Rietveld Profile Refinement Research Articles

Structure determination of KLaNb2O7 exhibiting ion exchange ability by X-ray powder diffraction

The crystal structure of KLaNb2O7 has been determined from X-ray powder diffraction data. The cell exhibits orthorhombic symmetry and belongs to C222 space-group with a=3.9060(1) Å, b=21.6031(7) Å, c=3.8879(1) Åand Z=2. The structure was refined using a Rietveld profile refinement method. The final reliability factors achieved were RWP=0.0954, RP=0.0676, RI=0.0290 and RF=0.0152. The structure is built up from double perovskite layers, LaNb2O7, interleaved by potassium atoms. The potassium ions were found to be statistically distributed at an interlayer site with an occupation factor of about 0.5. The ion exchange ability of this compound is discussed on the basis of the crystal structure.

Neutron powder diffraction study of the fast-ion transition in β-PbF 2

A high-resolution neutron powder diffraction study of the fluorite structured compound β-PbF 2 has been performed at ∼100 different temperatures in the range 273–1078 K. Rietveld profile refinement of the data confirms the presence of an anomaly in the thermal expansion coefficient at temperatures close to the fast-ion transition ( T c≈711 K) and gives detailed information concerning the extent of anion disorder within the fast-ion phase.

Neutron diffraction studies of the Zr 3P-D system

Products obtained by absorption of different amounts of deuterium in Zr 3P have been investigated by X-ray and neutron diffraction methods. Crystal structure analyses were performed using the Rietveld profile refinement technique on the neutron data. In one sample, an interstitial solid solution of deuterium in Zr 3P was identified. Its composition corresponded to the formula Zr 3PD 0.15, with the Zr 3P sublattice essentially being unchanged as compared to that of the pure phosphide (Ti 3P-type, space group P 4 2 n ). A second more deuterium-rich sample was found to contain two phases in nearly equal amounts. One of them was a solid solution of deuterium in Zr 3P of the approximate composition Zr 3PD 0.78, while the other phase was found to be isostructural with the Zr 3PD 3−x phase (sublattice of the β-V 3S-type, space group P 4 2 nbc , x ≈ 0.1) reported in a previous study, its composition according to the structure refinement being close to Zr 3PD 2.60. In both cases, all deuterium atoms are surrounded by four near-neighbour zirconium atoms in more or less distorted tetrahedral arrangements. Assuming the action of local deuterium ordering, no D-Zr, D-P and D-D distances shorter than 1.9, 2.7 and 2.3 Å were observed.

Neutron diffraction investigation of Si site preference in RENi 10Si 2 compounds

High resolution time-of-flight neutron powder diffraction has been performed on the intermetallic compound YNi 10Si 2 of the ThMn 12 (space group I 4/ mmm) tetragonal structure at room temperature. Rietveld profile refinement of the powder pattern gave detailed structural parameters. The Si atoms are found to preferentially occupy the 8⨍ site with some small occupancies also for both the 8 i and 8 j sites. A minority impurity phase has been identified as that of the cubic NaZn 13-type structure (space group Fm 3c) . This present neutron diffraction investigation of metalloid substitution on the three transition metal sublattices in this structure type, as well as previous neutron studies, confirm that both atomic size and enthalpy effects are important in determination of preferential site ordering.

Structural and Defect Study of Naß''-Gallatc by X-Ray Powder Diffraction Analysis

Abstract The Rietveld profile refinement has been applied to X-ray powder diffraction data on Naß''-gallate, a solid electrolyte isostructural with Naß''-alumina and characterized by different types of lattice defect. Several structural models have been refined and discussed in order to understand if a statistical comparison of the structure discrepancy factors can be used as a reliable criterion to retain or reject structural changes from one model to the other. It is shown that, due to the correlation between structural parameters and to the poor sensitivity of the structure discrepancy factors to chemical composition, some care is necessary to direct correctly the refinement and to obtain a reliable structural model

