Radiation X-ray Powder Diffraction Data Research Articles

Effect of chemical composition on mass transfer in Y<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> under oxygen potential gradient at high temperatures

The oxygen shielding properties and structural stability of polycrystalline Y2Ti2O7 (YT) solid solution wafers, which were cut from a sintered body and served as models for environmental barrier coatings, were evaluated by the oxygen permeation technique at high temperatures. Here, we examined the effect of compositions by using two samples with slightly different Y/Ti molar ratios. The oxygen permeability constants for YT increased dramatically with a slight increment in the Y/Ti molar ratio, and the activation energy for oxygen permeation markedly decreased. Oxygen permeation in the YT was controlled by the diffusion of oxide ions in the crystal. Almost no decomposition of the YT phase occurred due to migration of the constituent cations at high temperatures, indicating high structural stability under an oxygen potential gradient. The crystal structure was identified as the YT-based pyrochlore structure from Rietveld refinements of synchrotron radiation X-ray powder diffraction data, based on information on the composition dependence of the defect concentrations in the crystals estimated by first principles calculations. The diffusion path for oxide ions in the crystal at high temperature was also investigated based on a spatial distribution map described by the bond valence sum method. Finally, the mechanisms for the uptake of oxygen molecules on the YT surface exposed to a high oxygen partial pressure and the subsequent diffusion of oxide ions in the YT crystals were clarified.

Study of materials structure physics of isomorphic LiNbO3 and LiTaO3 ferroelectrics by synchrotron radiation X-ray diffraction

Electron charge density studies of stoichiometric LiNbO3 and LiTaO3 ferroelectrics have been carried out by analyzing the synchrotron radiation X-ray powder diffraction data using a combination of the Rietveld and maximum entropy methods. The clear relationships between the Nb(Ta)–O bond length, the electron charge density on the Nb(Ta)–O bond, and the phase transition temperature TC are revealed for isomorphic structures. Nb(Ta)–O bonding plays an important role in the elevation of TC. The TC in LiNbO3 being higher than that in LiTaO3 is attributed to the larger lattice distortion of the Nb–O oxygen octahedron in LiNbO3. The validity of estimating TC for LiNbO3 family crystals from the degree of lattice distortion in the ferroelectric structure is discussed.

Li5(BH4)3NH: Lithium-Rich Mixed Anion Complex Hydride

The Li5(BH4)3NH complex hydride, obtained by ball milling LiBH4 and Li2NH in various molar ratios, has been investigated. Using X-ray powder diffraction analysis the crystalline phase has been indexed with an orthorhombic unit cell with lattice parameters a = 10.2031(3), b = 11.5005(2), and c = 7.0474(2) A at 77 °C. The crystal structure of Li5(BH4)3NH has been solved in space group Pnma, and refined coupling density functional theory (DFT) and synchrotron radiation X-ray powder diffraction data have been obtained for a 3LiBH4:2Li2NH ball-milled and annealed sample. Solid-state nuclear magnetic resonance measurements confirmed the chemical shifts calculated by DFT from the solved structure. The DFT calculations confirmed the ionic character of this lithium-rich compound. Each Li+ cation is coordinated by three BH4– and one NH2– anion in a tetrahedral configuration. The room-temperature ionic conductivity of the new orthorhombic compound is close to10–6 S/cm at room temperature, with activation energy of 0...

Maximum Entropy Method Applied in the Experimental Visualization of Electron Density Distributions in BiFeO3

The BiFeO3 is a largely studied compound with magnetoeletric coupling at room temperature. This is due its potential technological applications that can be directly used in advanced technologies. BiFeO3 is an oxide with perovskite structure and R3c space group (rhombohedral symmetry). In this work, synchrotron radiation X-ray powder diffraction data was used to obtain a detailed description of BiFeO3 crystal structure. Thereafter, experimental electron density distribution calculations were obtained using by the maximum entropy method (MEM) statistical approach. The ionic/covalent nature of the bonding and the interaction between the ions are clearly revealed by constructing electron density maps. The Fe3+-O2− and Bi3+-O2− bonding were studied in 1 and 2 dimensional electron density distributions. Due the high resolution of the obtained maps, it can be regarded as the most precise electron density distributions seen inside the material.

