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Ruddlesden-Popper (RP) compounds of the general formula (AX)(ABX 3 ) n with their unique sequence of perovskite-like (ABX 3 ) and rock-salt-like units (AX) promise applications in diverse fields such as catalysis and superconductivity. Fluorination of RP oxides often leads to dramatic changes in the material properties, caused by differences in the atomic and electronic structure. While current research focuses on fluorination of n = 1 type RP oxides (A 2 BO 4 ), n = 3 RP oxyfluorides have remained elusive. We present the synthesis of the first iron-based n = 3 RP oxyfluoride, La 0.5 Sr 3.5 Fe 3 O 7.5 F 2.6 , which was obtained from an oxide precursor by topochemical fluorination with poly(vinylidene fluoride). Joint Rietveld refinements of neutron and powder X-ray diffraction data were used to determine the crystal structure. Best results were obtained in the space group Pbca (No. 61) with a = 5.5374(1) Å, b = 5.5441(1) Å, and c = 29.2541(2) Å. The effect of the aliovalent incorporation of fluoride ions is particularly evident with respect to changes in structure and magnetic properties. The magnetic behavior was studied using field- and temperature-dependent magnetization measurements, Mößbauer spectroscopy, and neutron diffraction. Additional magnetic Bragg reflections observed in the room-temperature neutron data were successfully refined in the space group Pbca (61.1.497 in Opechowski-Guccione notation), indicating a G-type antiferromagnetic ordering with a surprisingly high Néel temperature above 300 K. This strong increase of T N by several hundred Kelvin compared to the parent oxide is particularly remarkable.

We report a new nickel hydroxyfluoride diaspore Ni(OH)F prepared using hydrothermal synthesis from NiCl 2 ·6H 2 O and NaF. Magnetic characterization reveals that, contrary to other reported transition-metal hydroxyfluoride diaspores, Ni(OH)F displays weak ferromagnetism below the magnetic ordering temperature. To understand this difference, neutron diffraction is used to determine the long-range magnetic structure. The magnetic structure is found to be distinct from those reported for other hydroxyfluoride diaspores and shows an antiferromagnetic spin ordering in which ferromagnetic canting is allowed by symmetry. Furthermore, neutron powder diffraction on a deuterated sample, Ni(OD)F, reveals partial anion ordering that is distinctive to what has previously been reported for Co(OH)F and Fe(OH)F. Density functional theory calculations show that OH/F ordering can have a directing influence on the lowest energy magnetic ground state. Our results point toward a subtle interplay between the sign of magnetic exchange interactions, the electronic configuration, and anion disordering.

Europium tantalum perovskite oxynitrides were prepared by a new high-temperature solid-state synthesis under N 2 or N 2 /H 2 gas. The nitrogen stoichiometry was tuned from 0.63 to 1.78 atoms per Eu or Ta atom, starting with appropriate N/O ratios in the mixture of the reactants Eu 2 O 3 , EuN and Ta 3 N 5 , or Eu 2 O 3 and TaON, which was treated at 1200 °C for 3 h. Two phases were isolated with compositions EuTaO 2.37 N 0.63 and Eu 3 Ta 3 O 3.66 N 5.34 , showing different crystal structures and magnetic properties. Electron diffraction and Rietveld refinement of synchrotron radiation X-ray diffraction indicated that EuTaO 2.37 N 0.63 is a simple perovskite with cubic Pm 3̅ m structure and cell parameter a = 4.02043(1) Å, whereas the new compound Eu 3 Ta 3 O 3.66 N 5.34 is the first example of a triple perovskite oxynitride and shows space group P 4/ mmm with crystal parameters a = 3.99610(2), c = 11.96238(9) Å. The tripling of the c-axis in this phase is a consequence of the partial ordering of europium atoms with different charges in two A sites of the perovskite structure with relative ratio 2:1, where the formal oxidation states +3 and +2 are respectively dominant. Magnetic data provide evidence of ferromagnetic ordering developing at low temperatures in both oxynitrides, with saturation magnetization of about 6 μ B and 3 μ B per Eu ion for EuTaO 2.37 N 0.63 and the triple perovskite Eu 3 Ta 3 O 3.66 N 5.34 respectively, and corresponding Curie temperatures of about 7 and 3 K, which is in agreement with the lower proportion of Eu 2+ in the latter compound.

A-site ordered double perovskites with the general formula LnBaCo 2 O 6-δ (where Ln is a lanthanide element) present electrical and electrocatalytic properties that make them attractive as possible ceramic electrode materials for solid oxide cells or alkaline electrolyzers. The properties are highly influenced by the anion vacancy concentration, which is strongly related to the Co-oxidation state, and their location in the structure. Awareness of the stable phases is essential to synthesize, evaluate, and optimize the properties of LnBaCo 2 O 6-δ oxides at operating conditions in different applications. TbBaCo 2 O 6-δ are representative oxides of these layered perovskite systems. The present article reports a study of TbBaCo 2 O 6-δ by electron diffraction, high-resolution electron microscopy, and powder neutron diffraction experiments at different temperatures. The synthesis of TbBaCo 2 O 6-δ in air and slow cooling to room temperature (RT) at 5 °C h -1 leads to samples formed by distinct phases with different oxygen contents and crystal structures. The 122 and 112 phases (with a p × 2 a p × 2 a p and a being the lattice parameter of the simple cubic perovskite structure) are predominant in quasi-equilibrium prepared samples (cooled at RT at 1 °C h p × a p × 2 a p unit cells, respectively, with a p being the lattice parameter of the simple cubic perovskite structure) are predominant in quasi-equilibrium prepared samples (cooled at RT at 1 °C h -1 ) or prepared in Ar flow and quenched to RT. The evolution of the crystal structure of TbBaCo 2 O 6-δ during thermal oxygen release/uptaking consists of modulation from the 122 phase to the 112 phase (or vice versa during uptaking) by creation/occupation of anion vacancies within the TbO 1-δ planes. Anion vacancies are not detected in the oxygen crystallographic position different from those located within the TbO 1-δ planes even at the highest temperatures, supporting the 2D character of the high anion conduction of the LnBaCo 2 O 6-δ oxides.

