High-resolution neutron powder diffraction study on the structure of Sr2SnO4

From the high-resolution time-of-flight neutron powder diffraction data, the crystal structure of Sr₂SnO₄ at the temperature range between 4 and 300 K has been investigated. The Rietveld refinement has shown that Sr₂SnO₄ belongs to the space group Pccn, which can be derived from the tetragonal K₂NiF₄ structure by tilting the SnO₆ octahedra along the [100]_T- and [010]_T-axis, respectively, with non-equal tilts. The earlier reported first-order phase transition in Sr₂SnO₄, from Bmab to P4₂/ncm, has not been observed.     

Neutron powder diffraction study of phase transitions in Sr2SnO4

The phase transitions in Sr₂SnO₄ at high temperature have been studied using high resolution time-of-flight powder neutron diffraction. The room temperature structure of Sr₂SnO₄ is orthorhombic (Pccn), which can be derived from the tetragonal K₂NiF₄ structure by tilting the SnO₆ octahedra along the tetragonal [100]_T- and [010]_T-axes with non-equal tilts. At the temperature of about 423 K, it transforms to another orthorhombic structure (Bmab) characterized by the SnO₆ octahedral tilt around the [110]_T-axis. At still higher temperatures (∼573 K) the structure was found to be tetragonal K₂NiF₄-type (I4/mmm).     

SrMn3O6: an incommensurate modulated tunnel structure

The crystallographic structure of a mixed valent manganite SrMn₃O₆ with a 1D modulated structure is reported. The SrMn₃O₆ structure can be described with the basic subcell space group Pnma (a=9.1334(5) Å; b=2.8219(2) Å; c=12.0959(7)Å), but transmission electron microscopy revealed that the study of the real structure requires a 4D-formalism approach with superspace group P2₁/a(αβ0)00 (unique axis c), with a modulation wave vector q having the approximate components (0.52a*+0.31b*). The crystallographic structure is closely related to those of Na_xFe_xTi_2−xO₄ and Pb_1.5BaMn₆Al₂O₁₆, comprising of unusual “figure-of-eight”-shaped tunnels, made up of strings of edge- and corner-sharing (Fe/Ti)O₆ or (Mn/Al)O₆ octahedra, with the other cations situated in the tunnel cavities. Structural refinement was performed on X-ray and neutron powder diffraction data using the 4D formalism. All atoms in the crystal are affected by a displacive modulation wave, and a saw tooth function is employed to model the displacement and occupancy of the Sr sites. Magnetic susceptibility measurements reveal a sharp antiferromagnetic transition with T_N∼46 K.     

LiAl0.03Mn1.97O4的流变相辅助微波合成以及电化学性能

采用一种特殊微波合成法，流变相辅助微波合成法，制备了结晶度好、纯度高的尖晶石相的锂离子电池正极材料LiAl0.03Mn1.97O4。对其进行了XRD分析和SEM研究，并就结构、形貌与传统固相法制备的LiMn2O4、LiAl0.03Mn1.97O4进行了比较。采用这种流变相辅助微波合成法制备的LiAl0.03Mn1.97O4具有优良的电化学性能，电化学性能测试表明，这种材料具有比较高的首次放电容量（115mAh／g）以及良好的可逆性、优异的循环性能，25次循环结束比容量几乎不变，保持在115mAh／g左右，衰减性得到很好的改善。     

Thermische Dilatation und Hochdruckverhalten der Zintl-Phasen CaSn und BaSn

Thermal Dilatation and High Pressure Behaviour of the Zintl Phases CaSn und BaSn Data on the thermal dilatation and the compressibility of the compounds CaSn and BaSn are derived from x-ray measurements performed at high temperatures and high pressures in a diamond anvil device. At higher pressures BaSn undergoes a transition from the CaSi- to a CsCl-type structure. The field of existence in P and T is determined from the in situ measurements.     

