Dependent Neutron Powder Diffraction Research Articles

Observation of Anisotropic Thermal Expansion and the Jahn-Teller Effect in Double Perovskites Sr2-xLaxCoNbO6 Using Neutron Diffraction.

We use temperature dependent neutron powder diffraction (NPD) to investigate the structural changes and magnetic interactions in double perovskite Sr2-xLaxCoNbO6 (x = 0.4 and 0.6). A structural phase transition from tetragonal (I4/m) to monoclinic (P21/n) is observed between x = 0.4 and 0.6 samples. Interestingly, the temperature evolution of the unit cell parameters follows the Grüneisen approximation, and the analysis suggests an isotropic thermal expansion in the case of the x = 0.4 sample, whereas the x = 0.6 sample shows the anisotropy where the thermal expansion along the c-axis significantly deviates from the Grüneisen function. We observe the z-out Jahn-Teller distortion in the CoO6 and consequently z-in distortion in the adjacent NbO6 octahedra. With an increase in the La substitution, a decrease in the degree of octahedral distortion is evident from the significant reduction in the local distortion parameter Δ around the B-site atoms.

Structural, magnetic and magnetodielectric study in the layered oxide BiPbSr2MnO6

Layered oxide BiPbSr2MnO6 (BPSMO) is investigated from structural, magnetic and magnetodielectric perspective. Room temperature Rietveld refinement of neutron powder diffraction pattern shows that the compound crystallizes in orthorhombic ‘Amaa’ structure. Along with an antiferromagnetic ordering (TN) at 190 K, a weak ferromagnetic component is substantiated from magnetization isotherm measured at 175 K as well as temperature dependent neutron powder diffraction. Magnetic reflections were indexed in the nuclear lattice with propagation vector k = (0, 0, 0). Rietveld refinement of powder pattern at 2.8 K demonstrates that magnetic moments are almost normal to the a–b plane in an antiferromagnetic arrangement, but are canted slightly towards a–b plane. The Jahn–Teller distortion shows a substantial increase in the ordered state with decrease in temperature while the variation is trifling above the ordering temperature. The dielectric measurement shows a contrasting nature of the dielectric data for temperatures below and above TN, thus hinting a plausible magnetodielectric (MD) coupling. A confirmation to this coupling is seen in the temperature and field dependent magnetodielectric (MD) results. Substantial MD effect of ~ − 2% is observed at 175 K, near to the vicinity of magnetic transition.

Magnetic structure of ternary rare-earth alloy Ho1/3Tb1/3Er1/3

Temperature dependent neutron powder diffraction measurements have been performed on the ternary rare-earth alloy Ho1/3Tb1/3Er1/3. A dominant hexagonal closed packed structure is found, with a surface impurity phase that does not have any discernible magnetic signal. Three different magnetic phase transitions have been identified in the sample: (1) paramagnetic to helical k = (0,0,0.18) near T1 = 90 K, (2) addition of another helix k = (0,0,0.34) near T2 = 70 K, and (3) onset of ferromagnetism near T3 = 45 K. There is a degree of ambiguity in the direction of the ferromagnetic moments, but we argue that a c-axis ferromagnetic moment is most plausible. At T = 3.5 K the magnetic moments are found to be mc = 2.83 μB, mab[kc = 0.19] = 3.96 μB, and mab[kc = 0.37] = 3.17 μB.

Neutron diffraction studies on magnetic properties of Ca5Ni4V6O24

The temperature dependent neutron powder diffraction (NPD) and magnetization measurements of vanadium based garnet Ca5Ni4V6O24 (CNVO) have been performed to explore its crystal and magnetic structures. The magnetization results illustrate two magnetic anomalies at 7 and 4K. The Rietveld analysis of room temperature x-ray diffraction and NPD data confirms its Ia-3d crystal structure. The temperature dependent NPD shows the emergence of magnetic reflections below 7K whose intensity keeps on increasing with decreasing temperature down to 1.5K. The crystal structure remains cubic down to 1.5K. Rietveld analysis reveals that below 7K, the Ni moments in CNVO undergo a commensurate collinear A-type antiferromagnetic ordering with propagation vector k=(0,0,0) and the per site ordered moment of Ni2+ is 1.69±0.05μB. Absence of any additional magnetic or nuclear reflections below 4K confirms that the nuclear and magnetic structures remain invariant across the 4K magnetic anomaly.

