Tetrahedral Distortion Research Articles

Structure of β-AgGaO2; ternary I–III–VI2 oxide semiconductor with a wurtzite-derived structure

The structure of the wurtzite-derived β-AgGaO2 was refined by Rietveld analysis of high-resolution powder diffraction data obtained using synchrotron X-ray radiation. The space group of the crystal is Pna21 with lattice parameters of a0=5.56175Å, b0=7.14749Å, and c0=5.46875Å. The deviation of O–Ag–O and M–O–M bond angles from the regular tetrahedral angle of 109.5° was very large at ~8° and ~11°, respectively. The electronic structure of β-AgGaO2 is discussed based on its structure, and the indirect band gap of β-AgGaO2 was related to significant tetrahedral distortion. Although β-AgGaO2 decomposes into metallic silver and Ga2O3 at a high temperature in any atmosphere, β-AgGaO2 is stable up to 690°C under an O2 atmosphere. No direct transformation from the wurtzite-derived phase to a delafossite phase occurs in β-AgGaO2.

Ab-initio Prediction of Magnetoelectricity in Infinite-Layer CaFeO2and MgFeO2

Density functional based simulations are employed to explore magnetoelectric effects in iron-based oxides, showing a unique layered structure. We theoretically predict CaFeO2 to be a promising magnetoelectric, showing magnetically-controlled large electric polarization, possibly even above room temperature. The cross coupling between magnetic and dipolar degrees of freedom needs, as main ingredients, Fe-site spin-orbit coupling and a spin-dependent O p - Fe d hybridization, along with structural constraints related to the non-centrosymmetric point group and the peculiar geometry characterized by "flattened" FeO4 tetrahedrons. In order to enhance magnetoelectric effects, we performed a materials-design leading to a novel and optimized system, MgFeO2, where the larger O4 tetrahedral distortion leads to a stronger polarization.

Copper(II)‐Coordinated α‐Azophenols: Effect of the Metal‐Ion Geometry on Phenoxyl/Phenolate Oxidation Potential and Reactivity

AbstractTwo copper(II) complexes were synthesized from tridentate ligands involving a (Nquinoline,Nazo,Ophenol) donor set. The copper(II) ion is tetracoordinate with a chloride ion occupying the fourth position of the coordination sphere. Both X‐ray diffraction and electron paramagnetic resonance (EPR) spectroscopy reveal that the copper(II) ion geometry is square planar in 1. In contrast, significant tetrahedral distortions are observed in 2, as a result of the steric clash between the hydrogen atoms of the methyl substituent of the quinoline group and the chloride ion. Cyclic voltammetry curves of 1 and 2 in CH2Cl2 display a reversible oxidation wave at E1/2 = 0.59 V and 0.56 V versus ferrocenium/ferrocene, respectively, which was assigned to the phenoxyl/phenolate redox couple. Compounds 1+ and 2+ were generated and characterized by UV/Vis and EPR spectroscopy. Their reactivity with benzyl alcohol was investigated by kinetic measurements.

Crystal Chemistry and Surface Configurations of Two Iron-Bearing Trioctahedral Mica-1M Polytypes

