Distortion Of Crystal Structure Research Articles

Electronic correlations and crystal structure distortions in BaBiO3

BaBiO3 is a material where Bi4+ ions with half-filled 6s-states form an alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered insulator. The charge ordering is accompanied by breathing distortion of the BiO6 octahedra (extension and contraction of the Bi–O bond lengths). Standard density functional theory (DFT) calculations fail to obtain the crystal structure instability caused by the pure breathing distortions. Combining effects of the breathing distortions and tilting of the BiO6 octahedra allows DFT to reproduce qualitatively an experimentally observed insulator with monoclinic crystal structure but strongly underestimates the breathing distortion parameter and energy gap values. In the present work we reexamine the BaBiO3 problem within the GGA + U method using a Wannier function basis set for the Bi 6s-band. Due to the high oxidation state of bismuth in this material, the Bi 6s-symmetry Wannier function is predominantly extended spatially on surrounding oxygen ions and hence differs strongly from a pure atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with exclusion of high oxidation state compounds) where the major part of the d-band Wannier function is concentrated on the metal ion and a pure atomic d-orbital can serve as a good approximation. The GGA + U calculation results agree well with experimental data, in particular with experimental crystal structure parameters and energy gap values. Moreover, the GGA + U method allows one to reproduce the crystal structure instability due to the pure breathing distortions without octahedra tilting.

Uniaxial-Pressure Control of Magnetic Phase Transitions in a Frustrated Magnet CuFe1-xGaxO2 (x=0, 0.018)

We have investigated effects of applied uniaxial pressure ( p ) on magnetic phase transitions in a triangular lattice antiferromagnet CuFeO 2 (CFO) and a slightly Ga-substituted compound CuFe 1- x Ga x O 2 (CFGO) with x =0.018. We have performed neutron diffraction, spherical neutron polarimetry, magnetic susceptibility and pyroelectric measurements under p up to 100 MPa applied on the [1\bar10] surfaces of the single crystal samples. As a result, we have revealed that in both of CFO and CFGO ( x =0.018), the application of p significantly increases the transition temperature from the paramagnetic phase to a collinear incommensurate antiferromagnetic phase, at which these systems exhibit a crystal structural distortion associated with the magnetic ordering. This suggests that the application of p beaks equilateral symmetry of the triangular lattice above the original transition temperature, and partially relieves geometrical spin frustration, which generally suppresses long-range magnetic orderings even a...

Cd4Cu7As, The First Representative of a Fully Ordered, Orthorhombically Distorted MgCu2Laves Phase

The ternary Laves phase Cd(4)Cu(7)As is the first intermetallic compound in the system Cu-Cd-As and a representative of a new substitution variant for Laves phases. It crystallizes orthorhombically in the space group Pnnm (No. 58) with lattice parameters a = 9.8833(7) Å; b = 7.1251(3) Å; c = 5.0895(4) Å. All sites are fully occupied within the standard deviations. The structure can be described as typical Laves phase, where Cu and As are forming vertex-linked tetrahedra and Cd adopts the structure motive of a distorted diamond network. Cd(4)Cu(7)As was prepared from stoichiometric mixtures of the elements in a solid state reaction at 1000 °C. Magnetic measurements are showing a Pauli paramagnetic behavior. During our systematical investigations within the ternary phase triangle Cd-Cu-As the cubic C15-type Laves phase Cd(4)Cu(6.9(1))As(1.1(1)) was structurally characterized. It crystallizes cubic in the space group Fd3m with lattice parameter a = 7.0779(8) Å. Typically for quasi-binary Laves phases Cu and As are both occupying the 16c site. Chemical bonding, charge transfer and atomic properties of Cd(4)Cu(7)As were analyzed by band structure, ELF, and AIM calculations. On the basis of the general formula for Laves phases AB(2), Cd is slightly positively charged forming the A substructure, whereas Cu and As represent the negatively charged B substructure in both cases. The crystal structure distortion is thus related to local effects caused by Arsenic that exhibits a larger atomic volume (18 Å(3) compared to 13 Å(3) for Cu) and higher ionicity in bonding.