Combining different anions in one material allows tuning of its structural, magnetic, and electronic properties. We hereby present the mixed anion compound Sr 2 BN 2 H, expanding the less-known class of nitridoborate hydrides. Solid-state reaction of Sr 2 N, BN, and SrH 2 at 850 °C in a tube furnace yielded a gray, air- and moisture-sensitive powder of Sr 2 BN 2 H. It crystallizes as colorless platelets in the orthorhombic space group Pnma (no. 62) with a = 9.9164(2), b = 3.9079(1), and c = 10.1723(2) Å and Z = 4. An initial structural model was obtained from single-crystal X-ray diffraction data and corroborated by neutron powder diffraction data of the corresponding deuteride. Further validation by 1 H and 11 B MAS NMR, FTIR, and Raman spectroscopy complements the structural proof of anionic hydrogen present in the compound. Quantum chemical calculations support the experimental findings and reveal the electronic structure of Sr 2 BN 2 H.

Layered Sr 2 M 3 As 2 O 2 -type oxypnictides are composed of tetrahedral M 2 Pn 2 and square planar MO 2 layers, the building blocks of iron-based and cuprate superconductors. To further expand our understanding of the chemical and magnetic properties of the Sr 2 Cr 3- x Mn x As 2 O 2 solid solution, Sr 2 Cr 2 MnAs 2 O 2 has been synthesized. The compound crystallizes in the I 4/ mmm tetragonal space group with a refined stoichiometry of Sr 2 Cr 1.85 Mn 1.15 As 2 O 2 . The M(2) site within the M 2 Pn 2 slab is occupied by 42.7% Cr and 57.3% Mn, and the magnetic moments order antiferromagnetically below T N (M2) = 540 K with a C-type antiferromagnetic structure. The M(1) site within the MO 2 layers is fully occupied by Cr, and antiferromagnetic order is observed below T N (M1) = 200 K. Along c , there are two possible interplanar arrangements: ferromagnetic with the (1/2, 1/2, 0) propagation vector and antiferromagnetic with the (1/2, 1/2, 1/2) propagation vector. Magnetic phase separation arises so that both propagation vectors are observed below 200 K. Such magnetic phase separation has not been previously observed in Sr 2 M 3 As 2 O 2 phases (M = Cr, Mn) and shows that there are several competing magnetic structures present in these compounds.

Extended solid-state materials based on the hexagonal perovskite framework are typified by close competition between localized magnetic interactions and quasi-molecular electronic states. Here, we report the structural and magnetic properties of the new six-layer hexagonal perovskite Ba 3 CaMo 2 O 9 . Neutron diffraction experiments, combined with magnetic susceptibility measurements, show that the Mo 2 O 9 dimers retain localized character down to 5 K and adopt nonmagnetic spin-singlet ground states. This is in contrast to the recently reported Ba 3 SrMo 2 O 9 analogue, in which the Mo 2 O 9 dimers spontaneously separate into a mixture of localized and quasi-molecular ground states. Structural distortions in both Ba 3 CaMo 2 O 9 and Ba 3 SrMo 2 O 9 have been studied with the aid of distortion mode analyses to elucidate the coupling between the crystal lattice and electronic interactions in 6H Mo 5+ hexagonal perovskites.

We present the synthesis of a novel binary metal oxide material: Ba 7 Mn 4 O 15 . The crystal structure has been investigated by high-resolution powder synchrotron X-ray diffraction in the temperature range of 100-300 K as well as by powder neutron diffraction at 10 and 80 K. This material represents an isostructural barium-substituted analogue of the layered material Sr 7 Mn 4 O 15 that forms its own structural class. However, we find that Ba 7 Mn 4 O 15 adopts a distinct magnetic ordering, resulting in a magnetoelectric ground state below 50 K. The likely magnetoelectric coupling mechanisms have been inferred from performing a careful symmetry-adapted refinement against the powder neutron diffraction experiments, as well as by making a comparison with the nonmagnetoelectric ground state of Sr 7 Mn 4 O 15 .

Ba 3 VWO 8.5 is an oxide ion conductor with a bulk conductivity of 5.0 × 10 -5 S cm -1 at 600 °C. Ba 3 VWO 8.5 is anomalous to the other Ba 3 M'M″O 8.5 (M' = Nb; M″ = Mo, W) oxide ionic conductors, as it exhibits cation order with vanadium and tungsten on the M1 site only. Here, we report a variable temperature neutron diffraction study of Ba 3 VWO 8.5 , which demonstrates that cation order is retained up to 800 °C. We show for the first time that the structural rearrangements reported for hexagonal perovskite derivatives Ba 3 M'M″O 8.5 are dictated by water absorption. The significant water uptake in Ba 3 M'M″O 8.5 (M' = Nb; M″ = Mo, W) arises due to the flexibility of the crystal structure, whereby a fraction of the transition metal cations move from the M1 site to the octahedral M2 site upon absorption of water. The results presented here demonstrate that the presence of 50% V 5+ on the M1 site, which has a strong preference for tetrahedral geometry, is enough to disrupt the flexibility of the cation sublattice, resulting in the ordering of the cations exclusively on the M1 site and no significant water absorption.

