Fabio Denis Romero

SrFe0.5Ru0.5O2: Square-Planar Ru2+ in an Extended Oxide

Low-temperature topochemical reduction of the cation disordered perovskite phase SrFe(0.5)Ru(0.5)O(3) with CaH(2) yields the infinite layer phase SrFe(0.5)Ru(0.5)O(2). Thermogravimetric and X-ray absorption data confirm the transition metal oxidation states as SrFe(0.5)(2+)Ru(0.5)(2+)O(2); thus, the title phase is the first reported observation of Ru(2+) centers in an extended oxide phase. DFT calculations reveal that, while the square-planar Fe(2+) centers adopt a high-spin S = 2 electronic configuration, the square-planar Ru(2+) cations have an intermediate S = 1 configuration. This combination of S = 2, Fe(2+) and S = 1, Ru(2+) is consistent with the observed spin-glass magnetic behavior of SrFe(0.5)Ru(0.5)O(2).

Topochemical Fluorination of Sr3(M0.5Ru0.5)2O7 (M = Ti, Mn, Fe), n = 2, Ruddlesden–Popper Phases

Reaction of the appropriate Sr3(M(0.5)Ru(0.5))2O7 (M = Ti, Mn, Fe), n = 2, Ruddlesden-Popper oxide with CuF2 under flowing oxygen results in formation of the oxide-fluoride phases Sr3(Ti(0.5)Ru(0.5))2O7F2, Sr3(Mn(0.5)Ru(0.5))2O7F2, and Sr3(Fe(0.5)Ru(0.5))2O(5.5)F(3.5) via a topochemical anion insertion/substitution process. Analysis indicates the titanium and manganese phases have Ti(4+), Ru(6+) and Mn(4+), Ru(6+) oxidation state combinations, respectively, while Mössbauer spectra indicate an Fe(3+), Ru(5.5+) combination for the iron phase. Thus, it can be seen that the soft fluorination conditions employed lead to formation of highly oxidized Ru(6+) centers in all three oxide-fluoride phases, while oxidation states of the other transition metal M cations remain unchanged. Fluorination of Sr3(Ti(0.5)Ru(0.5))2O7 to Sr3(Ti(0.5)Ru(0.5))2O7F2 leads to suppression of magnetic order as the fluorinated material approaches metallic behavior. In contrast, fluorination of Sr3(Mn(0.5)Ru(0.5))2O7 and Sr3(Fe(0.5)Ru(0.5))2O7 lifts the magnetic frustration present in the oxide phases, resulting in observation of long-range antiferromagnetic order at low temperature in Sr3(Mn(0.5)Ru(0.5))2O7F2 and Sr3(Fe(0.5)Ru(0.5))2O(5.5)F(3.5). The influence of the topochemical fluorination on the magnetic behavior of the Sr3(M(0.5)Ru(0.5))2O(x)F(y) phases is discussed on the basis of changes to the ruthenium oxidation state and structural distortions.

Topochemical Reduction of the Ruddlesden–Popper Phases Sr2Fe0.5Ru0.5O4 and Sr3(Fe0.5Ru0.5)2O7

Reaction of the Ruddlesden-Popper phases Sr2Fe(0.5)Ru(0.5)O4 and Sr3(Fe(0.5)Ru(0.5))2O7 with CaH2 results in the topochemical deintercalation of oxide ions from these materials and the formation of samples with average compositions of Sr2Fe(0.5)Ru(0.5)O(3.35) and Sr3(Fe(0.5)Ru(0.5))2O(5.68), respectively. Diffraction data reveal that both the n = 1 and n = 2 samples consist of two-phase mixtures of reduced phases with subtly different oxygen contents. The separation of samples into two phases upon reduction is discussed on the basis of a short-range inhomogeneous distribution of iron and ruthenium in the starting materials. X-ray absorption data and Mössbauer spectra reveal the reduced samples contain an Fe(3+) and Ru(2+/3+) oxidation state combination, which is unexpected considering the Fe(3+)/Fe(2+) and Ru(3+)/Ru(2+) redox potentials, suggesting that the local coordination geometry of the transition metal sites helps to stabilize the Ru(2+) centers. Fitted Mössbauer spectra of both the n = 1 and n = 2 samples are consistent with the presence of Fe(3+) cations in square planar coordination sites. Magnetization data of both materials are consistent with spin glass-like behavior.

