Temperature dependent X-ray and neutron diffraction study of the liquid–solid and solid–solid equilibria in the Al 29.2Ga 27Zn 43.8 ternary alloy

We have investigated pressure-induced structural transitions in NaBH4 through density-functional theory calculations combined with X-ray and neutron diffraction experiments. Our calculations confirm that the cubic phase is stable up to 5.4 GPa and an orthorhombic phase occurs above 8.9 GPa, as observed in X-ray diffraction experiments. Both the calculations and X-ray diffraction measurements identify an intermediate tetragonal phase that appears between 6 and 8 GPa; that is, between the cubic and orthorhombic phases. This result is also confirmed by high-pressure neutron diffraction experiments performed on NaBD4. Our calculations and X-ray diffraction measurements show that the space group of the orthorhombic phase above 8.9 GPa is Pnma and the orthorhombic phase remains stable up to 30 GPa. The calculated equations of state are in excellent agreement with experiments.

Vibrational and elastic properties of the $R{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ skutterudites are investigated by, respectively, temperature $(T)$ dependent extended x-ray absorption fine structure (EXAFS) and pressure $(P)$ dependent x-ray diffraction (XRD) experiments. The Fe $K$-edge EXAFS experiments of the $R=\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, Ca, and Ba materials were performed in the $T$ interval $6<T<300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and XRD experiments of the $R=$ Na, K, Ca, Sr, and Ba materials were performed in the $P$ interval $1\phantom{\rule{4.pt}{0ex}}\text{atm}\phantom{\rule{4.pt}{0ex}}<P<16\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. From EXAFS, we obtained the correlated Debye-Waller parameters that were thus analyzed to extract effective spring constants connected with the Fe-$Y$ (where $Y=$ either $R$, Fe or Sb) scattering paths. Our findings suggest that in the case of the light cations, $R=\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ or Ca, the $R$ atoms are relatively weakly coupled to the cage, in a scenario reminiscent to the Einstein oscillators. From the XRD experiments, we obtained the bulk modulus ${B}_{0}$ for all $R=\mathrm{Na}$, K, Ca, Sr, and Ba materials, with values ranging from 77 GPa ($R=\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) to $R=99\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ ($R=$ Ba) as well as the compressibility $\ensuremath{\beta}$ as a function of $P$. The trend in $\ensuremath{\beta}$ as a function of the $R$ filler is discussed and it is shown that it does not correlate with simple geometrical considerations but rather with the filler-cage bonding properties.

Aqueous solutions of aluminum chloride exhibit a deep eutectic (-52˚C) that is likely due to solute-directed formation of complex network structures in the solvent. It is well known that the solute-bound solvent must have a distinct chemical environment from that of the bulk solvent. Commonly used vibrational spectroscopic measurements of aqueous salt solutions are not selective enough to directly identify distinct water environments in solution. However, variable-temperature 1H-NMR spectroscopy of aqueous solutions, specifically those with high charge density salts, can distinguish water directly bound to the cation from water of the bulk solution. This unique ability to resolve characteristic water peaks by NMR spectroscopy affords the opportunity to explore the hydration structure and dynamics of deep-eutectic aqueous solutions. The aqueous AlCl3 system is an excellent model to study solution structure since we can exploit the high charge density of the ions for which strong ion-dipole interactions are structure-directing to the solvent waters, allowing resolution of distinct water chemical environments. We conducted a thorough investigation of the composition- and temperature-dependent solution structure of the AlCl3 : water system using 1H-NMR spectroscopy, and X-ray and neutron diffraction. The room-temperature proton chemical shift of water proceeds downfield with increasing salt concentration until AlCl3•22H2O, the eutectic composition, after which further increasing concentration results in an upfield chemical shift (see Figure.) Using VT-NMR, we can resolve the first-nearest neighbor water signals from the bulk signal upon cooling. 1H-NMR T1 and T2 relaxation measurement of the distinct water environments reveal stark differences in proton mobility for compositions on the water-rich versus AlCl3-rich sides of the eutectic, implying significant changes in hydration structure. To evaluate the extent to which these changes are structural, a parallel set of composition- and temperature-dependent neutron and X-ray diffraction experiments demonstrate that both cations and anions exhibit significant structure-directing influences on the solution, which we hypothesize underlies deep eutectic behavior in aqueous solutions. Figure 1

