Sr2GaScO5, Sr10Ga6Sc4O25, and SrGa0.75Sc0.25O2.5: a play in the octahedra to tetrahedra ratio in oxygen-deficient perovskites

Three different perovskite-related phases were isolated in the SrGa(1-x)Sc(x)O(2.5) system: Sr(2)GaScO(5), Sr(10)Ga(6)Sc(4)O(25), and SrGa(0.75)Sc(0.25)O(2.5). Sr(2)GaScO(5) (x = 0.5) crystallizes in a brownmillerite-type structure [space group (S.G.) Icmm, a = 5.91048(5) ?, b = 15.1594(1) ?, and c = 5.70926(4) ?] with complete ordering of Sc(3+) and Ga(3+) over octahedral and tetrahedral positions, respectively. The crystal structure of Sr(10)Ga(6)Sc(4)O(25) (x = 0.4) was determined by the Monte Carlo method and refined using a combination of X-ray, neutron, and electron diffraction data [S.G. I4(1)/a, a = 17.517(1) ?, c = 32.830(3) ?]. It represents a novel type of ordering of the B cations and oxygen vacancies in perovskites. The crystal structure of Sr(10)Ga(6)Sc(4)O(25) can be described as a stacking of eight perovskite layers along the c axis ...[-(Sc/Ga)O(1.6)-SrO(0.8)-(Sc/Ga)O(1.8)-SrO(0.8)-](2).... Similar to Sr(2)GaScO(5), this structure features a complete ordering of the Sc(3+) and Ga(3+) cations over octahedral and tetrahedral positions, respectively, within each layer. A specific feature of the crystal structure of Sr(10)Ga(6)Sc(4)O(25) is that one-third of the tetrahedra have one vertex not connected with other Sc/Ga cations. Further partial replacement of Sc(3+) by Ga(3+) leads to the formation of the cubic perovskite phase SrGa(0.75)Sc(0.25)O(2.5) (x = 0.25) with a = 3.9817(4) ?. This compound incorporates water molecules in the structure forming SrGa(0.75)Sc(0.25)O(2.5)·xH(2)O hydrate, which exhibits a proton conductivity of ～2.0 × 10(-6) S/cm at 673 K.     

Ca(2.5)Sr(0.5)GaMn2O8: diamagnetic Ga in control of the structural and electronic properties of a bilayered manganate

The temperature dependence of the crystal structure and electronic properties of brownmillerite-like Ca(2.5)Sr(0.5)GaMn(2)O(8) has been studied by neutron powder diffraction and muSR spectroscopy. The results show that short-range 2D magnetic order begins to develop within the perovskite-like bilayers of MnO(6) octahedra approximately 50 K above the 3D Néel temperature of approximately 150 K. The bilayers show a structural response to the onset of magnetism throughout this temperature range whereas the GaO(4) layers that separate the bilayers only respond below the 3D ordering temperature. XANES spectroscopy shows that the sample contains Mn(3+) and Mn(4+) cations in a 1:1 ratio, and the behavior in the region of the Néel transition is interpreted as a local charge ordering. Electron diffraction and high-resolution electron microscopy have been used to show that the local microstructure is more complex than the average structure revealed by neutron diffraction, and that microdomains exist in which the GaO(4) tetrahedra show different orientations. It is argued that the bonding requirements of diamagnetic gallium control the electronic behavior within the perovskite-like bilayers.     

Pressure-induced intermetallic valence transition in BiNiO3

The valence state change of BiNiO3 perovskite under pressure has been investigated by a powder neutron diffraction study and electronic-state calculations. At ambient pressure, BiNiO3 has the unusual charge distribution Bi(3+)(0.5)Bi(5+)(0.5)Ni(2+)O3 with ordering of Bi(3+) and Bi(5+)charges on the A sites of a highly distorted perovskite structure. High-pressure neutron diffraction measurements and bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high-pressure phase is metallic Bi(3+)Ni(3+)O3. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable.     