AbstractThe crystal chemical features of the bulk and the uppermost (001) surface layers of freshly cleaved surfaces of two trioctahedral Fe-rich mica-1M (space group C2/m) polytypes, i.e. a tetraferriphlogopite from an alkaline-carbonatitic complex near Tapira, Belo Horizonte, Minas Gerais, Brazil, and an Fe2+-bearing phlogopite containing less tetrahedral Fe3+ from the Kovdor carbonatite-bearing, alkaline-ultrabasic complex, Kola Peninsula, Russia, are explored here. Mineral-surface effects were investigated by X-ray Photoelectron Spectroscopy (XPS) and compared to the bulk structure derived from single-crystal X-ray diffraction data. Based on microprobe analysis and the X-ray study, the chemical formulae are [XII](K0.99)[VI](Fe0.082+Fe0.153+Mg2.76Ti0.01)[IV](Fe0.823+Si3.18)O10.37F0.24(OH)1.39 and [XII](K0.94Na0.06)[VI](Fe0.172+Fe0.053+Mg2.75Mn0.01Ti0.05)[IV](Fe0.163+Al0.84Si3.00)O10.21F0.35(OH)1.44 for tetraferriphlogopite and Fe-bearing phlogopite, respectively. The tetrahedrally coordinated sites of the two minerals differ, where Fe-for-Si substitution is at 20.5% in tetra-ferriphlogopite and at 4% in Fe-bearing phlogopite.The bulk study showed that Fe3+ substitution increases the tetrahedral sheet thickness and the mean tetrahedral edge lengths in tetra-ferriphlogopite compared to Fe-bearing phlogopite. The tetrahedral rotation angle (α) changes remarkably from tetra-ferriphlogopite (α = 10.5°) to the Fe-bearing phlogopite (α = 8.5°), thus indicating a significantly greater initial lateral sheet misfit (leading to a greater tetrahedral ring distortion) between the tetrahedral and the octahedral sheets in the tetra-ferriphlogopite compared to Fe-bearing phlogopite. The Fe3+ substitution for Si and the differences in lateral dimensions of the tetrahedral and octahedral sheets affect the tetrahedral flattening angle (τ), with τ = 109.9° for tetraferriphlogopite and τ = 110.7° for Fe-bearing phlogopite.The binding energy (BE) of photoelectron peaks in XPS is dependent on the chemical state of atoms and on their local environment at the near surface. The Mg in both phlogopites is bonded to F, with the BE of Mg1s increasing as coordinated oxygen atoms are substituted by fluorine. For Fe-rich phlogopite (BE = 1306.8 eV), the binding energy is greater than for tetra-ferriphlogopite (BE = 1305.9 eV), and this is consistent with the bulk composition having greater F-for-OH substitution in Fe-rich phlogopite (F0.35vs. tetra-ferriphlogopite, F0.24 atoms per formula unit).

Preparation and Characterization of bis (4-methoxyanilinium) tetrachlorozincate

A new zinc complex of formula [C7H10NO]2 ZnCl4 has been prepared and characterized by X-ray diffraction, IR and UV-Visible spectroscopies. The complex crystallizes in the monoclinic space group P21/n with a minimal tetrahedral distortion of the ZnCl42- ion and with lattice parameters: a =12.054 (2) Å, b =7.129 (3) Å, c =23.480 (2) Å, β = 100.67 (2)°, V = 1983.03 (1) Å3 and Z = 4. The crystal structure was solved and refined to R = 0.080 and RW = 0.227 with 9611 independent reflections. It can be described by organic layers of p-anisidinium cations held together by C-H…O hydrogen bonds parallel to (010) plane, linked to the inorganic groups of ZnCl42- anions through N-H…Cl hydrogen bonds, electrostatic and Van Der Waals interactions, to form a three-dimensional network.

Rotational invariants of network former and modifier cations in silicate glasses

The structure of oxide glasses is described in terms of the local atomic environment of cations, and a key role is played by cation-oxygen coordination numbers (CN) and coordination polyhedra. There are no preferred axes in an isotropic material like a glass, so the description of coordination polyhedra must be rotationally invariant. Here a new analysis is presented using the second order rotational invariant Ql which are based on the spherical harmonic coefficients Clm of the coordination polyhedra. The Ql are related to crystal field strength parameters which are reported in the studies of rare earth luminescence. There are few previous studies of rotational invariants and they tend to focus on hard sphere and Lennard–Jones models, and to focus on Ql with l even. Here results are presented for Ql of Si, Na, Mg, Ca, Ba, and Eu cations in molecular dynamics models of silicate glasses including a 15,100 atom model of Eu-doped sodium silicate glass. For Si with CN=4 the Ql are very similar to those for tetrahedra, and variations in tetrahedral distortion are apparent in different glasses. For Na with CN=5 the Ql are similar to those for a random distribution, except for l=1 and 2 where the non-overlap of neighbouring atoms prevents a truly random distribution. The values of Ql for Mg cations with CN=4 and CN=5 show similarities to those for Al cations with CN=4 and Na cations with CN=5 respectively. The values of Ql for Ca and Ba cations with CN=6 differ from those for a random distribution for Ql with l≤4. For Eu with CN=6 the Ql are between those for octahedral and for a trigonal prism geometries. For all cations there are significant values for Ql with l odd.