The development of new synthetic methodologies of perovskite oxynitrides is challenging but necessary for the search of new compounds and the investigation of new properties. Here, we report a new method of preparation of the perovskite LaTaON 2 that has been investigated as a pigment and photocatalyst for water splitting. The synthesis proceeds through the solid-state reactions under N 2 at 1500 °C between La 2 O 3 , LaN, and Ta 3 N 5 or between LaN and TaON, which are completed after 3 h and lead to sintered, highly crystalline samples with particle sizes up to 1 μm. Nitrogen-deficient samples LaTaO 1+ x N 2- x with x ≤ 0.35 are prepared by changing the N/O ratio in the mixture of reactants. Electron diffraction, synchrotron diffraction, and neutron diffraction studies on stoichiometric and nitrogen-deficient compounds indicate that they crystallize in the monoclinic space group I 2/ m with lattice parameters for LaTaON 2 of a = 5.71458(7), b = 8.05987(10), c = 5.74772(6) Å, and β = 89.982(3)°. The three anion sites of the I 2/ m structure are partially occupied by oxygen and nitrogen, with a preference of nitride for two positions with occupancies of 77 and 88%. This anion distribution is different from that reported in previous studies of samples prepared by ammonolysis at lower temperature, suggesting that the synthesis conditions affect the anion order of this perovskite. Optical measurements indicate a band gap of about 1.9 eV, which is close to that observed in samples prepared by other methods. The determined dielectric permittivity for LaTaON 2 ε r ≈ 200, reported for the first time for a highly nitrided pseudocubic perovskite, is similar to that observed in perovskites with one nitrogen per formula.

We report two new rare-earth (R) ternary intermetallic compounds-Ho 2 Ni 0.8 T 1.2 with T = Si and Ge-that correspond to the R 5 Ni 2 T 3 phase earlier reported to form in Dy-Ni-T and Ho-Ni-T ternary systems. The compounds crystallize in a filled version of the orthorhombic Zr 2 Ni 1- x P-type structure with x = 0.52; their stoichiometry, determined from both single-crystal and powder X-ray diffraction data, is centered on Ho 2 Ni 0.8 T 1.2 with a narrow solid solubility range for the silicide, while the germanide appears to be a line phase. In addition to R = Dy and Ho, R 2 Ni 0.8 T 1.2 compounds also form for R = Y and Tb, representing the first examples of rare-earth-based compounds adopting the Zr 2 Ni 1- x P structural prototype. Bulk magnetization data reveal the main transitions of the ferrimagnetic or ferromagnetic type at T C = 38 K for Ho 2 Ni 0.8 Si 1.2 and T C = 37 K for Ho 2 Ni 0.8 Ge 1.2 , which are followed by subsequent magnetic reordering at lower temperatures. Neutron diffraction shows complex magnetic structures below T C with both ferromagnetic and antiferromagnetic components and magnetic propagation vector κ 1 = [0, 0, 0]. Below T N ≅ 24 K (22 K) for the silicide (germanide), an additional antiferromagnetic coupling following an incommensurate magnetic propagation vector κ 2 = [κ x , 0, 0] appears to coexist with the first magnetic structure.

Two M 2 (SeO 3 )F 2 fluoro-selenites (M = Mn 2+ , Ni 2+ ) have been synthesized using optimized hydrothermal reactions. Their 3D framework consists of 1D-[MO 2 F 2 ] 4- chains of edge-sharing octahedra with a rare topology of alternating O-O and F-F μ 2 bridges. The interchain corner-sharing connections are assisted by the mixed O vs F anionic nature and develop a complex set of M-X-M superexchanges as calculated by LDA+ U . Their interplay induces prominent in-chain antiferromagnetic frustration, while the interchain exchanges are responsible for the cycloidal magnetic structure observed below T N ≈ 21.5 K in the Ni 2+ case. For comparison the Mn 2+ compound develops a nearly collinear spin (canted) ordering below T N ≈ 26 K with ferromagnetic chain units.

Atomic arrangements in the mixed-valence oxide U 3 O 7 are refined from high-resolution neutron scattering data. The crystallographic model describes a long-range structural order in a U 60 O 140 primitive cell (space group P 4 2 / n ) containing distorted cuboctahedral oxygen clusters. By combining experimental data and electronic structure calculations accounting for spin-orbit interactions, we provide robust evidence of an interplay between charge localization and the magnetic moments carried by the uranium atoms. The calculations predict U 3 O 7 to be a semiconducting solid with a band gap of close to 0.32 eV, and a more pronounced charge-transfer insulator behavior as compared to the well-known Mott insulator UO 2 . Most uranium ions (56 out of 60) occur in 9-fold and 10-fold coordinated environments, surrounding the oxygen clusters, and have a tetravalent (24 out of 60) or pentavalent (32 out of 60) state. The remaining uranium ions (4 out of 60) are not contiguous to the oxygen cuboctahedra and have a very compact, 8-fold coordinated environment with two short (2 × 1.93(3) Å) "oxo-type" bonds. The higher Hirshfeld charge and the diamagnetic character point to a hexavalent state for these four uranium ions. Hence, the valence state distribution corresponds to 24/60 × U(IV) + 32/60 U(V) + 4/60 U(VI). The tetravalent and pentavalent uranium ions are predicted to carry noncollinear magnetic moments (with amplitudes of 1.6 and 0.8 μ B , respectively), resulting in canted ferromagnetic order in characteristic layers within the overall fluorite-related structure.

