Mechanism for the Combined Li-Na Ionic Conductivity in Sugilite (Fe2Na2KLi3Si12O30)-Type Compounds

This study explains the ionic conductivity in the mineral sugilite (idealized formula: Fe2Na2KLi3Si12O30) by resolving the dynamic disorder of both Li and Na cations using synchrotron X-ray single-crystal diffraction from 298 K to 1023 K. Non-zero anharmonic atomic displacement parameters at Na and Li sites at 1023 K adumbrated long-range charge transport routes for Li and Na cations commonly parallel to the (a-b) plane. Temperature-enhanced diffuse residuals in Fourier maps could unambiguously localize two interstitial sites suitable for Li, as well as three for Na. Each two-dimensional (2D) network of Li and Na interstitials was formed parallel to each other, providing Li and Na hopping pathways. The higher concentration of Na cations hopping in short distances of 2.0962(4)-2.3015(5) Å could be the main reason for the higher bulk conductivity values evaluated by impedance spectra of sugilite in comparison to those of its structural relatives with low Na contents, e.g., the mineral sogdianite ((Zr,Al,Fe)2Na0.36KLi3Si12O30). Bond valence sum landscape maps supported the critical role of dynamic disorder of Na+ over densely packed 2D interstitial networks for combined ionic conductivity along with mobile Li+ in sugilite-type compounds

Study of oxygen vacancy ordering in mullite at high temperatures

High temperature X-ray diffraction and quenching experiments of mullite single crystals with Al2O3:SiO2 ratio 2:1 have been performed to investigate the stability of the oxygen vacancy ordering close to the melting point of mullite. The experiments show that the structure of mullite exhibits an extremely stable, temperature-independent incommensurate modulation. Inspection of satellite reflections at different temperatures leads to the conclusion that the ordering scheme of oxygen vacancies after the crystallization of mullite persists to the melting point and does not show any disordering effects. The experimental results are in agreement with former theoretical calculations using a statistical mechanics approach which yield the critical temperature Tc > 3000°C

Monoammonium Trimetaphosphimate (NH₄)H₂(PO₂NH)₃

Trimetaphosphimates show a rich structural variability in both cation coordination and anion arrangement. They are precursors for crystalline, as well as amorphous oxonitridophosphates. The ammonium trimetaphosphimate (NH4)H2(PO2NH)3 is formed during the decomposition of the corresponding acid in solution. The monoclinic crystal structure of the monoammonium salt was elucidated by single crystal X-ray diffraction using synchrotron radiation. The trimetaphosphimate monoanions exhibit a twist conformation and form crankshaft-like stacks along [100], which have so far only been observed in (NH4)3(PO2NH)3·H2O and Ag3(PO2NH)3. (NH4)H2(PO2NH)3 decomposes at 170 °C, forming a poorly crystalline phase. Therefore, it is a model system and possible precursor for the synthesis of oxonitridophosphates

Precursors in lead phosphate-type ferroelastics: Diffuse X-ray scattering, group theory and modelling

A chain-adapted symmetry-mode analysis is presented for the symmetry change R 3 m > C12/c1 in ferroelastic lead phosphate. The primary and the secondary modes are derived for all relevant Wyckoff positions. The splitting schemes of the orbits and the patterns of the displacive modes are compatible with experimental diffraction studies performed in lead phosphate type crystals. The appearance of diffuse monoclinic X-ray diffraction signals above the ferroelastic transition point of lead phosphate is modelled using simple real structure simulations

Combined X-ray and neutron single-crystal diffraction in diamond anvil cells

It is shown that it is possible to perform combined X-ray and neutron single-crystal studies in the same diamond anvil cell (DAC). A modified Merrill–Bassett DAC equipped with an inflatable membrane filled with He gas has been developed. It can be used on laboratory X-ray and synchrotron diffractometers as well as on neutron instruments. The data processing procedures and a joint structural refinement of the high-pressure synchrotron and neutron single-crystal data are presented and discussed for the first time

The modulated low-temperature structure of malayaite, CaSnOSiO4

The crystal structure of the mineral malayaite has been studied by single-crystal X-ray diffraction at a temperature of 20 K and by calculation of its phonon dispersion using density functional perturbation theory. The X-ray diffraction data show first-order satellite diffraction maxima at positions q = 0.2606 (8)b*, that are absent at room temperature. The computed phonon dispersion indicates unstable modes associated with dynamic displacements of the Ca atoms. The largest-frequency modulus of these phonon instabilities is located close to a wavevector of q = 0.3b*. These results indicate that the malayaite crystal structure is incommensurately modulated by static displacement of the Ca atoms at low temperatures, caused by the softening of an optic phonon with Bg symmetry