Hierarchical dielectric orders in layered ferroelectrics Bi2SiO5.

Electric dipole engineering is now an emerging technology for high electron-mobility transistors, ferroelectric random access memory and multiferroic devices etc. Although various studies to provide insight into dipole moment behaviour, such as phase transition, order and disorder states, have been reported, macroscopic spontaneous polarization has been mainly discussed so far. Here, visualization of the electric dipole arrangement in layered ferroelectrics Bi2SiO5 by means of combined analysis of maximum entropy charge density and electrostatic potential distribution analysis based on synchrotron radiation X-ray powder diffraction data is reported. It was found that the hierarchical dipole orders, the weak-ferroelectric and ferroelectric configurations, were observed in the Bi2O2 and the SiO3 layers, respectively, and the ferrielectric configuration was realised by the interlayer interaction. This discovery provides a new method to visualize the local polarization in ferroelectric materials.

Evidence of electronic polarization of the As ion in the superconducting phase of F-doped LaFeAsO.

Understanding the nature of superconductivity in iron-based compounds is essential in the development of new strategies to increase T c. Using a charge density analysis based on synchrotron radiation X-ray powder diffraction data, we found that the charge carriers only accumulated in the iron layer of the superconducting phase of LaFeAsO1 - x F x at low temperatures. Analysis of the electrostatic potential distribution revealed the concerted enhancement of the electronic polarization of the As ions and the carrier redistribution. This suggests that the enhanced electronic polarization of the As ion plays an important role in inducing high T c superconductivity, and that the polaron concept, which has been previously regarded as an untenable mechanism, should be reconsidered for the description of the iron-arsenide superconducting phase.

Electronic Polarization in KNbO3 Visualized by Synchrotron Radiation Powder Diffraction

Electron charge density studies of a potassium niobate KNbO3 perovskite-type crystal have been carried out by analyzing synchrotron radiation X-ray powder diffraction data using a combination of the Rietveld method and the maximum entropy method. The crystal structures of the cubic, tetragonal, orthorhombic, and rhombohedral phases are accurately analyzed associated with the successive ferroelectric phase transitions by taking the multicomponent structures in the ferroelectric phases into account. The electrostatic potential analyses reveal the emergence of electronic polarization arising from the anisotropic covalent bonding on the Nb and O atoms, while no significant electronic polarization is detected on the isolated K ion. Thus, the spontaneous polarization is caused by the ionic displacements of the constituent atoms and the imbalance of the hybrid atomic orbitals on the Nb–O bonding.

Antimony desinsertion reaction from SbxCoSb3−x

The compound Sbx CoSb3-x was produced at 7.7 GPa and 550 ∘C in a self-insertion reaction from the binary skutterudite CoSb3. This self-insertion reaction is characterized by the collapse of some framework Sb atoms into the cages formed by the Co and Sb atoms in the skutterudite structure, as was further confirmed by Bader’s analysis of maximum-entropy charge density maps obtained from synchrotron radiation x-ray powder diffraction data. The opposite reaction (i.e., Sb desinsertion, Sbx CoSb3-x→CoSb3) occurs when Sbx CoSb3-x is heated above 180 ∘C at ambient pressure. This desinsertion reaction was followed by means of differential scanning calorimetry, x-ray diffraction, and electrical resistivity measurements. Differential scanning calorimetry measurements revealed the presence of two thermal events in samples rich in the Sbx CoSb3-x phase. An endothermic peak around 150 ∘C was assigned to a small change in the position of the guest Sb atoms inside the cages of Sbx CoSb3-x. This assignment was based on the analysis of charge density maps obtained from synchrotron x-ray diffraction measurements carried out both at room temperature and at 155 ∘C. Accordingly, the guest Sb atoms in Sbx CoSb3-x shift from the 12d (x,0,0) site of Im3¯ space group (in a position distant about 0.35 Å from the center of the cages), at room temperature, to the 2a (0,0,0) site (i.e., to the center of the cages) above 150 ∘C. An exothermic event starting at 180 ∘C is the thermal signature of the desinsertion of guest Sb atoms from the Sbx CoSb3-x skutterudite cages, as confirmed by x-ray diffraction analysis and further verified by electrical measurements. After heating to 350 ∘C, Sbx CoSb3-x samples fully convert back to CoSb3. The Sb desinsertion reaction from Sbx CoSb3-x follows a first order kinetics, with a transition enthalpy of approximately 21 kJ/mol and an activation energy of 83 kJ/mol.

