Understanding the Interplay of Vacancy, Cation, and Charge Ordering in the Tunable Sc2VO5+δ Defect Fluorite System.

We report the synthesis, structure, and redox behavior of the cation-ordered tetragonal Sc2VO5+δ defect fluorite superstructure previously thought to be the oxygen precise A3+2B4+O5 phase. Four synthesis routes in oxidative, reductive, and inert atmospheres are demonstrated. Ex situ and in situ powder X-ray and neutron diffraction analyses reveal vanadium disproportionation reactions. The structure-reaction map illustrates the oxygen-dependent competition between the tetragonal cation and anion ordered Sc2VO5+δ and the disordered cubic Sc2VO5+δ' (δ < δ' ≤ 0.5) phases as a function of temperature. Oxidation states and oxide stoichiometries were determined with DC magnetometry and XANES experiments. The tetragonal cation ordered Sc2VO5+δ phase with δ = -0.15(2) for as-synthesized samples reveals vanadium charge ordering. V3+ and V4+ cations occupy octahedral sites, whereas V5+ predominantly occupies a tetrahedral site. The paramagnetic 8g{V3+/4+}4 clusters are isolated by diamagnetic 2cV5+ cations. At temperatures below 500 °C the 8g{V3+/4+}4 clusters can be topotactically fine-tuned with varying V3+/V4+ ratios. Above 600 °C the tetragonal structure oxidizes to the cubic Sc2VO5+δ' fluorite phase-its disordered competitor. The investigation of the cation- and anion-ordered Sc-V-O phases, their formation, and thermal stability is important for the design of low-temperature solid state oxide ion conductors and vacancy structures.

The Highly Frustrated 5d2 Double Perovskite Doppelgängers, SrLaMgReO6 and SrLaLiOsO6. A Comparison including Isostructural La2LiReO6.

The synthesis and characterization of the double perovskite SrLaLiOsO6 is presented. It is isostructural (P21/n) and isoelectronic (5d2) with SrLaMgReO6, which has been reported previously. The cell volumes are the same to within 1.4%: i.e., these perovskites are doppelgängers. In a previous study SrLaMgReO6 showed no sign of spin order to 2 K. New data at lower temperatures disclose a maximum in the dc susceptibility near 1.5 K. As the Curie-Weiss (C-W) temperature (Θ) for this material is -161 K, an enormous frustration index, f ≈ 100, is implied (f = |Θ|/Tord). On the other hand, SrLaLiOsO6 does not follow the C-W law over the investigated susceptibility range, 2-300 K. Fitting with an added temperature independent term (TIP) gives µeff = 1.96 µB, Θ = -102 K, and TIP = 1.01 × 10-3 emu/mol. A clear zero-field-cooled (ZFC), field-cooled (FC) divergence in the dc data occurs at ∼10 K, suggesting a much reduced frustration index, f ≈ 10, relative to SrLaMgReO6. The real part of the ac susceptibility data, χ'max, shows a frequency shift that is consistent with a spin glass ground state according to the Mydosh criterion. Heat capacity data for SrLaLiOsO6 show no sign of a λ peak at 10 K and a linear dependence on temperature below 10 K, also supporting a spin glass ground state. A spin frozen ground state for SrLaMgReO6 could not be established from χ' data due to a much weaker signal. Nonetheless, the 10-fold difference in f between these doppelgänger materials is remarkable. It is possible that the enhanced covalency with the oxide ligands for Os6+ relative to Re5+ plays a major role here. Finally, a comparison with isostructural La2LiReO6 (with a much smaller f ≈ 4) is made and a correlation between the frustration level and the sense of the local distortion of the Re(Os)-O octahedron is pointed out.

Order/Disorder and in Situ Oxide Defect Control in the Bixbyite Phase YPrO3+δ (0 ≤ δ < 0.5).

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.

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, 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.

Valence tautomerism in a [2 × 2] Co4 grid complex containing a ditopic arylazo ligand.

We describe the structural and magnetic properties of a tetranuclear [2 × 2] Co4 grid complex containing a ditopic arylazo ligand. At low temperatures and in solution the complex is comprised of Co3+ and singly reduced trianion-radical ligands. In the solid state we demonstrate the presence of valence tautomerization via variable temperature magnetic susceptibility experiments and powder-pattern EPR spectroscopy. Valence tautomerism in polynuclear complexes is very rare and to our knowledge is unprecedented in [2 × 2] grid complexes.

Rhodium(ii) dimers without metal-metal bonds.

Oxidation of ((Me)BDI)Rh(cyclooctene) ((Me)BDI = [2,6-Me(2)C(6)H(3)NCMe](2)CH) with Br(2) or I(2) produces paramagnetic halide-bridged Rh(II) dimers [((Me)BDI)Rh](2)(µ-X)(2) without a direct Rh-Rh bond. Steric factors are proposed to play a key role in preventing the formation of Rh-Rh bonded alternative structures.

Observation of magnetic polarons in the magnetoresistive pyrochlore Lu2V2O7.

Materials that exhibit colossal magnetoresistance (CMR) have attracted much attention due to their potential technological applications. One particularly interesting model for the magnetoresistance of low-carrier-density ferromagnets involves mediation by magnetic polarons (MP)-electrons localized in nanoscale ferromagnetic 'droplets' by their exchange interaction. However, MP have not previously been directly detected and their size has been difficult to determine from macroscopic measurements. In order to provide this crucial information, we have carried out muon spin rotation measurements on the magnetoresistive semiconductor Lu(2)V(2)O(7) in the temperature range from 2 to 300 K and in magnetic fields up to 7 T. Magnetic polarons with characteristic radius R ≈ 0.4 nm are detected below about 100 K, where Lu(2)V(2)O(7) exhibits CMR; at higher temperature, where the magnetoresistance vanishes, these MP also disappear. This observation confirms the MP-mediated model of CMR and reveals the microscopic size of the MP in magnetoresistive pyrochlores.