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5 págs.; 4 figs.; Open Access funded by Creative Commons Atribution Licence 4.0, The composition of high-altitude ice clouds is still a matter of intense discussion. The constituents in question are ice and nitric acid hydrates, but the exact phase composition of clouds and its formation mechanisms are still unknown. In this work, conclusive evidence for a long-predicted phase, alpha-nitric acid trihydrate (alpha-NAT), is presented. This phase was characterized by a combination of X-ray and neutron diffraction experiments, allowing a convincing structure solution. Furthermore, vibrational spectra (infrared and inelastic neutron scattering) were recorded and compared with theoretical calculations. A strong interaction between water ice and alpha-NAT was found, which explains the experimental spectra and the phase-transition kinetics. On the basis of these results, we propose a new three-step mechanism for NAT formation in high-altitude ice clouds. Ó2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.KGaA., This research project has been supported (in part) by the European Commission under the 7th Framework Program“Strengthening the European Research Area, Research Infrastructures”. We acknowledge neutron beam time at ISIS and at the FRM II. TheX-ray measurements werecarried out within the X-ray Center of the Vienna University of Technology (XRC). H.G.and F.W. are grateful for financialsupport from the Austria Science Funds (FWF;P23027). O.G.acknowledges financial support from the Ministerio de Ciencia eInnovación (“Ramón y Cajal” program, FIS2010-16455 and CGL2013-48415). R.I. and M.J.R. would like toacknowledge generous funding from the Swiss National Science Foundation (grants 200020_125204 and200020_144431/1).

EGU2015, Vienna | Austria | 12 – 17 April 2015

In analogy to the synthesis of polycrystalline M2NaIO6 (M = Ca, Sr, Ba) by precipitation in water at 90 °C, the title compound was first prepared as a metastable compound. The stable modification of Pb2NaIO6 was obtained by a heat treatment to 400 °C followed by cooling to room temperature. The crystal structure was refined from powder diffraction data [space group P21/c (14), a = 5.9040(2), b = 5.7526(2), c = 10.1104(3) Å, β = 125.341(1)°]. On heating, at ca. 125 °C, a phase transition to a cubic high temperature modification was observed. The crystal structure was refined from XRD data measured at 200 °C [space group Fm3m (225), a = 8.2678(1) Å]. Depending on the precipitation temperature between 90 °C and 0 °C, several metastable modifications were obtained, which can be distinguished by significantly different lattice parameters. The XRD pattern of a powder precipitated at room temperature is pseudocubic. The crystal structure was refined at room temperature in P21/c with a = 5.8201(4), b = 5.8473(4), c = 10.0798(5) Å, β = 125.074(3)°. This modification behaves almost as a cubic lattice on heating as found from XRD and DSC measurements.

A comprehensive bulk and surface investigation of high-quality In2O3(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 +/- 0.5 x 10(5) Omega cm, which translates into a carrier concentration of approximate to 10(12) cm(-3). The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) angstrom from the previously accepted value of 10.117 angstrom. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 degrees C) surface show a step height of 5 angstrom, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 angstrom height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (E-F). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.

The effect on crystal structure and vibrational frequencies of physical pressure in BaFCl and chemical pressure in Ba1−xSrxFCl solid solutions is studied using periodic density-functional theory (DFT) calculations performed within the local-density approximation (LDA) and the generalized gradient approximation (GGA). These results are compared with previously published experimental data for BaFCl in conjunction with new experimental data for Ba1−xSrxFCl and show overall a good agreement with experiment. The GGA method outperforms the LDA method for the description of BaFCl under pressure. However, the two DFT methods perform equally well for the description of the solid solutions, which have been studied within the virtual-crystal approximation. They also give consistent values of the energy of formation of Ba1−xSrxFCl, which can be correlated with the experimentally observed melting points. The comparison of the calculated mode Grüneisen parameters shows that, for the investigated systems, the effect of the chemical pressure and that of the physical pressure are not identical.

A series of mixed crystals with general formula Ba7-xNayF12Cl2-zBrz in the ordered modification (space group P-6) has been studied by single crystal x-ray diffraction. Depending on synthesis conditions, the disorder in the channels (i.e. occupation of 0 0 z sites) can be changed. The disorder is found to be correlated with the refined Na content, and its effect on Ba-Cl(Br) bond length is discussed.

Mixed matlokite hosts of composition BaFBrxI1–x(0x1) (pure and doped with Sm2+, Eu2+) were studied with x-ray crystallography, luminescence, Raman, and electron paramagnetic resonance (EPR) spectroscopy. Results are presented for BaFBr0.5I0.5 which demonstrate that a ferrielectric domain structure is formed due to the fact that the heavy halogen ions form separate sublattices with randomly distributed domain walls. The space group of a domain is P4 mm (No. 99). The EPR data from Eu2+ allowed to determine the volume fraction of domains.

