High-temperature Electron Diffraction Research Articles

Controlling the nanostructure and thermal properties of double-perovskite rare-earth tantalates by elemental doping

Controlling the complex nanostructures that often form in cation-deficient oxides may provide an effective way to tailor their thermophysical properties, but this remains a challenging task. We report a study of un-doped and Hf-doped YTa3O9 with a Y-deficient double-perovskite structure using high-temperature X-ray diffraction, electron diffraction, and small-angle X-ray scattering. Undoped YTa3O9 has an orthorhombic structure with 90° and 180° domain walls and stacking faults observed at the micro and nano levels at room temperature. It undergoes a phase transition to a tetragonal phase, with an associated volume change, at around 700 K. Doping of the materials with Hf successfully stabilizes a tetragonal phase that includes a superstructure with periodic distances of a few nanometers at room temperature. The thermal conductivity of Hf-doped YTa3O9 is lower than that of the undoped material, suggesting that the interfaces between nanodomains interfere with diffusive thermal transport.

Reversible Phase Transformations in Novel Ce‐Substituted Perovskite Oxide Composites for Solar Thermochemical Redox Splitting of CO2

AbstractThermochemical splitting of CO2 and H2O via two‐step metal oxide redox cycles offers a promising approach to produce solar fuels. Perovskite‐type oxides with the general formula ABO3 have recently gained attention as an attractive redox material alternative to the state‐of‐the‐art ceria, due to their high structural and thermodynamic tunability. A novel Ce‐substituted lanthanum strontium manganite perovskite‐oxide composite, La3+0.48Sr2+0.52(Ce4+0.06Mn3+0.79)O2.55 (LSC25M75) is introduced, aiming to bridge the gap between ceria and perovskite oxide‐based materials by overcoming their individual thermodynamic constraints. Thermochemical CO2 splitting redox cyclability of LSC25M75 evaluated with a thermogravimetric analyzer and an infrared furnace reactor over 100 consecutive redox cycles demonstrates a twofold higher conversion extent to CO than one of the best Mn‐based perovskite oxides, La0.60Sr0.40MnO3. Based on complementary in situ high temperature neutron, synchrotron X‐ray, and electron diffraction experiments, unprecedented structural and mechanistic insight is obtained into thermochemical perovskite oxide materials. A novel CO2 splitting reaction mechanism is presented, involving reversible temperature induced phase transitions from the n = 1 Ruddlesden–Popper phase (Sr1.10La0.64Ce0.26)MnO3.88 (I4/mmm, K2NiF4‐type) at reduction temperature (1350 °C) to the n = 2 Ruddlesden–Popper phase (Sr2.60La0.22Ce0.18)Mn2O6.6 (I4/mmm, Sr3Ti2O7‐type) at re‐oxidation temperature (1000 °C) after the CO2 splitting step.

Controlling the Nanostructure and Thermal Properties of Double-Perovskite Rare-Earth Tantalates by Elemental Doping

Controlling the complex nanostructures that often form in cation-deficient oxides may provide an effective way to tailor their thermophysical properties, but this remains a challenging task. We report a study of un-doped and Hf-doped YTa3O9 with a Y-deficient double-perovskite structure using high-temperature X-ray diffraction, electron diffraction, and small-angle X-ray scattering. Undoped YTa3O9 has an orthorhombic structure with 90° and 180° domain walls and stacking faults observed at the micro and nano levels at room temperature. It undergoes a phase transition to a tetragonal phase, with an associated volume change, at around 700K. Doping of the materials with Hf successfully stabilizes a tetragonal phase that includes a superstructure with periodic distances of a few nanometers at room temperature. The thermal conductivity of Hf-doped YTa3O9 is lower than that of the undoped material, suggesting that the interfaces between nanodomains interfere with diffusive thermal transport.