Structural characteristics of an amorphous polymer melt, poly(propylene oxide) (PPO), have been studied by combining neutron and x-ray diffraction experiments and computer modeling using the reverse Monte Carlo (RMC) technique. The neutron diffraction experiments were performed on hydrogenous as well as deuterated samples. The experimentally determined nearest-neighbor distances were found to be in good agreement with literature data. The RMC modeling was applied for interpretation of the diffraction data to obtain more detailed structural information on bond angles, intermediate and long range correlations. For the intermediate range structure, the experimental structure factors demonstrate a first diffraction peak at about 1.45 Å−1, which from the RMC produced model can be related to the interchain distance of an almost random packing of the polymer chains. To investigate the chain conformation, partial atomic pair correlation functions have been calculated for atoms belonging to monomers close in sequence. The results show that the most probable conformation is a “stretched” trans conformation, where two consecutive methyl groups are pointing in almost opposite directions. Calculated bond and dihedral angle distributions support this finding and demonstrate the ability of the RMC method to produce polymer structures in good agreement with experimental results.

Herein we report the results of high pressure diffraction studies of zircon type ThGeO4. ThGeO4 exhibits anisotropic compressibility with the average compressibility along a-axis (20.8 × 10−4/GPa) larger than that along c-axis (9.98 × 10−4/GPa). Fitting the pressure dependence unit cell volume to 3rd order Birch-Murnaghan equation of states, the zero pressure bulk modulus (Ko) and volume (Vo) of 166(5) GPa and 341.6(3) Å3, respectively have been obtained. Preliminary studies on temperature dependent neutron and x-ray diffraction studies on ThGeO4 revealed anisotropic expansion behaviour with larger expansion coefficient along c-axis compared to a-axis. No structural transition under temperature or pressure is observed in between ambient pressure to 10 GPa and in the temperature range of 25 to 1273K.

Single crystals of 1:1:1 cocrystal solvate of caffeine, 4-chloro-3-nitrobenzoic acid and methanol are reported to demonstrate reversible bending up to large elastic strain at ambient conditions [1].The compound has orthorhombic space group symmetry Fdd2 [T = 100 K: a = 32.784(9), b = 55.541(15), c = 3.9564(12) , V = 7191(4) 3].Elastic bending in these crystals is governed by changing distances between molecules within stacks and molecular rotations [2].While combination of weak dispersive interactions viz.weak C-HO hydrogen bonds, -stacking and van der Waals forces between pseudo spherical functional groups aids flexibility, permanent plastic deformation in these crystals has been argued to be prevented by "interlocking"/ steric barriers in the supramolecular architecture [1,2].Upon heating at Tc1 = 333 K, the crystals lose flexibility and are mechanically brittle [1].Further heating leads to partial desolvation of methanol from their structure at Tc2 = 388 K [1].Using temperature dependent specific heat capacity and single crystal X-ray diffraction experiments, the phase transition at Tc1 is found to be continuous.Crystal structures above Tc1 suggest reorganization of the stacking arrangement between the caffeine-acid dimers with respect to the longest growth direction of the crystals.High temperature in situ powder X-ray diffraction experiments suggest that the compound undergoes a phase transition at a significantly lower temperature.Additional peaks are observed in the diffraction pattern.These peaks violate the Fcentred orthorhombic lattice.Alternatively, these peaks could be approximately described with an additional wave vector in (3+d) dimensions.

