Distortion Of Octahedron Research Articles

Properties of Undoped and Ca‐Doped LaMnO3 − Non‐Stoichiometry and Defect Structure

AbstractOxygen‐excessive LaMnO3+y samples with 0.02 ⩽ y ⩽ 0.26 have been prepared by thermal decomposition in air at 450 °C of a precursor obtained from an aqueous solution of lanthanum nitrate and manganese acetate (atomic ratio La/Mn = 1). The obtained solid is annealed at different temperatures from 700 to 1000 °C and under three oxygen pressures: 1.00, 0.21, and 10−4 atm. Samples with 0.14 ⩽ y ⩽ 0.26 are simple‐phased rhombohedral perovskites, the distortion of which decreases as the oxygen content (and Mn4+ content) increases. Samples with 0.02 ⩽ y ⩽ 0.09, obtained under low oxygen pressure (10−4 atm), have orthorhombic symmetry. In this case, the order of lattice parameters c/√2 < a < b, characteristic of O′‐perovskites, is a consequence of the Jahn−Teller distortion of the oxide octahedron around the d4 Mn4+ cation. The volume (Vf) per LaMnO3+y formula unit decreases regularly with increasing oxygen content (i.e., oxidation). Study has shown that the oxygen content is best described in terms of La and Mn vacancies, rather than of interstitial anions. Density measurements confirm the cation vacancy model involving equal amounts in La and Mn sub‐lattices. The infrared spectra of LaMnO3+y samples have been interpreted in connection with the structures of these solids. Correlations have been established between the spectral evolution and the crystal distortion of these compounds. La1−xCaxMnO3±y oxides with 0 < x ⩽ 1 have been synthesised by the same route, at 700 °C and under an oxygen pressure of 0.21 atm. It is shown that a negative correlation exists between the calcium content x and the degree of non‐stoichiometry: at low Ca content, the oxygen content is higher. The ability to form over‐stoichiometric compounds in air decreases with increasing Ca content and mostly disappears at x ⩾ 0.7. Samples with 0.1 ⩽ x ⩽ 0.4 are simple‐phased rhombohedral perovskites with constant Mn3+/Mn4+ ratios. Samples with 0.5 ⩽ x ⩽ 1 have orthorhombic symmetry. In this case, a linear variation of the Mn4+ content as a function of the Ca content is observed. The defect structure of these compounds is described in terms of La and Mn vacancies when 0 < x ⩽ 0.7 (0 ⩽ y ⩽ 0.19) and anion vacancies when 0.8 ⩽ x ⩽ 1 (−0.04 ⩽ y < 0). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Electron Spin Resonance of Cr3+ in K1–xTlxAl(SO4)2 · 12 H2O

The electron spin resonance (ESR) of Cr3+ in K1−xTlxAl(SO4)2·12H2O has been studied at 300 K and 9.45 GHz. The ESR spectrum for x between 30 and 90% shows several chromium complexes. The variation of the zero-field splitting parameter D suggests that monovalent ions play an important role in the trigonal distortion of the water octahedron around the trivalent metal ion in alums. - PASC: 76.30 F.

Hydrogen deficiency in Ti-rich biotite from anatectic metapelites (El Joyazo, SE Spain): Crystal-chemical aspects and implications for high-temperature petrogenesis

Titanium-rich biotites from graphite-bearing metapelitic xenoliths, equilibrated at ca. 850 °C and 7 kbar in the presence of a granitic melt, have been studied through complete chemical analysis and single-crystal XRD refinements. The chemical study combines EMP analyses, hydrogen determination by both SIMS and C-H-N elemental analysis, and Mössbauer spectroscopy. Biotites in the analyzed xenoliths have TiO 2 contents ranging from 4.5 to 4.9 wt% and an XFe of 0.67. Their F and Cl contents are negligible, and Fe 3+ /Fetot ranges from 10 to 16%. The H 2 O content of the biotites ranges from 2.4 to 2.8 wt%, and a small fraction of H is accommodated in the lattice as NH 4 . Based on these full chemical analyses, the calculated OH occupancy is 1.26 to 1.30 groups per formula unit, more than one third less than the stoichiometric value. The entrance of Ti in the octahedral site of biotite is consistent with the Ti-oxy exchange, whereas Ti-Tschermak or Ti-vacancy substitutions play a very minor role. The Fe 3+ -oxy exchange cannot account for the observed OH deficiency. From single-crystal XRD, biotites belong to the 1M polytype and contain variable amounts of stacking faults. The c cell parameter, K-O 4 and <K-O>outer distances provide an independent estimate of the OH content, which agrees with SIMS determinations. The linear relationship between VITi 4+ and the bond-length distortion of the cis-M2 octahedron reveals the partitioning of Ti 4+ in M2, and the Ti 4+ off-center shift toward O 4 supports the occurrence of the Ti-oxy exchange. The ordering of Ti 4+ over two non-equivalent M2 sites, which would be favored energetically, is in agreement with the evidence for a third octahedral site suggested by Mössbauer spectroscopy. The biotite dehydrogenation combined with the partitioning of Ti 4+ in M2 and the low thermal expansion of Ti 4+ - containing octahedra, are the keys to understanding the thermal stabilization of Ti substitution in biotites.