The antiferromagnetic behavior of Fe 3+ oxides of composition RE 1.2 Ba 1.2 Ca 0.6 Fe 3 O 8 , RE 2.2 Ba 3.2 Ca 2.6 Fe 8 O 21 , and REBa 2 Ca 2 Fe 5 O 13 (RE = Gd, Tb) is highly influenced by the type of oxygen polyhedron around the Fe 3+ cations and their ordering, which is coupled with the layered RE/Ba/Ca arrangement within the perovskite-related structure. Determination of the magnetic structures reveals different magnetic moments associated with Fe 3+ spins in the different oxygen polyhedra (octahedron, tetrahedron, and square pyramid). The structural aspects impact on the strength of the Fe-O-Fe superexchange interactions and, therefore, on the Néel temperature ( T N ) of the compounds. The oxides present an interesting transition from three-dimensional (3D) to two-dimensional (2D) magnetic behavior above T N . The 2D magnetic interactions are stronger within the FeO6 octahedra layers than in the FeO4 tetrahedra layers.

Mn 11 Ta 4 O 21 is presented as the first hexagonal A-site manganite. Based on simple rules, the structure is compatible with a 14H-layer ( cchchch ) 2 stacking sequence that is related to BaVO 3 and BaCrO 3 high-pressure polymorphs. The A-site overstoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at T N1 = 88 K and T N2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below T N1 and T N2 , respectively. A complementary description with 14H-( hhccccc ) 2 sequence of only Mn octahedra provides a direct comparison with BaMnO 3-δ hexagonal perovskites and naturally explains the AFM order. Below T N2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

The potential of the perovskite system Nd 1- x Sr x CoO 3-δ ( x = 1/3 and 2/3) as cathode material for solid oxide fuel cells (SOFCs) has been investigated via detailed structural, electrical, and electrochemical characterization. The average structure of x = 1/3 is orthorhombic with a complex microstructure consisting of intergrown, adjacent, perpendicularly oriented domains. This orthorhombic symmetry remains throughout the temperature range 373-1073 K, as observed by neutron powder diffraction. A higher Sr content of x = 2/3 leads to stabilization of the cubic perovskite with a homogeneous microstructure and with a higher oxygen vacancy content and cobalt oxidation state than the orthorhombic phase at SOFC operation temperature. Both materials are p-type electronic conductors with high total conductivities of 690 and 1675 S·cm -1 at 473 K in air for x = 1/3 and 2/3, respectively. Under working conditions, both compounds exhibit similar electronic conductivities, since x = 2/3 loses more oxygen on heating than x = 1/3, associated with a greater loss of p-type charger carriers. However, composite cathodes prepared with Nd 1/3 Sr 2/3 CoO 3-δ and Ce 0.8 Gd 0.2 O 2-δ present lower ASR values (0.10 Ω·cm 2 at 973 K in air) than composites prepared with Nd 2/3 Sr 1/3 CoO 3-δ and Ce 0.8 Gd 0.2 O 2-δ (0.34 Ω·cm 2 ). The high activity for the oxygen electrochemical reaction at intermediate temperatures is likely attributable to a large disordered oxygen-vacancy concentration, resulting in a very promising SOFC cathode for real devices.

Several different mechanisms of magnetoresistance (MR) have been observed in 1111 LnMnAsO 1- x F x oxypnictides (Ln = lanthanide) as a result of magnetic coupling between the Mn and Ln. Such phases also exhibit interesting magnetic phase transitions upon cooling. Sr 2 Mn 2 CrAs 2 O 2 has been synthesized to investigate if it is possible to observe MR and/or magnetic phase transitions as a result of magnetic coupling between the Mn and Cr. Sr 2 Mn 2 CrAs 2 O 2 crystallizes in the tetragonal space group I 4/ mmm containing alternating MO 2 2- and M' 2 As 2 2- layers, and neutron diffraction results demonstrate that the actual stoichiometry is Sr 2 Mn 2.23 Cr 0.77 As 2 O 2 . Cation order is present between Mn and Cr, with Cr predominantly occupying the square planar MO 2 2- site. Below 410 K, the magnetic moments of the Mn/Cr ions in the M' 2 As 2 2- sublattice exhibit G-type antiferromagnetic order. The Mn/Cr moments within the MO 2 layers from a G-type to a C-type antiferromagnetic arrangement. The results demonstrate that the superexchange interactions are finely balanced in Sr 2- layer order below 167 K with a K 2 NiF 4 -type antiferromagnetic structure that simultaneously induces a spin flip of the magnetic moments in the M' 2 As 2 2- layers from a G-type to a C-type antiferromagnetic arrangement. The results demonstrate that the superexchange interactions are finely balanced in Sr 2 Mn 2.23 Cr 0.77 As 2 O 2 . Sr 2 Mn 2.23 Cr 0.77 As 2 O 2 is semiconducting, and there is no evidence of MR.

Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr 2 YCu 2 O 7.85 cuprate, in which a long-range magnetic ordering of the high valent iron cations coexists with the superconducting interactions ( magnetic ordering temperature T N = 110 K > superconducting critical temperature T c = 70 K). The somewhat unexpected A-type AFM structure, with a μ(Fe) ∼ 2 μ B magnetic saturation moment associated with the hypervalent iron sublattice, suggests an unusual low spin state for the iron cations, while the low dimensionality of the magnetic structure results in a soft switching toward ferromagnetism under small external magnetic fields. The role of the crystal structure and of the high charge concentration in the stabilization of this unusual electronic configuration for the iron cations is discussed.

The BaZr 0.7 Ce 0.2 Y 0.1 O 3-δ -BaPrO 3-δ perovskite system, of interest for high-temperature electrochemical applications involving mixed protonic-electronic conductivity, forms a solid-solution with a wide interval of Ba substoichiometry in the range Ba(Ce 0.2 Zr 0.7 ) 1- x Pr x Y 0.1 O 3-δ , 0 ≤ x ≤ 1. Structural phase transitions mapped as a function of temperature and composition by high-resolution neutron powder diffraction and synchrotron X-ray diffraction reveal higher symmetry for lower Pr content and higher temperatures, with the largest stability field observed for rhombohedral symmetry (space group, R3̅ c). Rietveld refinement, supported by magnetic-susceptibility measurements, indicates that partitioning of the B-site cations over the A and B perovskite sites compensates Ba substoichiometry in preference to A-site vacancy formation and that multiple cations are distributed over both sites. Electron-hole transport dominates electrical conductivity in both wet and dry oxidizing conditions, with total conductivity reaching a value of ∼0.5 S cm -1 for the x = 1 end-member in dry air at 1173 K. Higher electrical conductivity and the displacement of oxygen loss to higher temperatures with increasing Pr content both reflect the role of Pr in promoting hole formation at the expense of oxygen vacancies. In more reducing conditions (N 2 ) and at low Pr contents, conductivity is higher in humidified atmospheres (∼0.023 atm pH 2 O) indicating a protonic contribution to transport, whereas the greater electron-hole conductivity with increasing Pr content results in lower conductivity in humidified N 2 due to the creation of protonic defects and the consumption of holes.

The oxide ionic conductor Ba 3 W 1.2 Nb 0.8 O 8.6 has been synthesized as part of an investigation into the new class of Ba 3 M'M''O 8.5 (M' = W, Mo; M'' = Nb) oxide-ion conducting hexagonal perovskite derivatives. The substitution of W 6+ for Nb 5+ in Ba 3 W 1+ x Nb 1- x O 8.5+ x/2 leads to an increase in the oxygen content, which enhances the low-temperature ionic conductivity. However, at 400 °C, the ionic conductivity of Ba 3 W 1.2 Nb 0.8 O 8.6 is still significantly lower than the molybdenum compound Ba 3 MoNbO 8.5 . Remarkably, at 600 °C the bulk oxide ionic conductivities of Ba 3 MoNbO 8.5 , Ba 3 WNbO 8.5 , and Ba 3 W 1.2 Nb 0.8 O 8.6 are very similar (σ b = 0.0022, 0.0017, and 0.0016 S cm -1 , respectively). The variable-temperature neutron diffraction results reported here demonstrate that Ba 3 W 1.2 Nb 0.8 O 8.6 undergoes a similar structural rearrangement to Ba 3 MoNbO 8.5 above 300 °C, but the ratio of (W/Nb)O 4 tetrahedra to (W/Nb)O 6 octahedra rises at a faster rate upon heating between 300 and 600 °C. There is a clear relationship between the ionic conductivity of Ba 3 M' 1+ x M'' 1- x O 8.5+ x/2 (M' = W, Mo; M'' = Nb) phases and the number of tetrahedrally coordinated M' and M ″ cations present within the crystal structure.

Layered perovskite titanium oxyhydrides have been prepared by low-temperature topochemical CaH 2 reduction from Ruddlesden-Popper Sr n+1 Ti n O 3 n+1 phases ( n = 1, 2) and structurally characterized by combined synchrotron X-ray and neutron diffraction data refinements. In the single-layered Sr 2 TiO 3.91(2) D 0.14(1) material, hydride anions are statistically disordered with oxides on the apical site only, as opposed to known transition-metal oxyhydrides exhibiting a preferred occupation of the equatorial site. This unprecedented site selectivity of H - has been reproduced by periodic DFT+ U calculations, emphasizing for the hydride defect a difference in formation energy of 0.24 eV between equatorial and apical sites. In terms of electronic structure, the model system Sr 2 TiO 3.875 H 0.125 is found to be slightly metallic and the released electron remains mostly delocalized over several Ti atoms. On the other hand, hydride anions in the double-layered Sr 3 Ti 2 O 6.20 H 0.12 material show a clear preference for the bridging apical site within the perovskite slabs, as confirmed by DFT calculations on the Sr 3 Ti 2 O 6.875 H 0.125 model system. Finally, the influence of the B-site chemical nature on the hydride site selectivity for early 3d transition metals is theoretically explored in the single-layered system by substituting vanadium for titanium. The V 3+ electronic polaron is suggested to play a role in stabilizing H - on the equatorial site in Sr 2 VO 4- x H x for x = 0.125.