Magnetism as a quantum phenomenon has received considerable attention not only in the past but also in the present. Low dimensionality of the crystal lattice on the one hand and small spins on the other hand enhance the quantum character of a system. This PhD work is concerned with the identification, synthesis, and evaluation of model spin system candidate compounds according to a given set of criteria with the aim to prepare a solid basis for follow-up investigations on their physical properties. Based on various experimental methods, mostly X-ray and neutron diffraction in combination with thermal and magnetic bulk property measurements, the structural and magnetic properties of the target compounds were probed and the results were evaluated in the context of the scientific questions of the three main projects. First, a series of quasi-one-dimensional hexagonal perovskites of the AMCl3 (A = Rb, Cs; M = Co, Ti, Sc) family was investigated. The focus was set on their magnetic properties regarding the characteristics of the respective M2+ ion as well as the dimensionality of the observed magnetic interactions. RbCoCl3 turns out to be a good realization of a Ising spin chain in the paramagnetic region with short-ranged antiferromagnetic correlations along the chains. Below TN1 = 28 K it undergoes two subsequent magnetic ordering transitions. The complex magnetic structures were successfully solved in a series of neutron diffraction experiments. The Ti-based isostructural compounds, in contrast, do not show magnetic long-range order down to 2 K and short-ranged antiferromagnetic correlations persist. Both RbTiCl3 and CsTiCl3 are highly anisotropic regarding the dominant magnetic exchange interaction. No structural phase transitions are observed. According to X-ray powder diffraction and magnetic bulk property measurements CsScCl3 is severely scandium deficient. Interestingly, the proposed homogeneity range between CsScCl3 and Cs3Sc2Cl9 was not confirmed despite intense synthetic efforts. Second, a group of quasi-two-dimensional ternary chlorides of the A2MCl4 (A = Rb, Cs; M = Cr, Mn) family was synthesized in order to study the critical perturbation of a weak 4-fold crystal field in the basal plane as a function of the external magnetic field and temperature. Neutron diffraction investigations on the Heisenberg ferromagnet Rb2CrCl4 are intrinsically difficult due to forces experienced by the ferromagnetic sample in a magnetic field. Up to 0.4 T no trend is established for the field-dependent evolution of the critical exponent and further experimental work is required. However, based on our diffraction data we can confirm space group I4/mmm as we do not observe any distortions in contrast to some literature propositions. For the first time, a series of magnetic susceptibility measurements investigates the magnetic hysteresis below 300 and 2 K. The antiferromagnetic Cs2MnCl4 undergoes a structural phase transition around 300�C upon cooling and only powder samples can be obtained of the I4/mmm-phase. However, for the first time the high-temperature crystal structure was solved on a metastable single crystal. It has the space group Pnma. A preparative procedure to produce large high-quality single crystals vii for planned neutron scattering experiments was successfully established for the isostructural but incongruently melting antiferromagnet Rb2MnCl4. Third, on the quest for kagome lattice type compounds, we identified the Na2M3Cl8 (M = Ti, Mn, Mg) family of incongruently melting ternary chlorides as promising candidates. The kagome type phase was investigated in a series of X-ray and neutron diffraction experiments. No magnetic long-range order is established in the Ti-based compound down liquid helium temperature. Single crystal magnetic susceptibility measurements display short-ranged antiferromagnetic correlations and a pronounced anisotropy. It structurally distorts upon cooling and the symmetry of the kagome type phase is lost in two successive phase transitions. For the first time, our comprehensive temperature-dependent single crystal X-ray diffraction data allows to acknowledge the intermediate phase as a crystallographically distinct phase. A qualitative structural model is proposed. For Na2Mn3Cl8, the symmetry of the lattice is preserved at least down to 100 K. Also at lower temperatures no features of magnetic long-range order or structural distortions are observed in the magnetic bulk data. Thus, the Mn-based compound is a promising candidate for follow-up investigations on the magnetic excitations in a kagome system. In the non-magnetic Mg-based compound, finally, a non-kagome type high-temperature phase was identified and the crystal structure was successfully solved on a metastable single crystal sample. Such crystal structure is observed for the first time in a halide compound. The experimental work was carried out between 2012 and 2016 at the Department of Chemistry and Biochemistry at the University of Bern, at the Paul Scherrer Institute, at the Institute Laue-Langevin, and at the University College London.

Enhancing the lithium-ion conductivity in Li2SrTa2-xNbxO7 (x = 0–2)

Hydrogenation of silicon-bearing hexagonal close-packed iron and its implications for density deficits in the inner core

Investigating disordered phases of C2Cl6 using an information theory approach

The temperature dependent structural studies carried out on the spin-correlated system A2FeCoO6 (A= Sm, Eu, Dy and Ho) or AFCO (A= Sm, E, D and H), using synchrotron radiation is presented. Owing to the large absorption cross-sections of the rare earths; Eu, Sm and Dy for neutrons, synchrotron radiation is one of the best available candidates for probing the system. The perovskite phase formation is inferred from laboratory XRD with Cu Kα source. The temperature dependent synchrotron X-ray diffraction (SXRD) experiments show the coexistence of monoclinic P21/n and orthorhombic Pbnm phases in Ho and Dy, while Eu and Sm are formed in single phase Pbnm. The temperature dependent DC magnetization measurements infer the presence of many interesting features such as thermal hysteresis, magnetic irreversibility, spin re-orientation, re-entrant magnetization and negative magnetization.