Experimental charge density of LiBD4from maximum entropy method

We report on maximum entropy method study of the experimental atomic and ionic charges of LiBD4 in its low-temperature orthorhombic phase. Synchrotron radiation x-ray powder diffraction data, neutron powder diffraction data, and density functional calculations were used. The atomic and ionic charges were determined for both experimental and theoretical results using the Bader analysis for atoms in molecules. The charge transfer from the Li cation to the BD4 anion is 0.86(+/- 9) e, which is in good agreement with the ab initio calculated value of 0.895 e. The experimental accuracy was determined considering the differences between results obtained for data collected at 10 and 90 K, different experimental setups (high-resolution diffractometer or image plate diffractometer), and different structural models used for the prior density distributions needed for accurate maximum entropy calculations (refined using only synchrotron radiation x-ray powder diffraction data or combined with neutron powder diffraction data).

Electron Charge Density Study on the Bonding Nature in MoO3

The accurate electron charge density distributions of MoO 3 have been investigated by analyzing the synchrotron radiation X-ray powder diffraction data using the MEM (Maximum Entropy Method)/Rietve...

Charge-Density Study of the High Temperature Orthorhombic Phase in Ferrielastic CsLiCrO 4

Electron charge density distribution of CsLiCrO 4 at 550 K in the high temperature orthorhombic phase has been obtained by analyzing the synchrotron radiation X-ray powder diffraction data using the MEM (maximum entropy method)/Rietveld method. Although the temperature factors of oxygen atoms are relatively large, definite evidence which shows the oxygen atoms to be in disorder state has not been obtained. It is found from the charge-density distribution that strong covalent bonding is formed between Cr and O atoms in CrO 4 tetrahedra, whereas Li atoms are regarded to be almost ionic as clear bonding electron distribution is not seen between Li and O atoms.

The crystal structure of La0.7Pr0.3Ba2Cu3Od ceramic compound

The structure of new La0.7Pr0.3Ba2Cu3Oy (LPBCO) compound was obtained at room temperature from synchrotron radiation X-ray powder diffraction data and refined by Rietveld technique. LPBCO has an isotypical structure with YBa2Cu3Oy (YBCO). The crystal data are: La0.7Pr0.3Ba2Cu3O6.96, Mr=716.16, orthorhombic system, space group Pmmm, a=3.9147(1) Å, b=3.8672(1) Å, c=11.7033(2) Å, V=177.177(6) Å3, Z=1, Dx=6.714 g/cm3; the structure was refined with 35 parameters to Rwp=7.41%, Rp=5.32%, and Rexp=3.07% for 5001 step intensities. Moreover, the total content of oxygen in a unit cell is refined as 6.96, which is less than that of the calculated one. We attribute the superconductivity-depression to the increase of the valence of copper.

Precise Charge Density Analysis of PbTiO 3

Precise charge-density distributions of cubic and tetragonal PbTiO 3 have been investigated associated with the ferroelectric phase transition at 763K, by means of the MEM/Rietveld analysis using synchrotron radiation X-ray powder-diffraction data. It has been clearly visualized that cubic PbTiO 3 consists of the three-dimensional covalent network of TiO 6 octahedra and isolated Pb 2+ ions, whereas tetragonal PbTiO 3 forms a layered structure with the two-dimensional covalent network of TiO 5 pyramids, to which Pb atoms are also covalently bonded. These findings on the binding nature of tetragonal PbTiO 3 give good agreement with theoretical predictions by the first-principles calculations.

Precise Charge Density Analysis of PbTiO 3

Precise charge-density distributions of cubic and tetragonal PbTiO 3 have been investigated associated with the ferroelectric phase transition at 763K, by means of the MEM/Rietveld analysis using synchrotron radiation X-ray powder-diffraction data. It has been clearly visualized that cubic PbTiO 3 consists of the three-dimensional covalent network of TiO 6 octahedra and isolated Pb 2+ ions, whereas tetragonal PbTiO 3 forms a layered structure with the two-dimensional covalent network of TiO 5 pyramids, to which Pb atoms are also covalently bonded. These findings on the binding nature of tetragonal PbTiO 3 give good agreement with theoretical predictions by the first-principles calculations.