The effect of substituting iron and zinc for cobalt in CaBaCo 4 O 7 was investigated using neutron diffraction and X-ray absorption spectroscopy techniques. The orthorhombic distortion present in the parent compound CaBaCo 4 O 7 decreases with increasing the content of either Fe or Zn. The samples CaBaCo 3 ZnO 7 and CaBaCo 4- x Fe x O 7 with x ≥ 1.5 are metrically hexagonal, but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe suppresses the ferrimagnetic ordering of the parent compound, and CaBaCo 4- x Fe x O 7 (0.5 ≤ x ≤ 2) samples are antiferromagnetically ordered following a new propagation vector k = (1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long-range magnetic interactions giving rise to a magnetic glass-like behavior in the CaBaCo 3 ZnO 7 sample. The oxidation states of iron and zinc are found to be 3+ and 2+, respectively, independently of the content, as confirmed by X-ray absorption spectroscopy. Therefore, the average Co oxidation state changes accordingly with the Fe 3+ or Zn 2+ doping. Also, X-ray absorption spectroscopy data confirm the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and X-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co 2+ /Co 3+ valence state or, alternatively, Co 2+ and Co 3+ ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO 4 tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder are observed only at the ferrimagnetic transition for CaBaCo 4 O 7 , revealing the occurrence of local magneto-elastic coupling.

The electrical and structural properties of the series Ba 3 Mo 1-x Nb 1+x O 8.5-x/2 (x = 0.0, 0.1, 0.2, 0.3) have been determined. Ba 3 Mo 1-x Nb 1+x O 8.5-x/2 crystallizes in a hybrid of the 9R hexagonal perovskite and palmierite structures, in which (Mo/Nb)O 4 and (Mo/Nb)O 6 units coexist within the structure. Nb substitutes preferentially at the octahedral site so that the ratio of (Mo/Nb)O 4 tetrahedra to (Mo/Nb)O 6 octahedra decreases with increasing x resulting in a reduction in the magnitude of the ionic conductivity from 1.3 × 10 -6 S cm -1 for x = 0.0 to 1.1 × 10 -7 S cm -1 for x = 0.3 at 300 °C. However, upon heating the conductivities of the solid solution converge, which suggests that the unusual thermal structural rearrangement previously reported for Ba 3 MoNbO 8 preserves the high temperature conductivity. The results demonstrate that the presence of (Mo/Nb)O 4 tetrahedra with nonbridging apical oxygen atoms is an important prerequisite for the ionic conduction observed in the Ba 3 MoNbO 8.5 system.

Members of the perovskite solid solution BaZr 1-x Pr x O 3-δ (0.2 ≤ x ≤ 0.8) with potential high-temperature electrochemical applications were synthesized via mechanical activation and high-temperature annealing at 1250 °C. Structural properties were examined by Rietveld analysis of neutron powder diffraction and Raman spectroscopy at room temperature, indicating rhombohedral symmetry (space group R3̅c) for members x = 0.2 and 0.4 and orthorhombic symmetry (Imma) for x = 0.6 and 0.8. The sequence of phase transitions for the complete solid solution from BaZrO 3 to BaPrO 3 is Pm3̅m → R3̅c → Imma → Pnma. The structural data indicate that Pr principally exists as Pr 4+ on the B site and that oxygen content increases with higher Pr content. Electrical-conductivity measurements in the temperature range of 250-900 °C in dry and humidified (pH 2 O ≈ 0.03 atm) N 2 and O 2 atmospheres revealed an increase of total conductivity by over 2 orders of magnitude in dry conditions from x = 0.2 to x = 0.8 (σ ≈ 0.08 S cm -1 at 920 °C in dry O 2 for x = 0.8). The conductivity for Pr contents x > 0.2 is attributable to positively charged electronic carriers, whereas for x = 0.2 transport in dry conditions is n-type. The change in conduction mechanism with composition is proposed to arise from the compensation regime for minor amounts of BaO loss changing from predominantly partitioning of Pr on the A site to vacancy formation with increasing Pr content. Conductivity is lower in wet conditions for x > 0.2 indicating that the positive defects are, to a large extent, charge compensated by less mobile protonic species. In contrast, the transport mechanism of the Zr-rich composition (x = 0.2), with much lower electronic conductivity, is essentially independent of moisture content.

Oxygen-deficient Sr 2 ScGaO 5 single crystals with a cubic perovskite structure were grown by the floating-zone technique. The transparent crystals of this pure 3D oxygen electrolyte are metastable at ambient temperature, showing one-sixth of all oxygen positions vacant. While neutron single-crystal diffraction, followed by maximum entropy analysis, revealed a strong anharmonic displacements for the oxygen atoms, a predominant formation of ScO 6 octahedra and GaO 4 tetrahedra is indicated by Raman spectroscopic studies, resulting in a complex oxygen defect structure with short-range order. Temperature-dependent X-ray powder diffraction (XPD) and neutron powder diffraction (NPD) studies reveal the cubic Sr 2 ScGaO 5 to be thermodynamically stable only above 1400 °C, while the stable modification below this temperature shows the brownmillerite framework with orthorhombic symmetry. Cubic Sr 2 ScGaO 5 remains surprisingly kinetically stable upon heating from ambient temperature to 1300 °C, indicating a huge inertia for the retransformation toward the thermodynamically stable brownmillerite phase. Ionic conductivity investigated by impedance spectroscopy was found to be 10 -4 S/cm at 600 °C, while oxygen 18 O/ 16 O isotope exchange indicates a free oxygen mobility to set in at around 500 °C.