La1 − xBaxCo0.2Fe0.8O3 − δ perovskites for application in intermediate temperature SOFCs

Physicochemical properties of La1−xBaxCo0.2Fe0.8O3−δ (LBCF) oxides are presented in aspect of their possible application as cathode materials in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). The obtained results show that the increase of concentration of Ba2+ leads to an increase of unit cell volume and to a decrease of crystal structure distortion, which is accompanied by an increase of the level of oxygen nonstoichiometry in the studied compounds. For samples with x≥0.6 cubic structure was observed, which is particularly worth noting for BaCo0.2Fe0.8O3−δ composition, as both BaCoO3−δ and BaFeO3−δ often, but depending on oxygen nonstoichiometry δ, adopt hexagonal-type structure. Electrical conductivity and Seebeck coefficient data of the materials are quite similar, comparing to La1−xSrxCo1−yFeyO3−δ oxides. For samples with barium content ≥0.6, at high temperatures, a clearly visible maximum on electrical conductivity dependence on temperature can be seen, which may be related to a significant increase of the oxygen nonstoichiometry. The highest conductivity in the intermediate temperature range (600–800°C) was observed for La0.6Ba0.4Co0.2Fe0.8O3−δ composition, however obtained values are significantly lower, comparing to strontium analog: La0.6Sr0.4Co0.2Fe0.8O3−δ. La0.6Ba0.4Co0.2Fe0.8O3−δ oxide was used as the cathode material in custom-made button-type IT-SOFCs, which properties were evaluated in 600–800°C temperature range. Additionally, chemical stability of that material in relation to Ce0.8Gd0.2O1.9 electrolyte was studied.

Neutron diffraction research of local nanodistortions of crystal lattice in diluted magnetic semiconductors Zn1‐xCox S and Zn1‐xCox Se (x =0.01)

AbstractLocal distortions of the sphalerite crystal structure of Zn1‐xCox S, Zn1‐xCox Se (x =0.01) semiconductor crystals were studied for the first time using thermal neutron diffraction at 300 and 78 K. The diffraction patterns of the crystals were revealed to contain diffuse scattering regions concentrated in the vicinity of the strong Bragg reflections. It was detected for both crystals that the intensity of the neutron diffuse scattering essentially decreases with cooling. Some arguments are given to explain the revealed properties of the diffuse scattering for these crystals in terms of a formation of the vibronic effect leading to the emergence of local nanosized distorted regions. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Sol–gel hydrothermal synthesis of strontium hexaferrite nanoparticles and the relation between their crystal structure and high coercivity properties

Hard magnetic strontium hexaferrite SrFe 12 O 19 nanoparticles were synthesized by the sol–gel hydrothermal method. The factors affecting the synthesized process, such as the mole proportion of the reactants, pH, temperature, the hydrothermal conditions and the calcination process, have been investigated. The crystal structures of these materials were refined by Rietveld method. The obtained materials have single crystal phase, equal nano-size, plate shape and high anisotropy. The high magnetic coercivity of 6.3 kOe with the magnetization at 11.1 kOe of 66 emu g −1 at room temperature was observed for the strontium hexaferrite nanoparticles. For other nanoparticles (SrLn x Fe 12-x O 19 and SrFe 12 O 19/CoFe 2 O 4) synthesized on the basis of SrFe 12 O 19 the complex completion of the crystal structure distortion and the interaction between magnetic phases were observed.

