility maximum occurs near 34 K, but long-range AF order sets in only below 22.4 K as determined by magnetization and heat capacity data. The entropy loss below 50 K is only 44% of the expected R ln 2, indicating the presence of short-range spin correlations over a wide temperature range. The Ru sublattice consists of centered, corner-sharing tetrahedra which can lead to geometric frustration if both the nearest-neighbor, J₁, and the next-nearest-neighbor, J₂, exchange constants are AF and of similar magnitude. A spin dimer analysis finds J₁/J₂ ≈ 25, indicating weak frustration, and a (d_z²)¹ ground state. A single, weak magnetic reflection was indexed as (110). The absence of the (002) magnetic reflection places the Ru moments parallel to the c axis. The Ru⁷⁺ moment is estimated to be 0.57(7) μ_B, reduced from an expected value near 1 μ_B. A recent computational study of isostructural, isoelectronic KOsO₄ predicts a surprisingly large orbital moment due to spin-orbit coupling (SOC). However, the free ion SOC constant for Ru⁷⁺ is only ∼30% that of Os⁷⁺, so it is unclear that this effect can be implicated in the low ordered moment for KRuO₄. The origin of the short-range spin correlations is also not understood.