Homogeneity range of the NaTl-type Zintl phase in the ternary system Li-In-Ag

The remarkably broad homogeneity range of the NaTl-type Zintl phase in the ternary phase diagram Li-In-Ag at room temperature was determined by structure evaluation using X-ray powder diffraction. The colours of the investigated Zintl phases correlate with the valence electron concentration (VEC) as already established for the quasibinary cut Li_0.5(In_xAg_1−x)_0.5 with 0.47?x?1.00, i.e. with decreasing VECs the colour changes from grey over reddish to bright yellow. All compounds in the new quasibinary cut Li_x(In_0.5Ag_0.5)_1−x with 0.47?x?0.60 appear free from vacancies in the Li-sublattice, even for Li-deficient compositions. The partial occupation of Li-sites by excess Ag and In instead is in full agreement with the behaviour of the binary NaTl-type Zintl phases Li_xZn_1−x and Li_xCd_1−x (0.47?x?0.54) with a low VEC about 1.5.     

High-resolution neutron powder diffraction study on the phase transitions in BaPbO3

Phase transitions that occurred in perovskite BaPbO₃ have been investigated using high-resolution time-of-flight neutron powder diffraction. The structure at room temperature is orthorhombic (space group Imma), which is derived from the cubic aristotype by tilting the PbO₆ octahedra around the two-fold axis (tilt system a⁰b⁻b⁻). The orthorhombic structure shows anisotropic line broadening attributed to the presence of micro twins. At above about 573 K, BaPbO₃ undergoes a discontinuous phase transition to a tetragonal structure (space group I4/mcm) with the tilting of the PbO₆ octahedra being about the four-fold axis of the cubic aristotype (tilt system a⁰a⁰c⁻). With further increasing the temperature, BaPbO₃ experiences a continuous phase transition to a simple cubic structure (space group Pm3¯m) at above about 673 K. The later phase transition is characterised by a critical exponent of β=0.36, depicted by the three-dimensional Heisenberg universality class. The earlier reported Imma→I2/m phase transition above room temperature has not been observed.     

Bulk and surface structure and high-temperature thermoelectric properties of inverse clathrate-III in the Si-P-Te system

The creation of thermoelectric materials for waste heat recovery and direct solar energy conversion is a challenge that forces the development of compounds that combine appreciable thermoelectric figure‐of‐merit with high thermal and chemical stability. Here we propose a new candidate for high‐temperature thermoelectric materials, the type‐III Si_172?xP_xTe_y cationic clathrate, in which the framework is composed of partially ordered silicon and phosphorus atoms, whereas tellurium atoms occupy guest positions. We show that the utmost stability of this clathrate (up to 1500 K) in air is ensured by the formation of a nanosized layer of phosphorus‐doped silica on the surface, which prevents further oxidation and degradation. As‐cast (non‐optimized) Si‐P‐Te clathrates display rather high values of the thermoelectric figure‐of‐merit (ZT=0.24–0.36) in the temperature range of 700–1100 K. These ZT values are comparable to the best values achieved for the properly doped transition‐metal‐oxide materials. The methods of the thermoelectric efficiency optimization are discussed.     

Synthesis and structure analysis of tetragonal Li7La3Zr2O12 with the garnet-related type structure

We have successfully synthesized a high-purity polycrystalline sample of tetragonal Li₇La₃Zr₂O₁₂. Single crystals have been also grown by a flux method. The single-crystal X-ray diffraction analysis verifies that tetragonal Li₇La₃Zr₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.134(4) Å and c=12.663(8) Å. The garnet-type framework structure is composed of two types of dodecahedral LaO₈ and octahedral ZrO₆. Li atoms occupy three crystallographic sites in the interstices of this framework structure, where Li(1), Li(2), and Li(3) atoms are located at the tetrahedral 8a site and the distorted octahedral 16f and 32g sites, respectively. The structure is also investigated by the Rietveld method with X-ray and neutron powder diffraction data. These diffraction patterns are identified as the tetragonal Li₇La₃Zr₂O₁₂ structure determined from the single-crystal data. The present tetragonal Li₇La₃Zr₂O₁₂ sample exhibits a bulk Li-ion conductivity of σ_b=1.63×10⁻⁶ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=5.59×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.54 eV in the temperature range of 300–560 K.     

