Jon P. Chapman

Structural phase transitions in the ordered double perovskite Sr2MnTeO6

The crystal structure of the ordered double perovskite Sr2MnTeO6 has been refined at ambient temperature from high resolution neutron and x-ray powder diffraction data in the monoclinic space group P 121/n 1 with a = 5.7009(1) A, b = 5.6770(1) A, c = 8.0334(1) A and β = 90.085(1)°. This represents a combination of in-phase (+) and out-of-phase (−) rotations of virtually undistorted MnO6 and TeO6 octahedra in the (−−+) sense about the axes of the ideal cubic perovskite. High temperature x-ray powder diffraction shows three structural phase transitions at approximately 250, 550 and 675 °C, each corresponding to the disappearance of rotations about one of these axes. The first transition was analysed by differential scanning calorimetry and showed a thermal hysteresis with an enthalpy of 0.55 J g−1. We propose the () sequence of structural transitions which has not been previously reported for a double perovskite oxide.

Factors determining the effect of Co(II) in the ordered double perovskite structure: Sr2CoTeO6

Double perovskites are of interest due to their diverse properties that are of potential use in technological applications. In order to determine the effect of Co(II) in the double perovskite structure in the absence of other paramagnetic species, mixed valency or mis-site disorder, we have studied the ordered perovskite Sr2CoTeO6 synthesised by the freeze-drying method. The room temperature monoclinic P21/n symmetry (a = 5.6437(2) A, b = 5.6096(2) A, c = 7.9271(2) A, β = 90.059(2)°) is maintained down to 4 K with structural transitions to I2/m then Fmm at 373 K (100 °C) and 773 K (500 °C). Below TN = 18 K, the Type I antiferromagnetic structure is observed with magnetic moments of magnitude 2.25(3) μB rotated 58° out of the ab plane. AC magnetic susceptibility and specific heat data show maxima at 19 K, associated with long-range antiferromagnetic order. EPR and diffuse reflectance spectroscopies confirm that Co is in the +2 oxidation state, in highly regular octahedral coordination and is highly ionic. A small magnetic irreversibility, with Hc and Mr of 36 Oe and 0.5 emu mol−1, is observed at low temperature but this is not due to conventional spin glass or cluster glass behaviour. Calculations from specific heat give a magnetic entropy of 4.2 J mol−1 K−1, close to the theoretical value for the S′ = ½ state of the Co2+ cation at low temperature. Distortions of the structure, demonstrated to be primarily rotations of highly regular octahedra, change the geometry of the magnetic exchange pathways but are insufficient to explain the variation in ordering temperatures and magnetic structure types observed, with orbital energies within the exchange pathways having a significant influence on the properties of these and similar technologically important materials.

Spin-glass behaviour in the double perovskite “Sr2FeTeO6” due to mis-site disorder and cation vacancy formation

The double perovskite of nominal composition Sr2FeTeO6 has been prepared by the freeze-drying method. Simultaneous Rietveld refinement of X-ray and high resolution D2B neutron diffraction data has shown it to have space group I12/m1 but with a significant degree of pseudo-tetragonal symmetry with a = 5.6080(3) A, b = 5.5952(3) A, c = 7.9081(5) A and β = 90°, and a far more complex cation distribution over the B- and B′-sites. Refinement of Fe and Te fractions over these sites yielded the composition (Fe0.84□0.16)B(Te0.87Fe0.13)B′. This, and the absence of any significant formation of oxygen vacancies, yields an oxidation state for Fe close to +3, confirmed by EPR measurements (g ∼ 2.00). The disorder found over the B- and B′-sites implies that each Fe3+O6 octahedron is coordinated to six species, each of which can be Te, □ or another Fe, producing a statistical distribution of exchange pathways. The presence of several signals in the room temperature 57Fe Mossbauer spectrum and the broadening of the ESR signals can be related to this statistical distribution of the coordination environments around Fe3+. Magnetic DC susceptibility measurements indicate that the most important interactions are of antiferromagnetic nature. Absence of magnetic peaks in the 4 K neutron diffraction data, divergence below 40 K of field-cooled and zero-field-cooled DC susceptibilities, the frequency dependence of χ′ and χ″ magnetic AC susceptibilities and the lack of a three-dimensional λ-type peak in the specific heat measurements are all consistent with spin-glass type behaviour, as expected from the variety of frustrated magnetic interactions arising from the observed mis-site disorder and vacancies.

Phase separation in manganites induced by orbital–ordering strains

Doped manganite perovskites AMnO(3) exhibit a rich variety of electronic properties, resulting from the interplay of charge (Mn(3+)/Mn(4+)), spin (Mn magnetic moment) and orbital (Mn(3+) Jahn-Teller distortion) degrees of freedom. Magnetisation measurements and ESR spectra have been used to study a series of eight AMnO(3) perovskites, in which the A cation sites are occupied by a distribution of 70% trivalent lanthanide and 30% divalent Ca, Sr or Ba ions. These all have a mean A cation radius of 1.20 Angstrom but different values of the cation size variance sigma(2). A change from orbital disorder to order (cooperative Jahn-Teller distortions) was previously found in the insulating regime at sigma(2) = approximately 0.005 Angstrom(2). This work has shown that co-existence of the orbitally ordered and disordered phases is found in sigma(2)= 0.0016-0.0040 Angstrom(2) samples, with a difference of 40 K between their Curie temperatures. This is ascribed to competition between orbital ordering and microstructural lattice strains. At larger sigma(2) > 0.005 Angstrom(2), the orbital ordering strains are dominant and only this phase is observed. This intermediate temperature phase segregation is one of many strain-driven separation phenomena in manganites.