REMGa3Ge and RE3Ni3Ga8Ge3 (M = Ni, Co; RE = rare-earth element): new intermetallics synthesized in liquid gallium. X-ray, electron, and neutron structure determination and magnetism

New quaternary intermetallic phases REMGa(3)Ge (1) (RE = Y, Sm, Tb, Gd, Er, Tm; M = Ni, Co) and RE(3)Ni(3)Ga(8)Ge(3) (2) (RE = Sm, Gd) were obtained from exploratory reactions involving rare-earth elements (RE), transition metal (M), Ge, and excess liquid Ga the reactive solvent. The crystal structures were solved with single-crystal X-ray and electron diffraction. The crystals of 1 and 2 are tetragonal. Single-crystal X-ray data: YNiGa(3)Ge, a = 4.1748(10) A, c = 23.710(8) A, V = 413.24(2) A(3), I4/mmm, Z = 4; Gd(3)Ni(3)Ga(8)Ge(3), a = 4.1809(18) A, c = 17.035(11) A, V = 297.8(3) A(3), P4/mmm, Z = 1. Both compounds feature square nets of Ga atoms. The distribution of Ga and Ge atoms in the REMGa(3)Ge was determined with neutron diffraction. The neutron experiments revealed that in 1 the Ge atoms are specifically located at the 4e crystallographic site, while Ga atoms are at 4d and 8g. The crystal structures of these compounds are related and could be derived from the consecutive stacking of disordered [MGa](2) puckered layers, monatomic RE-Ge planes and [MGa(4)Ge(2)] slabs. Complex superstructures with modulations occurring in the ab-plane and believed to be associated with the square nets of Ga atoms were found by electron diffraction. The magnetic measurements show antiferromagnetic ordering of the moments located on the RE atoms at low temperature, and Curie-Weiss behavior at higher temperatures with the values of mu(eff) close to those expected for RE(3+) free ions.     

Transmission electron microscopy studies of NaLaMgWO6: spontaneous formation of compositionally modulated stripes

Transmission electron microscopy studies of the perovskite NaLaMgWO 6 reveal the formation of a complex, compositionally modulated structure. Annular dark field scanning transmission electron microscopy images and scanning transmission electron microscopy-electron energy-loss spectroscopy scans show that this modulation involves a repeating pattern of La-rich and La-poor stripes, each stripe 6 a p or approximately 24 A wide (where a p is the edge length of the simple cubic perovskite unit cell). High-resolution transmission electron microscopy images clearly show, and electron diffraction patterns confirm, a periodicity of 12 a p along either the [100] p or [010] p direction. Available evidence suggests a spontaneous separation into stripes that possess the nominal stoichiometry, NaLaMgWO 6, alternating with Na-poor/La-rich stripes that have a stoichiometry of (La x Na 1-3 x )LaMgWO 6. X-ray powder diffraction measurements are insensitive to this intricate structural complexity, which may be a more widespread feature of (A (+)Ln (3+))MM'O 6 perovskites than previously appreciated.     

Control of magnetic ordering by Jahn--Teller distortions in Nd(2)GaMnO(6) and La(2)GaMnO(6).

