C. McCammon

Oxygen vacancy ordering in CaTiO3–CaFeO2.5 perovskites: From isolated defects to infinite sheets

Abstract The oxygen vacancy ordering process in the system CaTiO3-CaFeO2.5 has been studied as a function of composition and temperature. The samples (CaTi1−x Fe x O3−x/2 with 0 ⩽x⩽ 1) were annealed at temperatures between 900°C and 1400°C and characterized, after drop quench, by means of XRD, Mossbauer spectroscopy and TEM. Vacancy ordering starts from isolated oxygen vacancies randomly distributed in the lattice. They first cluster into chains of finite length-which increases with increasing Fe content or decreasing temperature. Eventually, the chains become infinitely long and arrange in planes with tetrahedrally coordinated cations that alternate with layers of octahedrally coordinated ones. Long-range order of T (tetrahedral) and O (octahedral) planes develops in samples with x ⩾ 0.50. Mossbauer area ratio data indicate that at low Fe contents Ti must be present, partially, in pentacoordinated sites while at higher x this cation must be distributed over octahedral as well as tetrahedral sites.

Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition

We discover that hcp phases of Fe and Fe(0.9)Ni(0.1) undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mössbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.

Displacive phase transitions and spontaneous strains in oxygen deficient CaFexTi1-xO3-x/2 perovskites (0lexle0.40)

The symmetry of CaFex Ti1-x O3-x /2 perovskites (0 x 0.40) has been studied by means of room temperature and high temperature x-ray powder diffraction. At room temperature the perovskites undergo at least two displacive phase transitions with increasing Fe content: from the CaTiO3 Pbnm structure to the tetragonal I 4/mcm polymorph at x = 0.205±0.017 followed by transformation to the cubic Pm 3m structure at x = 0.251±0.029. A strain analysis of the unit cell demonstrates that the orthorhombic to tetragonal transition is first order in character while the tetragonal to cubic one is second order. An in situ high temperature XRD study on compositions with x = 0.109 and x = 0.188 shows that both samples undergo the same set of phase transitions at high temperature as found in the CaFex Ti1-x O3-x /2 samples with increasing Fe content at room temperature. The transition temperatures indicate that the phase boundaries in the temperature-composition phase diagram may be non-linear.

Iron oxidation state of FeTiO 3 under high pressure

The oxidation state of iron in

Order–disorder–reorder process in thermally treated dolomite samples: a combined powder and single-crystal X-ray diffraction study

{text{FeTiO}}_{3}

Effect of Lone-Electron-Pair Cations on the Orientation of Crystallographic Shear Planes in Anion-Deficient Perovskites

under high pressure was investigated by combining x-ray diffraction, Mossbauer spectroscopy, x-ray-absorption spectroscopy, and density-functional theory based calculations. Our results demonstrate that the ilmenite-to-perovskite phase transition occurs above 20 GPa at room temperature and on compression two phases coexist to 40 GPa. The

Effect of high pressure on the crystal structure and electronic properties of magnetite below 25 GPa

{text{Fe}}^{3+}/ensuremath{Sigma}text{Fe}

Iron–magnesium alloying at high pressures and temperatures

ratio increases up to 16 GPa, probably attributed to the

FeO and MnO high-pressure phase diagrams: relations between structural and magnetic properties

d

In situ high-pressure study of LiNbO3-type FeTiO3: X-ray diffraction and Mössbauer spectroscopy

-electron drifting of cations via the oxygen bridge in the adjacent octahedral, then decreases at higher pressure due to the ilmenite-to-perovskite phase transition accompanied by a slight decrease in iron valence state. Our ab initio calculations further show that the most significant changes in the charge distribution in