Structure and Magnetism of the Topotactically Reduced Oxychloride Sr4Mn3O6.5Cl2

Reaction of the n = 3 Ruddlesden-Popper oxychloride Sr(4)Mn(3)O(7.5)Cl(2) with calcium hydride yields the topotactically reduced phase Sr(4)Mn(3)O(6.5)Cl(2). The deintercalation of oxide ions from the central MnO(1.5) layer of the starting phase is accompanied by a rearrangement of the anion lattice, resulting in a layer of composition MnO(0.5) in the reduced material, consisting of chains of MnO(4) tetrahedra connected by edge and corner sharing. Magnetization and low-temperature neutron diffraction data are consistent with antiferromagnetic coupling of manganese spins, but no long-range magnetic order is observed down to 5 K, presumably due to the large interlayer separation in the reduced phase. The influence of anion substitution on the structural selectivity of low-temperature reduction reactions is discussed.

Two Charge Ordering Patterns in the Topochemically Synthesized Layer-Structured Perovskite LaCa2Fe3O9 with Unusually High Valence Fe3.67+

A-site-ordered layer-structured perovskite LaCa2Fe3O9 with unusually high valence Fe3.67+ was obtained by low-temperature topochemical oxidation of the A-site layer-ordered LaCa2Fe3O8. The unusually high valence Fe3.67+ in LaCa2Fe3O9 shows charge disproportionation of Fe3+ and Fe5+ first along the layer-stacking ⟨010⟩ direction below 230 K. Fe3+ is located between the La3+ and Ca2+ layers, while Fe5+ is between the Ca2+ layers. The two-dimensional electrostatic potential due to the A-site layered arrangement results in the quasi-stable ⟨010⟩ charge ordering pattern. Below 170 K, the charge ordering pattern changes, and the 2:1 charge-disproportionated Fe3+ and Fe5+ ions are ordered along the ⟨111⟩ direction. The ground-state charge ordering pattern is stabilized primarily by the electrostatic lattice energy, and the Fe5+ ions are arranged to make the distances between the nearest neighboring Fe5+ as large as possible.

Successive Charge Transitions of Unusually High‐Valence Fe3.5+: Charge Disproportionation and Intermetallic Charge Transfer

A perovskite-structure oxide containing unusually high-valence Fe3.5+ was obtained by high-pressure synthesis. Instability of the Fe3.5+ in Ca0.5 Bi0.5 FeO3 is relieved first by charge disproportionation at 250 K and then by intermetallic charge transfer between A-site Bi and B-site Fe at 200 K. These previously unobserved successive charge transitions are due to competing intermetallic and disproportionation charge instabilities. Both transitions change magnetic and structural properties significantly, indicating strong coupling of charge, spin, and lattice in the present system.

Local structure study of the orbital order/disorder transition in LaMnO3

Orbital degrees of freedom are a key ingredient in unconventional physics, including colossal magnetoresistance (CMR). When ordered, orbital arrangements can be characterized using conventional crystallographic approaches. Yet CMR emerges from states of orbital disorder, for which the experimental signature is much more ambiguous. Here, the authors study the CMR parent compound LaMnO

Hexagonal Perovskite Ba4Fe3NiO12 Containing Tetravalent Fe and Ni Ions

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Charge Disproportionation in Sr0.5Bi0.5FeO3 Containing Unusually High Valence Fe3.5

, using total scattering to understand its orbital order/disorder transition. They find a discontinuous change in local structure that indicates a fundamental change in the type of orbital arrangement at the transition. The analysis highlights the difficulty of discriminating between local structural models when static and dynamic disorder are strongly coupled.

Strontium Vanadium Oxide–Hydrides: “Square‐Planar” Two‐Electron Phases

BaFe xNi1- xO3 with end members of BaNiO3 ( x = 0) and BaFeO3 ( x = 1), which, respectively, adopt the 2H and 6H hexagonal perovskite structures, were synthesized, and their crystal structures were investigated. A new single phase, Ba4Fe3NiO12 ( x = 0.75), that adopts the 12R perovskite structure with the space group R3̅ m ( a = 5.66564(7) Å and c = 27.8416(3) Å), was found to be stabilized. Mössbauer spectroscopy results and structure analysis using synchrotron and neutron powder diffraction data revealed that nominal Fe3+ occupies the corner-sharing octahedral site while the unusually high valence Fe4+ and Ni4+ occupy the face-sharing octahedral sites in the trimers, giving a charge formula of Ba4Fe3+Fe4+2Ni4+O11.5. The magnetic properties of the compound are also discussed.