Variable Temperature Neutron Diffraction Study Research Articles

A variable temperature neutron diffraction study of dual ion conducting Sr3V2O8

Sr3V2O8 exhibits sizeable oxide ion and proton conductivity at 800 °C. In order to investigate any correlation between the crystal structure and electrical properties of Sr3V2O8, a variable temperature neutron diffraction study has been performed. Results show that there is no change in crystal symmetry upon heating. However, the VO4 tetrahedra are observed to become more distorted upon heating and more oblate in shape. The distortion arises due to a shortening of the apical V–O1 bond length and accompanying increase of the V–O2 bond length with temperature. Bond valence site energy analysis show that there is a reduction in the energy of the migration pathways for movement of the oxide ions as the Sr–V–Sr bottlenecks expand upon heating.

The Ambiguous Origin of Thermochromism in Molecular Crystals of Dichalcogenides: Chalcogen Bonds versus Dynamic Se─Se/Te─Te Bonds.

We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High-pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism - especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se─Se and Te─Te bonds. Further, a co-crystal of dpdSe with iodine (dpdSe-I2) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling.

The Ambiguous Origin of Thermochromism in Molecular Crystals of Dichalcogenides: Chalcogen Bonds versus Dynamic Se−Se/Te−Te Bonds

AbstractWe report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High‐pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism—especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se−Se and Te−Te bonds. Further, a co‐crystal of dpdSe with iodine (dpdSe‐I2) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling.

Variable Temperature Neutron Diffraction Study of the Oxide Ion Conductor Ba3VWO8.5.

Ba3VWO8.5 is an oxide ion conductor with a bulk conductivity of 5.0 × 10-5 S cm-1 at 600 °C. Ba3VWO8.5 is anomalous to the other Ba3M'M″O8.5 (M' = Nb; M″ = Mo, W) oxide ionic conductors, as it exhibits cation order with vanadium and tungsten on the M1 site only. Here, we report a variable temperature neutron diffraction study of Ba3VWO8.5, which demonstrates that cation order is retained up to 800 °C. We show for the first time that the structural rearrangements reported for hexagonal perovskite derivatives Ba3M'M″O8.5 are dictated by water absorption. The significant water uptake in Ba3M'M″O8.5 (M' = Nb; M″ = Mo, W) arises due to the flexibility of the crystal structure, whereby a fraction of the transition metal cations move from the M1 site to the octahedral M2 site upon absorption of water. The results presented here demonstrate that the presence of 50% V5+ on the M1 site, which has a strong preference for tetrahedral geometry, is enough to disrupt the flexibility of the cation sublattice, resulting in the ordering of the cations exclusively on the M1 site and no significant water absorption.

Structural studies of N-(methoxysalicylidene)-fluoroaniline, N-(methoxysalicylidene)-chloroaniline and N-(methoxysalicylidene)-bromoaniline derivatives

Twenty seven N-(methoxysalicylidene)-haloaniline (halo = F, Cl or Br) compounds were synthesized. The crystal structures of all 27 compounds have been determined at low temperature and are reported herein, along with a variable-temperature neutron diffraction study on two of the compounds. New polymorphs were identified for two of the compounds along with a temperature-induced phase transition for one of the other compounds. Visual observations on the thermochromism of the 27 compounds are also reported. The interplanar angle between the two aromatic rings and the intermolecular interactions in the structures are examined and linked to the visual observations on the thermochromism.

The Elusive Structural Origin of Plastic Bending in Dimethyl Sulfone Crystals with Quasi‐isotropic Crystal Packing

AbstractBending in molecular crystals is typically associated with the anisotropy of intermolecular interactions. The intriguing observation is reported of plastic bending in dimethyl sulfone, which exhibits nearly isotropic crystal packing and interaction topology, defying the known structural models of bending crystals. The origin of the bending phenomenon has been explored in terms of intermolecular interaction energies, experimental X‐ray charge density analysis, and variable temperature neutron diffraction studies. H⋅⋅⋅H dihydrogen interactions and differences in electrostatic complementarity between molecular layers are found to facilitate the bending behavior.