Temperature-dependent emission spectra of Sm[2+]-doped SrMgF4 have been obtained in the temperature range from 50 to 300 K. At 50 K, six bands are observed for the very strong [5]D0→[7]F0 transition, in agreement with the reported sixfold crystal superstructure. The overall splitting of more than 70 cm[−1] highlights the important structural differences of the six Sr sites. Upon heating progressively to room temperature, the spectra change progressively with a more pronounced change between 270 and 300 K. These observations suggest the possibility of a complex structural behavior for SrMgF[4] which will require new experiments.

Crystals of ordered and disordered Ba7F12Cl2 were prepared by flux growth and solid state reactions. These new structures were characterized by single crystal and powder X-ray diffraction. The disordered variant which shows disorder on one of the cation sites was obtained from a BaF2 + BaCl2 + NaCl/NaF flux. It has hexagonal space group P63/m (176) with one formula unit per unit cell. The lattice constants are a = b = 1059.55(5) pm and c = 420.10(4) pm (at 21 °C). The structure was refined to R(Rw) = 0.026(0.030) for 346 independent reflections and 26 parameters. Slow cooling of a mixture of BaF2 and LiCl yields the ordered variant. This one crystallizes in the hexagonal space group P6 (174) with one formula per unit cell. Lattice constants at 21 °C are a = b = 1063.46(2) pm and c = 417.52(1) pm. The structure was refined to R(Rw) = 0.017 (0.017) for 638 independent reflections and 45 parameters. The structural arrangement and the interatomic distances of the two variants are mutually similar. The barium atoms have coordination number nine. Propeller-type arrangements with a chloride ion on the axis and the fluoride ions as blades are observed. These latter ones are interconnected into channels' of tricapped fluoride prisms. Occupation disorder of the barium sites in the channels of the disordered variant makes the main difference between the two. An unexpectedly high X-ray density obtained for both variants of Ba7F12Cl2 can be correlated to the density of other barium fluorohalides having a coordination number of nine for the barium ion.

Crystals of composition Ba[7]F[12]Cl[2] were obtained by areaction at room temperature between Ba[2+]/Cl[-]/F[-]in a gel of agar-agar/water. The hexagonal crystals have space group P6, a=1064.69(8), c=417.89(5)pm, V=410.24(8) 10[6] pm[3] and Z=l. The anions form a propeller type network located in tunnels parallel to the c[hex] axis; the chloride ions are located at the center on the propeller axes. The Ba[2+] ions are coordinated by a (distorted) tricapped trigonal environment of fluoride and chloride anions. Disorder is present for one particular Ba[2], site. The average structure is isotypic with the structure of Pb[7]F[12]Cl[2].

The structure of the complex perovskite PbFe0.5Ta0.5O3 (PFT), which belongs, according to the literature, to the ferroic species m̅3mF3m, was refined by single-crystal and powder x-ray diffraction at room temp., i.e. within the range of stability of the cubic phase. Attention was paid to the possible presence of tetragonal spontaneous strain, suggested by the anomalous uniaxial birefringence of each (100)cub pyramidal growth sector. A crystal cut from a single growth sector was measured and compared with an as-grown one. Several models, both cubic (Pm̅3m) and tetragonal (P4/mmm), these latter used in refining the single growth sector, are compared. At. coordinates are given. The main feature of all the refined structures is the positional disorder at the Pb site, the importance of which in connection with the ferroic transitions is briefly discussed.

Birefringence; Crystal structure; Ferroelastic domain; Ultraviolet and visible spectra (optical studies of ferroelastic domain structures of LaGaO3 and NdGaO3) gallium oxide lanthanum neodymium ferroelastic domain; visible gallium oxide lanthanum neodymium ferroelastic; birefringence gallium oxide lanthanum neodymium; structure gallium oxide lanthanum neodymium ferroelastic

Crystal of KCoPO4 were obtained by growth in a gel of tetramethoxysilane/H2O at 64 Deg. The hexagonal crystals have space group P63, a 18.206(1), c 8.5135(8) .ANG., Z = 24. At. coordinates are given. The structure is isotypic with a-KZnPO4. It comprises an ordered three dimensional network of alternating CoO4 and PO4 tetrahedra, which has rings of six tetrahedra of six tetrahedra in the xy plane. These rings from tunnels in the [001] direction, where the K ions are located. Upon heating, the compd. undergoes a phase transition at .apprx.565 Deg; on cooling, the phase transition occurs at 449 Deg, where it transforms into another structure which is yet unknown. The hexagonal structure is a metastable phase.

Two phase transitions of K3Fe5F15, at about 290 and 120 K, have been identified by polarized light microscopy and magnetic measurements on a single crystal grown from high temp. soln. with subsequent quenching from 973 K. The former one at about 290 K is a new second-order structural transition from the orthorhombic to a monoclinic phase, as shown by the appearance of new types of ferroelastic domain walls and a rotation of the optical indicatrix around the pseudo-tetragonal principal axis. The transition at about 120 K is second order and magnetic nature, accompanied by anomalies of spontaneous linear birefringence and antiferromagnetic type magnetic susceptibility. The temp. dependence of the magnetization suggests that a spontaneous ferromagnetic component appears in the magnetically ordered phase below 120 K.