Synchrotron Radiation Powder X-ray Diffraction Research Articles

The Metal–Insulator Transition in Y1-xCaxTiO3Caused by Phase Separation

In order to explore the origin of the metal–insulator (M–I) transition, the precise crystal structures of the hole-doped Mott insulator system, Y 1- x Ca x TiO 3 ( x =0.37, 0.39 and 0.41), are studied for the temperature range between 20 K and 300 K by synchrotron radiation (SR) X-ray powder diffraction. For both Y 0.63 Ca 0.37 TiO 3 and Y 0.61 Ca 0.39 TiO 3 compositions, the orthorhombic ( P b n m ) – monoclinic ( P 2 1 / n ) structural phase transition occurs at approximately 230 K, which is much higher than their individual M–I transition temperatures, i.e., 60 K and 130 K, respectively. For these compositions, significant phase separation (low-temperature orthorhombic + monoclinic) is also found in the vicinity of the M–I transition temperature. On the other hand, Y 0.59 Ca 0.41 TiO 3 , which does not exhibit M–I transition and preserves a metallic behavior down to 1.5 K, is in a two phase state from 20 K to 300 K. It is concluded that the existence of the phase separation causes the M–I transition in Y 1- x Ca x TiO 3 , and the low-temperature orthorhombic phase contributes to the metallic property of this system.

MnO6distortion and spin ordering inNd1−xSrxMnO3(x=0.55and 0.65)

Interrelation between the distortion of the ${\mathrm{MnO}}_{6}$ octahedra and spin ordering was investigated for ${\mathrm{Nd}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_{3}$ and ${\mathrm{Nd}}_{0.35}{\mathrm{Sr}}_{0.65}{\mathrm{MnO}}_{3}$ by means of synchrotron-radiation x-ray powder diffraction and neutron powder diffraction. The ground state of the former (latter) compounds is the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}{(d}_{{3z}^{2}\ensuremath{-}{r}^{2}})$ orbital-ordered state. A strong orbital-spin coupling is observed in ${\mathrm{Nd}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{MnO}}_{3},$ making a sharp contrast with the weak coupling in ${\mathrm{Nd}}_{0.35}{\mathrm{Sr}}_{0.65}{\mathrm{MnO}}_{3}.$ We interpret the strong coupling in terms of the kinetic energy gain of the ${e}_{g}$ carriers in the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbital-ordered state.

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.

High-pressure structural analysis of(Nd,Sm)1/2Sr1/2MnO3:Origin for pressure-induced charge ordering

Pressure effects on the structural parameters are investigated in a half-doped manganites $({\mathrm{Nd}}_{0.125}{\mathrm{Sm}}_{0.875}{)}_{1/2}{\mathrm{Sr}}_{1/2}{\mathrm{MnO}}_{3},$ by means of the synchrotron-radiation x-ray powder diffraction measurement. The compound shows a pressure-induced metal-insulator transition due to the subtle competition between the double-exchange interaction and the charge-ordering instability [Y. Tokura et al., Phys. Rev. Lett. 76, 3184 (1996)]. We have found that application of pressure induces the four-long and two-short deformation of the ${\mathrm{MnO}}_{6}$ octahedra. We present a possible scenario for the pressure-induced charge-ordering.

Elastic Moduli and Hardness of Cubic Silicon Nitride

The bulk modulus B0= 290(5) GPa and its first pressure derivative B′0= 4.9(6) were obtained for c‐Si3N4 from volume versus pressure dependence. Measurements were performed under quasi‐hydrostatic conditions in a diamond anvil cell to 53 GPa using synchrotron radiation and energy dispersive X‐ray powder diffraction. This combined with nanoindentation measurements determined the shear modulus G0 of c‐Si3N4 to be 148(16) GPa. The Vickers microhardness HV(0.5) for dense, oxygen‐free c‐Si3N4 was estimated to be between 30 and 43 GPa. Both the elastic moduli and microhardness of c‐Si3N4 exceed those of the hexagonal counterparts, α‐ and β‐phases.

Optical properties of nonlinear potassium lithium niobate crystals

Crack-free potassium lithium niobate (KLN) single crystals of different compositions have been grown by the resistance-heating Czochralski technique. The crystal structure was studied using synchrotron radiation X-ray powder diffraction, and the crystal compositions were determined by X-ray fluorescent spectroscopy. Homogeneous and crack-free samples of about 6×6×7 mm 3 were used to study the lattice vibration spectra by Raman and infrared reflection spectroscopy. The results show a strong influence of Li ions in the C sites of the KLN crystal on the characteristic Raman and infrared reflectivity spectra of the Nb–O octahedral ions. In particular, the symmetric bend vibrational mode, ν 5, is split into three Raman peaks; the antisymmetric bend vibrational mode, ν 4, in the infrared spectrum is split into three peaks as well. The antisymmetric stretch vibrational mode, ν 3, is broadened greatly and slightly split. The crystals obtained have good quality and improved second harmonic generation properties.

Chemical pressure control of exchange interaction in Mo pyrochlore

Structural parameters are systematically investigated for Mo pyrochlore ${R}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ $(R=\mathrm{Dy},$ Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius ${r}_{\mathrm{R}}$ of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with ${r}_{\mathrm{R}}$ from $129.7\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{Dy}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ (SG) to $131.5\ifmmode^\circ\else\textdegree\fi{}$ for ${\mathrm{Nd}}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.