The substitution of Ga(3+) into the Jahn--Teller distorted, antiferromagnetic perovskites LaMnO(3) and NdMnO(3) strongly affects both the crystal structures and resulting magnetic ordering. In both compounds the Ga(3+) and Mn(3+) cations are disordered over the six coordinate sites. La(2)GaMnO(6) is a ferromagnetic insulator (T(c) = 70 K); a moment per Mn cation of 2.08(5) mu(B) has been determined by neutron powder diffraction at 5 K. Bond length and displacement parameter data suggest Jahn--Teller distortions which are both coherent and incoherent with the Pnma space group symmetry of the perovskite structure (a = 5.51122(4) A, b = 7.80515(6) A, c = 5.52947(4) A) at room temperature. The coherent distortion is strongly suppressed in comparison with the parent LaMnO(3) phase, but the displacement ellipsoids suggest that incoherent distortions are significant and arise from local Jahn--Teller distortions. The preparation of the new phase Nd(2)GaMnO(6) has been found to depend on sample cooling rates, with detailed characterization necessary to ensure phase separation has been avoided. This compound also adopts the GdFeO(3)-type orthorhombically distorted perovskite structure (space group Pnma, a = 5.64876(1) A, b = 7.65212(2) A, c = 5.41943(1) A at room temperature). However, the B site substitution has a totally different effect on the Jahn--Teller distortion at the Mn(3+) centers. This phase exhibits a Q(2) mode Jahn--Teller distortion similar to that observed in LaMnO(3), although reduced in magnitude as a result of the introduction of Ga(3+) onto the B site. There is no evidence of a dynamic Jahn-Teller distortion. At 5 K a ferromagnetically ordered Nd(3+) moment of 1.06(6) mu(B) is aligned along the y-axis and a moment of 2.8(1) mu(B) per Mn(3+) is ordered in the xy plane making an angle of 29(2) degrees with the y-axis. The Mn(3+) moments couple ferromagnetically in the xz plane. However, along the y-axis the moments couple ferromagnetically while the x components are coupled antiferromagnetically. This results in a canted antiferromagnetic arrangement in which the dominant exchange is ferromagnetic. Nd(2)GaMnO(6) is paramagnetic above 40(5) K, with a paramagnetic moment and Weiss constant of 6.70(2) mu(B) and 45.9(4) K, respectively. An ordered moment of 6.08(3) mu(B) per Nd(2)GaMnO(6) formula unit was measured by magnetometry at 5 K in an applied magnetic field of 5 T.     

Reduction of the host cationic framework charge by isoelectronic substitution: synthesis and structure of Hg7Ag2P8X6 (X = Br, I) and Hg6Ag4P8Br6

The first compounds, Hg(7)Ag(2)P(8)X(6) (X = Br, I) and Hg(6)Ag(4)P(8)Br(6), featuring the partial isoelectronic substitution of Hg(2+) for Ag(1+) in mercury-pnicogen frameworks have been obtained and structurally characterized. The new compounds are the supramolecular assemblies built of the covalently bonded metal-pnicogen frameworks trapping guests of different complexity. The frameworks feature the perfect ordering of Hg(2+) and Ag(1+) cations and contain P(2)(4)(-) and P(6)(6)(-) phosphorus clusters. The substitution of Hg(2+) with Ag(1+) leads to the reduction in charge of the host cluster-containing cationic matrix and concomitant replacement of the monatomic X(-) guest by a lesser amount of the AgBr(3)(2)(-) anions.     

Structural tuning of charge,orbital, and spin ordering in double-cell perovskite series between NdBaFe(2)O(5) and HoBaFe(2)O(5)

Charge, orbital, and magnetic ordering of NdBaFe(2)O(5) and HoBaFe(2)O(5), the two end-members of the double-cell perovskite series RBaFe(2)O(5), have been characterized over the temperature range 2-450 K, using differential scanning calorimetry, neutron thermodiffractometry and high-resolution neutron powder diffraction. Upon cooling, both compounds transform from a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions, through a "premonitory" charge ordering into a class-II MV compound, and finally to a class-I MV phase at low-temperature. The latter phase is characterized by Fe(2+)/Fe(3+) charge ordering as well as orbital ordering of the doubly occupied Fe(2+) d(xz) orbitals. The relative simplicity of the crystal and magnetic structure of the low-temperature charge-ordered state provide an unusual opportunity to fully characterize the classical Verwey transition, first observed in magnetite, Fe(3)O(4). Despite isotypism of the title compounds at high temperature, neutron diffraction analysis reveals striking differences in their phase transitions. In HoBaFe(2)O(5), the Verwey transition is accompanied by a reversal of the direct Fe-Fe magnetic coupling across the rare earth layer, from ferromagnetic in the class-II and -III MV phases to antiferromagnetic in the low-temperature class-I MV phase. In NdBaFe(2)O(5), the larger Nd(3+) ion increases the Fe-Fe distance, thereby weakening the Fe-Fe magnetic interaction. This decouples the charge and magnetic ordering so that the Fe-Fe interaction remains ferromagnetic to low temperature. Furthermore, the symmetry of the charge-ordered class-I MV phase is reduced from Pmma to P2(1)()ma and the magnitude of the orbital ordering is diminished. These changes destabilize the charge-ordered state and suppress the temperature at which the Verwey transition occurs. A comparison of the magnetic and structural features of RBaFe(2)O(5) compounds is included in order to illustrate how structural tuning, via changes in the radius of the rare-earth ion, can be used to alter the physical properties of these double-cell perovskites.     