Antiferroelectric properties and site occupations of R3+ cations in Ca8MgR(PO4)7 luminescent host materials

Ca8MgR(PO4)7 (R = La, Pr, Nd, Sm−Lu, and Y) phosphates with a β-Ca3(PO4)2 related structure were prepared by a standard solid-state method in air. Second-harmonic generation, differential scanning calorimetry, and dielectric measurements led to the conclusion that all Ca8MgR(PO4)7 are centrosymmetric and go to another centrosymmetric phase in the course of a first-order antiferroelectric phase transition well above room temperature (RT). High-temperature electron diffraction showed that the symmetry changes from R3¯c to R3¯m during the phase transition. Structures of Ca8MgR(PO4)7 at RT were refined by the Rietveld method in centrosymmetric space group R3¯c. Mg2+ cations occupy the M5 site; the occupancy of the M1 site by R3+ cations increases monotonically from 0.0389 for R = La to 0.1667 for R = Er-Lu, whereas the occupancy of the M3 site by R3+ cations decreases monotonically from 0.1278 for R = La to 0 for R = Er-Lu. In the case of R = Er-Lu, the M3 site is occupied only by Ca2+ cations. P1O4 tetrahedra and cations at the M3 site are disordered in the R3¯c structure of Ca8MgR(PO4)7. Using synchrotron X-ray powder diffraction, we found that annealing conditions do not significantly affect the distribution of Ca2+ and Eu3+ cations between the structure positions of Ca8MgEu(PO4)7. Luminescent properties of Ca8MgEu(PO4)7 powder samples were investigated under near-ultraviolet (n-UV) light. Excitation spectra of Ca8MgEu(PO4)7 show the strongest absorption at about 395 nm that matches with commercially available n-UV-emitting GaN-based LED chips. Emission spectra show an intense red emission due to the 5D0 → 7F2 transition of Eu3+.

Strong charge ordering above room temperature in B-site disordered electron-doped manganite SrMn0.875Mo0.125O3−δ

Low- as well as high-temperature electron and x-ray diffraction studies have been carried out on a rare-earth free B-site disordered electron-doped manganite SrMn0.875.Mo0.125O3−δ in the temperature range of 83–637 K. These studies reveal the occurrence of strong charge ordering (CO) at room temperature in a pseudo-tetragonally distorted perovskite phase with space group Pmmm. A non-integral modulation vector of 8.95 times along [−110] indicates a charge density wave-type modulation. The CO phase with basic perovskite structure Pmmm transforms to a charge disorder cubic phase through a first-order phase transition at 355 K. Supporting temperature-dependent measurements of resistance and magnetization show metal–insulator and antiferromagnetic transitions across 355 K with a wide hysteresis ranging from 150 K to 365 K. The occurrence of pseudo-tetragonality of the basic perovskite lattice with c/a < 1 together with charge-ordered regions with 2-dimensional modulation have been analyzed as the coexistence of two CO phases with -type and orbital ordering.

High-temperature stability, structure and thermoelectric properties of [formula omitted] phases

Polycrystalline perovskite-type CaMn 1 - x Nb x O 3 phases (with x = 0.02 , 0.05 , 0.08 and 0.10) were investigated with regard to their structure, microstructure and thermal stability as a function of temperature. The studied phases revealed a complex microstructure at room temperature with 90 ° twinned domains. At high temperatures, the manganate phases underwent a structural transition from orthorhombic to cubic symmetry, as confirmed by in situ high-temperature X-ray powder diffraction and electron diffraction data. Thermogravimetric heating/cooling studies showed a reversible thermal reduction/reoxidation process that occurred above a defined transition temperature. A possible mechanism relating the high-temperature structural transition and the thermal reduction process of slightly substituted CaMnO 3 phases was proposed. The thermal reduction process resulted in a change in the Mn 3 + / Mn 4 + concentrations in the Mn sublattice, and therefore in a modification of the transport properties. A comprehensive study examined the impact of both phenomena on the electrical and thermal transport properties.

Structure, force fields, and vibrational spectra of cerium tetrahalides

The geometrical structure, force fields, and vibrational spectra of CeX4 (X = F, Cl, Br, I) were investigated by second, third, and fourth order Moller-Plesset perturbation theory, CISD+Q configuration interaction method, and the CCSD(T) coupled cluster method. Calculations on CeF4 were also performed by multiconfiguration second order perturbation theory MCQDPT2/CASSCF. The wave function of the ground state of CeX4 molecules was found to be appreciably non-one-configurational; this property increases from cerium fluorides to iodides and leads to the divergence of the series of Moller-Plesset perturbation theory. The calculated data point to a tetrahedral equilibrium nuclear configuration in CeX4 molecules. The energy barriers to the inversion of the tetrahedral CeX4 molecules via the square configurations are high enough, 74–89 kJ/mol. The calculated vibration frequencies, effective internuclear distances, and mean amplitudes of nuclear vibrations in CeF4 agree with IR and Raman spectroscopic and high-temperature gas-phase electron diffraction data.

