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The gadolinium pyrochlores, Gd2B2O7, are amongst the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd2Pt2O7, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd2Pt2O7 undergoes an antiferromagnetic ordering transition at TN = 1.6 K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a non-divergent relaxation rate in µSR. The form of the heat capacity below TN suggests that the ground state is an anisotropic collinear antiferromagnet with an excitation spectrum that is gapped by 0.245(1) meV. The ordering temperature in Gd2Pt2O7, TN = 1.6 K, is a substantial 160% increase from other gadolinium pyrochlores, which have been found to order at 1 K or lower. We attribute this enhancement in TN to the B-site cation, platinum, which, despite being non-magnetic, has a filled 5d t2g orbital and an empty 5d eg orbital that can facilitate superexchange. Thus, the magnetic frustration in Gd2Pt2O7 is partially "relieved", thereby promoting magnetic order.

The crystal structure of KRuO 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, I4/a, is confirmed at both temperatures. Atomic positions of greater accuracy than the original 1954 X-ray study are reported. The rare Ru ion resides in a site of distorted tetrahedral symmetry with nominal electronic configuration 4d(e). 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, J, and the next-nearest-neighbor, J, exchange constants are AF and of similar magnitude. A spin dimer analysis finds J/J ≈ 25, indicating weak frustration, and a (d) 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 Ru moment is estimated to be 0.57(7) μ, reduced from an expected value near 1 μ. A recent computational study of isostructural, isoelectronic KOsO predicts a surprisingly large orbital moment due to spin-orbit coupling (SOC). However, the free ion SOC constant for Ru is only ∼30% that of Os, so it is unclear that this effect can be implicated in the low ordered moment for KRuO. The origin of the short-range spin correlations is also not understood.

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Exhibiting rich magnetic behaviour and potentially multiferroic properties, the dugganites, a Te(6+) containing subgroup of the langasite series, are an attractive family of compounds for future study. It was recently shown that Pb-bearing members of the dugganite series undergo distortions away from the P321 symmetry that is characteristic of the langasites. Here, we detail the consequences these distortions have on the magnetism exhibited by Pb3TeCo3V2O14 and Pb3TeCo3P2O14, solving the magnetic structures of both compounds with respect to a new supercell. Using neutron scattering and magnetic susceptibility measurements, we show that small applied magnetic fields can seriously perturb the delicate magnetic states in both of these systems. This is further demonstrated by presenting how doping P(5+) onto the nonmagnetic V(5+) site completely changes the magnetic structure from either of the end series members. Finally, it is shown using inelastic neutron scattering and magnetic susceptibility measurements that Pb3TeCo3V2O14 can be characterized using a model for isosceles trimers, which do not exist in the previously reported P321 subcell.

The YPrO3+δ system is a nearly ideal model system for the investigation of oxide defect creation and annihilation in oxide ion conductor related phases with potential applications as solid state electrolytes in solid oxide fuel cells. The formation, structure, high temperature reactivity, and magnetic susceptibility of phase pure YPrO3+δ (0 ≤ δ ≤ 0.46) are reported. The topotactic reduction and oxidation of the YPrO3+δ system was investigated by powder X-ray in situ diffraction experiments and revealed bixbyite structures (space group: Ia3̅) throughout the series. Combined neutron and X-ray data clearly show oxygen uptake and removal. The research provides a detailed picture of the Y(3+)/Pr(3+)/Pr(4+) sublattice evolution in response to the redox chemistry. Upon oxidation, cation site splitting is observed where the cation in the ((1)/4, (1)/4, (1)/4) position migrates along the body diagonal to the (x, x, x) position. Any oxygen in excess of YPrO3.0 is located in the additional 16c site without depopulating the original 48e site. The in situ X-ray diffraction data and thermal gravimetric analysis have revealed the reversible topotactic redox reactivity at low temperatures (below 425 °C) for all compositions from YPrO3 to YPrO3.46. Magnetic susceptibility studies were utilized in order to further confirm praseodymium oxidation states. The linear relation between the cubic unit cell parameter and oxygen content allows for the straightforward determination of oxygen stoichiometry from X-ray diffraction data. The different synthesis strategies reported here are rationalized with the structural details and the reactivity of YPrO3+δ phases and provide guidelines for the targeted synthesis of these functional materials.