The Elusive Structural Origin of Plastic Bending in Dimethyl Sulfone Crystals with Quasi-isotropic Crystal Packing.

Bending in molecular crystals is typically associated with the anisotropy of intermolecular interactions. The intriguing observation is reported of plastic bending in dimethyl sulfone, which exhibits nearly isotropic crystal packing and interaction topology, defying the known structural models of bending crystals. The origin of the bending phenomenon has been explored in terms of intermolecular interaction energies, experimental X-ray charge density analysis, and variable temperature neutron diffraction studies. H⋅⋅⋅H dihydrogen interactions and differences in electrostatic complementarity between molecular layers are found to facilitate the bending behavior.

Investigation of the Relationship between the Structure and Conductivity of the Novel Oxide Ionic Conductor Ba3MoNbO8.5

A variable temperature neutron diffraction study of the novel oxide ion conductor Ba3MoNbO8.5 has been performed between 25 and 600 °C. Nonmonotonic behavior of the cell parameters, bond lengths, and angles are observed indicating a structural rearrangement above 300 °C. The oxygen/vacancy distribution changes as the temperature increases so that the ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra increases upon heating above 300 °C. A strong correlation between the oxide ionic conductivity and the number of (Mo/Nb)O4 tetrahedra within the average structure of Ba3MoNbO8.5 is observed. The increase in the number of (Mo/Nb)O4 tetrahedra upon heating from 300–600 °C most likely offers more low energy transition paths for transport of the O2– ions enhancing the conductivity. The unusual structural rearrangement also results in relaxation of Mo(1)/Nb(1) and Ba(2) away from the mobile oxygen, increasing the ionic conductivity. The second order Jahn–Teller effect most likely further enhances the distortion ...

Synthesis, anion order and magnetic properties of RVO3−xNx perovskites (R = La, Pr, Nd; 0 ≤ x ≤ 1)

The rare earth vanadium oxynitride perovskites RVO3−xNx (R = La, Pr, Nd; 0 ≤ x ≤ 1) have been prepared by treatment of RVO4 precursors in NH3. The ammonolysis reactions proceed through an initial reduction of RVO4 precursors to RVO3 perovskites that are progressively nitrided and reoxidised to RVO3−xNx materials with varying proportions of V4+/V3+. Variable temperature neutron diffraction studies show that NdVO2N and PrVO2.24N0.76 have orthorhombic Pbnm symmetry between 3.5 and 300 K, but LaVO2.11N0.89 undergoes a broad Pbnm to Rc structural transition over this temperature range. In LaVO3−xNx samples the proportion of the Rc phase at room temperature increases with the nitrogen content. The partial anion order indicative of planes of cis-VN2 chains found in NdVO2N is also present in PrVO2.24N0.76, and is thus robust to the disorder created by non-stoichiometry. Magnetic measurements reveal spin freezing transitions at low temperatures in LaVO2.09N0.91 and PrVO2.24N0.76, but NdVO2N is paramagnetic.

Variable temperature neutron diffraction study of crystal structure and transport pathways in oxide ion conductors Bi12.5Ln1.5ReO24.5 (Ln=Lu, Er)

Abstract Samples of highly conducting Bi 12.5 Lu 1.5 ReO 24.5 and Bi 12.5 Er 1.5 ReO 24.5 have been studied by neutron powder diffraction at room temperature for both phases and at 25 °C ≤ T ≤ 500 °C in the case of Bi 12.5 Er 1.5 ReO 24.5 . Both materials crystallize in the cubic δ-Bi 2 O 3 related system, space group Fm-3m. Changes in the oxygen sublattice at 25 °C ≤ T ≤ 500 °C have been investigated by the Rietveld and maximum entropy methods.