Superionic phase transitions in anti-fluorite structures

Fast ion conductors attract continuous and increasing interest in view of possible applications in battery technology. Early examples of superionic phase transitions in anti-fluorite type structures, where the small cations reside in a tetrahedral cage of large anions include Ag2Te [1]. At elevated temperatures anharmonic atom thermal displacements induce cation diffusion towards the large void formed by the central anion octahedron. Of the anti-fluorite structure type compounds Li2 X, where X=(O, S, Se, Te), the compounds Li2O and Li2S showed diffuse transitions to a superionic phase. Very recent advances in battery technology of these compounds [2] motivated us to investigate the end member Li2Te [3] by temperature dependent neutron powder diffraction. The quasi-harmonic temperature dependence of the Li thermal displacement factor shows a distinct steepening of slope around 4000C, indicating a phase transition to a superionic phase. Analysis of derived probability density functions and atom potentials again reveal a corresponding increase of anharmonic, anisotropic Li-ion motion towards the octahedral void. This indicates opening up of Li-ion diffusion pathways at the phase transition. The superionic phase transitions of the Li2X anti-fluorite type structures are steered by their cation-anion distance ratio, which in turn determines their respective transition temperatures. The superionic phase transitions mark the onset of cation sublattice melting, where these transition temperatures are proportional to the melting temperatures of the entire compounds.

Complex structural phase transitions in slightly Ca modified Na0.5Bi0.5TiO3

A temperature dependent neutron powder diffraction study, in conjunction with dielectric and ferroelectric characterization, of slightly Ca modified Na0.5Bi0.5TiO3 (NBT) revealed an instability with regard to a non-polar orthorhombic (Pbnm) distortion above room temperature. This intermediate orthorhombic phase has earlier been reported for unmodified NBT by electron diffraction studies, but has never been captured by global (x-ray/neutron) diffraction techniques. Calcium substitution seems to amplify the magnitude of this intermediate orthorhombic distortion thereby making the corresponding superlattice reflections become visible in the neutron diffraction pattern. The study revealed the following sequence of very complex structural evolution with temperature: .

Phase Behaviour of CoCl2-MnCl2Mixed Crystals

AbstractThermal properties of CoCl2, MnCl2and their mixed crystals are investigated by means of temperature dependent neutron powder diffraction and time-resolved small angle neutron scattering. The coefficients of thermal expansion of these crystals have been determined as a function of temperature. The temperature-induced structural variations can be explained by distortions of CoCl6/MnCl6octahedra and changes in Co/Mn–Cl bond lengths. CoCl2and MnCl2form homogeneous solid solutions in the entire temperature range between the solidus and 50 K. There is no indication of any miscibility gap as predicted from thermodynamic calculations.

Magnetic structure and magneto-elastic-structural coupling in Cr-modified SrRuO3: A neutron powder diffraction study

Temperature dependent neutron powder diffraction and magnetization studies have been carried out on a Cr-substituted SrRuO3 close to the solubility limit, that is, SrRu0.88Cr0.12O3 (SRC-12). Evidence of ferromagnetic ordering below 200 K was deduced from magnetization studies as well as from neutron powder diffraction. The magnetic lattice coincides with the chemical one (k = 0) and the major component of the ordered magnetic moment has been found along the c-axis. The system exhibits anomalous variation in the cell volume close to the magnetic ordering temperature. The spontaneous magnetostrictive strain below the ordering temperature, estimated by fitting the temperature dependence of the lattice parameters and the cell volume using Debye–Grüneisen function, revealed that the main contribution to the volume magnetostriction comes from the c-axis. The magneto-elastic behavior is further shown to be coupled to the octahedral tilt about the c axis. The analysis of magnetic structure and the magneto-elastic strain suggests c axis to be the easy axis of magnetization for this system.