Redox chemistry and magnetism of LaSrM(0.5)Ru(0.5)O(4±δ) (M = Co, Ni and Zn) Ruddlesden-Popper phases

The n = 1 Ruddlesden-Popper (RP) phases LaSrM(0.5)Ru(0.5)O(4±δ) (M = Co, Ni and Zn) have been prepared by solid state reactions and structurally characterized by powder X-ray and electron diffraction. All the samples adopt the tetragonal I4/mmm space group with random M and Ru cation occupation on the B-sites. The potential causes of no cation ordering are discussed. A combined analysis of the tolerance factors, the distortion of the octahedral coordination of M and Ru cations and the magnetic interactions between M and Ru cations provide a better understanding for forming a phase with 3D cation ordering on the B-sites in the n = 1 RP phases. The investigation of XPS spectra suggests that the transition element species exist as mixed ion pairs, Ru((4-δ)+)-Ru(4+)? Co(2+)-Co(3+) in LaSrCo(0.5)Ru(0.5)O(4), and Ru(4+)-Ru((4+δ)+)? Ni(+)-Ni(2+) in LaSrNi(0.5)Ru(0.5)O(4), which is consistent with cation disorder over the B sites. LaSrCo(0.5)Ru(0.5)O(4) shows a weakly ferromagnetic behaviour below 50 K; LaSrNi(0.5)Ru(0.5)O(4) is evidenced by the presence of long-range magnetic ordering at a Néel temperature of 125 K, and LaSrZn(0.5)Ru(0.5)O(4) exhibits a paramagnetic behaviour down to 5 K. Due to atomic disorder, Ru4d, O2p covalent coupling is weakened, strengthening the intraatomic spin-spin coupling among the π* electrons. Charge transfer between Ru and Co or Ru and Ni, as well as the increasing overlap of both nearest-neighbour and next-nearest-neighbour Ru 4d electrons due to atomic disorder, favour the formation of ferromagnetic interactions. Although antiferromagnetism is dominant, particularly in LaSrNi(0.5)Ru(0.5)O(4), ferromagnetic interactions are stronger in the title compounds than in the related La(2)MRuO(6) (M = Co, Ni) double perovskites where the B-site cations are ordered.     

The concentration of surface VO species on oxidized vanadium oxide catalysts

The hollandite-type phase K_1.33Mg_3.11Sb_4.89O₁₆ has been studied by X-ray and electron diffraction as well as high resolution electron microscopy at 500 kV. This material was found to adopt the tetragonal hollandite structure, space group $\text{I4}\text{m}$, with a = 10.315 (4)Å; c = 3.080 (4) Å. The formation of a 3c body-centered supercell was observed and this was shown to be due to ordering of potassium cations within the tunnel sites. Computer image simulations established that ordering of the tunnel cations alone rather than the octahedrally coordinated framework cations was responsible for superlattice formation. In some crystals the supercell ordering appeared to occur in domains.     