Variable temperature neutron diffraction study of Bi3ReO8 oxide ion conductor

The crystal structure of fluorite-related Bi3ReO8 containing isolated ReO4 tetrahedra has been studied by powder X-ray and neutron diffraction at elevated temperatures. The phase is cubic and undergoes a phase transition between T ~150°C and 250°C passing from space group P213 at 25°C to I213 at higher temperature. The material is an oxide ion conductor and presents a sharp increase in conductivity at T ~450°C, with σ ~8×10−3Scm−1 above 500°C. Two different long range oxide ion diffusion mechanisms involving vacancy and interstitial defects are proposed. Rotation of isolated ReO4 tetrahedra seem to facilitate the oxide ion conduction.

A variable-temperature neutron diffraction study of ussingite; a strong asymmetric hydrogen bond in an aluminosilicate framework

A powder neutron diffraction study of ussingite, Na2AlSi3O8(OH), over the temperature range 4–850 K has been undertaken. The strong hydrogen bond that exists in this mineral has been accurately determined with the O–H distance at 1.070(8) A and an O(donor)–O(acceptor) separation of 2.481(5) A at 4 K, The distribution of hydrogen along the O–O direction remains asymmetric between 4 and 850 K with the H atom being fully ordered at a single site, rather than partially disordered over two sites of a double-potential well, as in serandite. A gradual increase in the bonded O–H distance at higher temperatures is observed, indicative of a broadening of the potential well in which the hydrogen atom is sited. Below 50 K, the material shows negative thermal expansion, likely to be associated with reduced bending motion of the O–H bond.

Variable temperature neutron diffraction studies of single crystals of LiND 2

We have synthesized a single crystal of lithium amide (LiNH 2, LiND 2) by melting method, and performed neutron diffraction of the single crystal at variable temperature. LiND 2 is tetragonal structure and I–4 space group. Lattice parameters and unit cell volume of LiND 2 at room temperature, 50 °C, 100 °C, 150 °C and 200 °C were determined. Both of the lattice parameters and the unit cell volume increase with increase of temperature. From these results, we have estimated coefficient of volumetric thermal expansion α V of LiND 2 to be 222 × 10 −6/K. With increase of temperature, all thermal ellipsoids gradually expand because of thermal vibration.

Spin, Orbital and Lattice Coupling in the Double Perovskite Ba2154SmMoO6

AbstractAn unexpectedly high antiferromagnetic transition temperature (TN = 130 K) has been observed in the double perovskite Ba2154SmMoO6 as a result of a strong interplay between spin, orbital and lattice degrees of freedom. A variable temperature neutron diffraction study of Ba2154SmMoO6 demonstrates that the orbital order below TN is as a result of a Jahn-Teller distortion of the MoO6 octahedra.

YBaMn 2O 5: crystal and magnetic structure reinvestigation

The ordered oxygen-deficient perovskite YBaMn 2O 5 has been synthesized as a monophasic sample. A variable temperature neutron diffraction study of this phase has been carried out. The crystal structure of this manganite was solved in the space group P4/ nmm ( a = 5.547 Å and c = 7.649 Å). This oxide, which is closely related to the YBaCuFeO 5 structure, forms an alternate stacking of yttrium and barium layers along the c axis. It differs from the latter, however, by the existence of two distinct sites for manganese, corresponding to an ordering of the Mn 3+ and Mn 2+ cations, respectively. Elucidation of the magnetic structure of YBaMn 2O 5, from low temperature PND (powder neutron diffraction) data, leads to a G-type antiferromagnetic model; the superimposed Mn 2+/Mn 3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements.

A variable temperature neutron diffraction study of the layered perovskite YBaMn2O5

A variable temperature neutron diffraction study has been carried out on the layered perovskite YBaMn2O5 between 100 and 300 K. A broad peak indexed as (1/2, 1/2, 1) on the nuclear cell is consistent with short range MnII /MnIII valence ordering. The magnetic structure below the ferrimagnetic ordering temperature of 167 K has been determined and is in agreement with a previously proposed model. Changes in lattice parameters, bond lengths and angles show evidence of an exchange striction at TC .