Magnetic field dependent neutron powder diffraction studies of Ru0.9Sr2YCu2.1O7.9

Temperature and magnetic field dependent neutron diffraction has been used to study the magnetic order in Ru0.9Sr2YCu2.1O7.9. The appearance of (1/2, 1/2, 1/2), (1/2, 1/2, 3/2), and (1/2, 1/2, 5/2) peaks below TM=140 K manifests the antiferromagnetic order. Neutron diffraction patterns measured in applied magnetic fields from 0 to 6 T show the destruction of the antiferromagnetic order with increasing field. There is no evidence of spontaneous or field-induced long range ferromagnetic order. This latter result contradicts the vast majority of other experimental observations for this system.

Structure and phase transition ofNa0.5La0.5TiO3

A temperature dependent neutron powder diffraction study was carried out onNa0.5La0.5TiO3 (NLT) from 298 K up to 1373 K. Rietveld analysis using high resolution x-ray and neutronpowder diffraction data confirmed the rhombohedral structure (space group ) of this compound. The rhombohedral phase transforms to cubic () above 873 K. The temperature variation of the tilt angle, a continuous change of thespecific volume across the transition, and the strain analysis suggest a second order natureof the cubic–rhombohedral phase transition, which is in conformity with the grouptheoretical prediction based on symmetry considerations.

Neutron diffraction analysis of magnetic ordering in YSr2Cu2RuO8

The superconducting ruthenocuprates RESr2Cu2RuO8 (where RE is a rare earth or Y) have been interpreted as ferromagnetic superconductors, based on a strong response to applied field at the Ru Néel temperature (around 130K). However, neutron diffraction measurements on the Gd compound by Lynn et al. [Phys. Rev. B 61, 1214964 (2000)] (using a separated isotope) show evidence only of antiferromagnetic order, while very limited data by Takagiwa et al. [J. Phys. Soc. Jpn. 70, 333 (2001)] on the Y compound show one antiferromagnetic Bragg peak and possible coexisting ferromagnetism. We have studied a sample of YSr2Cu2.1Ru0.9O7.9 prepared by a high pressure-high temperature route. Temperature dependent neutron powder diffraction shows antiferromagnetic order, manifested by the observation of the 12, 12, 12 and 12, 12, 32 reflections, which appear at 135K [as suggested by the superconducting quantum interference device (SQUID) data]. Their intensities at the lowest temperature are consistent with a c-axis orientation of the Ru magnetic moments. Although the lowest order nuclear Bragg peak shows a small intensity increase as the temperature is lowered, this is attributed to magnetic ordering of the Cu planes and not to the presence of a ferromagnetic component, in agreement with the polarized neutron data of Lynn et al. [Phys. Rev. B. 61, 1214964 (2000)] Magnetic resonance and SQUID data on the same sample (to be reported elsewhere) show a complex picture, associated with Cu magnetic ordering.

