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From our material exploration study in wide pressure and temperature conditions, we found a new 6H polymorph of BaRhO3 was stabilised under high pressure conditions from 14 to 22 GPa. The material crystallised in the monoclinic 6H hexagonal perovskite structure in space group C2/c. The 4H BaRhO3 polymorph was stabilised at lower pressures, but the 3C cubic BaRhO3 likely requires pressures greater than 22 GPa. Both 6H and 4H polymorphs contain Rh2O9 dimers and the large 4d Rh orbital spatial diffusivity in these dimers leads to Pauli paramagnetic and metallic ground states, which are also supported by first-principles electronic structure calculations. High Wilson ratios of approximately 2 for either compound indicate strong electron correlation.

From our material exploration study in wide pressure and temperature conditions, we found a new 6H polymorph of BaRhO

A fully oxygenated hexagonal 12-layer perovskite $\mathrm{BaFe}{\mathrm{O}}_{3}$ with nominal $\mathrm{F}{\mathrm{e}}^{4+}$ was obtained by high-pressure synthesis. The structure consists of face-sharing $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{9}$ octahedral trimers and corner-sharing octahedra. A remarkable series of electronic transitions that relieve the electronic instability of $\mathrm{F}{\mathrm{e}}^{4+}$ are discovered on cooling; $2\mathrm{F}{\mathrm{e}}^{4+}\ensuremath{\rightarrow}\mathrm{F}{\mathrm{e}}^{3+}+\mathrm{F}{\mathrm{e}}^{5+}$ charge disproportionation and ordering over two sites accompanied by a structural distortion at 500 K; charge disproportionation of the remaining $\mathrm{F}{\mathrm{e}}^{4+}$ with magnetic order at 280 K; and an unprecedented charge interchange where $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{F}{\mathrm{e}}^{5+}$ states are exchanged between two sites with spin orientation change at 50 K. These properties demonstrate the presence of multiple competing ground states in this complex, highly condensed network of corner- and face-sharing octahedra.

BaFexNi1–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 R3m (a = 5.66564(7) A and c = 27.8416(3) A), was found to be stabilized. Mossbauer 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.

The perovskite Ca0.5Bi0.5FeO3 containing unusually high valent Fe3.5+ undergoes sequentially charge disproportionation (CD) of the Fe centres and intersite charge transfer (CT) between Bi and Fe. From structural, magnetic, and transport property characterization, we found that substitution of Fe with Co occurs isoelectronically to form Ca0.5Bi0.5 3+(Fe1−xCox)3.5+O3 and destabilizes the CD state. This results in materials exhibiting only intermetallic charge transfer behaviour in the region 0.01 < x < 0.67. The CT transitions for these materials only involves Fe3.5+ whereas Co remains in the 3.5+ oxidation state at all temperatures. The doped Co3.5+ ions give Pauli-paramagnetic like conducting behavior. The Co-doping effect is very different from that observed in CaFe1−xCoxO3. The charge transition behavior of Fe3.5+ in the present system are also in contrast to those in Ca0.5Bi0.5FeO3 and Sr0.5Bi0.5FeO3

A Sr analogue of Ca0.5Bi0.5FeO3, Sr0.5Bi0.5FeO3, containing unusually high valence Fe3.5+ ions was synthesized by using a high-pressure technique. It relieves the electronic instability due to the unusually high valence of Fe3.5+ by a single charge disproportionation (CD) transition (Fe3.5+ -> 0.75Fe(3+) + 0.25Fe(5+)) rather than the successive CD and intermetallic charge transfer (CT) transitions seen in Ca0.5Bi0.5FeO3. Conduction-band narrowing due to the significant bend in the Fe-O-Fe bond in the rhombohedral R (3) over barc crystal structure stabilized the charge-disproportionated state at low temperatures. Most importantly, Bi3+ ions in Sr0.5Bi0.5FeO3 do not act as countercations accepting oxygen holes as they do in Ca0.5Bi0.5FeO3, resulting in the absence of the intermetallic CT transition. The large cavity of the A-site Sr ions prevents the charge-transferred Bi5+ from being stabilized. In the charge-disproportionated state the nearest-neighbor Fe3+ spins align antiferromagnetically and one-fourth of the Fe3+ spins are randomly replaced by Fe5+ spins coupled ferromagnetically with the neighboring Fe3+ spins.

High-pressure, high-temperature reaction conditions can be useful to stabilize metastable polymorphs of complex transition metal oxides. We successfully prepare a new defect pyrochlore Pb

A perovskite-structure oxide containing unusually high-valence Fe3.5+ was obtained by high-pressure synthesis. Instability of the Fe3.5+ in Ca0.5Bi0.5FeO3 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.

Perovskite materials containing unusually high valent Fe cations are prone to undergoing charge transitions in order to relieve their inherent electronic instability. The charge transitions are often accompanied by significant changes to the structural transport, and magnetic properties. The distribution of charge in disproportionated and transferred perovskites is susceptible to small structural and electronic differences, leading to complex structural and magnetic behaviour. Here, we review the structural and magnetic behaviour of perovskites containing unusually high-valent Fe cations.