Recent progress of quantum crystallography enables us to determine quantum mechanical information from diffraction data [1].Analytical techniques for both charge density [2,3] and thermal displacements [4,5] have been developed and utilized for the static and dynamical crystallography.Accurate data for these developed techniques are strongly required to exert full abilities of the methods.Recent progress of synchrotron radiation X-ray single crystal diffraction including beam stability, a state-of-the-art detector system enables us to measure such accurate data for quantum crystallography.In the present study, we measured the high-quality temperature dependent single crystal X-ray diffraction data of simple amino acids, taurine and L-alanine for an accurate quantum crystallography.So far, simple amino acids are often used in the development of the method for quantum crystallography [4,5].Synchrotron radiation single crystal X-ray diffraction (SRXRD) experiments were carried out at SPring-8 BL02B1 beamline.The data of L-alanine were measured at 40 K and 100 K.The data of taurine were measured at 40, 100, 150 and 200K.A He gas flow low temperature device was used for the measurements of 40 K data.The other data were measured using a N2 gas flow low temperature device.Data reduction for all the data was carried out using Program CrysAlis Pro.Typical values of the completeness and redundancy for the processed data were approximately 100 % and more than 10.Initial structures were solved and refined by SHELX suite implemented in the Olex2 system.The extracted intensities and structures were used for the following quantum crystallographic studies.Theoretical charge density and structure factors were prepared by CRYSTAL14.The determined independent atom structure models and structure factors were used in the further analysis using XD2016.The charge density distribution by Hansen-Coppens multipole model and thermal displacement factors including anharmonic thermal vibrations were determined in the refinement.Significant changes in thermal displacement factors were observed in the temperature dependence data.The high-quality data for both materials enables us to perform the refinement of not only anisotropic thermal displacement parameters but also anharmonic thermal displacement parameters by Gram-Charlier expansion up to 4 th order terms.Reliability factors, R and wR for L-alanine 100K with 0.31 Å resolution were 3.1% and 8.6%, respectively.

Recent progress in impurity-doped topological insulators has shown that the gap at the Dirac point shrinks with reducing temperature. This is an obstacle for experimental realization of the quantum anomalous Hall effect at higher temperature due to the requirement of a larger energy gap. In order to solve this puzzle, we study the gap at the Dirac point by performing temperature-dependent photoemission spectroscopy and X-ray diffraction experiments in Cr-doped Bi2Se3. Our valence band photoemission study revealed that the gap alters with temperature due to residual gas condensation on the sample surface with cooling. Residual gas on the sample surface creates an electron doping effect that modifies the chemical potential of the system resulting in the change of the gap size with variable temperature. Furthermore, such electron doping can weaken the ferromagnetism and lead to a bulk band contribution in the transport measurements. Therefore, such effects can hinder the existence of the quantum anomalous Hall state at higher temperatures. Hence, this work can pave the way for future studies towards a high-temperature quantum anomalous Hall effect.

The nature of the pre-morphotropic phase boundary (MPB) cubic-like state in the lead-free piezoelectric ceramics (1−x)Na1/2Bi1/2TiO3-(x)BaTiO3 at x ∼ 0.06 has been examined in detail by electric field and temperature dependent neutron diffraction, x-ray diffraction, dielectric and ferroelectric characterization. The superlattice reflections in the neutron diffraction patterns cannot be explained with the tetragonal P4bm and the rhombohedral (R3c) phase coexistence model. The cubic like state is rather a result of long ranged modulated complex octahedral tilt. This modulated structure exhibits anomalously large dielectric dispersion. The modulated structure transforms to a MPB state on poling. The field-stabilized MPB state is destroyed and the modulated structure is restored on heating the poled specimen above the Vogel-Fulcher freezing temperature. The results show the predominant role of competing octahedral tilts in determining the nature of structural and polar states in Na1/2Bi1/2TiO3-based ferroelectrics.

Structural and optical properties of double sodium–bismuth molybdate NaBi(MoO4)2 semiconductor compound was investigated by x-ray diffraction, Raman and transmission experiments. From the x-ray diffraction experiments, the crystal that has tetragonal structure was obtained. Vibrational modes of the crystal were found from the Raman experiments. Transmission experiments were performed in the temperature range of 10–300 K. Derivative spectroscopy analysis and absorption spectrum analysis were performed to get information about the change in band gap energy of the crystal with temperature. It was observed that the band gap energies of the crystal at different temperatures obtained from these techniques are well consisted with each other. By the help of absorption spectrum which was obtained from transmission measurements performed at varying temperatures, absolute zero value of the band gap and average phonon energy as 3.03 ± 0.02 eV and Eph = 24 ± 0.2 meV, respectively. Moreover, based on absorption spectrum analysis the Urbach energy of the crystal was obtained as 0.10 eV.