Structure and Magnetic Properties of KRuO 4 .

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 (dz 2 )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) μB , reduced from an expected value near 1 μB . 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.