Synthesis and structure of an ABW type thallium aluminosilicate

TlABW, TlAlSiO 4 was prepared hydrothermally in presence of Li + , Na + , and K + ions, and its structure was determined from X-ray powder diffraction data. The unit cell is a = 8.297 (1), b = 9.417 (1), c = 5.413 (1) A, and the space group is Pna2 1 , No. 33, Z = 4 . The structure was refined using powder diffraction data by the Rietveld profile refinement technique. The anion framework has an ordered Si/Al distribution. The extraframework cations (Tl + ) occupy three partially populated positions. The structure is described, and the synthesis of TlABW in presence of lithium, sodium, or potassium ions is discussed.

Mössbauer and magnetic studies of the R 2Fe 17N x (R = Lu, Sm) intermetallic compounds

Interstitial ternary nitrides R 2Fe 17N x have been prepared for R=Lu and Sm. The compound with Lu has the Th 2Ni 17 type structure whereas the compound with Sm has the Th 2Zn 17 type structure. The crystallographic constants were determined by means of Rietveld's profile refinement technique. Magnetic measurements and Mossbauer spectra of the compounds R 2Fe 17N x were obtained between 4.2 and 300 K and compared with the non nitrogenated ones. The Curie temperatures of these compounds are 753 K for R=Sm and 683 K for R=Lu.

Rietveld refinement of Na 1.8Be 0.9Si 1.1O 4; an ionic conductor with a cristobalite-related structure

The structure of the ionic conductor Na 1.8Be 0.9Si 1.1O 4 has been refined with the Rietveld Profile Refinement technique, based on X-ray powder diffraction data. The unit cell parameters were found to be a=4.920, b=9.876 and c=13.922 A ̊ . With space group symmetry Pcba assumed, the refinement yielded R F=0.049 and R wp=0.088. The cristobalite-related structure is built up of corner-shared [MeO 4]-tetrahedra, with Me=Si and Be. The Si 4+ and Be 2+ ions are statistically distributed and the Na + ions, located in the cavities formed by the tetrahedra, are found to be five-and four-coordinated. The findings are discussed in relation to the structure of Na 2BeSiO 4 and migration pathways for the Na + ions.

Synthesis and crystal structure of lithium phosphorus nitride Li 7PN 4: The first compound containing isolated PN 4-tetrahedra

Single phase Li 7PN 4 was synthesized under nitrogen atmosphere at 620°C by solid state reaction of Li 3N and P 3N 5 with molar ratio 7:1. Li 7PN 4 has cubic symmetry, P 43n, a = 936.48(3) pm, Z = 8 . (Rietveld profile refinement method; X-ray powder diffraction data; R(wp) = 0.099 and R( I, hkl) = 0.0744; 77 nonzero reflections observed, 9 positional parameters refined.) Li 7PN 4 is an ionic solid containing both lithium cations and isolated PN 4 anions. Atomic arrangement in the solid derives from an anti-fluorite type of structure. The PN bond lengths (average: 171 pm) correspond to a bond order of approximately 1 1 4 in the PN 4-tetrahedra. In the cubic unit cell the phosphorus atoms resemble β-tungsten analogous arrangement.

Neutron diffraction and transmission electron microscopy study of hydrogen-induced phase transformations in Zr3Al

Hydrogen-induced phase transformations of the equilibrium intermetallic compound Zr3 Al have been studied by in situ elastic neutron scattering, x-ray diffraction, and transmission electron microscopy (TEM). TEM observations reveal two distinct modes of amorphous phase formation in Zr3 Al upon hydrogenation, namely, heterogeneous nucleation at preexisting grain boundaries, and homogeneous nucleation within single-crystal grains. In situ neutron diffraction reveals a phase separation between a hydrogen (deuterium) poor and a hydrogen (deuterium) rich crystalline phase. Rietveld profile refinement of the neutron diffraction data indicates predominant hydrogen (deuterium) occupation of the octahedral interstitial sites in the crystalline matrix that have only Zr nearest-neighbor atoms. The two different modes of amorphous phase nucleation are directly related to the degree of hydrogen dissolution in the host crystalline matrix.