Rietveld Refinement of Tetragonal V-ZrO2 Solid Solutions Obtained from Gels by X-ray Powder Diffraction

The crystal structure of three tetragonal V_xZr_1−xO₂ solid solutions, with x=0.025, 0.05, and 0.075, prepared by heating dried gel precursors at 450°C in air atmosphere, have been determined by Rietveld refinement on the basis of powder X-ray powder diffractometer data. They contain V⁴⁺ cations surrounded by eight oxygens, four at a distance between 2.079 and 2.093 Å and another four at longer distances between 2.369 and 2.348 Å. The estimation of the crystal average oxygen position from the X-ray lattice parameter of V_xZr_1−xO₂ conform with the relationship proposed by Howard et al. (J. Am. Ceram. Soc. 81, 241 (1998)).     

Crystal structure of Ca2Ln3Sb3O14 (Ln=La, Pr, Nd and Y): A novel variant of weberite

The crystal structures of Ca₂Ln₃Sb₃O₁₄ (Ln=La, Pr, Nd and Y) and Ca₂Sb₂O₇ at room temperature were refined by the Rietveld method using combined X-ray and neutron powder diffraction data. Ca₂Sb₂O₇ adopts the weberite structure having the space group Imma. The structures of Ca₂Ln₃Sb₃O₁₄ are, however, neither the orthorhombic nor the tetragonal chiolite as has been suggested previously. They crystallize in the monoclinic space group I2/m11 belonging to a hitherto unknown type of deformation of the parent (orthorhombic) weberite structure.     

The crystal structure of 3-methyluracil from X-ray powder diffraction data

The crystal structure of 3-methyluracil has been determined ab initio by conventional monochromatic X-ray powder diffraction data. The crystal data are: orthorombic, a=6.6294(1), b=13.1816(3), c=6.53938(9) (Å), V=571.45(3) (ų), space group Pbnm, Z=8. The structure was solved by direct methods and the final Rietveld refinement converged to R_p=0.0398, R_wp=0.0528, R_Bragg=0.0294. The crystal structure exhibits endless chains of planar molecules, connected via head-to-tail N-H?O hydrogen bonds.     

Neutron powder diffraction study of tetragonal Li7La3Hf2O12 with the garnet-related type structure

We have successfully synthesized a polycrystalline sample of tetragonal garnet-related Li-ion conductor Li₇La₃Hf₂O₁₂ by solid state reaction. The crystal structure is analyzed by the Rietveld method using neutron powder diffraction data. The structure analysis identifies that tetragonal Li₇La₃Hf₂O₁₂ has the garnet-related type structure with a space group of I4₁/acd (no. 142). The lattice constants are a=13.106(2) Å and c=12.630(2) Å with a cell ratio of c/a=0.9637. The crystal structure of tetragonal Li₇La₃Hf₂O₁₂ has the garnet-type framework structure composed of dodecahedral La(1)O₈, La(2)O₈ and octahedral HfO₆. Li atoms occupy three types of crystallographic site in the interstices of this framework structure, where Li(1) atom is located at the tetrahedral 8a site, and Li(2) and Li(3) atoms are located at the distorted octahedral 16f and 32g sites, respectively. These Li sites are filled with the Li atom. The present tetragonal Li₇La₃Hf₂O₁₂ sample exhibits bulk Li-ion conductivity of σ_b=9.85×10⁻⁷ S cm⁻¹ and grain-boundary Li-ion conductivity of σ_gb=4.45×10⁻⁷ S cm⁻¹ at 300 K. The activation energy is estimated to be E_a=0.53 eV in the temperature range of 300-580 K.     

