Perovskite Host Research Articles

Topochemical Strategies for the Formation of Alkali-metal Halide Arrays within Perovskite Hosts

AbstractMultistep topochemical reactions can be used to construct alkali-metal halide arrays within layered perovskite hosts. Combinations of ion exchange and reductive intercalation (A = Li) or reductive and oxidative intercalation (A = Rb) allow one to prepare the compounds such as (A2Cl)LaNb2O7. These products consist of perovskite blocks separated by double alkali-metal halide layers where the local layer structure is dependent on the size of the alkali cation. Details on the synthesis and structures of these materials are presented, and the general utility of the topochemical strategies used in their preparation is discussed.

Third-order contributions to the 8S7/2→ 6P7/2, 6P5/2 two-photontransitions of Eu2+ in KMgF3

The line strengths in four polarization directions of the incident radiation of the, Γ8 and , Γ7,Γ8 two photon absorption(TPA) transitions of Eu2+ in the perovskite KMgF3 host have been calculated using the third-order Judd–Pooler (JP) formalism. In general,the calculated relative intensities are in good agreement with experiment, except for theratios between the linear and circular polarizations in the transitions. In particular, the scalar operator term in the third-order spin–orbitalcorrection formula successfully explains nine transitions with different linear polarizations.

Mössbauer study of charge disproportionate BaFeO 3− y and Ba 0.9Sr 0.1FeO 3− y with 6H-perovskite structure

BaFeO 3- y and Ba 0.9Sr 0.1FeO 3− y have been synthesized under 100 MPa O 2 and investigated by Mössbauer spectroscopy down to 5 K. Both compounds have been confirmed to be 6H-type hexagonal structures; Mössbauer spectra at 5 K for both compositions consist of two magnetic hyperfine sextets with different center shifts (0.48–0.46 and −0.11 to −0.14 mm s −1) and hyperfine fields (49–50 and 24–25 T), which clearly indicates charge disproportionation reaction of Fe. Mössbauer parameters of Fe in perovskite hosts are discussed.

Formation of metal-anion arrays within layered perovskite hosts. Preparation of a series of new metastable transition-metal oxyhalides, (MCl)LaNb(2)O(7) (M = Cr, Mn, Fe, Co).

A new series of transition-metal oxyhalides (MCl)LaNb(2)O(7) (M = Cr, Mn, Fe, Co) have been prepared by a simple topochemical route. Layered perovskite hosts (ALaNb(2)O(7), A = Li, Na, K or Rb) were reacted with the corresponding anhydrous metal halides under mild reaction conditions (<400 degrees C). The compounds were examined by X-ray powder diffraction; the series appears to be isostructural with (CuCl)LaNb(2)O(7), and the layer spacings, with the exception of M = Co, follow the trend expected from transition-metal cationic radii. Thermal analysis with differential scanning calorimetry (DSC) shows the materials to be metastable where all four compounds decompose exothermically above 690 degrees C.

Construction of copper halide networks within layered perovskites. Syntheses and characterization of new low-temperature copper oxyhalides.

The construction of two-dimensional (2D) copper halide networks within a variety of perovskite hosts by a low-temperature topochemical method is demonstrated. Ion exchange between some layered perovskite oxides of the type A'[An - 1(M,M')nO3n + 1] (A' = alkali metal, H, NH4; A = alkaline earth, rare earth, or Bi; M,M' = Nb, Ta, Ti; n = 2, 3) with CuX2 (X = Cl, Br) results in the oxyhalides (CuX)[An - 1(M,M')nO3n + 1]. Rietveld refinements from X-ray powder diffraction data show that the structures of these new copper oxyhalides contain edge-sharing CuO2X4 octahedra sandwiched between the M/M'O6 octahedra of the perovskite slabs. The compounds are low-temperature phases that decompose well below 700 degrees C. The copper oxyhalides exhibit antiferromagnetic ordering resulting from the magnetic exchange interactions within the planar Cu-X networks.