Powder X-ray diffraction studies of a low-temperature phase transition in the n-hexadecane/urea inclusion compound

Powder X-ray diffraction studies of the n-hexadecane/urea inclusion compound have been carried out between 296 and 46 K to investigate structural aspects of the host (urea) structure, particularly in relation to a low-temperature phase transition in this compound. Our results show that, at ca. 150 ± 1 K, the symmetry of the host structure changes from hexagonal (high-temperature phase; space group P6122) to orthorhombic (low-temperature phase; probable space group P212121). This assessment of the transition temperature is in good agreement with previous measurements using other experimental approaches, and our interpretation of the structural change associated with the transition confirms earlier speculations based upon single-crystal X-ray diffraction investigations. Although the transition involves a major change of crystal symmetry, there is only a relatively minor change in the actual distribution of scattering matter within the crystal, and we outline, in general terms, a systematic approach towards rationalizing changes in a powder diffractogram arising from such a transition. No satisfactory determination of the host structure within the low-temperature phase has been reported previously, and since multiple crystal twinning is thought to accompany the phase transition on passing from the high-temperature phase to the low-temperature phase, powder X-ray diffraction represents a more viable route to this information than single-crystal X-ray diffraction. Structure refinement from the powder X-ray diffraction data collected at 120 K has been attempted using the Rietveld profile refinement method.

ChemInform Abstract: The Structures of Lanthanum Hydroxide Nitrates Investigated by the Rietveld Profile Refinement Technique.

AbstractLa(OH)2NO3 crystallizes in space group P21/m with Z=2 and La(OH)2NO3·H2O crystallizes in C2/m with Z=4.

Crystal structure of Li2TbF6 and magnetic study under high magnetic Field

The crystal structure of Li2TbF6 was solved ab-initio at 300 and 5 K using X-ray and neutron powder diffraction data respectively. This compound crystallizes in the monoclinic system, space group P21/C (n° 14), Z = 4, with a = 7.585(1) Å, b = 4.965(1) Å, c = 11.116(1) Å and β = 106.96(1)° at 300 K.The structure was solved and refined using direct methods and Rietveld profile refinement techniques. Li2TbF6 exhibits a completely new structural type for A2MF6 compounds with Tb4+ in bicapped trigonal prisms. Lithium ions in this structure adopt two types of coordination polyhedra : octahedra and unusual square pyramids. Relationships with γ-Na2-UF6 and CaTa2O6 structural type will be presented.The saturation magnetization measured in a 200 kθe magnetic field at 4.2 K has been found to be equal to 6.9 μB, what provides confirmation of the degree 4+ of the terbium which is isoelectronic with Gd3+ and exhibits a 8Ssol72 ground state.The Curie law is not found to be obeyed at low temperatures. This behaviour will be discussed in terms of magnetic interactions or splitting of the ground state level in zero-field.

The magnetic structure of Pd 3MnD 0.53

The magnetic structure of ordered Pd 3MnD 0.53 has been determined using the Rietveld profile refinement method on neutron powder data collected at 4.2 K. The tetragonal crystal structure is described in space group I4/ mmm. A colinear antiferromagnetic structure with a magnetic moment of 4.8 μB/Mn was found. The direction of the magnetic moments forms an angle of 55° with the tetragonal axis.

Effect of argon-annealing and subsequent oxygen-annealing on the superconductivity and structure of Tl2CaBa2Cu2O8-x

Abstract Annealing the single-phase Tl 2 CaBa 2 Cu 2 O 8 superconductor ( T c =108 K ) in argon environment at 950 K for 24 h brings the T c down to ≅80 K. Re-annealing the samples in oxygen under similar conditions restores the T c to 105±5 K. Rietveld profile refinement of the neutron diffraction spectra of the argon-annealed samples indicates that some Tl-ions, presumably Tl 1+ , move from the Tl double-layers to occupy Ca-sites. Upon subsequent annealing in oxygen, however, these Tl-ions escape from the lattice. Implications of this and other structural features are described.