On the effect of 4f electrons on the structural characteristics of lanthanide trihalides: Computational and electron diffraction study of dysprosium trichloride

The molecular and electronic structure of dysprosium trichloride, DyCl(3), was calculated by high-level quantum chemical methods in order to learn about the effect of the partially filled 4f subshell and of the possible spin-orbit coupling on them. High-temperature electron diffraction studies of DyCl(3) were also carried out so that we could compare the computed geometry with the experimental one, after thermal corrections on the latter. Dysprosium monochloride, DyCl, and the dimer of dysprosium trichloride, Dy(2)Cl(6), were also investigated by computation. We found that the electron configuration of the 4f subshell does not influence the geometry of the trichloride monomer molecule as the ground state and first excited state molecules have the same geometry. Nonetheless, taking the 4f electrons into account in the calculation, together with the 5s and 5p electrons, is important in order to get geometrical parameters consistent with the results from experiment. Based on electron diffraction and different levels of computation, the suggested equilibrium bond length (r(e)) of DyCl(3) is 2.443(14) A, while the thermal average distance (r(g)) from electron diffraction is 2.459(11) A. The molecule is trigonal planar in equilibrium. Although the ground electronic state splits due to spin-orbit coupling, the lowering of the total electronic energy is very small (about 0.025 hartree) and the geometrical parameters are not affected. In contrast with the monomeric trichloride molecule, the bond angles of the dimer seem to be different for different electronic states, indicating the influence of the 4f electronic configuration on their structure. We carried out an anharmonic analysis of the out-of-plane vibration of the trichloride monomer and found that the vibration is considerably anharmonic at 39.5 cm(-1), compared with the 30.5 cm(-1) harmonic value.

Quasilinear Molecule par Excellence, SrCl2: Structure from High‐Temperature Gas‐Phase Electron Diffraction and Quantum‐Chemical Calculations—Computed Structures of SrCl2×Argon Complexes.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Quasilinear Molecule par Excellence, SrCl2: Structure from High‐Temperature Gas‐Phase Electron Diffraction and Quantum‐Chemical Calculations—Computed Structures of SrCl2⋅Argon Complexes

The molecular geometry of strontium dichloride has been determined by high-temperature electron diffraction (ED) and computational techniques. The computation at the MP2 level of theory yields a shallow bending potential with a barrier of about 0.1 kcal mol(-1) at the linear configuration. The experimentally determined thermal average Sr--Cl bond length, r(g), is 2.625+/-0.010 A and the bond angle, angle-spherical(a), is 142.4+/-4.0 degrees . There is excellent agreement between the equilibrium bond lengths estimated from the experimental data, 2.607+/-0.013 A, and computed at different levels of theory and basis sets, 2.605+/-0.006 A. Based on anharmonic analyses of the symmetric and asymmetric stretching as well as the bending motions of the molecule, we estimated the thermal average structure from the computation for the temperature of the ED experiment. In order to emulate the effect of the matrix environment on the measured vibrational frequencies, a series of complexes with argon atoms, SrCl(2)Ar(n) (n=1-7), with different geometrical arrangements were calculated. The complexes with six or seven argon atoms approximate the interaction best and the computed frequencies of these molecules are closer to the experimental ones than those computed for the free SrCl(2) molecule.

Thermally Induced Solid-State Phase Transition of Bis(triisopropylsilylethynyl) Pentacene Crystals

Bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) is a functionalized pentacene derivative designed to enhance both the solution solubility and solid-state packing of pentacene. In this paper, we report our observations of a solid-state phase transition in TIPS pentacene crystals upon heating or cooling. Evidence from differential scanning calorimetry (DSC), hot-stage optical microscopy, as well as high-temperature X-ray and electron diffraction are presented. A reasonable match with experimental data is obtained with molecular modeling. Our results reveal that the transition is associated with a conformational reorganization of the TIPS side groups, accompanied by a slight decrease in the acene-to-acene spacing and a shift of the overlap between the neighboring pentacene units. The observed cracking should be avoided or minimized in TIPS pentacene-based thin film transistors to maintain their relatively high charge carrier mobility.