Elaboration and characterization of glassy cobalt phosphomolybdates

Selecting two series of P 2O 5-based oxide glasses containing at the same time the MoO 3 and CoO oxides, an investigation of structural properties has been undertaken by infrared (IR) and electron paramagnetic resonance (EPR) spectroscopies. The structure of glasses inside the CoO–MoO 3–P 2O 5 system may be considered as made of several types of molybdenum and phosphorous structural groups, the ratios of which depend on composition. The EPR spectra of Mo(V) ions are characterized by the large anisotropy of g- and A-tensors along the local z axis. The values of the covalency parameters k ‖ and k ⊥ determined for some glasses are correlated to the axial distortion of (Mo VO 6) octahedron along the z axis. EPR spectra carried out at lower temperatures show additional broad line near g≈4 and a narrow one at g≈2. The former is attributed to the HS form of cobalt(II) in octahedral site. The latter is associated with LS electron configuration of Co 2+ ions and/or Mo 5+ centers. The existence of Co 2+ in octahedral environment in both HS and LS forms is interpreted in term of a spin-crossover for Co(II) in these glasses.

Tetragonal structure of CaSiO3 perovskite above 20 GPa

High‐resolution synchrotron X‐ray diffraction measurements were performed at 20–46 GPa on CaSiO3 perovskite (Ca‐pv) synthesized in a laser heated diamond cell. After heating, we observed peak splittings for the cubic 200 and 211 diffraction lines, attributable to a tetragonal distortion of Ca‐pv. The bulk modulus of the tetragonal phase is 255 ± 5 GPa for K0T′ = 4. The observed intensities and d‐spacings of the split lines indicate that the c‐axis is shorter than the a‐axes (c/a = 0.993 − 0.996). This is compatible with either SiO6 octahedron rotation around the a‐axes or SiO6 octahedron distortion (shorter Si‐O bonds along the c‐axis than the a‐axes), provided that the observed phase has a tetragonal unit cell. These structural distortions in Ca‐pv can affect its physical properties and capacity for large cations.

Raman phonons as a probe of disorder, fluctuations, and local structure in doped and undoped orthorhombic and rhombohedral manganites

We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular, we study $R{\mathrm{MnO}}_{3}$ $(R=\mathrm{La},$ Pr, Nd, Tb, Ho, Er, Y, and Ca) and the different phases of Ca- or Sr-doped $R{\mathrm{MnO}}_{3}$ compounds as well as cation deficient $R{\mathrm{MnO}}_{3}.$ The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of ${\mathrm{MnO}}_{6}$ octahedra and independently of the average structure obtained by diffraction techniques. One type of spectra corresponds to the orthorhombic $\mathrm{Pbnm}$ space group for octahedra with cooperative or dynamic Jahn-Teller distortions, with stretching modes as the main features and whose frequencies correlate to Mn-O distances. The other spectrum is associated to regular but tilted octahedra whose modes can be described in the rhombohedral $R\overline{3}c$ structure, where only bending and tilt modes are observed. The main peaks of compounds with regular ${\mathrm{MnO}}_{6}$ octahedra, such as ${\mathrm{CaMnO}}_{3},$ highly Ca-doped ${\mathrm{LaMnO}}_{3},$ or the metallic phases of Ca- or Sr-doped ${\mathrm{LaMnO}}_{3},$ are bending and tilt ${\mathrm{MnO}}_{6}$ octahedra modes which correlate to $\mathrm{R}\ensuremath{-}\mathrm{O}(1)$ bonds and Mn-O-Mn angles, respectively. In low and optimally doped manganites, the intensity and width of the broad bands are related to the amplitude of the dynamic fluctuations produced by polaron hopping in the paramagnetic insulating regime. The activation energy, which is proportional to the polaron binding energy, is the measure of this amplitude. This study permits to detect and confirm the coexistence, in several compounds, of a paramagnetic matrix with lattice polaron together with regions without dynamic or static octahedron distortions, identical to the ferromagnetic metallic phase. We show that Raman spectroscopy is an excellent tool to obtain information on the local structure of the different microphases or macrophases present simultaneously in many manganites.