Charge ordering in Ag 2BiO 3

In order to resolve discrepancies between the valence state +IV as assigned to bismuth in Ag 2BiO 3 based on structural arguments, and its physical properties, we have reinvestigated this compound. By single crystal structure analyses at different temperatures in combination with neutron powder diffraction, systematic twinning was recognized. Thus, the former model describes the average structure of the twinned crystal. By refining the data set assuming an inversion twin, the correct crystallographic description has been achieved (Ag 2BiO 3, P n n 2 , a = 5.9830 ( 8 ) , b = 6.3239 ( 7 ) , c = 9.5762 ( 13 ) Å , Z = 4 ). In this model, there are two distinct crystallographic sites for bismuth, allowing for a charge ordering according to Ag 2Bi 3+Bi 5+O 3. Average distances in BiO 6-octahedra are 2.34 and 2.13 Å, for Bi 3+ and Bi 5+, respectively. The bond valence sums are calculated as 3.08 and 5.06. This charge ordering at room temperature fully explains the semiconducting behavior and diamagnetism of the compound. Moreover, temperature dependent neutron powder diffraction (2–298 K) revealed a monoclinic distortion of the lattice upon cooling that was confirmed independently by single crystal investigations (100 K). The structure was refined in Pn ( a = 5.95492 ( 10 ) , b = 6.31001 ( 9 ) , c = 9.58082 ( 13 ) Å , β = 92.4823 ( 2 ) ° , Z = 4 , T = 100 K ). Except for a tilting of the BiO 6-octahedra with oxygen atoms at common vertices acting as hinges, no but slight structural changes have been observed. The bismuth charge ordering is unaffected by the orthorhombic/monoclinic phase transition.

Structural and magnetic properties of LaFe 0.5Cr 0.5O 3 studied by neutron diffraction, electron diffraction and magnetometry

The structural and magnetic properties of the perovskite type compound LaFe 0.5Cr 0.5O 3 have been studied by temperature dependent neutron powder diffraction and magnetization measurements. Rietveld refinement of the neutron diffraction data shows that the compound crystallizes in an orthorhombic perovskite structure with a random positioning of the Fe and Cr cations at the B sublattice. The magnetic structure at 10 K is a collinear antiferromagnetic one with the magnetic moment per site being equal to 2.79(4) μ B. Magnetisation measurements confirm the overall antiferromagnetic behaviour. Moreover, it indicates a weak uncompensated magnetic moment close to the transition temperature T N ≈ 265 K. This moment can be described by a magnetic cluster state, which remains up to 550 K. Electron diffraction patterns along with high-resolution transmission electron microscopy images reveal that the crystallites are composed by domains of different orientation, which share the same cubic perovskite sub-cell reflections.

Temperature Dependent Neutron Powder Diffraction Study of the Laves Phase Compound TbCo2.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Temperature dependent neutron powder diffraction study of the Laves phase compound TbCo 2

The structure, magnetization and magnetostriction of Laves phase compound TbCo 2 are investigated by temperature dependent high resolution neutron powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure above its Curie temperature T C and exhibits a rhombohedral distortion (space group R 3 ¯ m ) below T C. By an appropriate extrapolation of the temperature factor of Co atom above T C, the Rietveld refinement of the neutron powder diffraction data of the rhombohedral structure converges satisfactorily and reveals that the moments of Co1(3b) and Co2(9e) are almost equal. Tb moment follows well the Brillouin function. The total magnetic moment of TbCo 2 is about 5.8 μ B/f.u., the anisotropic magnetostriction constant λ 111 is about 4.6 × 10 −3 and the volume magnetostriction ω s is about 8.7 × 10 −3 at 14 K.

Formation of a cubic Sr 2MnWO 6 phase at elevated temperature; a neutron powder diffraction study

In a temperature dependent neutron powder diffraction (NPD) study we observed the high temperature cubic phase at 973 K in the polycrystalline double perovskite Sr 2MnWO 6. Rietveld analysis of the NPD data shows that the room temperature tetragonal phase exists up to 573 K (space group P4 2/ n, a=8.0119 (4) Å, c=8.0141(8) Å). At 773 K, the primitive tetragonal symmetry change to body-centred tetragonal (space group I4/ m, a=5.6935(5) Å, c=8.077(1) Å) and finally at 973 K it becomes face-centred cubic (space group Fm-3 m, a=8.0864(8) Å). The changes in the structural symmetry are connected to the small distortion of the B-site octahedra, which are insensitive to the Differential Thermal Analysis (DTA) signal.