An Intricate Molecule: Aluminum Triiodide. Molecular Structure of AlI3 and Al2I6 from Electron Diffraction and Computation.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

An Intricate Molecule: Aluminum Triiodide. Molecular Structure of AlI3 and Al2I6 from Electron Diffraction and Computation

The molecular structure of aluminum triiodide was investigated in the gas phase by high-temperature gas-phase electron diffraction and high-level computations. The geometries of monomeric, AlI3, and dimeric, Al2I6, molecules were determined from two separate experiments carried out under carefully controlled conditions to prevent decomposition. This is the first experimental determination of the dimer structure by modern techniques. The computed geometrical parameters strongly depend on the applied methods and basis sets as well as on core-valence correlation effects. The electron diffraction thermal average bond length, r(g), of AlI3 at 700 K is 2.448(6) A; while those of Al2I6 at 430 K are 2.456(6) A (terminal) and 2.670(8) A (bridging). The equilibrium geometry of the monomer molecule is planar with D(3h) symmetry. The dimer molecule is extremely floppy, and it is difficult to determine the symmetry of its equilibrium geometry by computation, as it is sensitive to the applied methods. MP2 and CCSD calculations find the Al2I6 molecule puckered with C(2v) symmetry (although with a very small barrier at planarity), while density functional methods give a structure with a planar central ring of D(2h) symmetry. Comparison of the computed vibrational frequencies with the gas-phase experimental ones favors the D(2h) symmetry structure.

High-Temperature Polymorphism in Metastable BiMnO3

The multiferroic perovskite BiMnO3, synthesized under high-pressure conditions, decomposes if heated at room-pressure in the temperature range of 500−650 °C. Comparative studies by high-temperature X-ray diffraction, electron diffraction, thermal analysis, and magnetic investigation revealed the existence of a complex pathway to decomposition, depending on the heating rate, pressure, and atmosphere that involves different metastable phases. In particular the as-prepared monoclinic phase (I) transforms to a second monoclinic form (II) at 210 °C and then to an orthorhombic phase (III) at 490 °C. These phase transitions, fast and reversible, occur on heating with a drop in volume and are moved at higher temperatures when pressure is decreased. The transition from II to III, typically observed in inert atmosphere, can be detected also in air when the heating rate is kept sufficiently high. When III is heated in an oxygen-containing atmosphere a slow irreversible transition to variants IV and then V takes place with kinetics depending on temperature, heating rate, and oxygen partial pressure. Both IV and V are oxidized ferromagnetic phases containing Mn4+ characterized by a modulated structure based on fundamental triclinic perovskite cells. Their magnetic behavior shows a strong analogy with thin films of BiMnO3, suggesting for the latter an oxidized nature and for the former a possible multiferroic behavior.

Ferroelectric Phase Transition in the Whitlockite‐Type Ca9Fe(PO4)7; Crystal Structure of the Paraelectric Phase at 923 K.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Experimental investigation and thermodynamic calculation of the phase equilibria in the Cu-Sn and Cu-Sn-Mn systems

The phase equilibria in the Cu-rich portion of the Cu-Sn binary and Cu-Sn-Mn ternary systems have been determined using the diffusion-couple method, differential scanning calorimetry (DSC), high-temperature electron diffraction (HTED), and high-temperature X-ray diffraction (HTXRD) techniques. The present experimental results on the binary Cu-Sn system show the presence of the two-stage, second-order reaction A2 → B2 → D03 in the bcc-phase region, rather than a two-phase equilibrium between the disordered bcc (A2) and the ordered bcc (D03) phases, as reported before. Phase equilibria in the Cu-Sn-Mn ternary system in the composition range of 0 to 30 at. pct Sn and 0 to 30 at. pct Mn at 550 °C, 600 °C, 650 °C, and 700 °C have been determined, and a ternary compound (Cu4MnSn) with a very small solubility has been detected. A thermodynamic analysis of the Cu-Sn-Mn ternary system including the Cu-Sn and Mn-Sn binary systems has also been carried out by the CALPHAD (Calculation of Phase Diagrams) method, in which the Gibbs energy of the bcc phase is described by the two-sublattice model in order to take into account the second-order A2/B2 ordering reaction. A consistent set of optimized thermodynamic parameters for the Cu-Sn-Mn system for describing the Gibbs energy of each phase results in a better fit between calculation and experiment.