Synthesis, structure and electronic structure of a new polymorph of CaGe2

Reported are the flux synthesis, the crystal structure determination, the properties and the band structure calculations of a new polymorph of CaGe₂, which crystallizes with the hexagonal space group P6₃mc (no. 186) with cell parameters of a=3.9966(9) and c=10.211(4) Å (Z=2; Pearson's code hP6). The structure can be viewed as puckered layers of three-bonded germanium atoms, , which are stacked along the direction of the c-axis in an ABAB-fashion. The germanium polyanionic layers are separated by the Ca cations. As such, this structure is closely related to the structure of the other CaGe₂ polymorph, which crystallizes with the rhombohedral CaSi₂ type in the R3¯m space group (No. 166), where the layers are arranged in an AA′BB′CC′-fashion, and are also interspaced by Ca²⁺ cations. LMTO calculations suggest that in spite of the formal closed-shell configuration for all atoms and the apparent adherence to the Zintl rules for electron counting, i.e., Ca²⁺[3b-Ge¹⁻]₂), the phase will be a poor metal due to a small Ca-3d-Ge-4p band overlap. Magnetic susceptibility measurements as a function of the temperature indicate that the new CaGe₂ polymorph exhibits weak, temperature independent, Pauli-paramagnetism.     

The structure of CeAlO3 by Rietveld refinement of X-ray powder diffraction data

The room temperature structure of perovskite CeAlO₃ has been reinvestigated by X-ray powder diffraction. The Rietveld refinement has confirmed the tetragonal symmetry; but revealed a super cell, a=5.32489(6) Å and c=7.58976(10) Å, with the space group I4/mcm. In CeAlO₃, the distortion from the ideal cubic perovskite is caused by the cooperative tilting of the AlO₆ octahedra around the primitive cubic [001]_p-axis.     

The Hydrogenation of the Zintl Phase NdGa Studied by in situ Neutron Diffraction

The hydrogenation of the Zintl phase NdGa was studied by in situ neutron powder diffraction. We find a compositional range of 0.1 < x < 0.8 in NdGaH_1+x. Hydrogen atoms are located in two different positions, in HNd₄ tetrahedra, and close to the polyanionic chains. For the latter, the Ga–H distance in NdGaH_1.66 is quite long (ca. 200 pm) with a trigonal bipyramidal Nd₃Ga₂ surrounding of hydrogen atoms. Hydrogen poor NdGaH_<1 phases as known for similar systems were not observed. The changing hydrogen content shows no measureable effect on the unit cell volume, but on lattice parameter ratios. Superstructures occur for 0.53 < x < 0.66 and 0.73 < x < 0.8, leading to a doubling or tripling of the lattice parameter a. They are probably caused by partial hydrogen ordering. The threefold superstructure contains a ¹_∞[(Ga–H–Ga–H–Ga)^6–] moiety with hydrogen bridging two gallium chains.     

The Superstructure of Lead Tetragonal Tungsten Bronze

Electron-optical studies of the superstructure of lead tetragonal tungsten bronze (lead-TTB) are presented. Samples were synthesized for a range of lead compositions and synthesis conditions. For lead-TTB synthesized for very short reaction times, compositional analysis combined with electron diffraction revealed these specimens to also contain significant levels of an intergrowth tungsten bronze (ITB) phase. It was proposed that ITB was formed during the reaction as an intermediate between the tungsten oxide reagent and lead-TTB. Electron-diffraction investigations of lead-TTB over a range of specimen compositions determined that the large majority of crystallites examined exhibited a well-ordered 2√2a_TTB×√2b_TTB×2c_TTB supercell periodicity, although evidence of a larger a-axis repeat was also observed. No evidence for a variation in the supercell with lead composition was observed. A model for the supercell was generated from consideration of stacking sequences of correlated ordered arrays of lead and tilted octahedra and it was demonstrated that larger superstructures could be generated using alternative stacking sequences.