Search

Methylammonium (MA) lead trihalide perovskites, CH3NH3PbX3 (X = I, Br, Cl), have emerged as a new class of light-absorbing materials for photovoltaic applications, reaching efficiencies of 23% when implemented in solar cell heterojunctions. In particular, MAPbBr3 is a promising member with a large bandgap that gives rise to a high open circuit voltage. Here we present a structural study from neutron diffraction (ND) data of an undeuterated MAPbBr3 specimen, carried out to follow its crystallographic behaviour in the 2–298 K temperature range. Besides the known crystallographic phases, i.e. the high-temperature Pm 3 ¯ m cubic structure, the intermediate I4/mcm tetragonal symmetry and the low-temperature Pnma orthorhombic phase, we additionally identified, from a detailed sequential ND analysis, a novel intermediate phase within the 148.5–154.0 K temperature range as an orthorhombic Imma structure, early associated with a coexistence of phases. Moreover, our ND data allowed us to unveil the configuration of the organic MA units and their complete localization within the mentioned temperature range, thus improving the crystallographic description of this compound. The evolution with temperature of the H-bonds between the organic molecule and the inorganic cage is also followed. A deep knowledge of the crystal structure and, in particular, the MA conformation inside the perovskite cage seems essential to establish structure–property correlations that may drive further improvements. [ABSTRACT FROM AUTHOR]

The previously unattained fabrication of single phase Ln2ZrO5 (Ln = Sm, Eu, Gd, and Tb) compounds via relatively low sintering temperature (1400°C) is achieved in this study using a coprecipitation method. The crystal structures have been investigated by neutron, synchrotron X‐ray powder diffraction, and electron diffraction techniques. While the general long‐range structure may be well described by the defect‐fluorite type structure with Fm3¯$\bar{3}$m symmetry, electron diffraction has highlighted a complex underlying modulated structure that varies between each compound. These compounds have been tested for ion‐irradiation response using in situ 1 MeV krypton ions and transmission electron microscopy characterization. None of the compounds undergo a crystalline to amorphous transition, even holding at 50 K. Both the underlying fluorite and modulated superstructures are little affected by the irradiation. However, some atomic rearrangements are observed in the postirradiated electron diffraction patterns for the Sm2ZrO5 specimen. [ABSTRACT FROM AUTHOR]

The hybrid perovskites are coordination frameworks with the same topology as the inorganic perovskites, but with properties driven by different chemistry, including host-framework hydrogen bonding. Like the inorganic perovskites, these materials exhibit many different phases, including structures with potentially exploitable functionality. However, their phase transformations under pressure are more complex and less well understood. We have studied the structures of manganese and cobalt guanidinium formate under pressure using single-crystal X-ray and powder neutron diffraction. Under pressure, these materials transform to a rhombohedral phase isostructural to cadmium guanidinium formate. This transformation accommodates the reduced cell volume while preserving the perovskite topology of the framework. Using density-functional theory calculations, we show that this behaviour is a consequence of the hydrogenbonded network of guanidinium ions, which act as struts protecting the metal formate framework against compression within their plane. Our results demonstrate more generally that identifying suitable host-guest hydrogen-bonding geometries may provide a route to engineering hybrid perovskite phases with desirable crystal structures. [ABSTRACT FROM AUTHOR]

The structure of fully hydrated grossular, or katoite, contains an unusual arrangement of four O-H bonds within each O4 tetrahedra. Neutron and X-ray total scattering from a powdered deuterated sample have been measured to investigate the local arrangement of this O4D4 cluster. The O-D bond length determined directly from the pair distribution function is 0.954 Å, although the Rietveld-refined distance between average O and D positions was slightly smaller. Reverse Monte Carlo refinement of supercell models to the total scattering data show that other than the consequences of this correctly determined O-D bond length, there is little to suggest that the O4D4 structure is locally significantly different from that expected based on the average structure determined solely from Bragg diffraction. [ABSTRACT FROM AUTHOR]

We revisit the phase diagram of the relaxor ferroelectric PMN-PT using neutron powder diffraction to test suggestions that residual oxygen vacancies and/or strain affect the ground state crystal structure. Powdered samples of PMN-PT were prepared with nominal compositions of, 0.20, 0.30, and 0.40 and divided into two identical sets, one of which was annealed in air to relieve grinding-induced strain and to promote an ideal oxygen stoichiometry. For a given composition and temperature the same structural phase is observed for each specimen. However, the distortions in all of the annealed samples are smaller than those in the as-prepared samples. Further, the diffraction patterns for, 0.20, and 0.30 are best refined using the monoclinicspace group. By comparing our neutron diffraction results to those obtained on single crystals having similar compositions, we conclude that the relaxor skin effect in PMN-PT vanishes on the Ti-rich side of the morphotropic phase boundary. [ABSTRACT FROM PUBLISHER]