Structural features of Ag[AuF4] and Ag[AuF6] and the stuctural relationship of Ag[AgF4]2 and Au[AuF4]2 to Ag[AuF4]2.

Synchrotron radiation X-ray powder diffraction data (SPDD) have been obtained for Ag[AgF4]2, Au[AuF4]2, Ag[AuF4], and Ag[AuF6]. Ag[AgF4]2 and Au[AuF4]2 are isostructural with Ag[AuF4]2, space group (SG) P2(1)/n, Z = 2, with the following: for Ag[AgF4]2 a = 5.04664(8), b = 11.0542(2), c = 5.44914(9) A, beta = 97.170(2) degrees; for Au[AuF4]2 a = 5.203(2), b = 11.186(3), c = 5.531(2) A, beta = 90.55(2) degrees. The structure of Ag[AgF4]2 was refined successfully (SPDD) applying the Rietveld method, yielding the following interatomic distances (A): AgII-F = 2.056(12), 2.200(13), 2.558(13); AgIII-F = two at 1.846(12), others = 1.887(12), 1.909(13), 2.786-(12), 2.796(12), 2.855(12). AgAuF4, like other AA'F4 salts (A = Na-Rb; A' = Ag, Au), crystallizes in the KBrF4 structure type, SG I4/mcm (140), Z = 4 with a = 5.79109(6), c = 10.81676(7) A. SPDD gave (in A) four AuIII-F = 1.89(1) and eight AgI-F = 2.577(7). SPDD for AgAuF6 confirmed that it has the LiSbF6 structure, SG R3, Z = 3, with a = 5.2840(2), c = 15.0451(6) A.

Nature of the Hydrogen Bond in Tetragonal KDP (KH2PO4) Observed by Maximum Entropy Method

In order to explore the nature of the hydrogen bond, the electron density distribution of KH 2 PO 4 (KDP) at room temperature was obtained by the maximum entropy method (MEM) using synchrotron radiation X-ray powder diffraction data. In the obtained electron density distribution maps, the contour lines linked the two oxygen atoms of neighboring PO 4 groups and a hydrogen-bonding network was clearly seen. However, no local maxima of electron density due to hydrogen atoms were recognized. This figure of hydrogen bonding in KDP is similar to that in ice (I h ), which is the only hydrogen-bonded compound previously analyzed by MEM. In both cases, the hydrogen atoms are believed to be disordered between the two oxygen atoms. The present study shows that the character of the hydrogen bond in crystalline materials can be described clearly by MEM, even in the case where heavier atoms such as K and P exist besides O atoms.

Structural Characterization of the High-Temperature Phase Transitions in Ca8[Al12O24](MoO4)2Aluminate Sodalite Using X-ray Powder Diffraction

The structures are reported of the room-temperature and the three high-temperature phases of Ca8[Al12O24](MoO4)2aluminate sodalite CAM. Structure refinements have been performed with the Rietveld method using synchrotron radiation X-ray powder diffraction data. The cubic phase has symmetryI43mwitha=9.29377(4) Å. The tetragonal phases have symmetryP4c2, and their unit cells corresponds to a2a×2a×csupercell of the cubic phase. The second tetragonal phase (T2) exists for 614<T<624 K and has lattice parametersa=13.14536(6) Å andc=9.29224(8) Å. The first tetragonal (T1) phase is stable for 590<T<614 K and has lattice parametersa=13.12263(5) Å andc=9.32081(5) Å, The orthorhombic phase has symmetryAba2 witha=26.14683(8) Å,b=13.07061 Å, andc=9.31413(2) Å. The transition (on decreasing temperatures) atTc≈624 K, from the cubic to the T2 phase, is of second order and is found to be related to change in the orientational order of the cage anions MoO2−4. The T2 to T1 transition atTc≈614 K is of first order, and it corresponds to a displacement of the cage anions to an off-center position in the cages, while keeping orientational disorder over at least two orientations. The transition atTc≈590 K, from the T1 phase to the orthorhombic phase, is again a first-order transition, and it is related to a complete ordering of the cage anions. A detailed description is given of the structural distortions of the framework, accompanying the phase transitions.