Synthesis and structure refinement of layered perovskites Ba5−xLaxNb4−xTixO15 (x=0, 1, 2, 3 and 4) solid solutions

Ba_5−xLa_xNb_4−xTi_xO₁₅ solid solutions were prepared by solid state reaction method. Structural analysis for the stoichiometric phases was performed for x=0, 1, 2 and 3 by Rietveld analysis of neutron powder diffraction data. The x=0, 1 and 2 members could be refined in the space group P-3m1 (stacking sequence chhcc, polytypoid 5 H). There is a decrease in cell volume as x increases. La³⁺ occupies preferentially the A2 site (Wyckoff site 2d) and Ti⁴⁺ the B2 site (Wyckoff site 2c). As x increases there is an increase of the global instability index (GII) (which is a measure of the extent to which the BVS rule is violated over the whole structure) indicating the presence of intrinsic strains large enough to cause instability at room temperature. This strain is responsible for a structural change for the member with x=3, which could be refined in the space group P-3c1 (stacking sequence (chhcc)₂, polytypoid 2×5H=10H). This change in space group is associated with a cooperative rotation of (Nb/Ti)O₆ octahedra around the c-axis, necessary to accommodate the smaller La³⁺ ion in the cuboctahedral cavity.     

A high temperature powder neutron diffraction structural study of the apatite-type oxide ion conductor, La9.67Si6O26.5

High-resolution neutron powder diffraction studies of the oxide ion conductor La_9.67Si₆O_26.5 are reported for temperatures ranging between 25 and 900 °C. The best fit to the data was obtained for space group P6₃ and there was no evidence for any change in symmetry over the temperature range studied. Interstitial oxide ions are identified lying in sites similar to those predicted by previous computer modelling studies, and in agreement with structural studies on related materials. Furthermore, occupancy of these sites is enhanced by Frenkel-type disorder from neighbouring positions. The results thus add further weight to the interpretation that, in these apatite-type systems, the silicate substructure is important for the accommodation of interstitial oxide ions and their migration.     

Combined neutron and synchrotron X-ray diffraction study of Sr/Mg-doped lanthanum gallates up to high temperatures

Combined neutron diffraction and high-resolution synchrotron X-ray powder diffraction methods have been used to examine the crystal structures of two sample sets of Sr/Mg-doped Lanthanum gallate with the compositions La_0.9Sr_0.1Ga_1−yMg_yO_{3−0.5(0.1+y)} (y=0, 0.1, 0.2) and La_0.8Sr_0.2Ga_1−yMg_yO_{3−0.5(0.2+y)} (y=0.15, 0.2) up to 900 °C. At room temperature all samples of the first series exhibit orthorhombic structures with space group Imma: La_0.9Sr_0.1GaO_2.95: , , ; La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9: , , ; La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85: , , . The samples of the second series have the cubic perovskite structure with space group at room temperature: La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825: ; La_0.8Sr_0.2Ga_0.8Mg_0.20O_2.80: . Samples of the first series transform from the orthorhombic to a rhombohedral (Imma→) structure at ∼170 °C for La_0.9Sr_0.1GaO_2.95, at ∼430 °C for La_0.9Sr_0.1Ga_0.9Mg_0.1O_2.9, and between 600 and 700 °C for La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85. Both La_0.8Sr_0.2Ga_0.85Mg_0.15O_2.825 and La_0.8Sr_0.2Ga_0.8Mg_0.2 show no structural deviations from the cubic aristotype over the whole temperature range. The room temperature Imma structures of the first series are justified by a domain model and are rationalized in terms of static disorder increasing with Mg content, thus driving the phase transition temperatures to higher values in agreement with tolerance factor considerations. The distortion of the rhombohedral high-temperature phases (octahedra tilting and compression) and the effect of phase transitions on the ionic conductivity are discussed.     

The stability of Na-doped Bi2(NbCr)O7−x pyrochlores: The non-existence of “(BiNa)(NbCr)O6”

We show that the pyrochlore described as “(BiNa)(NbCr)O₆” is in fact a non-stoichiometric pyrochlore with an approximate composition of (Bi_1.33Na_0.67)(Nb_1.33Cr_0.67)O_7−x. Refinement of this structure using constant wavelength powder neutron diffraction data reveals the presence of vacancies in the anion sites coupled with displacive disorder of the Bi and Na cations. This is necessary to achieve a satisfactory bonding arrangement for both the Bi and Na cations that occupy the pyrochlore A-type sites. Attempts to prepare other pyrochlores in the series Bi_2−xNa_x(NbCr)O₆ were unsuccessful and it appears that the pyrochlore is only stable only over a very narrow composition range. The structure of the pure Bi pyrochlore Bi₂(NbCr)O₇ is also described.     