Assembly of Metal−Anion Arrays within a Perovskite Host. Low-Temperature Synthesis of New Layered Copper−Oxyhalides, (CuX)LaNb2O7, X = Cl, Br

A low-temperature topotactic route is used to assemble metal−anion arrays within a perovskite host. Ion exchange between RbLaNb2O7 and CuX2 (X = Cl, Br) results in a new set of layered copper−oxyhalide perovskites, (CuX)LaNb2O7. Rietveld structural analysis of X-ray powder diffraction data confirms the formation of a two-dimensional copper−halide network in the double-layered perovskite interlayer. This new structure type contains unusual CuO2X4 octahedra that corner-share with NbO6 octahedra from the perovskite slab and edge-share with each other along all four equatorial edges. Magnetic susceptibility measurements show that both products exhibit antiferromagnetic transitions below 40 K. Additionally, these materials are found to be low-temperature phases, decomposing completely by 700 °C. The synthetic approach described in this work is significant in that it demonstrates how host structures can be used as templates in the directed low-temperature assembly of extended metal−anion arrays.

Catalytic properties of SrSn1−xSbxO3 in methanol oxidation

Ceramic model catalyst materials of general formula SrSn1−xSb x O3 are synthesised by solid state reaction. X-ray diffraction shows the materials to be single phase over the composition range 0 ⩽x⩽0.085, with a second phase, identified as SrSb2O5, being produced at higher antimony doping levels. XPS, Auger electron spectroscopy and valence band photoemission show strong segregation of Sb to the surface of the material even within the single phase regime. The catalytic properties of the materials are examined using methanol oxidation as a test reaction. The activity and selectivity to formaldehyde production shown by these materials is found to be strongly correlated with the attainment of the bulk solubility limit of Sb in the SrSnO3 perovskite host.

Electronic structure of the copper(II) ion doped in cubic KZnF3

The absorption and magnetic circular dichroism spectra of 1%–3% copper(II) doped into the cubic perovskite host KZnF3 are measured over the temperature range 1.8–300 K. Sharp magnetic dipole allowed transitions 2Eg(Γ8)→2T2g(Γ7,Γ8) are observed together with accompanying vibrational fine structure. The spectra are interpreted on the basis of a tetragonally elongated CuF4−6 ground state geometry which arises from strong Jahn–Teller coupling. This results in a statistical distribution of three equivalent elongations which can undergo reorientation on the electron paramagnetic resonance time scale. The Jahn–Teller coupling within the 2T2g multiplet is partially quenched by spin–orbit coupling and the lowest Kramers doublet Γ7 has an octahedral geometry, while the higher lying Γ8 state has a small tetragonal distortion. This system presents an excellent opportunity to study the spectroscopy of the copper(II) ion in a cubic environment.

Axial electron-channelling analysis of perovskite: II. Site identification of Sr, Zr and U impurities

Abstract The orientation dependence of characteristic X-ray emission (the Borrmann effect) under near-zone-axis diffraction conditions has been used to identify the site preferences of strontium, zirconium and uranium impurities within a CaTiO3 (perovskite) host structure. As characteristic emission lines from these impurities occur at both higher and lower energies than the calcium or titanium K-shell excitations, effects of delocalization are clearly measureable, and are used as a tool in the ALCHEMI analysis. It is found that strontium and uranium strongly partition into calcium sites, whereas zirconium occupies titanium sites.

Thermal quenching of chromium photoluminescence in ordered perovskites. I. Temperature dependence of spectra and lifetimes.

The temperature dependence of chromium photoluminescence spectra and lifetimes have been measured in ordered perovskite (${\mathrm{Cs}}_{2}$${\mathrm{NaYCl}}_{6}$, ${\mathrm{K}}_{2}$${\mathrm{NaScF}}_{6}$, and ${\mathrm{K}}_{2}$${\mathrm{NaGaF}}_{6}$) host crystals. Analysis of lifetime data yields thermal-quenching activation energies \ensuremath{\Delta}E of 4250, 7270, and 9240 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The Mott theory, applied to a single-configuration-coordinate, linear-coupling model with parameters inferred from emission spectra, fails by 20 orders of magnitude to explain the observed radiationless-transition rate.

EPR studies of some f n and d n electronic impurities in KTaO3 single crystals

Single crystals of the cubic perovskite host, potassium tantalate (KTaO3), doped with iron group, lanthanide, and actinide impurities have been investigated using the technique of electron paramagnetic resonance (EPR). The EPR spectra of Yb3+ and U5+ have been observed for the first time in potassium tantalate by employing crystals co-doped with both impurities. Multiple doping of the material during the crystal growth process avoided the production of semiconducting KTaO3 and resulted in the incorporation of adequate concentrations of the trivalent lanthanide ion Yb3+. The EPR results indicate that Yb3+ occupies a site in which the local symmetry is axial as a result of nearby charge compensation. Pentavalent uranium is found to occupy a substitutional cubic symmetry site. EPR investigations of Cu2+, Co2+, Mn2+, Ni3+, and Fe3+ were also carried out.