Atomic and Magnetic Long-Range Orderings in BaLaMRuO6 (M=Mg and Zn)

The crystal structures of double perovskite BaLaMRuO₆ (M=Mg, Zn) obtained from the refinements on both X-ray and neutron diffraction data, different from those reported previously that used either X-ray or neutron diffraction data alone, are reported. The room temperature X-ray and neutron data were refined with a model in the tetragonal space group I4/m (a=5.6230(4), c=7.964(1) Å, V=251.81(4) ų for M=Mg; a=5.6521(3), c=7.9987(9) Å, V=255.53(3) ų for M=Zn). The low-temperature neutron diffraction data of the two compounds are also refined in the same space group (a=5.6156(4), c=7.953(1) Å, V=250.80(4) ų for M=Mg at 13 K; a=5.6418(4), c=7.981(1) Å, V=254.03(4) ų for M=Zn at 10 K). Both compounds show almost complete ordering of B-site atoms (M/Ru). For both compounds, the low-temperature neutron diffraction data below about 20 K showed magnetic diffraction peaks that could be accounted for with a Type I antiferromagnetic ordering of Ru spins in an atomically ordered double perovskite structure. These compounds showed discrepancies between field cooled and zero field cooled magnetization data below the antiferromagnetic ordering temperatures.     

Cation-controlled assembly of Na+-linked lacunary alpha-Keggin tungstosilicates

Two novel salts of lacunary tungstosilicates with guanidinium and alkali metal cations, (CH6N3)7Na[SiW11O39].(CH3)2CO.8H2O (1) and (CH6N3)K6Na[SiW11O39].11.5H2O (2), have been synthesized and their crystal structures have been determined by synchrotron X-ray diffraction. In both crystals, the Na+ cations link the lacunary Keggin-type tungstosilicate anions into linear structures. The neighboring [SiW11O39]8- anions are related by two-fold screw and translational operations in compounds 1 and 2, respectively. Second harmonic generation was observed for compound 1.     

Crystal chemistry of melilite [CaLa]2[Ga]2[Ga2O7]2: a five dimensional solid electrolyte

Melilite-type [A(2)](2)[B(I)](2)[B(II)(2)O(7)](2) gallates are promising ion conducting electrolytes for deployment in solid oxide fuel cells. Single crystals of [CaLa](2)[Ga](2)[Ga(2)O(7)](2), grown in an optical floating zone furnace, were investigated using a combination of transmission electron microscopy and single crystal X-ray diffraction. Strong anisotropic displacements of oxygen arise from the structural misfit between the interlayer Ca/La cations and the [Ga]-[Ga(2)O(7)] tetrahedral layers. A model employing two-dimensional modulation achieves bond lengths and bond angles that preserve satisfactory bond valence sums throughout the structure. The melilite belongs to the tetragonal superspace group P42(1)m(α, α, 0)00s(α, α, 0)000, α = 0.2160(5), with a subcell metric of a = 7.9383(2) ?, c = 5.2641(3) ?, onto which modulation vectors are superimposed: q(1) = α (a* + b*), q(2) = α (-a* + b*). Both displacive (cation and anion) and occupational (cation) modulations contribute to incommensuration. The analysis of structural adjustments that accompany changes in temperature and composition provides assurance that the crystal chemical model is correct. By better understanding the flexibility of this modulated structure a rational approach toward crystallochemical optimization of electrolyte performance by enhancing oxygen mobility becomes feasible.     

Modular construction of oxide structures--compositional control of transition metal coordination environments

