  High purity polycrystalline Na0.5La0.5RuO3 was synthesized by a solid state
method, and its properties were studied by magnetic susceptibility, heat
capacity and resistivity measurements. We find it to be a tetragonal
perovskite, in contrast to an earlier report, with random La/Na mixing. With a
Curie-Weiss temperature of -231 K and effective moment of 2.74 uB/mol-Ru, there
is no magnetic ordering down to 1.8 K. A broad hump at 1.4 K in the heat
capacity, however, indicates the presence of a glassy magnetic transition,
which we attribute to the influence of the random distribution of Na and La on
the perovskite A sites. Comparison to CaRuO3, a structurally ordered ruthenate
perovskite with similar properties, is presented. First-principle calculations
indicate that the Na-La distribution determines the local magnetic exchange
inter-actions between Ru ions, favoring either antiferromagnetic or
ferromagnetic coupling when the local environment is Na or La rich. Thus our
data and analysis suggest that mixing cations with different charges and sizes
on the A site in this perovskite results in magnetic frustration through a
balance of local magnetic exchange interactions.