Structural studies of rhodium doped Sr2RuO4

The influence of Rh doping on the structure of Sr₂RuO₄ has been investigated using neutron powder diffraction methods. The metallic Ru rich compounds adopt a regular K₂NiF₄-type structure, space group I4/mmm, with Ru-O-Ru bond angles of 180°. The structures of the nonmetallic Rh rich compounds crystallise in space group I4/acd and are characterised by tilting of the MO₆ octahedra reducing the Ru-O-Ru angle to about 160°. Irrespective of Rh content the MO₆ polyhedra are not regular octahedra but are elongated along the c direction. The temperature dependence of the structure of Sr₂Ru_0.9Rh_0.1O₄ was investigated and revealed this elongation to be weakly temperature dependent.     

Structure of the low-temperature phase of Rb3D(SeO4)2 by single-crystal neutron diffraction

Crystal structure of Rb₃D(SeO₄)₂ has been investigated at 25 K (below the transition temperature T_c=95.4 K) by single-crystal neutron diffraction. Accompanying the transition, the SeO₄ groups, which are all equivalent in the phase above the transition (space group A2/a), split into eight nonequivalent groups in a superlattice (a×2b×2c, space group A2) in the low-temperature phase. Based on the D atom positions obtained, each of the SeO₄ groups was identified to be in the state closer to a HSeO₄⁻ ion or to a SeO₄²⁻ ion and the dipole arrangement of SeO₄-D-SeO₄ dimer was revealed. This dipole arrangement has ‘ferri’ structure along the polar b-axis, but ‘antiferro’ structure in the plane perpendicular to the b-axis. These results are consistent with the characteristics found in the earlier dielectric measurements.     

CO2 motion in carbon dioxide deuterohydrate determined by applying maximum entropy method to neutron powder diffraction data

Crystal structures of carbon dioxide deuterohydrate were studied by neutron powder diffraction at temperatures from 10 to 200 K. Maps of scattering length density distribution were obtained using a maximum entropy method (MEM), which clarified the motion of CO₂ molecules in the hydrate. In small cages, the carbon atom of the CO₂ molecule is at the center of the cage, and the oxygen atoms of CO₂ revolve freely around the carbon atom. In large cages, the carbon atom also is at the center of the cage, but the oxygen atoms tend to revolve around the carbon atom along the plane parallel to the hexagonal facets of the cage.     

Phase transitions in BaCeO3: neutron diffraction and Raman studies

Polycrystalline samples and small single crystals of the perovskite BaCeO₃ were studied by neutron diffraction and Raman spectrometry between 300 and 1200 K. The controversy about the phase transitions originally deduced from our previous Raman study and those observed since by neutron diffraction by Knight has stimulated this work. Pretransitional effects which are detected by Raman much before long-range ordering takes place can partly explain the above disagreement. A continuous monitoring of the structural changes by neutron diffraction and by Raman spectroscopy including polarization analysis has allowed discussion of the transition mechanisms: The first transition Pnma–Imma takes place at 573 K and is of second order. Although some modes soften when the temperature is raised as in many of these perovskite compounds the transition is likely partly displacive partly order–disorder. The Raman modes which disappear transform in modes at the X point of the Brillouin zone of the Imma phase. The second transition Imma–R c takes place at 673 K and is first order. The last transition R c–Pm3m occurs above 1200 K and the transition temperature which can be deduced by extrapolation to zero Raman intensity is in good agreement with neutron results. This second order transition is progressive and begins at about 400 K, the intermediate R c structure appearing as an attempt for slowing down the structural evolution toward the cubic perovskite form.     

Evolution from relaxor-like dielectric to ferroelectric in Ba[(Fe0.5Nb0.5)1−xTix]O3 solid solutions

Ba[(Fe_0.5Nb_0.5)_1−xTi_x]O₃ (x=0.2,0.4,0.6,0.8,0.85,0.9 and 0.95) solid solutions were synthesized by a standard solid-state reaction technique. X-ray diffraction at room temperature and dielectric characteristics over a broad temperature and frequency range were evaluated systematically. The structure of Ba[(Fe_0.5Nb_0.5)_1−xTi_x]O₃ solid solutions changed from cubic to tetragonal with increasing x. A Debye-like dielectric relaxation following the Arrhenius law similar to that in Ba(Fe_0.5Nb_0.5)O₃ was observed at lower temperature in the composition range 0.2≤x≤0.8, while the relaxor ferroelectric, diffused ferroelectric and normal ferroelectric behavior were observed for x=0.85,0.9 and 0.95, respectively. The process of the evolution of relaxor-like dielectric to ferroelectric suggested the changing from dilute polar micro-domains to polar micro-domains, polar micro/macro-domains and then polar macro-domains in the present ceramics.     