Electronic structure, ground terms and crystal field transitions in cobalt doped cubic perovskite hosts

Abstract The energies of some terms relevant to Co(II), Co(III) and Co(IV) ions in cubic oxides have been calculated in order to determine the ground state as a function of the unit cell parameter of the crystal host.

Point defects and semiconducting behaviour of Fe doped titanates deduced from molecular orbital diagrams

Abstract The electronic levels relevant to Fen+ and Fen+_VO centers trapped in a cubic perovskite hosts are reported for nominal ionicities n varying from 1 to 5 and the possible semiconducting behaviours of these centers are discussed.

Electronic structures of Co(II) and Co(III) impurities in cubic perovskite hosts

The spin-polarized option of the multiple-scattering $X\ensuremath{\alpha}$ method is used to obtain the electronic structures of divalent and trivalent cobalt ions in SrTi${\mathrm{O}}_{3}$ and BaTi${\mathrm{O}}_{3}$ samples. Various clusters account for the different charge-compensation mechanisms (oxygen vacancies, fluorine ions) and different spin configurations are considered for Co(II) and Co(III) centers. Transition-state computations are carried out to get the various multiplet levels of Co(II) and CO(III) cubic centers. The high-spin $^{5}T_{2g}$ and the low-spin $^{1}A_{1g}$ terms are, respectively, the Co(III) ground terms in BaTi${\mathrm{O}}_{3}$ and SrTi${\mathrm{O}}_{3}$ hosts. The 10Dq crystal-field parameter of Co(III) increases from 1.9 eV (BaTi${\mathrm{O}}_{3}$) to 2.1 eV (SrTi${\mathrm{O}}_{3}$). In the light of $X\ensuremath{\alpha}$ energy diagrams, the possible interpretations of the absorption spectra relevant to Reimeika BaTi${\mathrm{O}}_{3}$: Co and SrTi${\mathrm{O}}_{3}$: Co crystals are reviewed.

High resolution MCD and phosphorescence excitation spectroscopy of Mn2+in KMgF3and KZnF3

A combination of high resolution MCD and phosphorescence excitation techniques has been used to determine, for the first time, the optical spectra of single Mn2+ ions in the cubic perovskite hosts KMgF3 and KZnF3. Particular attention has been paid to transition mechanisms, spin-orbit effects and vibronic structure, and the experimental results have been compared with theoretical crystal field calculations based on the complete d 5 configuration. All of the states arising from the 4 G, 4 P and 4 D terms have been observed and a dynamic Jahn-Teller effect has been observed in the 4 T 1g a state. Both magnetic dipole and electric dipole (vibrationally induced) absorption processes occur and the former is most important for transitions to the 4 T 1g a , 4 A 1g , 4 T 1g b states. Calculated and observed magnetic dipole intensities are in good agreement. A complete analysis of the zero phonon (magnetic dipole) lines and vibronic (magnetic and electric dipole) side-bands associated with transitions to 4 A 1g an...

First- and Second-Shell Hyperfine Interactions in Iron-Group-Doped Perovskite Fluorides

An electron-spin-resonance (ESR)/ electron-nuclear double-resonance (ENDOR) investigation of KZn${\mathrm{F}}_{3}$: $M (M={\mathrm{Mn}}^{2+}, {\mathrm{Fe}}^{3+}, {\mathrm{Co}}^{2+}, {\mathrm{Ni}}^{2+})$ has been carried out with a view toward obtaining an improved understanding of the second-neighbor fluorine (F2) hyperfine interaction in perovskite hosts. The isotropic part of the F2 hyperfine interaction is calculated using the model of Davies and improved fluorine-ion wave functions. The results are compared with both the present experiment and recently published data for ${\mathrm{V}}^{2+}$, ${\mathrm{Cr}}^{3+}$, ${\mathrm{Mn}}^{2+}$, and ${\mathrm{Cr}}^{1+}$ in KMg${\mathrm{F}}_{3}$. This comparison indicates that the overlap model by itself is inadequate and that covalency effects are of comparable importance for the F2 ions. In addition to the F2 results for ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$, we report ESR and precise ENDOR measurements of the nearest-neighbor fluorine, the $^{55}\mathrm{Mn}$, and the $^{59}\mathrm{Co}$ hyperfine interactions. The modified nuclear $g$ factor found for $^{59}\mathrm{Co}^{2+}$ is shown to be consistent with theory.