Comparison of the temperature- and pressure-dependent behavior of the crystal structure of CrAs

The crystal structure of CrAs was investigated using synchrotron X-ray single- crystal diffraction for separate dependences on temperature (30–400 K) and on pressure (0–9.46 GPa). The isosymmetrical magnetostructural phase transition at TN = 267 K can induce a change in the microstructure by twinning due to a crossing of the orthohexagonal setting of the unit-cell parameter ratio c/b. Within the crystal structure, one particular Cr–Cr distance exhibits anomalous behavior in that it is nearly unaffected by temperature and pressure in the paramagnetic phase, which is stable above 267 K and at high pressures. The distinction of this shortest Cr–Cr distance might be of importance for the superconducting properties of CrAs

Bimetallic Organoplatinum(II)‐Ag(I) Cluster Cations with Ag−Pt Interactions Unsupported by Conventional Bridging Ligands

The organoplatinum complex of a primary amine and AgClO$_4$ aggregate in methanol to oligonuclear mixed-metal species. A perchlorate salt 3 with the empirical formula C$_{96}$H$_{146}$Ag$_7$Cl$_{13}$N$_{12}$O$_{62}$Pt$_6$ could be isolated and structurally characterized with the help of X-ray diffraction at the DESY synchrotron. The product contains both a penta- and a tetra-nuclear complex cation in which Ag(I) and Pt(II) alternate, held together without any conventional bridge. The cationic species feature short Ag⋅⋅⋅Pt distances between 2.7 and 2.9 Å and contacts longer than 2.4 Å between Ag and C atoms in aromatic rings. In addition to the interactions with Ag(I) cations, Pt(II) is in a square-planar arrangement, coordinated by a chelating and a terminal amine and an aqua ligand. The central Ag(I) in the pentanuclear cation is located on a twofold crystallographic axis and not involved in any obvious coordinative bond; it exclusively shows short contacts to the neighboring Pt(II) ions and the Pt-bonded, formally anionic carbon atoms of the cyclometallated organic ligand. Powder diffraction shows that 3 melts and re-solidifies without decomposition

Superspace approach helps: determination of proton dynamics in the phase transition of modulated supramolecular ferroelectrics: 5,5′-dimethyl-2,2′-bipyridine and bromanilic acid

Temperature dependent crystal structures are reported for the co-crystal of 5,5′-dimethyl-2,2′-bipyridine (55DMBP) and bromanilic acid (H$_2$ba) across its phase transitions. 55DMBP–H$_2$ba is ferroelectric (FE) below T = 245 K and remains paraelectric (PE) at higher temperatures up to 360 K, but passes through two PE–PE phase transitions. X-ray diffraction data at 120 K reveals a ferroelectric phase (FE-I phase), which can be described as a commensurately modulated structure with superspace group P[1 with combining macron]$(σ_1σ_2σ_3)_0$ with modulation wave vector q = 0.5, 0.5, 0.5. At 250 K, the crystal transforms into the paraelectric phase PE-II, which possesses the same modulation wave vector. Above 320 K the modulation wave vector becomes incommensurate, q = (0.5000, 0.4944, 0.5221), while the superspace group remains the same in the FE-I, PE-II and PE-IC phases. Different choices of the phase of the modulation wave allow the PE-II to FE-I phase transition to be described by a phase shift in superspace. Above 338 K the satellite Bragg reflections disappear. The crystal structure at 346 K of this PE-III phase is periodic with space group P[1 with combining macron] and a unit cell that acts as basic structure for the modulated phases. Peak profiles become very broad at 350 K and at 360 K the crystal disintegrates, and the material becomes amorphous. Anharmonic atomic displacements are found for the Br atoms in the PE-IC and PE-III phases. The FE-I phase is ferroelectric due to proton transfer within part of the O–H⋯N intermolecular hydrogen bonds, a mechanism similar to that of phenazine-chloranilic acid. The PE-IC phase involves modulations of the proton between two tautomeric forms of H$_2$ba, thus leading to an exchange between O1–H1⋯N1 and O$_2$–H1⋯N1 hydrogen bonds. This mechanism is essentially different from the incommensurability in phenazine-chloranilic acid