Observation of Griffiths phase in antiferromagnetic La0.32Eu0.68MnO3

We report the Griffiths phase (GP) scenario for isovalent doping in antiferromagnetic La0.32Eu0.68MnO3. Rietveld refinement of structural data displays strong crystal structural distortion. The dc and ac magnetic studies nicely demonstrate unambiguous aspects of robust GP. The presence of ferromagnetically correlated spin clusters is found above the transition temperature, TN. The disorder-driven phase inhomogeneity is correlated to strong structural distortion, giving rise to the GP. This is an unique example in manganite where double exchange interaction does not play any role for the observed phenomenon.

The low and high temperature electrochemical performances of Li 3V 2(PO 4) 3/C cathode material for Li-ion batteries

Li 3V 2(PO 4) 3/C cathode material is synthesized by a carbon-thermal reduction method using polyvinyl alcohol as carbon source at 700 °C. The Li 3V 2(PO 4) 3/C electrode presents a high initial discharge capacity of 84.3, 111.1, 128.7, 129.2 and 132.1 mAh g −1 at −20, 0, 25, 40 and 65 °C between 3.0 and 4.3 V, and 118.9, 132.1, 187.6, 180.3 and 172.2 mAh g −1 between 3.0 and 4.8 V at 0.1 C, respectively. However, the electrode only delivers small discharge capacities at −20 °C at 10 C rate. The capacity fade at low temperatures is mainly attributed to the reduced ionic and electronic conductivity of the electrolyte, the increased impendence of solid electrolyte interface (SEI) and charge-transfer resistance on the electrolyte–electrode interfaces. At higher temperatures, the capacity increases with increasing temperature between 3.0 and 4.3 V, but decreases between 3.0 and 4.8 V. In the potential range of 3.0–4.8 V, the larger crystal structural distortion and non-uniformity of SEI layer at high temperatures may be the main reasons for the capacity loss.

SPONTANEOUS POLARIZATION IN UNCONVENTIONAL Ta2O5 DIELECTRIC CERAMICS BY LASER SINTERING

Spontaneous polarization was found in Ta2O5 dielectric ceramics prepared by laser sintering technique. After heat corrosion, 90° and 180° domains in the ceramics was observed by field emission scanning electron micrographs. The crystal structure of the laser-sintered ceramics at RT was determined as monoclinic phase, which indicates the lattice distortion existed in the ceramics. The variation of O-Ta-O bending vibration mode was deduced from the variation of phonon mode at the low wave-number (57 cm-1 and 250 cm-1). The enhancement of effective electric field for tantalum atoms due to the lattice distortion was determined by calculation of charge distribution coefficients. The distortion of crystal structure due to the phase transformation from higher symmetric orthorhombic phase to lower symmetric monoclinic phase explained spontaneous polarization in the laser-sintered Ta2O5 ceramics.

Time-domain analysis of optically controllable biphotonic gratings in azo-dye-doped cholesteric liquid crystals

This study investigated optically controllable biphotonic gratings (BGs) in azo-dye-doped cholesteric liquid crystals. The BGs were formed under the illumination of one green beam with the simultaneous irradiation of an interference field generated by two coherent red beams. This study ascribes the formation of the BGs to the green-beam-induced dye reorientation and elongation of the helical pitch through trans-cis isomerization and red-beam-induced suppression of dye reorientation and elongation of the helical pitch by cis-trans back isomerization. The diffraction characteristics strongly depended on the helical pitch of the cholesteric structure, the polarization state of the probe beam, and the relative intensity of the green and red beams. Application of the finite-difference time-domain method demonstrated that the model of photoinduced distortion of the cholesteric liquid crystal structure satisfactorily explains this dependence.

Floating zone crystal growth and structural distortion of Pb 2V 3O 9

Single crystals of the spin dimer compound Pb 2V 3O 9 were grown for the first time by the floating zone method. Room temperature single-crystal X-ray diffraction measurements confirm their crystallinity and structure. The crystal growth process is discussed in detail and high temperature differential scanning calorimetry measurements that show a structural phase transition from a monoclinic to a triclinic unit cell at 759(1) K are presented.