Synthesis, characterization, biological activity of binuclear Co(II), Cu(II) and mononuclear Ni(II) complexes of bulky multi-dentate thiosemicarbazide

The chelation behavior of 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarbonyl) bis (N-ethylhydrazine-1-carbothioamide) (H6ETS)(1) towards Co2+, Ni2+and Cu2+ have been studied. The spectral data revealed that the ligand acts as a bi- and/or mono-negative multi-dentate. The isolated Ni(II) and Cu(II) complexes are square-planar while the Co(II) is tetrahedral. EPR spectrum of Cu(II) complex confirmed simulated an axial spin-Hamiltonian exhibiting a four-line pattern with nitrogen super-hyperfine couplings originating from imine hydrazinic nitrogen atoms and possess a significant amount of tetrahedral distortion leading to a pseudo-square-planar geometry with unpaired electron has dx2-y2 ground state. Also, the thermal behavior and kinetic parameters were determined. Furthermore, the title compounds were investigated for their antibacterial activity using inhibition zone diameter and for DNA degradation, superoxide-scavenging activity as well as hydroxyl radicals that generated by the oxidation of cytochrome c in L-ascorbic acid/CuSO4–cytochrome c system.

Single Crystal X‐ray Structure of Cu3TaSe4 and a Comparative Study of Cu3MX4 (M = V, Nb, Ta; X = S, Se, Te)

AbstractSingle crystals of Cu3TaSe4 were obtained by chemical vapor transport with iodine as transport agent. Cu3TaSe4 crystallizes in the sulvanite structure type with space group P$\bar{4}$3m and a = 5.6613(3) Å at room temperature. Structure refinements against single‐crystal X‐ray diffraction data result in a Cu–Se distance slightly longer than the Ta–Se distance, whereas previous Rietveld refinements obtained a Cu–Se distance shorter than the Ta–Se distance [G. E. Delgado et al., J. Alloys Comp. 2007, 439, 346]. The structural variations of sulvanite type compounds Cu3MX4 (M = V, Nb, Ta; X = S, Se, Te) are discussed in view of small distortions of CuX4 and MX4 tetrahedra and the atomic valences.

Deprotonation of a hydrophosphorane and synthesis of the Ni(II) metallophosphorane [NiCl{P(OC6H4N(CH3))2}(P(CH3)3)2

The hydrophosphorane [HP(OC6H4N(CH3))2] has been deprotonated using lithium tetramethylpiperidide to produce Li[P(OC6H4N(CH3))2] (1). Compound 1 reacts further with NiCl2(PMe3)2 to form the Ni(II) phosphoranide complex [NiCl{P(OC6H4N(CH3))2}(P(CH3)3)2] (2). The crystal structures of both 1 and 2 were determined. The structure of 1 is dimeric with bridging lithium cations and also features one THF solvent molecule coordinated to each lithium cation; the geometry about the lithium atom is tetrahedral. The structure of 2 displays tetrahedral distortion from an idealized square-planar geometry. Both 1 and 2 characterized by 1H and 31P NMR, with 13C NMR for 2 also. DFT calculations (B3LYP/6-311++G(2df,2p)) were also done on 2; both the NMR and DFT studies indicate π-donation from the metal to the pentacoordinate phosphorus.