Local electronic and geometrical structure ofLaNi1−xMnxO3+δperovskites determined by x-ray-absorption spectroscopy

Local and electronic structures of Ni and Mn atoms in the ${\mathrm{LaNi}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{3+\ensuremath{\delta}}$ series were studied by means of x-ray-absorption spectroscopy. The Ni and Mn K edges and Mn ${L}_{3}{/L}_{2}$ edges were analyzed. The local structure, determined by extended x-ray-absorption fine-structure spectroscopy, shows a distorted ${\mathrm{MnO}}_{6}$ octahedron in ${\mathrm{LaMnO}}_{3}.$ This distortion decreases when Ni replaces Mn in the unit cell. In samples with the same Mn/Ni ratio, the octahedron distortion is smaller in oxidized samples. This result shows that the holes induced by oxidation belong mainly to the Mn sublattice. In the ${\mathrm{LaNi}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{3+\ensuremath{\delta}}$ series, a contraction of the ${\mathrm{MnO}}_{6}$ octahedron is coupled to an expansion of the ${\mathrm{NiO}}_{6}$ octahedron. This result is well correlated with the changes in the oxidation states deduced from x-ray-absorption near-edge spectroscopy (XANES). The Mn valence state continuously changes from the formal 3+ state in ${\mathrm{LaMnO}}_{3}$ to nearly a 4+ state in ${\mathrm{LaNi}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_{3}.$ The Ni valence state instead shifts from ${\mathrm{Ni}}^{3+}$ in ${\mathrm{LaNiO}}_{3}$ to ${\mathrm{Ni}}^{2+}$ in ${\mathrm{LaNi}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_{3}.$ Therefore, ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ are present in ${\mathrm{LaNi}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_{3+\ensuremath{\delta}}.$ Moreover, the detailed analysis of the XANES spectra points to an important mixing between the $3d$ orbitals of both cations and the $2p$ orbitals of oxygen atoms.

Lattice-spin mechanism in colossal magnetoresistive manganites.

We present a single-orbital double-exchange model, coupled with cooperative phonons (the so called breathing modes of the oxygen octahedra in manganites). The model is studied with Monte Carlo simulations. For a finite range of doping and coupling constants, a first-order metal-insulator phase transition is found, which coincides with the paramagnetic-ferromagnetic phase transition. The insulating state is due to the self-trapping of every carrier within an oxygen octahedron distortion.

POLYHEDRON DISTORTIONS IN TOURMALINE

Distortion parameters [� , � 2 , DI(Y‐O), DI(O‐ Y‐O), DI(O‐O)] have been calculated for cation polyhedra in tourmalines of different chemical compositions. Tourmalines with greater amounts of small, highly charged ions in the YO6 octahedron display greater bond-length distortion in Y. The size and charge of the occupants at the Y site have been included in an empirically determined formula intended to evaluate the bond-length distortion of the YO6 octahedron. This equation demonstrates that the bond-length distortion in the YO6 octahedron is a function of the size and charge of the Y-site occupants, and is predictable from those occupants (r = 0.972 for observed versus calculated distortion). The distortion of the Z octahedron in a tourmaline is largely a function of the of that tourmaline [r = ‐0.985 for all tourmalines where O3 is occupied by 3 (OH) apfu], although the occupant of the O3 site also affects that distortion. The bond-length distortion of the TO4 tetrahedron is small, but examination of the angle distortion of the tetrahedron demonstrates a strong covariance with the charge at the X site (r = 0.892). Finally, distortions in the XO9 polyhedron were found to correlate with the charge of the occupants at the Y site and the fluorine content ( r = 0.881).

