New Oxidation States and Defect Chemistry in the Pyrochlore Structure

Abstract

The introduction of anion vacancies into complex metal oxides is a powerful method of controlling structure and properties by tailoring metal coordination environment and oxidation state. The hydride anion can effect reductive transformations of oxides inaccessible to conventional metal getters or gaseous reducing agents.[1] The A 2 B 2 O 7 pyrochlore structure [2] is important for properties as diverse as anion and mixed conduction, [3] superconductivity, [4] and colossal magnetoresistance, [5] but, in contrast to ABO 3 perovskite, there is little chemistry established for the low-temperature introduction of anion vacancies and generation of associated defect structures. Pyrochlore is well known for stabilizing high oxidation states on the octahedral B site; A ). Here we report the systematic low-temperature synthesis of pyrochlore systems with up to 90 % titanium(iii) on the octahedral sites.

Phase Separation during the Topotactic Reduction of the Pyrochlore Y2Ti2O7

“…The weak magnetic behavior of Y 2 Ti 2 O 7 - x is in strong contrast to that reported for Lu 2 Ti 2 O 7 - x , which exhibits a magnetic susceptibility consistent with strongly antiferromagnetically coupled local moments for both x = 0.9 and 0.5 (θ = 176 and 227 K for x = 0.9 and 0.5, respectively) …”

“…At convergence the two-phase models provided a relatively good fit to the diffraction data; however, there were a number of statistically significant deviations particularly in fits to the neutron diffraction data. The recent report on the analogous reduced lutetium and ytterbium titanium pyrochlores Lu 2 Ti 2 O 6.10 and Yb 2 Ti 2 O 6.45 observed a small amount of metal antisite disorder accompanied by the partial occupation of 48f and 32e defect anion sites. These features were added to the structural models with significant improvements to the fitting statistics of all three samples.…”

“…Phase 2 has a more complicated structure due to the presence of metal antisite disorder and the associated occupation of defect anion positions labeled O(3) and O(4) according to the scheme presented by Blundred et. al . In this more reduced phase between 5 and 7% of the metal cations are on the “wrong” crystallographic site and the occupation of the two defect oxygen sites can be rationalized on the basis of the coordination requirements of the two different metal cations.…”

Ordered Rock-Salt Related Nanoclusters in CaMnO2

“…), have been used to prepare a large number of metastable reduced phases with perovskite-type structure. − This is the case of the Sr−Fe−O system where, for many years, the brownmillerite Sr 2 Fe 2 O 5 was the most reduced phase . However, by using calcium hydride at low temperatures as a reductant, , a stable SrFeO 2 phase with Fe 2+ in an unexpected square planar oxygen coordination has been recently stabilized by Tsuyimoto et al More recently, a new reduced 4H-BaSrMnO 2+δ with Mn 2+ in tetrahedral coordination has been isolated from the reduction process of 4H-BaSrMnO 3 with lithium hydride . These examples show that by using binary metal hydrides as reducing agents, the reduction process takes place at remarkably low temperatures (280−300 °C) and metastable phases can be obtained.…”

Spin‐Ladder Iron Oxide: Sr3Fe2O5

“…However, this restriction was recently overcome [9] by using calcium hydride at low temperatures as a reductant, as initiated and developed by Hayward, Rosseinsky and co-workers. [10,11] Low-temperature reaction of cubic perovskite SrFe IV O 3 with CaH 2 led to stable SrFe II O 2 with a square-planar oxygen coordination environment around the high-spin Fe 2+ ion. The structure is isostructural with the "infinite"-layer cupric oxides.…”