Structural, optical and magnetic properties of sol–gel derived ZnO:Co diluted magnetic semiconductor nanocrystals: an EXAFS study

Structural, local structural, optical and magnetic properties of sol–gel derived Zn1−xCoxO (0 ≤ x ≤ 0.04) nanoparticles have been studied. The crystallite structure, size, and lattice strain have been estimated by X-ray diffraction (XRD) with Rietveld refinement and high-resolution transmission electron microscopy (HRTEM). The small linear increase in lattice parameter ‘a’ and decrease in lattice parameter ‘c’ have been observed which can be attributed to the small distortion of Zn tetrahedron. Extended X-ray Absorption Fine Structure (EXAFS) measurements show that Co-doping creates oxygen vacancies without causing any significant change in the host lattice structure. X-ray Absorption Near Edge Structure (XANES) measurements rule out the presence of metallic Co clusters in the samples. Raman spectroscopy has been employed to study the crystalline quality, structural disorder, and defects in the host lattice. The tetrahedral coordination of the oxygen ions surrounding the zinc ions and wurtzite structure has been studied by FTIR analysis. UV-Vis measurements have been used to study the effect of Co-doping on absorption spectra and hence on the band gap. The band gap initially decreases for low Co-concentration and increases with higher Co-concentration. The PL spectra show six peaks out of which the peak in the ultraviolet (UV) region has been assigned to the near band edge excitonic emission (NBE) and other peaks are related to different defect states. Room temperature ferromagnetism (weak) is observed and magnetization increases with increasing Co-concentration. The grain boundaries, oxygen vacancy and bound magnetic polarons (BMPs) jointly may be responsible for this room temperature ferromagnetism. Variation of resistivity with temperature shows that a thermally activated conduction (Arrhenius) mechanism is valid in the high temperature region whereas Mott's variable-range hopping (VRH) mechanism is valid in the low temperature region.

Local lattice distortions in oxygen deficient Mn-doped ZnO thin films, probed by electron paramagnetic resonance

The structure and functional properties of metal oxide films for device applications are largely affected by oxygen vacancies. While the macroscopic relationship between functionality and oxygen supply during growth is easy to access, the local influence of changing oxygen content on the incorporated metal atoms has been rarely investigated. As a model system, we use Mn as a local probe in hetero- and homoepitaxial ZnO thin films for electron paramagnetic resonance (EPR). Mn is expected to be incorporated as Mn2+ in the Zn-lattice site of ZnO films grown in a wide range of oxygen partial pressures. The zero field splitting (ZFS) parameter D depends on the crystallographic c/a ratio of ZnO : Mn lattice constants as it measures the trigonal distortion of oxygen tetrahedra at the Zn2+ site with respect to the Mn2+ site. The ZFS parameter D correlates linearly with displacement of Mn2+ ions along the c-axis in the MnO4 tetrahedra and the corresponding bond lengths between the Mn2+ ions and the axial oxygen ion.

A discrete neutral transition-metal citrate cubane with an M₄O₄ core; coordinative versatility of the [M(II)₄(citrate)₄]⁸⁻ fragment.

The neutral cluster [Mn(II)8(citrate)4(H2O)18] is formed by the [M4(citrate)4](8-) fragment, with an Mn4O4 cubane core, which bonds four peripheral aquomanganese units--two [Mn(H2O)4](2+) and two [Mn(H2O)5](2+)--through a total of six metal-ligand bonds, giving a discrete neutral compound. The compound presents a unique coordination mode in which the citrate cubane acts as a chelate to each of the two peripheral [Mn(H2O)4](2+) (tetra-aquo) units. A detailed analysis of the central and peripheral geometries is given in terms of the tetrahedral distortions of key structural features. A reversible dehydration-rehydration process has been observed in a polycrystalline sample of the complex, whose structure lacks pores or channels.