First-Principles Study of Hydrogen-Doping Effects on Iron-Based Superconductors

We investigate effects of hydrogen doping on iron-based superconductors with the use of first-principles density functional theory. The most stable location of hydrogen atoms in LaFeAsO is found to be near Fe sites, which is consistent with NMR experiments. We also evaluate optimized crystal structure distortions and their electronic states. An origin of the critical temperature enhancement is probably caused by the lattice distortions appropriate for superconductivity. Electron doping due to hydrogen may be also related to the enhancement.

PdZn or ZnPd: Charge Transfer and Pd–Pd Bonding as the Driving Force for the Tetragonal Distortion of the Cubic Crystal Structure

AbstractThe intermetallic compound PdZn was reported to have two modifications, the tetragonal CuAu‐type low‐temperature phase and the cubic CsCl‐type high‐temperature phase. The existence of the cubic high‐temperature phase at the equiatomic composition could not be proven experimentally. Calculations of the total energy reveal a minimum at c/a = 1.155, clearly excluding the cubic structure at the equiatomic composition in the system Pd‐Zn. Analysis of the chemical bonding applying the electron localizability approach reveals charge transfer from zinc to palladium and proves direct Pd–Pd interactions in the (001) plane to be the main reason for the tetragonal distortion of the cubic CsCl‐type structural pattern for ZnPd.

London penetration depth in the tight binding approximation: orthorhombic distortion andoxygen isotope effects in cuprates

We present a simple derivation of an expression for the superfluid density in superconductors with the tight binding energy dispersion. The derived expression isdiscussed in detail because of its distinction from the known expressions for ordinarysuperconductors with parabolic energy dispersion. We apply this expression for theexperimental data analysis of the isotope effect in London penetration depth parameterλ in the BiSrCuO and YBaCuO family compounds near optimal doping, taking intoaccount the orthorhombic distortion of crystal structure, and estimate the isotopicchange of hopping parameters from the experimental data. We point out that1/λ2 temperature behaviour is very sensitive to the ratio2Δm(T = 0)/kBTc and estimate this quantity for a number of compounds.

Upconversion Luminescence Properties of Er3+–Bi3+ Codoped CaSnO3 Nanocrystals with Perovskite Structure

New phosphor nanocrystals of ASnO3:Er3+ (A = Ca, Sr, and Ba) and CaSnO3:Er(3+)-Bi3+ were prepared by a hydrothermal method. Their structures were examined by X-ray diffraction and Raman spectroscopy. Upconversion luminescence properties of the Er3+ ions in these host materials were investigated upon excitation at 980 nm. Green emission bands of Er3+ around 528 and 545 nm were more favorable in CaSnO3 than in SrSnO3 and BaSnO3. This was interpreted in terms of a crystal structural distortion and configuration coordinate model. The investigation of the upconversion luminescence of Er(3+)-Bi3+codoped CaSnO3 nanocrystals indicates that Bi3+ ions could largely sensitize the emission of Er3+ ions which was attributed to the efficient energy transfer from Bi3+ to Er3+ ions and the modification of the local symmetry of Er3+ ions. The results show that the CaSnO3:Er(3+)-Bi3+ nanocrystals have potential application for fabricating visible upconversion devices.

Chemical demineralization of different metamorphic grade coals

The paper analyzes a process of deep mineralization of various metamorphic grade coals pre-ground in different destructive units, namely, in centrifugal-planetary mill and disintegrator. Coal dispergation in higher energy intensive mills greatly enhances inorganic component extraction to acidic solutions. This is explained by distortion of crystal structure and amorphization of minerals under various kinds and different intensity mechanical actions.