The effects of reaction temperature and pO2 were investigated on a series of (Ba,Ca,Nd)FeO3-delta perovskite systems in order to isolate phases containing ordered arrangements of the distinct vacancy and cation ordering patterns identified in less compositionally complex iron oxide systems. Initial synthesis in air at high temperature yields cubic perovskite phases (I) with average iron oxidation states higher than 3; selected area electron diffraction together with diffuse features observed in the synchrotron X-ray diffraction (SXRD) patterns of these materials show evidence of small domains of short-range cation and vacancy order. Annealing these materials in nitrogen or in a sealed tube in the presence of an NiO/Ni buffer yielded the Fe(3+) phase Ca2Ba2Nd2Fe6O16 (II), closely related to Sr2LaFe3O8 but with partial cation order as well as anion order present the larger Ba cations are largely present in the 12-coordinate site between the octahedral iron layers, and Ca is largely present in 10-coordinate sites between octahedral and tetrahedral sites. Further reduction of Ca2Ba2Nd2Fe6O16 using a Zr getter yields the mixed-valence phase Ca2Ba2Nd2Fe6O15.6 (III). The structure of III was solved by maximum entropy analysis of XRD data coupled with analysis of high-temperature neutron diffraction data and refined against combined SXRD and high-Q ambient-temperature neutron data. This material crystallizes in a 20-fold perovskite super cell (Imma, a approximately square root(2 x a(p), b approximately 10 x a(p), c approximately square root(x 2a(p)) and can be visualized as an intergrowth between brownmillerite (Ca2Fe2O5) and the YBa2Fe3O8 structure. There are three distinct iron coordination environments, octahedral (O), square-pyramidal (Sp), and trigonal planar (Tp, formed by distorting the tetrahedral site in brownmillerite), which form a Sp-O-Tp-O-Sp repeat. Bond valence calculations indicate that Tp is an Fe(2+) site, while the O and Sp sites are Fe(3+). The A-site cations are also partially ordered over three distinct sites: 8-coordinate between the Sp layers, 10-coordinate between Tp and O layers, and 12-coordinate between Sp and O layers. Mossbauer spectroscopy, magnetometry, and variable-temperature neutron diffraction show that the material undergoes two magnetic transitions at approximately 700 and 255 K.     

An incommensurately modulated structure of η'-phase of Cu(3+x)Si determined by quantitative electron diffraction tomography

The diffraction data of η'-Cu(3+x)(Si,Ge) were collected by 3D quantitative electron diffraction tomography on a submicrometer-sized sample, and the structure was solved by the charge-flipping algorithm in superspace. It is shown that the structure is trigonal, and it is incommensurately modulated with two modulation vectors q(1) = (α, α, 1/3) and q(2) = (-2α, α, 1/3), superspace group P31m(α, α, 1/3)000(-2α, α, 1/3)000. The modulation functions of some atoms are very complicated and reach amplitudes comparable with the unit cell dimensions. The modulated structure can be described as sheets of Cu clusters separated by honeycomb layers of mixed Si/Ge positions. The shape of the Cu clusters in the sheets strongly varies with the modulation phase, and the predominant form is an icosahedron. The striving of the Cu layers to form icosahedral clusters is deemed to be the main driving force of the modulation. The combination of methods used in this work can be applied to other structures that are difficult to crystallize in large crystals and opens new perspectives, especially for investigations of aperiodic or otherwise complex metallic alloys.     

Synthesis, structure, negative thermal expansion, and photocatalytic property of Mo doped ZrV2O7

A new series of compounds identified in the phase diagram of ZrO(2)-V(2)O(5)-MoO(3) have been synthesized via the solution combustion method. Single crystals of one of the compounds in the series, ZrV(1.50)Mo(0.50)O(7.25), were grown by the melt-cool technique from the starting materials with double the MoO(3) quantity. The room temperature average crystal structure of the grown crystals was solved using the single crystal X-ray diffraction technique. The crystals belong to the cubic crystal system, space group Pa3 (No. 205) with a = 8.8969 (4) ?, V = 704.24 (6) ?(3), and Z = 4. The final R(1) value of 0.0213 was achieved for 288 independent reflections during the structure refinement. The Zr(4+) occupies the special position (4a) whereas V(5+) and Mo(6+) occupy two unique (8c) Wyckoff positions. Two fully occupied O atoms, (24d) and (4b), one partially occupied O atom (8c) have been identified for this molybdovanadate, which is a unique feature for these crystals. The structure is related to both ZrV(2)O(7) and cubic ZrMo(2)O(8). The temperature dependent single crystal studies show negative thermal expansion above 370 K. The compounds have been characterized by powder X-ray diffraction, solid-state UV-vis diffuse reflectance spectra, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The photocatalytic activity of these compounds has been investigated for the degradation of various dyes, and these compounds show specificity toward the degradation of non-azoic dyes.     