Structural controls on the anisotropy of tetrahedral frameworks: the example of monoclinic feldspars

The structural variation of monoclinic C2/m alkali feldspars has been analysed, based on 50 previously-determined structures and 10 new structure refinements including three determinations of the structure of an Or90 orthoclase at temperatures of 600 K, 900 K and 1075 K. The influence of temperature, composition and state of Al,Si order on the overall average tetrahedral bond lengths of these structures is statistically insignificant. Analysis of the structures in terms of the tilts of the tetrahedra shows that the tilts evolve uniformly with unit-cell volume irrespective of whether the volume is changed by temperature or exchange of the extra-framework cation. There is a small but systematic decrease in the distortion of the T1 tetrahedron with increasing unit-cell volume. Comparisons of the refined structures with the results of geometric modelling show that volume changes are driven by exchange of the extra-framework cation, or by an increase in its thermal vibrations upon heating. The extreme anisotropy of the changes in the unit-cell parameters of monoclinic feldspars is not due to anisotropic interaction of the extra-framework cation with the anions of the framework, but due to the tilting of the tetrahedra controlled in part by O-O interactions. The anisotropy and tilting is not significantly modified by either Al,Si ordering or the distortions of the tetrahedra provided that the latter remain constant. The monoclinic feldspars show a slightly reduced anisotropy of strains as the unit-cell volume increases as a result of a decrease in the angular distortion of the T1 tetrahedron. The general conclusion is drawn that the pattern of anisotropy of the elastic properties (thermal expansion, compressibility, elastic compliances) and the cell-parameter changes of a tetrahedral framework structure with changing extra-framework species are intrinsic to the topology of the framework. The relative insensitivity of the anisotropy of the strains induced by volume changes to distortions of the tetrahedra also means that framework models which incorporate regular tetrahedra can be safely used to predict anisotropy, provided that the tetrahedral distortions do not change. If, as in the monoclinic feldspars, such a model does not reproduce exactly the observed anisotropy of the real structures, then this immediately indicates that there is a significant change in the distortion of the framework tetrahedra.

Synthesis and Characterization of Nickel Compounds with Tetradentate Thiolate–Thioether Ligands as Precursors for [NiFe]–Hydrogenase Models

AbstractSynthesis and X‐ray structural analyses of three NiS4 compounds with aliphatic tetradentate thiolato/thioether ligands―[Ni(xch)], [Ni(pdtch)], and [Ni(pdtdm)], in which H2xch, H2pdtch, and H2pdtdm are α,α′‐bis(trans‐2‐mercapto‐1‐thiacyclohexyl)‐o‐xylene, 1,3‐bis(trans‐2‐mercapto‐1‐thiacyclohexyl)propane, and 1,9‐dimercapto‐3,7‐dithia‐2,2,8,8‐tetramethylnonane―are reported and further investigated by NMR spectroscopy, electronic absorption spectroscopy, and cyclic voltammetry. The molecular and electronic structures of these nickel complexes are described. All complexes are square planar and show only a small tetrahedral distortion, consistent with a low‐spin nickel(II) center (S = 0). Cyclic voltammetry data reveal that for all compounds both one‐electron oxidation and reduction are irreversible, which is explained by means of DFT calculations. The presented complexes are discussed with regard to their suitability as precursors for dinuclear [NiFe]–hydrogenase model compounds.