High pressure X-ray diffraction study of ReS2

The high-pressure behavior of rhenium disulfide (ReS₂) has been investigated to 51.0 GPa by in situ synchrotron X-ray diffraction in a diamond anvil cell at room temperature. The results demonstrate that the ReS₂ triclinic phase is stable up to 11.3 GPa, at which pressure the ReS₂ transforms to a new high-pressure phase, which is tentatively identified with a hexagonal lattice in space group P6?m2. The high-pressure phase is stable up to the highest pressure in this study (51.0 GPa) and not quenchable upon decompression to ambient pressure. The compressibility of the triclinic phase exhibits anisotropy, meaning that it is more compressive along interlayer directions than intralayer directions, which demonstrates the properties of the weak interlayer van der Waals interactions and the strong intralayer covalent bonds. The largest change in the unit cell angles with increasing pressures is the increase of β, which indicates a rotation of the sulfur atoms around the rhenium atoms during the compression. Fitting the experimental data of the triclinic phase to the third-order Birch-Murnaghan EOS yields a bulk modulus of K_OT=23±4 GPa with its pressure derivative K_OT′= 29±8, and the second-order yields K_OT=49±3 GPa.     

Heat capacity anomaly in UO2 in the vicinity of 1300 K: an improved description based on high resolution X-ray and neutron powder diffraction studies

X-ray and neutron powder diffraction studies of UO₂ were performed under controlled oxygen partial pressure between room temperature and 1673 K. More than 40 neutron diffraction patterns were recorded. The thermal expansion coefficient of UO₂ and the temperature dependence of Debye-Waller factors for oxygen and uranium atoms were determined. The dependence of Debye-Waller factors as a function of temperature is linear and the thermal expansion coefficient follows the classical Debye regime within the temperature range 300-1000 K. Above 1200 K, a departure from this quasi-harmonic behavior is clearly observed. Both an abnormal increase of the thermal expansion and of the oxygen sublattice disorder are evidenced. The departure of the lattice parameter from a linear thermal variation is found to be thermally activated with an effective activation energy close to 1 eV, very similar to the activation energy already found for the electrical conductivity. This new result suggests that polarons may affect the mean lattice parameter. A new thermodynamic model is then proposed to explain the heat capacity thermal variation by only three contributions: harmonic phonons, thermal expansion and polarons.     

Electroresistance effect in La1−xCaxMnO3 (0<x<1) ceramics

Electroresistance (ER) effects were investigated for a full series of manganite ceramics La_1−xCa_xMnO₃ (0<x<1), synthesized by solid state reaction. The results indicate that while the ER effects are large only in the presence of electrically active, high E-field boundaries, the equilibrium or metastable electronic-magnetic states in the adjoining domains are also significant, as a large ER occurs only at x=0.51 and x=0.17; those compositions are both near a two-phase coexistence region, i.e. close to a compositional regime where equilibrium insulating/metallic phase domains and interfaces would occur spontaneously.     

Shock-induced phase transitions in the MgO-FeO system to 200 GPa

We report new shock-compression data for single-crystal MgO at 114 and 192 GPa. Our data together with the existing shock-wave data revealed a volume discontinuity at 170±10 GPa along with the MgO Hugoniot. The discontinuity gives a volume increase of 1.9%, indicating a possible phase transition from a rock-salt structure (B1) to a high-temperature phase along with the MgO Hugoniot. We re-examined the Hugoniot data on polycrystalline sample (Mg_0.6, Fe_0.4)O up to 200 GPa [M.S. Vassiliou, T.J. Ahrens, The equation of state of Mg_0.6Fe_0.4O to 200 GPa, Geophys. Res. Lett. 9 (1982) 127-130], which showed similar discontinuity with a 2.2% volume increase at 135±10 GPa. Our results add to fundamental understandings of the behavior of MgO and the lower mantle mineral magnesiowüstite (Mg, Fe)O at ultrahigh pressure and temperature.     

Mesoporous (ZrO2-TiO2)/Al2O3 ternary carriers as hydrodesulfurization catalysts support

Ternary systems at various compositions were synthesized by coprecipitating Zr and Ti (to get a ZrO₂-TiO₂ 40-60 mol%) chlorides in aqueous basic media (provided by urea thermal decomposition) over an alumina substrate. Materials characterization included N₂ physisorption, X-ray diffraction, thermal analysis, high-resolution electron microscopy and Raman and UV-vis spectroscopies. High interaction among components was clearly evidenced by various techniques. Textural properties of ternary oxides could be tuned depending on composition of formulations. Mixed oxides with 10 or 20 mol% of ZrO₂-TiO₂ (at 40-60% mol, in turn) had the most suitable combination of textural properties (surface area, average pore diameter and pore volume) for the intended application (support of catalyst for hydrodesulfurization de oil-derived middle distillates). The suitability of those ternary supports was demonstrated in the dibenzothiophene hydrodesulfurization where the corresponding supported MoS₂ catalysts (at 2.8 atom Mo nm⁻²) were much more active (on a per mass of catalyst basis) than when impregnated over either alumina or zirconia-titania oxides.     