Disappearance of the Co magnetic moment in LaCoO3

In this contribution we discuss the origin of the intriguing non-magnetic ground state observed in LaCoO3. We show that the non-magnetic state of the Co3+ ion can be understood within the Quantum Atomistic Solid-State theory (QUASST) and is caused by the off-cubic distortion of the local crystal field related to the distortion of the local octahedron. In formation of the non-magnetic state the relatively small intra-atomic spin-orbit coupling is essentially important. We point out that in LaCoO3 exists the fine meV-scale electronic structure associated with the atomic-like discrete energy states.

Phonon Vibration and Conduction of La2 − xSrxCuO4 − δ and La1.85 − xSr0.15 + xCu1 − xCoxO4 − δ

Polycrystalline samples La2 − xSr x CuO4 − δ (0.06 ≤ x ≤ 0.5) and La1.85 − xSr0.15 + xCu1 − xCo x O4 − δ (0 ≤ x ≤ 1) were synthesized by a conventional solid state reaction method. The structure, phonon vibration, and conduction were investigated by means of XRD, infrared (IR) spectra, and resistance. It is found that the increase of itinerant hole carriers could mask the in-plane \({\text{Cu}}{\kern 1pt} - {\kern 1pt} {\text{O(1)}}\) stretching vibration mode (689 cm−1). The softening of the \({\text{Cu}}{\kern 1pt} - {\kern 1pt} {\text{O(1)}}\) phonon vibration mode gives an index of the weakening of \({\text{Cu}}{\kern 1pt} - {\kern 1pt} {\text{O}}\) hybridization. The distortion of CuO6 octahedron and the microstructural inhomogeneity induced by Co doping lead to the widening of the IR absorption peaks.

Explanation of the EPR g factors for Co 2+ impurities in trigonal Cd 2P 2S 6 crystal

By extending the theory of Abragam and Pryce, the formulas of EPR parameters g ∥ and g ⊥ for 3d 7 ion in trigonal octahedral crystals are established from a cluster approach. In these formulas, not only the configuration interaction effect, but also the covalency effect are considered and the parameters related to both effects and the trigonal distortion are determined from the optical spectra and the structural data of the crystal under study. Based on these formulas, the g ∥ and g ⊥ of Cd 2P 2S 6 : Co 2+ crystal are reasonably explained from its structural data. The relationship between the sign of Δ g (= g ∥− g ⊥) and the trigonal distortion (elongated or compressed) of ligand octahedron, which is opposite to that obtained in the previous paper, is given and the results are discussed.

Local Lattice Distortion near Co2+ in SrLaAlO4 Crystal

Abstract The local tetragonal distortion in the vicinity of substitutional Co2+ impurities in SrLaAlO4 crystal is studied by fitting the calculated EPR parameters g\\, g⟂, A\\ and A⟂ to the observed values. The result shows that the local distortion is mainly due to the elongation of metal-ligand distance R± perpendicular to the C4 axis. This point is consistent with the expectation based on the consideration of ionic radius sum. This smaller tetragonal distortion of the oxygen octahedron surrounding the Co2+ impurity than that in the host crystal is supported by the result obtained in Cr3+-doped SrLaAlO4 crystal. -P a cs: 61.70Rj; 76.30Fc; 71.70Ch

Geometric local structure at the Mn site in charge-ordered mixed valence manganites.

An extensive study of Mn K-edge absorption sepctroscopy has been carried out in La(1-x)Ca(x)MnO3 and Tb(1-x)Ca(x)MnO3 (x=0, 0.33, 0.5, 0.67, 1) series as a function of temperature. The EXAFS analysis for the charge ordering compounds (x > or = 0.5) shows the presence of a static distortion of the MnO6 octahedron in the whole temperature range. However, this local distortion is lower than the distortion found for Jahn-Teller compounds. Magnetoresistive compounds (x=0.33) show a regular octahedron in the low temperature metallic phase. Instead, no changes are found for the first coordination shell (Mn-O) across the charge ordering transition while Mn-Mn distribution shows minor variations with the temperature. The structural analysis of the second coordination shell suggests that the Mn-O-Mn angle is the most relevant parameter to describe the electric and magnetic behaviour of these compounds.

In situ XAFS study on cathode materials for lithium-ion batteries.

Ni and Co K-edge X-ray absorption spectra of LiNi0.8Co0.2O2 have been collected using in situ coin cells. To investigate the electronic and structural changes accompanied by the capacity fading during electrochemical cycling and keeping batteries at high temperatures, the cells with different cycling states and keeping conditions (temperature, time) were prepared. Upon charging the cell, the Ni and Co K absorption edge shifted towards higher energy, and the good correlation between the range of chemical shifts upon charging and the capacity of the cell was observed. From quantitative analysis of EXAFS data, it was revealed that the capacity fading is closely related to the Jahn-Teller distortion of the NiO6 octahedron.