Mössbauer study of new vanadate glass with large charge-discharge capacity

Charge-discharge capacity and cyclicity of lithium ion battery (LIB) was evaluated in which 15Li2O·10Fe2O3·xSnO2·5P2O5·(70–x)V2O5 glass (x = 0 and 20 in mol%, abbreviated as xLFSPV) was used as a cathode. A local structure of xLFSPV glass before and after charging was investigated by 57Fe- and 119Sn-Mossbauer spectroscopies. 57Fe-Mossbauer spectrum of xLFSPV glass with ‘x’ of 20 was composed of a doublet with isomer shift (δ) of 0.35±0.02 mm s − 1 and quadrupole splitting (Δ) of 0.88±0.03 mm s − 1 due to distorted FeIIIO4 tetrahedra. 119Sn-Mossbauer spectrum of this glass consisted of a doublet with δ of 0.08±0.01 and Δ of 0.52±0.01 mms − 1 due to distorted SnVIO6 octahedra. After discharging the battery from 4.5 to 1.0 V, larger δ of 0.40±0.03 mm s − 1 and Δ of 0.94±0.04 mm s − 1 were obtained, indicating that both iconicity of Fe-O bonds and local distortion of FeIIIO4 tetrahedra were increased. On the contrary, identical δ of 0.09±0.01 mm s − 1 and Δ of 0.50±0.01 mm s − 1 were observed in the 119Sn-Mossbauer spectrum of 20LFSPV glass after the discharge, indicating that chemical environment of SnIVO6 octahedra was not affected after the discharge. Charge-discharge curve of LIB containing 20LFSPV glass as a cathode active material recorded under the current density of 8.3 mA g − 1 (0.011 mA cm − 2) between 1.0 and 4.5 V showed a large initial charge capacity of 431.1 mAh g − 1 and discharge capacity of 382.3 mAh g − 1, respectively. These results indicate that 20LFSPV glass could be a new cathode active material for LIB.

Enhancement of electrical conductivity and chemical durability of 20R2O•10Fe2O3•xWO3•(70 − x)V2O5 glass (R = Na, K) caused by structural relaxation

A relationship between local structure, thermal endurance, chemical durability and electrical conductivity of alkaline iron tungsten vanadate glass, 20R2O•10Fe2O3•xWO3•(70−x)V2O5, R=Na and K, x=0–50mol.%, abbreviated as RFWV was investigated by 57Fe-Mössbauer spectroscopy, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), differential thermal analysis (DTA), leaching test using 20vol.% HCl and dc-probe method. Mössbauer spectra of 20Na2O•10Fe2O3•xWO3•(70−x)V2O5 glass (NFWV) consisted of one paramagnetic doublet with a stable isomer shift (δ) of 0.40mms−1 and an increasing quadrupole splitting (Δ) from 0.68 to 0.82mms−1 with the increase of “x” from 0 to 50mol.%, indicating that local distortion of FeIIIO4 tetrahedra increased with WO3 content. After isothermal annealing at 500°C for 100min, Δ decreased to 0.62, 0.59, 0.67 and 0.67mms−1 for NFWV glass with “x” of 0, 10, 20 and 30, showing that local distortion of FeIIIO4 tetrahedra was reduced due to the structural relaxation. Original NFWV glass exhibited a slightly decreasing electrical conductivity (σ) from 2.9×10−6 to 5.5×10−8Scm−1 with the increase of “x” from 0 to 50. The isothermal annealing of NFWV glass caused a remarkable increase in σ of 4.5×10−3, 6.2×10−4 and 9.8×10−4Scm−1 with “x” of 0, 10 and 20, all of which showed a small activation energy for electron hopping (WH) of 0.21, 0.20 and 0.15eV, respectively. On the other hand, increases in glass transition temperature (Tg), crystallization temperature (Tc) and activation energy for crystallization (Ea) were observed for NFWV glass with WO3 content, proving that the tungsten ion contributes to enhancement of chemical bond strength of NFWV glass. Similar results of an increase in σ and a decrease in dissolution rate due to annealing were observed for KFWV glass. It is concluded that structure, physical and chemical properties of RFWV glass can be controlled by introduction of WO3.

Determination of formal redox potentials in aqueous solution of copper(II) complexes with ligands having nitrogen and oxygen donor atoms and comparison with their EPR and UV–Vis spectral features