BaFe

The coupling between spins and charge disproportionation states has been investigated in the $\mathrm{LaC}{\mathrm{a}}_{2}\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{9}$ oxide with neutron powder diffraction. This $A$-site layer ordered triple perovskite $\mathrm{LaC}{\mathrm{a}}_{2}\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{9}$ undergoes charge disproportionation on cooling and shows two different charge ordering patterns. At 230 K, $\mathrm{F}{\mathrm{e}}^{3.67+}$ disproportionates into a 2:1 ratio of $\mathrm{F}{\mathrm{e}}^{3+}:\mathrm{F}{\mathrm{e}}^{5+}$, which order in a layered manner along the $\ensuremath{\langle}010\ensuremath{\rangle}$ direction of the pseudocubic unit cell. At lower temperatures $(Tl170\phantom{\rule{0.16em}{0ex}}\mathrm{K})$, the charge ordering pattern changes to a layered arrangement along the $\ensuremath{\langle}111\ensuremath{\rangle}$ direction. Neutron powder diffraction data show that in the intermediate temperature range ($170\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl230\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) the spins order into a cycloidal structure on the $ac$ plane for the $\mathrm{F}{\mathrm{e}}^{3+}$ cations while the $\mathrm{F}{\mathrm{e}}^{5+}$ cations remain paramagnetic. For the lowest temperature range ($2\phantom{\rule{0.16em}{0ex}}\mathrm{K}lTl190\phantom{\rule{0.16em}{0ex}}\mathrm{K}$), the spin structure follows the charge ordering and evolves to a $\ensuremath{\langle}111\ensuremath{\rangle}$ layered magnetic structure.

We use a combination of neutron and X-ray total scattering measurements together with pair distribution function (PDF) analysis to characterise the variation in local structure across the orbital order--disorder transition in LaMnO$_3$. Our experimental data are inconsistent with a conventional order--disorder description of the transition, and reflect instead the existence of a discontinuous change in local structure between ordered and disordered states. Within the orbital-ordered regime, the neutron and X-ray PDFs are best described by a local structure model with the same local orbital arrangements as those observed in the average (long-range) crystal structure. We show that a variety of meaningfully-different local orbital arrangement models can give fits of comparable quality to the experimental PDFs collected within the disordered regime; nevertheless, our data show a subtle but consistent preference for the anisotropic Potts model proposed in \emph{Phys Rev.\ B} {\bf 79}, 174106 (2009). The key implications of this model are electronic and magnetic isotropy together with the loss of local inversion symmetry at the Mn site. We conclude with a critical assessment of the interpretation of PDF measurements when characterising local symmetry breaking in functional materials., 14 pages, 8 figures, 3 tables

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.

A-site-ordered layer-structured perovskite LaCa

A perovskite-structure oxide containing unusually high-valence Fe

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.

SrVO2H, Sr2VO3H, and Sr3V2O5H2 are prepared by topochemical replacement of oxide ions in SrVO3, Sr2VO4, and Sr3V2O7, resp., by hydride ions using CaH2 (600—630 °C).

A series of strontium vanadium oxide-hydride phases prepared by utilizing a low-temperature synthesis strategy in which oxide ions in Sr(n+1)V(n)O(3n+1) (n=∞, 1, 2) phases are topochemically replaced by hydride ions to form SrVO2H, Sr2VO3H, and Sr3V2O5H2, respectively. These new phases contain sheets or chains of apex-linked V(3+)O4 squares stacked with SrH layers/chains, such that the n=∞ member, SrVO2H, can be considered to be analogous to "infinite-layer" phases, such as Sr(1-x)Ca(x)CuO2 (the parent phase of the high-T(c) cuprate superconductors), but with a d(2) electron count. All three oxide-hydride phases exhibit strong antiferromagnetic coupling, with SrVO2H exhibiting an antiferromagnetic ordering temperature, T(N)>300 K. The strong antiferromagnetic couplings are surprising given they appear to arise from π-type magnetic exchange.

The title compound is prepared by reduction of SrFe0.5Ru0.5O3 with CaH2 at 400 °C for 96 h.

Sr3 (Ti0.5Ru0.5)2O7F2 (I), Sr3 (Mn0.5Ru0.5)2O7F2 (II), and Sr3 (Fe0.5Ru0.5)2O5.5F3.5 (III) are obtained from Sr3(M0.5Ru0.5)2O7 (M: Ti, Mn, Fe) by a solid state topochemical anion insertion/substitution process using CuF2 under flowing O2 (250 °C, 3 times 8 h).

The title compound is prepared by topochemical reduction of Sr3Co2O5Cl2 with NaH as solid state reducing agent (grinding, 200 °C, 2 x 2 d).

Topochemical reduction of Sr(3)Co(2)O(5)Cl(2) with NaH at 200 °C yields Sr(3)Co(2)O(4)Cl(2), a phase consisting of infinite double sheets of corner-linked Co(II)O(4) square planes stacked with SrCl rocksalt layers. At 298 K, the structure of Sr(3)Co(2)O(4)Cl(2) is described in the tetragonal space group I4/mmm [a = 4.007(1) Å, c = 22.282(1) Å]; however, on cooling below 200 K, the structure undergoes a lattice distortion to adopt a structure with orthorhombic symmetry in space group Immm [a = 3.9757(5) Å, b = 4.0294(5) Å, and c = 22.147(3) Å at 5 K]. The structural distortion can be considered Jahn-Teller-like as it lifts the orbital degeneracy of the square planar, d(7), Co(II) centers, demonstrating a strong coupling between the electronic configuration and the structural lattice of this oxychloride phase. On cooling below 50 K, Sr(3)Co(2)O(4)Cl(2) adopts a canted antiferromagnetically ordered state. All magnetization data show that the local cobalt moment is much greater than would be expected for a simple spin-only S = 3/2 center, indicating a strong orbital contribution to the magnetic behavior.

The title compound is prepared by reduction of Sr4Mn3O7.5Cl2 with CaH2 (Parr vessel that can withstand the evolving H2 pressure, 350 °C, 8 d) and characterized by powder X-ray and neutron diffraction, and magnetic measurements.

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.