A-site-ordered perovskite Ce1/2Cu3Ti4O12 has been found to crystallize in two different forms, one with random and the other with ordered Ce/vacancy distribution at the A site of the prototype AA'3B4O12 structure. The random phase is isostructural with CaCu3Ti4O12, and the ordered phase is a new ordered derivative of the AA'3B4O12-type perovskite with two crystallographically distinct Cu sites. Although both phases form a G-type antiferromagnetic arrangement of Cu(2+) spins below 24 K, their magnetisms are quite different. A typical antiferromagnetic transition is observed in the random phase, whereas a small ferromagnetic moment appears below 24 K in the ordered phase, which rapidly decreases upon further cooling. A mean-field approximation approach revealed that this unusual behavior in the ordered phase is an L-type ferrimagnetism driven by the nonequivalent magnetizations of the two ferromagnetic Cu(2+) spin sublattices in the G-type spin structure. This unusual ferrimagnetism is a direct consequence of the Ce/vacancy ordering.

The synthesis and structural and magnetic characterizations of K3Fe6F19, a new iron potassium fluoride with a complex tungsten bronze related structure, are presented. This phase was found during the investigation of relatively low-temperature (600 °C) synthesis conditions of classical tetragonal tungsten bronze (TTB) fluorides and can be considered an intermediate that forms at this temperature owing to faster crystallization kinetics. The K3Fe6F19 compound has an orthorhombic structure (space group Cmcm (63), a = 7.6975(3) Å, b = 18.2843(7) Å, c = 22.0603(9) Å) related to the TTB one, where the perovskite cage is substituted by a large S-shaped channel simultaneously occupied by two potassium atoms. The magnetic structure, characterized by magnetization measurements on an oriented single crystal and powder neutron diffraction, is dominated by the presence of interconnected double stripes of antiferromagnetic triangular exchange interaction patterns alternately rotated in clock- and anticlockwise fashion. The magnetic order takes place in a wide temperature range, by increasing progressively the interaction dimensionality.

The crystal structure and magnetic properties of the RbMnPO4 zeolite-ABW-type material have been studied by temperature-dependent neutron powder diffraction, low-temperature magnetometry, and heat capacity measurements. RbMnPO4 represents a rare example of a weak ferromagnetic polar material, containing Mn(2+) ions with TN = 4.7 K. The neutron powder diffraction pattern recorded at T = 10 K shows that the compound crystallizes in the chiral and polar monoclinic space group P2(1) (No. 4) with the unit cell parameters: a = 8.94635(9), b = 5.43415(5), and c = 9.10250(8) Å and β = 90.4209(6)°. A close inspection of the crystal structure of RbMnPO4 shows that this material presents two different types of zigzag chains running along the b axis. This is a unique feature among the zeolite-ABW-type materials exhibiting the P2(1) symmetry. At low temperature, RbMnPO4 exhibits a canted antiferromagnetic structure characterized by the propagation vector k1 = 0, resulting in the magnetic symmetry P2(1)'. The magnetic moments lie mostly along the b axis with the ferromagnetic component being in the ac plane. Due to the geometrical frustration present in this system, an intermediate phase appears within the temperature range 4.7-5.1 K characterized by the propagation vector k2 = (kx, 0, kz) with kx/kz ≈ 2. This ratio is reminiscent of the multiferroic phase of the orthorhombic RMnO3 phases (R = rare earth), suggesting that RbMnPO4 could present some multiferroic properties at low temperature. Our density functional calculations confirm the presence of magnetic frustration, which explains this intermediate incommensurate phase. Taking into account the strongest magnetic interactions, we are able to reproduce the magnetic structure observed experimentally at low temperature.

The crystal and magnetic structures of the organic-inorganic hybrid compound Cr(II) ammoniumethylphosphonate chloride monohydrate, Cr[D(3)N-(CH(2))(2)-PO(3))(Cl)(D(2)O)] (1), have been studied by temperature-dependent neutron powder diffraction and superconducting quantum interference device (SQUID) magnetometry. The compound represents a rare example of a magnetoelectric polar organic-inorganic hybrid solid, containing high spin Cr(2+) ions (S = 2) and is a canted antiferromagnet (weak ferromagnet) below T(N) = 5.5 K. The neutron powder diffraction pattern recorded at T = 10 K, shows that the partially deuterated compound crystallizes in the same non centrosymmetric monoclinic space group P2(1) (No. 4) with the following unit-cell parameters: a = 5.24041(4) Å, b =13.93113(8) Å, c = 5.26081(4) Å, and β = 105.4347(5)°. Powder neutron diffraction of a partially deuterated sample has enabled us, for the first time, to locate the water molecule. At low temperature, the compound presents a canted antiferromagnetic state characterized by k = 0 resulting in the magnetic symmetry P2(1)'. This symmetry is in agreement with the previously reported large magnetodielectric effect. The crystal structure of (1) can be described as being built up of triangular lattice planes made up of [Cr(II)O(4)Cl] square pyramids which are separated by ammonium ethyl groups along the b axis. The transition from paramagnetic to weakly ferromagnetic state results from super-superexchanges only. Surprisingly, while the overall magnetic behavior is antiferromagnetic, the Cr(II)O(4)Cl planes are ferromagnetic, and the strongest antiferromagnetic coupling is via the ammonium ethyl groups. Our density functional calculations confirm these aspects of the spin exchange interactions of (1) and that the spin exchange interactions between Cr(II) ions are considerably weak compared with the single-ion anisotropy of Cr(II).