Beyond the structure-property relationship paradigm: influence of the crystal structure and microstructure on the Li+ conductivity of La2/3Li(x)Ti(1-x)Al(x)O3 Oxides

The crystal structures of several oxides of the La(2/3)Li(x)Ti(1-x)Al(x)O(3) system have been studied by selected-area electron diffraction, high-resolution transmission electron microscopy, and powder neutron diffraction, and their lithium conductivity has been by complex impedance spectroscopy. The compounds have a perovskite-related structure with a unit cell radical2 a(p)x2 a(p)x radical2 a(p) (a(p)=perovskite lattice parameter) due to the tilting of the (Ti/Al)O(6) octahedra and the ordering of lanthanum and lithium ions and vacancies along the 2 a(p) axis. The Li(+) ions present a distorted square-planar coordination and are located in interstitial positions of the structure, which could explain the very high ionic conductivity of this type of material. The lithium conductivity depends on the oxide composition and its crystal microstructure, which varies with the thermal treatment of the sample. The microstructure of these titanates is complex due to formation of domains of ordering and other defects such as strains and compositional fluctuations.     

High-temperature behavior and polymorphism in novel members of the perovskite family Pb2LnSbO6 (Ln=Ho, Er, Yb, Lu)

The synthesis, crystal structure, and dielectric properties of four novel members of the family of double perovskites Pb(2)LnSbO(6) are described. The room-temperature crystal structures were refined from neutron powder diffraction (NPD) data in the monoclinic C2/c (No. 15) space group. They contain a completely ordered array of alternating LnO(6) and SbO(6) octahedra sharing corners, tilted in antiphase along the three pseudocubic axes, with a a(-)b(-)b(-) tilting scheme, which is very unusual in the crystallochemistry of perovskites. The lead atoms occupy highly asymmetric voids with 8-fold coordination due to the stereoactivity of the Pb(2+) electron lone-pair. Several trends are observed for the entire family of compounds upon heating. The Ln = Lu, Yb, and Er oxides display three successive phase transitions in a narrow temperature range, as shown by differential scanning calorimetry (DSC) data, while the Ln = Ho shows only two transitions. Different crystal structure evolutions have been found from temperature-dependent NPD and DSC, following the space-group sequence C2/c → P2(1)/n → R ?3 → Fm ?3m for Ln = Lu and Yb, the sequence C2/c → unknown → P2(1)/n → Fm ?3m for Ln = Er, and C2/c → P2(1)/n → Fm ?3m for Ln = Ho. The Ln/Sb long-range ordering is preserved across the consecutive phase transitions. Dielectric permittivity measurements indicate the presence of a paraelectric/antiferroelectric transition (associated with the last structural transition), as suggested by the negative Curie temperature from the Curie-Weiss fit of the reciprocal permittivity.     

High dilution of anionic vacancies in Sr(0.8)Ba(0.2)Fe(O,F)(~2.5)

The (Ba,Sr)FeO(3-δ) system is known for its strong tendency for oxygen and vacancies to order into several forms including fully ordered pseudobrownmillerites, hexagonal perovskites with segregation of the vacancies in particular anionic layers and low deficient (pseudo)cubic compounds (generally δ < 0.27, Fe(3/4+)). We show for the first time, using a simple chemical process, the easy access to a large amount of vacancies (δ ≈ 0.5, Fe(3+)) within the room-temperature stable tetragonal (pseudocubic) Sr(0.8)Ba(0.2)FeF(~0.1)(O,F)(~2.5.) The drastic effect of the incorporation of a minor amount of fluoride passes through the repartition of local O/F/□ constraints shifting the tolerance factor into the pseudocubic range for highly deficient compounds. It is stable up to 670 K, where an irreversible reoxidation process occurs, leading to the cubic-form. The comparison with the cubic oxide Sr(0.8)Ba(0.2)FeO(~2.7) shows the increase of the resistivity (3D-VRH model) by two decades due to the almost single valent Fe(3+) of the oxofluoride. In addition, the G-type magnetic ordering shows relatively weak moment for Fe(3+) cations (M(Fe) ≈ 2.64(1) μB at room temperature) attributed to incoherent magnetic components expected from local disorder in such anionic-deficient compounds.