Ferroelectric phase transition in the whitlockite-type Ca 9Fe(PO 4) 7; crystal structure of the paraelectric phase at 923 K

A newly discovered phase transition in Ca 9Fe(PO 4) 7 is studied by high-temperature X-ray powder diffraction (XRD) and electron diffraction (ED), high-resolution electron microscopy, second-harmonic generation (SHG), differential scanning calorimetry, dielectric and electrical conductivity measurements. The phase transition temperature, T c, is 890±10 K. Dielectric measurements reveal the ferroelectric nature of the phase transition. A spontaneous polarization of about 2–3 μC cm −2 is estimated from SHG. The phase transition from a polar ferroelectric form β (space group R3 c), to a centrosymmetric paraelectric form β′ (space group R 3c ), is reversible and of first order. The evolution of the crystal structure and lattice parameters is studied by high-temperature XRD and ED. The structural parameters of β′-Ca 9Fe(PO 4) 7 are refined by the Rietveld method from XRD data at 923 K in the space group R 3c ( Z=6) with lattice parameters a=10.3878(2) Å and c=37.8387(4) Å ( R wp=1.48% and R I=2.16%). Both β- and β′-Ca 9Fe(PO 4) 7 are related to the whitlockite-type structure. The β→ β′ phase transition in Ca 9Fe(PO 4) 7 is accompanied by a disordering of the Ca 2+ ions at the Ca3 sites and orientational disordering of the P1O 4 tetrahedra.

Phase transitions and twin lamellae in the (Bi,Pb)2223 phase: in situ high temperature electron diffraction studies

Phase transitions and twin lamellae of the (Bi,Pb)2223 phase have been studied by in situ heating experiments in a transmission electron microscope over a range of temperatures 293–800 K. In some crystal grains of the (Bi,Pb)2223 phase a modulation with the modulation wave vector q ≅0.137 b ∗ (Pb-type) was observed with 1st order satellites associated with the [0 0 1] zone. Furthermore, a monoclinic distortion was observed, as indicated by splitting of the reflections in the b ∗ direction. The monoclinicity was confirmed by dark-field images, which showed (1 0 0) twin lamellae. Diffuse streaks along 〈 h h 0〉 are present in some crystals already at room temperature, but appear in others at temperatures below 365 K. Their intensities sharpen with increasing temperature, and at 630 K a 2 a ×2 b superstructure appears. At the same temperature the monoclinic distortion and the twin lamellae disappear gradually. The new 2 a ×2 b superstructure and Pb-type modulations coexist until the phase starts to decompose. The change of these structural modulations and twin transformations are discussed in connection with oxygen ordering within the different structural layers.

Molecular Geometry of Monomeric and Dimeric Yttrium Trichloride from Gas‐Phase Electron Diffraction and Quantum Chemical Calculations.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Molecular Geometry of Monomeric and Dimeric Yttrium Trichloride from Gas-Phase Electron Diffraction and Quantum Chemical Calculations

The molecular geometry of yttrium trichloride has been determined by high-temperature gas-phase electron diffraction. The vapor phase consisted of about 87% monomeric and 13% dimeric species. High-level quantum chemical calculations have also been carried out for both the monomer and dimer of yttrium trichloride, and their geometries, harmonic force fields, and vibrational frequencies have been determined. The monomer YCl3 molecule was found to be planar (D3h symmetry) both by experiment and by computation. The bond length of YCl3 from electron diffraction is 2.450(7) (rg) or 2.422(12) (re) Å. It proved remarkably difficult to obtain a converged theoretical prediction for the bond length; large polarization bases are needed, and the published bases accompanying the pseudopotentials used appear to be overcontracted. The SCF method predicts bonds that are too long by some 0.043 Å, whereas the B3LYP method overestimates by about 0.03 Å. The B3PW91 prediction is almost within the experimental uncertainty for re. Among the traditional correlated methods, the MP2 distance with an infinitely large basis is probably indistinguishable from the experimental value, given the combined uncertainties, whereas the estimated CCSD(T) result of 2.423(10) Å is astonishingly close to the experimental result. The out-of-plane bending motion for YCl3 is noticeably anharmonic, with the result that straightforward quantum predictions of its frequency are lower than the value observed in the gas phase at high temperature.