Formal redox potentials in aqueous solution were determined for copper(II) complexes with ligands having oxygen and nitrogen as donor atoms. All the chosen copper(II) complexes have well-known stereochemistries (pseudo-octahedral, square planar, square-based pyramidal, trigonal bipyramidal or tetrahedral) as witnessed by their reported spectroscopic, EPR and UV–visible (UV–Vis) features, so that a rough correlation between the measured redox potential and the typical geometrical arrangement of the copper(II) complex could be established. Negative values have been obtained for copper(II) complexes in tetragonally elongated pseudo-octahedral geometries, when measured against Ag/AgCl reference electrode. Copper(II) complexes in tetrahedral environments (or flattened tetrahedral geometries) show positive redox potential values. There is a region, always in the field of negative redox potentials which groups the copper(II) complexes exhibiting square-based pyramidal arrangements. Therefore, it is suggested that a measurement of the formal redox potential could be of great help, when some ambiguities might appear in the interpretation of spectroscopic (EPR and UV–Vis) data. Unfortunately, when the comparison is made between copper(II) complexes in square-based pyramidal geometries and those in square planar environments (or a pseudo-octahedral) a little perturbed by an equatorial tetrahedral distortion, their redox potentials could fall in the same intermediate region. In this case spectroscopic data have to be handled with great care in order to have an answer about a copper complex geometrical characteristics.

Ab Initio Calculations of Possible γ-Gauche Effects in the 13C-NMR for Methine and Carbonyl Carbons in Precise Polyethylene Acrylic Acid Copolymers

The impacts of local polymer chain conformations on the methine and carbonyl 13C-NMR chemical shifts for polyethylene acrylic acid p(E-AA) copolymers were predicted using ab initio methods. Using small molecular cluster models, the magnitude and sign of the γ-gauche torsional angle effect, along with the impact of local tetrahedral structure distortions near the carbonyl group, on the 13C-NMR chemical shifts were determined. These 13C-NMR chemical shift variations were compared to the experimental trends observed for precise p(E-AA) copolymers as a function acid group spacing and degree of zinc-neutralization in the corresponding p(E-AA) ionomers. These ab initio calculations address the future ability of 13C-NMR chemical shift variations to provide information about the local chain conformations in p(E-AA) copolymer materials.

Molecular Dynamics in Paraelectric Phase of KH2PO4Crystals Studied by Single Crystal NMR and MAS NMR

The temperature dependences of the NMR spectrum and the spin-lattice relaxation times in <TEX>$KH_2PO_4$</TEX> were investigated via single-crystal NMR and MAS NMR. The stretched-exponential relaxation that occurred because of the distribution of correlation times was indicative of the degree of the distribution of the double-well potential on the hydrogen bond. The behaviors responsible for the strong temperature dependences of the <TEX>$^1H$</TEX> and <TEX>$^{31}P$</TEX> spin-lattice relaxation times in the rotating frame <TEX>$T_{1{\rho}}$</TEX> in <TEX>$KH_2PO_4$</TEX> are likely related to the reorientational motion of the hydrogen-bond geometry and the <TEX>$PO_4$</TEX> tetrahedral distortion.

Impact of Mn3+ upon Structure and Magnetism of the Perovskite Derivative Pb2–xBaxFeMnO5 (x ∼ 0.7)

On the basis of the Mn(3+) for Fe(3+) substitution in Pb(2-x)Ba(x)Fe2O5, a novel oxide Pb1.3Ba0.7MnFeO5 has been synthesized at normal pressure. Though it belongs to the same structural family, the mixed "MnFe" oxide exhibits a very different structural distortion of its framework compared to the pure "Fe2" oxide, due to the Jahn-Teller effect of Mn(3+). Combined neutron diffraction, high resolution electron microscopy/high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) investigations allow the origin of this difference to be determined. Here we show that the MO6 octahedra of the double perovskite layers in the "MnFe" structure exhibit a strong tetragonal pyramidal distortion "5 + 1", whereas the "Fe2" structure shows a tetrahedral distortion "4 + 2" of the FeO6 octahedra. Similarly, the MO5 polyhedra of the "MnFe" structure tend toward a tetragonal pyramid, whereas the FeO5 polyhedra of the "Fe2" structure are closer to a trigonal bipyramid. Differently from the oxide Pb(2-x)Ba(x)Fe2O5, which is antiferromagnetic, the oxide Pb1.3Ba0.7MnFeO5 exhibits a spin glass behavior with Tg ∼ 50 K in agreement with the disordered distribution of the Mn(3+) and Fe(3+) species.