posted on 2016-12-01, 19:34authored byCasey A. Marjerrison, Cole Mauws, Arzoo Z. Sharma, Christopher R. Wiebe, Shahab Derakhshan, Chad Boyer, Bruce D. Gaulin, John E. Greedan
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 (dz2)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.