The crystal structure
of KRuO<sub>4</sub> 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, <i>I</i>4<sub>1</sub>/<i>a</i>, is confirmed at both temperatures. Atomic positions of greater
accuracy than the original 1954 X-ray study are reported. The rare
Ru<sup>7+</sup> ion resides in a site of distorted tetrahedral symmetry
with nominal electronic configuration 4d<sup>1</sup>(e<sup>1</sup>). 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 <i>R</i> 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, <i>J</i><sub>1</sub>, and the next-nearest-neighbor, <i>J</i><sub>2</sub>, exchange constants are AF and of similar magnitude.
A spin dimer analysis finds <i>J</i><sub>1</sub>/<i>J</i><sub>2</sub> ≈ 25, indicating weak frustration,
and a (d<sub><i>z</i></sub><sup>2</sup>)<sup>1</sup> 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 <i>c</i> axis. The Ru<sup>7+</sup> moment is estimated
to be 0.57(7) μ<sub>B</sub>, reduced from an expected value
near 1 μ<sub>B</sub>. A recent computational study of isostructural,
isoelectronic KOsO<sub>4</sub> predicts a surprisingly large orbital
moment due to spin–orbit coupling (SOC). However, the free
ion SOC constant for Ru<sup>7+</sup> is only ∼30% that of Os<sup>7+</sup>, so it is unclear that this effect can be implicated in
the low ordered moment for KRuO<sub>4</sub>. The origin of the short-range
spin correlations is also not understood