Evolution of the atomic and magnetic structure of Sr 3Fe 2WO 9: A temperature dependent neutron powder diffraction study

Both the nuclear and magnetic structure of a complex perovskite with composition Sr 3Fe 2WO 9 (SFWO) has been studied by neutron powder diffraction (NPD) at different temperatures. The most striking feature of this type of complex perovskite is the coexistence of ferrimagnetic and antiferroelectric types of ordering below T = 373 K. The NPD data at 10 and 300 K were refined with a model in the tetragonal space group I4/m. SFWO has a partially ordered perovskite structure, in which Fe and W cations distribute regularly to some degree at the B-sites in the ABO 3 perovskite lattice. For both temperatures magnetic diffraction peaks were registered, and a possible model for the magnetic structure was proposed in accordance with the ferrimagnetic properties of SFWO.

Interplay of spin and orbital ordering in the layered colossal magnetoresistance manganiteLa2−2xSr1+2xMn2O7(0.5<~x<~1.0)

The crystallographic and magnetic phase diagram of the n=2 layered manganite La2-2xSr1+2xMn2O7 in the region x=>0.5 has been studied using temperature dependent neutron powder diffraction. The magnetic phase diagram reveals a progression of ordered magnetic structures generally paralleling that of 3-D perovskites with similar electronic doping: A (0.5<=x<=0.66) --> C (0.75<=x<=0.90) --> G (0.90<=x<=1.0). However, the quasi-2-D structure amplifies this progression to expose features of manganite physics uniquely accessible in the layered systems: (a) a "frustrated" region between the A and C regimes where no long-range magnetic order is observed; (b) magnetic polytypism arising from weak inter-bilayer magnetic exchange in the Type-C regime; and (c) a tetragonal to orthorhombic phase transition whose temperature evolution directly measures ordering of d3y2-r2 orbitals in the a-b plane. This orbital-ordering transition is precursory to Type-C magnetic ordering, where ferromagnetic rods lie parallel to the b-axis. These observations support the notion that eg orbital polarisation is the driving force behind magnetic spin ordering. Finally, in the crossover region between Type-C and Type-G states, we see some evidence for the development of local Type-C clusters embedded in a Type-G framework, directly addressing proposals of similar short-range magnetic ordering in highly-doped La1-xCaxMnO3 perovskites.

Temperature dependent neutron powder diffraction study of Bi 3(GaSb 2)O 11

The structure of Bi 3(GaSb 2)O 11 as a function of temperature (10≤ T≤295 K) has been refined from powder neutron diffraction data using a Rietveld analysis. No structural phase transitions were observed in the range 10–295 K. The structure of Bi 3(GaSb 2)O 11 was refined in space group Pn 3 ̄ . The Atomic Displacement Parameters (ADP) of the one of the Bi atoms, Bi(1), were both large and anisotropic. It was determined from the temperature variation that this is due to a positional site disorder and the open geometry. A disordered model, involving splitting the Bi(1) atoms around its 3 fold axis, was found to significantly improve the structural refinements.

Neutron diffraction study, magnetic structure and thermodiffractometry study of the β″ Na(H2O, H3O+) ferrite

Temperature dependent neutron powder diffraction has been used to study the β″ Na(H2O, H3O+) ferrite compound from room temperature to 830°C. The following phenomena were observed: 1.(i) The existence of two superimposed magnetic modes, one ferromagnetic described with the propagation vector k = [0 0 0], the other one antiferromagnetic which is described with the propagation vector k = [0 032]. The antiferromagnetic (AF) model arrangement is in fact an extrapolation of Gorter's (AF) collinear model proposed for the potassium β-ferrite. The two magnetic modes are in agreement with the representation theory analysis.2.(ii) The existence of two transitions one at T = 250°C (of magnetic origin), the other one (of nuclear or magnetic origin) at T = 450°C.3.(iii) From the variation of cell parameters with temperature there is evidence of the complete desorption of water at T = 220°C.