The nature of the apparently continuous structural phase transition at 1,049 K in the perovskite-structured, MgSiO isomorph, neighborite (NaMgF), from the orthorhombic ( Pbnm) hettotype phase to the cubic ( $$ Pm\overline{3} m $$ ) aristotype structure, has been re-investigated using high-resolution, time-of-flight neutron powder diffraction. Using data collected at 1 K intervals close to the nominal phase transition temperature, the temperature dependence of the intensities of superlattice reflections at the M point $$ \left( {\frac{2\pi }{a}\left[ {\frac{1}{2},\frac{1}{2},0} \right]} \right) $$ and the R point $$ \left( {\frac{2\pi }{a}\left[ {\frac{1}{2},\frac{1}{2},\frac{1}{2}} \right]} \right) $$ of the pseudocubic Brillouin zone indicate the existence of a new intermediate tetragonal phase in space group P4/ mbm, with a narrow phase field extending from ~1,046.5 to ~1,048.5 K, at ambient pressure. Group theoretical analysis shows that the structural transitions identified in this study, Pbnm- P4/ mbm, and P4/ mbm- $$ Pm\overline{3} m $$ , are permitted to be second order. The observation of the tetragonal phase resolves the longstanding issue of why the high-temperature phase transition, previously identified as Pbnm- $$ Pm\overline{3} m $$ , and which would be expected to be first order under Landau theory, is in fact found to be continuous. Analysis of the pseudocubic shear strain shows it to vary with a critical exponent of 0.5 implying that the phase transition from Pbnm to P4/ mbm is tricritical in character. The large librational modes that exist in the MgF octahedron at high temperature, and the use of Gaussian probability density functions to describe atomic displacements, result in apparent bond shortening in the Mg-F distances, making mode amplitude determination an unreliable method for determination of the critical exponent from internal coordinates. Crystal structures are reported for the three phases of NaMgF at 1,033 K ( Pbnm), 1,047 K ( P4/ mbm) and 1,049 K ( $$ Pm\overline{3} m $$ ). [ABSTRACT FROM AUTHOR]

Copper–manganese oxides were analyzed by in situ high-temperature powder neutron and X-ray diffraction to investigate their crystal structure. Cu–Mn spinel was found to form a continuous solid solution with cubic symmetry between Mn3O4 and Cu2MnO4. A high-temperature phase with approximate composition Cu5Mn4O9 was shown to have hexagonal symmetry. The cation distribution and lattice parameters of Cu–Mn spinel were resolved through Rietveld refinement of in situ neutron diffraction data. The results demonstrated that the Cu ion has a lower octahedral site preference than manganese ions, and quenching is not a reliable method to determine the equilibrium structure in the system. [ABSTRACT FROM AUTHOR]

Abstract  The cell dimensions and crystal structures of the fluoroperovskite NaMgF3 (neighborite), synthesized by solid state methods, have been determined by powder neutron diffraction and Rietveld refinement over the temperature range 300–3.6 K using Pt metal as an internal standard for calibration of the neutron wavelength. These data show that Pbnm NaMgF3 does not undergo any phase transitions to structures of lower symmetry with decreasing temperature. The cell dimensions and atomic coordinates together with polyhedron volumes and distortion indices are given for Pbnm NaMgF3 at 25 K intervals from 300 to 3.6 K. Decreases in the a and c cell dimensions reach a saturation point at 50 K, whereas the b dimension becomes saturated at 150 K. The distortion of the structure of Pbnm NaMgF3 from the aristotype cubic $$ Pm\ifmmode\expandafter\bar\else\expandafter\=\fi{3}m $$ structure is described in terms of the tilting of the MgF6 octahedra according to the tilt scheme a − a − c . With decreasing temperature the antiphase tilt (a −) increases from 14.24� to 15.39�, whereas the in-phase tilt (c ) remains effectively constant at ∼10.7�. Changes in the tilt angles are insufficient to cause changes in the coordination sphere of Na that might induce a low temperature phase transition. The structure of Pbnm NaMgF3 is also described in terms of normal mode analysis and displacements of the condensed normal modes are compared with those of Pbnm KCaF3. [ABSTRACT FROM AUTHOR]