(CaMn)MnTeO - An Anomalously Stable High-Pressure Double Perovskite.

High pressure high temperature treatments of the composition CaMnMnTeO are found to yield only an ABB'O-type double perovskite (CaMn)MnTeO, rather than a AA'BB'O double double perovskite with A- and B- site cation order as found in analogs CaMnMnReO and CaMnMnWO with similar cation sizes. Double perovskite (CaMn)MnTeO adopts a monoclinic structure in space group P2/n with a framework of highly tilted MnO and TeO octahedra enclosing disordered Ca and Mn cations. Magnetic measurements show that (CaMn)MnTeO is a highly frustrated spin glass with a freezing transition at 5 K, and no long-range spin order is apparent by neutron diffraction at 1.

Pauli-paramagnetic and metallic properties of high pressure polymorphs of BaRhO oxides containing RhO dimers.

From our material exploration study in wide pressure and temperature conditions, we found a new 6H polymorph of BaRhO was stabilised under high pressure conditions from 14 to 22 GPa. The material crystallised in the monoclinic 6H hexagonal perovskite structure in space group C2/c. The 4H BaRhO polymorph was stabilised at lower pressures, but the 3C cubic BaRhO likely requires pressures greater than 22 GPa.

Studies of the 4d and 5d 6H perovskites BaBMO, B = Ti, Zn, Y; M = Ru, Os, and cubic BaBRuO polymorphs stabilised under high pressure.

The synthesis, structures and magnetism of six mixed 3d-5d oxides Ba3BM2O9 (B = Ti, Y, Zn; M = Ru, Os) are described. When prepared at ambient pressure the six oxides display a 6H type perovskite structure comprised of corner sharing BO6 and face sharing M2O9 motifs. Synchrotron X-ray diffraction reveals a small monoclinic distortion in Ba3ZnRu2O9; the remaining oxides exhibit a hexagonal structure.

Structural and Magnetic Studies of O-Type Ruthenium and Osmium Oxides.

Oxides of the form O with = K, Rb, Cs and = Ru and Os have been synthesized and characterized by diffraction and magnetic techniques. For = K the oxides adopted the tetragonal (4/) scheelite structure. RbOsO, which crystallizes as a scheelite at room temperature, underwent a continuous phase transition to 4/ near 550 K.

Crystal structures and phase transition behaviour in the 5d transition metal oxides AReO (A = Ag, Na, K, Rb, Cs and Tl).

The structures of the six perrhenates (AReO4 A = Ag, Na, K, Rb, Cs and Tl) at room temperature have been established using powder neutron diffraction methods. These demonstrate the rigid nature of the ReO4 tetrahedra, with the Re-O distances decreasing very slightly and the O-Re-O bond angles approaching the regular tetrahedron value of 109.5° as the size of the A-type cation increases.

Structures and Phase Transitions in Pertechnetates.

The temperature dependence of the structures of four pertechnetates (TcO = Ag, Tl, Rb, Cs) from 90 K to their melting points is described. Synchrotron X-ray diffraction measurements show that RbTcO undergoes a 4/ to 4/ transition near 530 K that is associated with a change in the orientation of the TcO tetrahedra about the scheelite axis. AgTcO also exhibits a tetragonal scheelite type structure, and this is retained between 90 and 750 K, above which it melted.

Structural and magnetic studies of KOsO, a 5d quantum magnet oxide.

The quantum magnet KOsO4 has been characterized by a combination of X-ray and neutron diffraction techniques. The tetrahedrally coordinated Os7+ 5d1S = 1/2 cations were determined to order antiferromagnetically along the c axis below 35 K. A miniscule ordered magnetic moment of 0.

Magnetic and Structural Studies of Sc Containing Ruthenate Double Perovskites AScRuO (A = Ba, Sr).

Ruthenium-containing double perovskites AScRuO have been synthesized as polycrystalline powders and structurally characterized using a combination of synchrotron X-ray and neutron powder diffraction methods. When A = Ba, a hexagonal 6L perovskite structure is obtained if the synthesis is conducted at ambient pressure and a rock-salt ordered cubic structure is obtained if the sample is quenched from high pressures. The Sr oxide SrScRuO is obtained with a rock-salt ordered corner sharing topology.

Structural and Magnetic Properties of the Osmium Double Perovskites BaSrYOsO.

The crystal and magnetic structures of double perovskites of the type BaSrYOsO were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO and OsO octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3̅m (x ≈ 0.

Thermal expansion in BaRuO perovskites - an unusual case of bond strengthening at high temperatures.

The temperature dependences of the structures of three polytypes of BaRuO have been investigated between room temperature and 1000 °C using high resolution synchrotron X-ray diffraction. The structural studies reveal a systematic decrease of the Ru-Ru distance as the pressure required to prepare the polytype increases. The O-O distance across the shared face increases as the Ru-Ru separation decreases.