Microdomain Structure of Cr-Doped Manganites: Nd1/2Ca1/2(Mn,Cr)O3

Crystal and magnetic structures of Cr-doped manganites, Nd 1/2 Ca 1/2 Mn 1- y Cr y O 3 ( y =0.00 and 0.03), have been investigated by synchrotron radiation (SR) x-ray powder diffraction as well as neutron powder diffraction measurements. A detailed analysis of the high-resolution x-ray profile has revealed that the Cr-doped compound exhibits broad extra reflections, suggesting the formation of microdomains below the charge-ordering temperature T CO . The origin of the microdomain structure is discussed in terms of the charge separation.

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.

Phase transformation and conductivity in nanocrystal PbS under pressure

The grain-size effect on the phase transition induced by pressure in PbS was studied by in situ high-pressure electrical resistance and synchrotron radiation x-ray powder diffraction measurements. The mean transition pressure of the B1-to-B16 phase transformation was found to be 6.3±1.3 GPa in 8±1 nm PbS while it is 3.1±0.7 GPa for 10 μm PbS. The resistivity of the B16 PbS phase decreases exponentially with pressure in both samples at ambient temperature. They follow R∝exp(−CP), where C=−0.64 GPa−1 for 10 μm PbS and C=−0.34 GPa−1 for 8±1 nm PbS. These results are discussed in terms of a decrease of energy band gap with increasing pressure.

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.

Time-Resolved X-ray Powder Diffraction Using a Large-Area CCD-Based Detector and Rietveld Refinement: Solid-State Polymerization of S2N2 to (SN)x

A kinetic study of the solid-state polymerization of disulfur dinitride (S2N2) to polysulfur nitride [(SN)x] has been performed, combining monochromatic high-energy (lambda = 0.3263 A) synchrotron radiation X-ray powder diffraction, a large-area (ø = 220 mm) CCD-based X-ray image-intensifier detector and Rietveld refinement. Recently developed techniques for detector calibration and reduction of two-dimensional images to one-dimensional diffraction patterns have been employed for data processing/analysis. Good fits were obtained after Rietveld refinement [Rp = 8.4%, wRp = 9.4%, sin(theta(max))/lambda = 0.585 A(-1)] of diffraction patterns of S2N2 from images with 2 s exposure time. The solid-state polymerization of S2N2 to (SN)x, was followed at a maximum rate of two diffraction images per minute. Scale factors and cell parameters for S2N2 and beta-(SN), as functions of time were readily obtained after Rietveld refinement of the diffraction patterns obtained from each individual image throughout the polymerization. The polymerization was preceded by a lattice distortion of S2N2, and at 50% conversion the a axis had decreased by about 1% and the c axis had increased about 1%. The polymerization yielded not only the expected polymer beta-(SN)x, but also a small amount of a compound that could be another phase of (SN)x.

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.

Equation of state of cobalt up to 79 GPa.

Angle-dispersive synchrotron-radiation x-ray powder-diffraction experiments on cobalt were performed by using a diamond-anvil cell and an imaging plate. The hcp phase remained stable up to the highest pressure of 79 GPa. The bulk modulus and its pressure derivative are determined as ${\mathit{B}}_{0}$=199\ifmmode\pm\else\textpm\fi{}6 GPa and ${\mathit{B}}_{0}^{\ensuremath{'}}$=3.6\ifmmode\pm\else\textpm\fi{}0.2, respectively. The axial ratio c/a continuously decreases under pressure. \textcopyright{} 1996 The American Physical Society.

Stability and the equation of state of alpha -manganese under ultrahigh pressure.

We have performed angle dispersive synchrotron-radiation x-ray powder-diffraction experiments on \ensuremath{\alpha}-Mn up to 190 GPa at room temperature by using a diamond-anvil cell and an imaging plate. We have obtained evidence for a structural phase transition in Mn above 165 GPa. A new diffraction peak appeared above 165 GPa, which can be most likely indexed to the 110 reflection of a body-centered-cubic structure. The volume reduction associated with the structural phase transition was estimated to be 2%. The bulk modulus and its pressure derivative of \ensuremath{\alpha}-Mn were determined as ${\mathit{B}}_{0}$=158 GPa and ${\mathit{B}}_{0}^{\ensuremath{'}}$=4.6, respectively. The pressure dependence of the bulk modulus indicates the decrease of the magnetic moments of Mn under pressure.

The structure of cimetidine (C10H16N6S) solved from synchrotron-radiation X-ray powder diffraction data

The crystal structure of cimetidine (C10H16N6S) has been solved and refined from high-resolution synchrotron X-ray powder data collected on station 8.3 at Daresbury Laboratory. The structure is monoclinic, P21/n, Z = 4, a = 10.7029 (3), b = 18.8262 (1), c = 6.8266 (2) Å, β = 111.306 (2)°. The data were auto-indexed and the integrated intensities were obtained by pattern decomposition. The structure was solved by direct methods followed by iterative cycles of least-squares refinement and Fourier syntheses. All 17 non-H atoms were located. The structure was then refined using the method of Rietveld [J. Appl. Cryst. (1969), 2, 65–71]. A difference Fourier synthesis after the convergence of the non-H-atom model showed positive electron density at the expected positions of all 16 H atoms, ten of which were significantly above the background. The H atoms were allowed to refine unconstrained and the final agreement factors were RI = 1.9, Rwp = 8.5 and Rex = 6.9%; the equivalent R-factors without the H atoms were RI = 10.3 and Rwp = 16.2%, demonstrating the significant contribution of the H atoms to the profile.