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, <i>I</i>4₁/<i>a</i>, 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 <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>₁, and the next-nearest-neighbor, <i>J</i>₂, exchange constants are AF and of similar magnitude. A spin dimer analysis finds <i>J</i>₁/<i>J</i>₂ ≈ 25, indicating weak frustration, and a (d_<i>z</i>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 <i>c</i> 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

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, <i>I</i>4₁/<i>a</i>, 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 <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>₁, and the next-nearest-neighbor, <i>J</i>₂, exchange constants are AF and of similar magnitude. A spin dimer analysis finds <i>J</i>₁/<i>J</i>₂ ≈ 25, indicating weak frustration, and a (d_<i>z</i>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 <i>c</i> 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

Cubic Re⁶⁺ (5d¹) Double Perovskites, Ba₂MgReO₆, Ba₂ZnReO₆, and Ba₂Y_2/3ReO₆: Magnetism, Heat Capacity, μSR, and Neutron Scattering Studies and Comparison with Theory

Double perovskites (DP) of the general formula Ba₂MReO₆, where M = Mg, Zn, and Y_2/3, all based on Re⁶⁺ (5d¹, t_2g¹), were synthesized and studied using magnetization, heat capacity, muon spin relaxation, and neutron-scattering techniques. All are cubic, <i>Fm</i>3̅<i>m</i>, at ambient temperature to within the resolution of the X-ray and neutron diffraction data, although the muon data suggest the possibility of a local distortion for M = Mg. The M = Mg DP is a ferromagnet, <i>T</i>_c = 18 K, with a saturation moment ∼0.3 bohr magnetons at 3 K. There are two anomalies in the heat capacity: a sharp feature at 18 K and a broad maximum centered near 33 K. The total entropy loss below 45 K is 9.68 e.u., which approaches <i>R</i> ln 4 (11.52 e.u.) supporting a <i>j</i> = 3/2 ground state. The unit cell constants of Ba₂MgReO₆ and the isostructural, isoelectronic analogue, Ba₂LiOsO₆, differ by only 0.1%, yet the latter is an anti-ferromagnet. The M = Zn DP also appears to be a ferromagnet, <i>T</i>_c = 11 K, μ_sat(Re) = 0.1 μ_B. In this case the heat capacity shows a somewhat broad peak near 10 K and a broader maximum at ∼33 K, behavior that can be traced to a smaller particle size, ∼30 nm, for this sample. For both M = Mg and Zn, the low-temperature magnetic heat capacity follows a <i>T</i>^3/2 behavior, consistent with a ferromagnetic spin wave. An attempt to attribute the broad 33 K heat capacity anomalies to a splitting of the <i>j</i> = 3/2 state by a crystal distortion is not supported by inelastic neutron scattering, which shows no transition at the expected energy of ∼7 meV nor any transition up to 100 meV. However, the results for the two ferromagnets are compared to the theory of Chen, Pereira, and Balents, and the computed heat capacity predicts the two maxima observed experimentally. The M = Y_2/3 DP, with a significantly larger cell constant (3%) than the ferromagnets, shows predominantly anti-ferromagnetic correlations, and the ground state is complex with a spin frozen component <i>T</i>_g = 16 K from both direct current and alternating current susceptibility and μSR data but with a persistent dynamic component. The low-temperature heat capacity shows a <i>T</i>¹ power law. The unit cell constant of <i>B</i> = Y_2/3 is less than 1% larger than that of the ferromagnetic Os⁷⁺ (5d¹) DP, Ba₂NaOsO₆