RESUMO
The synthesis of polycrystalline samples of B-site doped SrRu1-xMxO3 with x≤ 0.2 by solid state methods is described for a number of dopants (M = Mg, Mn, Fe, Co, Ni, Cu, or Zn) and the structures of these established using Synchrotron X-ray powder diffraction, and for SrRu0.8Cu0.2O3 high resolution neutron diffraction. With the exception of M = Cu, samples with x = 0.2 form an orthorhombic Pbnm type perovskite structure at room temperature and these exhibit a sequence of phase transitions upon heating associated with the gradual reduction in the cooperative tilting of the corner sharing octahedra. SrRu0.8Cu0.2O3 forms a unique monoclinic structure at low temperatures and this transforms to the cubic Pm3[combining macron]m structure via an I4/mcm intermediate upon heating. The magnetic and electronic properties of the samples have been studied. Doping results in a decrease in the Curie temperature and at x = 0.2 all the samples are insulators. This is a consequence of the partial oxidation of the Ru cation that narrows the Ru 4d bands coupled with the suppression of the itinerant nature of the Ru 4d electrons due to the random distribution of the dopant cations. Ru L3-edge X-ray absorption spectroscopy of the Cu doped samples reveal a gradual increase in the average Ru oxidation sate upon doping. Electrical resistivity measurements show that doping increases the resistivity of the samples, and the temperature dependence of the resistivities are consistent with Arrhenius-type charge conduction.
RESUMO
The long range average crystal structure, as well as the short and medium range structural features, of the A-site deficient and oxygen deficient perovskite Sr11Mo4O23 have been determined. Rietveld refinement of synchrotron X-ray and neutron powder diffraction data show that this compound is cubic with space group Fd3[combining macron]m and a lattice parameter of a = 16.4108 Å. These findings contradict earlier reports of a tetragonal crystal structure. Sr11Mo4O23 appears to be isostructural with Ba11W4O23, except that the disordered coordination environment around one of the Mo sites seems to be a mixture of octahedral and square pyramidal instead of octahedral and tetrahedral. The short and medium range structural features have been inspected using the neutron pair distribution function (PDF). Short range correlations between the oxygen polyhedra surrounding the Mo(2) atom exist to avoid short O-O contacts. A model has been constructed which contains such correlations and is verified by reverse Monte Carlo (RMC) modeling of the PDF. The RMC refinements also give the distribution of inter-atomic distances in this compound which reveals how the various atomic positions are correlated and over what length scales. These results are important for understanding the ionic conduction pathways.
RESUMO
High resolution powder neutron diffraction measurements on polycrystalline SrSnO3 between 8 and 350 K are described. SrSnO3 retains the orthorhombic Pbnm structure over this temperature range. Examination of the thermal expansion of the individual lattice parameters reveals an anomaly near 230 K that may reflect the presence of polar nanodomains associated with local disorder of the octahedral tilts.
RESUMO
The structural, magnetic and electrical properties of the mixed Ti-Mn oxides Sr(1-x)La(x)Ti(0.5)Mn(0.5)O3 (0 ≤ x ≤ 0.5) are reported. At room temperature the oxides have a cubic structure in space group Pm3m for x ≤ 0.25 and rhombohedral in R3c for 0.3 ≤ x ≤ 0.50. X-ray absorption spectroscopic measurements demonstrate the addition of La(3+) is compensated by the partial reduction of Mn(4+) to Mn(3+). Variable temperature neutron diffraction measurements show that cooling Sr(0.6)La(0.4)Ti(0.5)Mn(0.5)O3 results in a first order transition from rhombohedra to an orthorhombic structure in Imma. Complex magnetic behaviour is observed. The magnetic behaviour of the mixed valent (Mn(3+/4+)) examples is dominated by ferromagnetic interactions, although cation disorder frustrates long range magnetic ordering.
RESUMO
Oxides of the type LaCr(1-y)Cu(y)O3 have been prepared using solid-state methods and their crystal structures refined using synchrotron X-ray powder diffraction. The solubility limit of Cu was found to be around y = 0.2, and such oxides are orthorhombic in space group Pbnm. X-ray absorption spectroscopy measurements at the Cr and Cu L-edges demonstrated that the Cr remains trivalent upon Cu doping, with the Cu being present as Cu(III). The oxides are found to be antiferromagnets, and the Néel temperature, TN, decreases as the Cu content is increased. The crystal and magnetic structures of one example La(Cr0.85Cu0.15)O3 have been investigated between 3 and 350 K by neutron powder diffraction. The samples are semiconductors.
RESUMO
Neutron diffraction methods are used to refine the structure of SrRu(0.8)Ni(0.2)O(3) at room temperature and 4 K. X-ray absorption measurements at the Ru L-edge demonstrate that partial oxidation of the Ru(4+) to Ru(5+) occurs upon introducing Ni into the SrRuO(3) structure to form SrRu(0.8)Ni(0.2)O(3). Whilst the diffraction measurements show that SrRu(0.8)Ni(0.2)O(3) is isostructural with SrRuO(3), the Ni doped compound exhibits an unusual first order orthorhombic-cubic phase transition near 670 K as revealed by x-ray diffraction. The Curie temperature in SrRu(0.8)Ni(0.2)O(3) is lower than that found in SrRuO(3) as a consequence of local distortions reducing the p-d hybridization.