Synthesis of Cu2ZnSnSe4 compound powders by solid state reaction using elemental powders

The solid state reaction method was used to synthesize single phase and near stoichiometric Cu₂ZnSnSe₄ compound from elemental Cu, Zn, Sn and Se powders in a quartz tube furnace under an Ar flow at atmospheric pressure. These elemental powders were initially milled using zirconia balls. The α-CuSe phase was present in all of the milled powders because of the mechanical alloying effect between the Cu and Se powders. The solid state reaction mechanism was examined for the synthesis process. The phase analysis suggested that the Cu₂ZnSnSe₄ powder crystallized into the stannite phase with a high degree of crystallinity after near stoichiometric molar ratios of the powders was reacted at 500 °C for 6 h. This study showed that the solid state reaction method was a straightforward technique for the synthesis of the Cu₂ZnSnSe₄ compound powders from the elemental powders.     

STM spectroscopy on superconducting FeSe1−xTex single crystals at 300 mK

We report cryogenic scanning tunneling spectroscopy measurements on single crystals of superconducting FeSe_1−xTe_x, at doping levels of x=0.5 and 0.7, with critical temperatures . Atomically resolved topographic images were obtained, showing large-scale density-of-state clustering which appears to have no periodicity and to vary with the doping. Conductance spectra taken at 300 mK showed a generally asymmetric V-shaped background, along with a sharp dip structure within . These spectra appeared to vary over ∼nm length scale, and not correlated with the topography. The overall spectral evolution versus temperature is consistent with the dip structure arising from a superconducting energy gap which closes above T_c, and with the spectral background having a non-superconducting origin. The persistence of finite zero-bias conductance down to 300 mK, well below T_c, indicates the presence of low-energy quasiparticles on parts of the Fermi surface. We discuss our data in light of some other recent spectroscopic measurements of FeSe_1−xTe_x, and in terms of its characteristic band structure.     

Synthesis and characterization of a new monophosphate [2,5-(CH3)2C6H3NH3]H2PO4

Chemical preparation, calorimetric studies, crystal structure and spectroscopic investigations are given for a new noncentrosymmetric organic cation monophosphate [2,5-(CH₃)₂C₆H₃NH₃]H₂PO₄. This compound is orthorhombic P2₁2₁2₁ with the following unit-cell parameters: a=5.872(4), b=20.984(3), c=8.465(1) Å, Z=4, V=1043.0(5) Å³ and D_x=1.396 g cm⁻³. Crystal structure has been solved and refined to R=0.048 using 2526 independent reflections. Structure can be described as an inorganic layer parallel to (a,b) planes between which organic groups [2,5-(CH₃)₂C₆H₃NH₃]⁺ are located. Multiple hydrogen bonds connecting the different entities of compound thrust upon three-dimensional network a noncentrosymmetric configuration.     

The effect of doping LiMn2O4 spinel on its use as a cathode in Li-ion batteries: neutron diffraction and electrochemical studies

A series of compounds Li_1+yMn_2−xM′_xO₄ (x≤0.1;y≤0.02), have been synthesised by doping the parent LiMn₂O₄ spinel with various metal ions of variable oxidation state. Powder neutron diffraction data has been collected on these samples alongside a series of electrochemical experiments in order to elucidate the relationship between structure on the performance of these systems as Li batteries. Doping the LiMn₂O₄ spinel with a small amount of metal ions has a remarkable effect on the electrochemical properties. Whereas the capacity of the spinels doped with trivalent ions is much greater, the cycling fading properties are much enhanced with using divalent ions as dopants. The underlying reasons for this are discussed, and it is suggested that the occupancy of the tetrahedral site with divalent ions to form a more compact structure offers an improved structural stability to support greater Li insertion/extraction, but which ultimately prevents the free movement of Li also sited on the tetrahedral site of the lattice.     

Study of quantum effects on atomic displacements in quartz

The crystal structure of quartz (SiO₂) was analyzed by neutron powder diffraction at several temperatures in the range of 10-250 K. The temperature dependence of the structure parameters was consistent with our previous results obtained using single-crystal X-ray diffraction above room temperature. Atomic displacements are order parameters for displacive structural phase transitions. The temperature evolution of Si atomic displacement in quartz was analyzed by studying the quantum expansion of the Landau potential. The expansion was found to accurately describe the evolution of the atomic displacement over the entire temperature interval. To the best of our knowledge, such a verification of atomic displacement is the first of its kind. A proportional relationship between spontaneous strain and the square of the atomic displacement was observed over the entire temperature interval. The validity of the obtained characteristic temperature for the quantum effect is discussed and compared with the results of previous Raman-scattering studies.