The structures of the doubly ordered perovskites NaCeMnWO(6) and NaPrMnWO(6), with rock salt ordering of the Mn(2+) and W(6+)B-site cations and layered ordering of the Na(+) and (Ce(3+)/Pr(3+)) A-site cations, have been studied by transmission electron microscopy, electron diffraction, neutron and synchrotron X-ray powder diffraction. Both compounds possess incommensurately modulated crystal structures. In NaCeMnWO(6) the modulation vector (with reference to the ideal ABX(3) perovskite subcell) is q ≈ 0.067a* (∼58.7 Å) and in NaPrMnWO(6)q ≈ 0.046a* (∼85.3 Å). In both compounds the superstructures are primarily the two-dimensional chessboard type, although some crystals of NaCeMnWO(6) were found with one-dimensional stripes. In some crystals of NaPrMnWO(6) there is a coexistence of chessboards and stripes. Modeling of neutron diffraction data shows that octahedral tilting plays an important role in the structural modulation.

Fluor-chlorellestadite solid solutions Ca(10)(SiO(4))(3)(SO(4))(3)Cl(2-x)F(x), serving as prototype crystalline matrices for the fixation of hazardous fly ash, were synthesized and characterized by powder X-ray and neutron diffraction (PXRD and PND), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR). The lattice parameters of the ellestadites vary linearly with composition and show the expected shrinkage of unit cell volume as fluorine (IR = 1.33 Å) displaces chlorine (IR = 1.81 Å). FTIR spectra indicate little or no OH(-) in the solid solutions. All compositions conform to P6(3)/m symmetry where F(-) is located at the 2a (0, 0, (1)/(4)) position, while Cl(-) is displaced out of the 6h Ca(2) triangle plane and occupies 4e (0, 0, z) split positions with z ranging from 0.336(3) to 0.4315(3). Si/S randomly occupy the 6h tetrahedral site. Ellestadites rich in Cl (x ≤ 1.2) show an overall deficiency in halogens (<2 atom per formula unit), particularly Cl as a result of CaCl(2) volatilization, with charge balance achieved by the creation of Ca vacancies (Ca(2+) + 2Cl(-) →□(Ca) + 2□(Cl)) leading to the formula Ca(10-y)(SiO(4))(3)(SO(4))(3)Cl(2-x-2y)F(x). For F-rich compositions the vacancies are found at Ca(2), while for Cl-rich ellestadites, vacancies are at Ca(1). It is likely the loss of CaCl(2) which leads tunnel anion vacancies promotes intertunnel positional disorder, preventing the formation of a P2(1)/b monoclinic dimorph, analogous to that reported for Ca(10)(PO(4))(6)Cl(2). Trends in structure with composition were analyzed using crystal-chemical parameters, whose systematic variations served to validate the quality of the Rietveld refinements., (© 2011 American Chemical Society)

One of the key points of interest in pyrochlore materials containing bismuth derives from the dielectric properties of some such materials that are linked to the displacements of the bismuth atoms from the ideal site. This study uses high pressure to probe the variations in, and causes of, these displacements. Under compression Bi(2)Ti(2)O(7) does not undergo any phase changes, but Bi(2)Sn(2)O(7) undergoes a similar series of changes to those observed during heating. The trigonal β-Bi(2)Sn(2)O(7) structure is solved from high temperature powder neutron diffraction data and hence the sequence of phases observed in Bi(2)Sn(2)O(7) is discussed for the first time. The variation in Bi displacements can be considered in terms of the frustration of the tetrahedral lattice that accommodates them. It can also be inferred that the main driver for Bi displacement is a deficiency in the bond valence sum of bismuth.

New ternary and quaternary nitride halides, Ba(2)N(X,X') (X = F, Cl, Br; X' = Br, I), have been synthesized from the high temperature reactions of barium subnitride with the respective barium halides under an inert atmosphere. The former include the first fully characterized barium nitride halides for X other than F, and the latter are the first examples of barium nitride mixed halides. The variation in structure with composition has been investigated by powder X-ray and powder neutron diffraction techniques. The heavier ternary and quaternary nitride halides (X, X' = Cl, Br, I) crystallize in the hexagonal space group R3m, with the anti-α-NaFeO(2) structure. Ba(2)NF forms with both an anti-α-NaFeO(2) structure, in which N(3-) and F(-) are ordered and an anion-disordered simple rock salt structure. The hexagonal polymorph of Ba(2)NF is the only example to date of a nitride fluoride adopting this layered structure. Both the ternary and the quaternary compounds display very weak, temperature independent paramagnetism.

Polycrystalline samples of Ln18Li8Rh5O39 (Ln = La, Pr) have been synthesized by the ceramic method and characterized by X-ray and neutron diffraction. The compounds crystallize in the cubic space group Pmn, with a0 approximately 12.1 Angstroms. The unit cell contains four intersecting 111 chains, each comprised of an alternating sequence of face-sharing RhO6 octahedra and LiO6 trigonal prisms. The octahedra located at the points of intersection contain Rh4+, whereas the remainder contain Rh3+; the compounds thus contain a charge-ordered arrangement of the two cations. The polyhedral chains are enclosed in tunnels formed by the Ln-O sublattice. The magnetic properties of the two new compounds are discussed briefly: both are paramagnetic over the temperature range 5 < TK < 300.