Electronic structure of theR2Ti2O7(R=Sm–Er, Yb, Lu) oxides

We present the results of the electronic structure investigation for the compounds ${R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ $(R=\mathrm{Sm}$--Er, Yb, Lu) with the pyrochlore-type crystal structure. The methods of x-ray photoelectron and emission spectroscopies, as well as the first-principles band-structure calculations have been employed. A good agreement between theoretical and experimental results was observed. We found considerable influence of the R $5p$ - O $2s$ interaction on the formation of the subvalent bands that has to be taken into account while interpreting experimental data. Estimated values of the optical gaps are closely related to the degree of ${\mathrm{TiO}}_{6}$ octahedron distortion.

G Factors and the tetragonal distortion of ligand octahedron for Co 2+ in Rb 2MgF 4 crystal

The calculation formulas of g-factors g // and g ⊥ for 3d 7 ion in tetragonal octahedral crystals are established from a cluster approach. In these formulas, the parameters related to covalency effect, configuration interaction and low-symmetry crystal field can be determined from the optical spectra and the structural data of the studied system. Based on these formulas, the structural parameters of ligand octahedra of Co 2+ in Rb 2MgF 4 crystal are obtained by fitting the calculated g // and g ⊥ to the observed values. The result suggests that the CoF 6 (and hence MgF 6) octahedra in Rb 2MgF 4:Co 2+ are tetragonal compressed. The relationship between the sign of Δ g(= g ⊥− g //) and the sign of distortion (elongated or compressed) of ligand octahedron and the causes of the mistakes of octahedron distortion for Rb 2MgF 4:Co 2+ in the previous papers are discussed.

Explanation of the g-factors and Hyperfine Structure Constants of Co2+ in Tetragonal K2ZnF4

Abstract The formulas of the g-factors gII , g┴ and the hyperfine structure constants AII.A┴ for 3d7 ions in tetragonal octahedral crystals are established from a cluster approach. Differing from previous formulas, in these formulas the role of configuration interaction (CI)-and covalency (CO)-effects is considered, and the parameters related to both effects are obtained from the optical spectra and the structural parameters of the studied crystal. From these formulas, the EPR parameters gi and Ai for K2 ZnF4 :Co2+ are calculated. The results show good agreement with the observed values. The contributions to the EPR parameters gi and Ai, from the CI and CO effects, and the relationship between the sign of Δg(=g┴ - gII) and the tetragonal distortion (elongated or compressed) of the ligand octahedron are discussed

Re-Interpretation of EPR Spin-Hamiltonian Parameters bm2 for Gd3+-O2- Center in the Tetragonal Phase of Perovskite-Type Compound Rb1-xKxCaF3 with x = 0.05

The spin-Hamiltonian parameters b(2)(2), b(2)(-1) and b(2)(0) for Gd3+-O2- center in the tetragonal phase of Rb0.95K0.05CaF3 perovskite-type compound are studied by the superposition model which considers the (GdOF5)(4-) octahedron distortion related to the elongation or release of the metal-ligand bond lengths in the octahedron. The parameters b(2)(m) can be reasonably explained, and information for the local phase transition behaviour is obtained.

XANES Study of Cu2+-Binding Sites in Aquatic Humic Substances

The structure and composition of the inner shell of copper (II) complexes with aquatic humic substances (HS) were studied by X-ray absorption near-edge structure (XANES) spectroscopy. The interpretation of the XANES data was based on FEFF8 code real-space multiple-scattering calculations for the prototype [Cu(H2O)6]2+ complex. These calculations reproduced the tetragonal distortion of the CuO6 octahedron representing the inner shell. They also demonstrated the polarization nature of this effect's manifestations (“splitting” of the derivative) in XANES. The extension of this interpretation to the Cu−HS system confirmed suppression of the tetragonal distortion of the inner shell in these species established by independent EXAFS experiments. XANES measurements for Cu/C ratios varying from 0.0005 to 0.03 demonstrated the nonuniform nature of copper binding sites. Experiments with Cu−ethylenediamine and Cu−glycine model systems and FEFF8 calculations indicated that for low copper concentrations (Cu/C < 0.005) ...