Crystal structure of β-VO(HPO 4) · 2H 2O solved from X-ray powder diffraction

The crystal structure of β-VO(HPO 4) · 2H 2O has been determined from X-ray powder diffraction data. The cell is triclinic (space group P 1, Z = 4 ) with a = 5.659(2)Å, b = 7.578(4) Å, c = 12.623(5) Å, α = 89.66(2)°, β = 102.14(2)°, and γ = 92.23(2)°. Starting positional parameters were obtained by direct methods applied to 640 reflections which are derived from a modified Rietveld analysis working without structural model. The structure was refined using Rietveld profile refinement principles. The final reliability factors were R I = 0.039, R p = 0.076, and P wp = 0.092. The structure is built up of chains of corner-shared VO 4(H 2O) 2 octahedra and (HPO 4) tetrahedra alternated along the c crystallographic axis. These chains are interconnected through HPO 4 groups forming layers stacked in the [100] direction. In contrast to α-VO(HPO 4) · 2H 2O, there are no isolated water molecules between the layers.

A structural study of the sodium (V) uranate, NaUO 3, by time-of-flight powder neutron diffraction

Powder neutron diffraction using the Rietveld profile refinement method modified for time-of-flight data was used to investigate the structure of the distorted perovskite, NaUO 3 (space group Pbnm; a = 5.779(4), b = 5.907(3), and c = 8.283(4) Å). The octahedron of oxygen atoms around the pentavalent uranium atom in this compound is slightly distorted with four of six oxygen atoms at 2.152 Å and the other two at 2.145 Å.

Thermal Diffuse Scattering in Neutron Time-of-Flight Powder Patterns

ABSTRACTInelastic scattering from lattice phonons contributes a significant fraction of the observed Bragg intensity in time of flight neutron powder patterns of transition metal monoxides at elevated temperatures. Ignoring this thermal diffuse scattering (TDS) leads to errors in the site occupations of defects present in these materials. The intensity from one phonon TDS has been calculated as a function of time-of-flight and used to correct the measured intensities. The effect of this correction on the results of Rietveld profile refinement is discussed.

The structure of Na 2BeSiO 4 obtained by the Rietveld profile refinement technique

Polycrystalline Na 2BeSiO 4 was prepared by solid state reaction. The crystal structure of the compound was determined by X-ray diffraction methods. The space group is Pca2 1 and the cell dimensions, a = 9.8658 (8) A ̊ , b = 4.9109 (6) A ̊ , c = 13.8879 (15) A ̊ ; and V = 672.87 A ̊ 3 . The structure was refined by the Rietveld full-profile technique. R F and R WP values of 0.057 and 0.118 respectively were obtained. The structure was found to be related to cristobalite. The framework consists of strings of corner-sharing SiO 4 and BeO 4 tetrahedra. Two Na atoms are four-coordinated and two five-coordinated. Furthermore, the Na atoms are ordered on an almost perfect diamond net. The structure is compared with these of other alkali beryllium silicates.

Evidence for K-substitution in the Tl-sites of superconducting Tl 2CaBa 2Cu 2O χ: Neutron diffraction studies

A comparison of the Rietveld profile refinement of neutron diffraction data of the superconducting compounds Tl-2122 (i.e. Tl 2CaBa 2Cu 2O χ) and Tl 1K 1Ca 1Ba 2Cu 2O y provides evidence that nearly half of the Tl-sites belonging to the pure Tl-2122 structure get substituted by K-ions in Tl 1K 1Ca 1Ba 2Cu 2O y . The consequence of this is reflected in a substantial movement of Ba-ions towards the plane of the O(2)-ions in the K-substituted compound.