Syntheses, structures, and geometric and spectral characteristics of the hexachlorodicuprate(II) complexes with the protonated 4-azafluoren-9-one derivatives: The crystal and molecular structures of bis(4-azafluoren-9-onium) hexachlorodiaquadicuprate(II) dihydrate

Three complexes containing hexachlorodicuprate(II) anions and protonated 4-azafluorenone derivatives as counterions were isolated. The crystal and molecular structures of 4-azafluoren-9-one, (HL1)2[{CuCl2(H2O)}2(μ-Cl)2] · 2H2O (L1), and spectral characteristics of the complexes were determined. Relationships relating the degree of distortion of the crystal structure of the hexachlorodicuprate(II) anions to the spectral characteristics of the compounds were proposed on the basis of the experimental and published data.

Tricolor luminescence in rare earth doped CaZrO 3 perovskite oxides

Intense tricolor photoluminescence under ultraviolet excitation was observed from Eu–Mg codoped CaZrO 3 and Tb–Mg codoped CaZrO 3. The RGB tricolor was obtained from a single host lattice of perovskite-type CaZrO 3 by just varying a doped rare earth ion and its concentration. Red luminescence was observed in Eu–Mg codoped CaZrO 3, and green and blue luminescence was observed in Tb–Mg codoped CaZrO 3. The color of the luminescence in Tb–Mg codoped CaZrO 3 changed with Tb concentration due to the cross relaxation in Tb ions; green color was seen at high concentration and blue color at low concentration. In comparison with other perovskite host lattices, these zirconates showed more intense luminescence than hafnates and stannates. In addition, Mg codoping with the rare earth ions markedly increased the luminescence intensity. It is considered that the intense luminescence in the zirconates is attributed to the large crystal structural distortion of CaZrO 3 and the local lattice distortion around the rare earth ions enhanced by Mg substitution at Ca sites.

Effect of pH on atmospheric scorodite synthesis by oxidation of ferrous ions: Physical properties and stability of the scorodite

In the scorodite novel synthesis reaction, where ferrous ions were oxidized by oxygen gas in the presence of a high concentration of arsenic(V), the effect of sodium ion and reaction pH was investigated. The results indicate a prominent pH effect but a relatively minor influence of sodium ion. The pH range suitable for the synthesis of stable scorodite was pH 0.3 to 1.0. For solution pH higher than 1.2, the precipitates were very fine and the amounts of scorodite and arsenic that dissolved increased. The leaching profile showed a striking change at pH 1.2 that has not been adequately explained by previous literature. Below pH 1.0, the precipitates contained more sulfur and sodium as the pH decreased and had Fe/(As + S) molar ratios below 1. The precipitates obtained from the reaction with a final pH of around 0 had increased arsenic mobility. Consequently, the pH range of 0.3 to 1.0 was judged to be favorable for synthesizing stable scorodite that dissolves < 0.1 mg/L arsenic. The arsenic leaching profile was related to Fe/(As + S) molar ratio, which indicates the degree of distortion of the scorodite crystal structure. A simple substitution model of AsO 4 3− for SO 4 2− does not explain the stoichiometric shortage of iron but no crystalline structure other than scorodite was identified by XRD.

Stress-Induced Chemical Detection Using Flexible Metal−Organic Frameworks

In this work we demonstrate the concept of stress-induced chemical detection using metal-organic frameworks (MOFs) by integrating a thin film of the MOF HKUST-1 with a microcantilever surface. The results show that the energy of molecular adsorption, which causes slight distortions in the MOF crystal structure, can be converted to mechanical energy to create a highly responsive, reversible, and selective sensor. This sensor responds to water, methanol, and ethanol vapors, but yields no response to either N2 or O2. The magnitude of the signal, which is measured by a built-in piezoresistor, is correlated with the concentration and can be fitted to a Langmuir isotherm. Furthermore, we show that the hydration state of the MOF layer can be used to impart selectivity to CO2. Finally, we report the first use of surface-enhanced Raman spectroscopy to characterize the structure of a MOF film. We conclude that the synthetic versatility of these nanoporous materials holds great promise for creating recognition chemistries to enable selective detection of a wide range of analytes.