CaF2-type Structure Research Articles

Hochdruck-Hochtemperatursynthese und kristallstruktur von Al 11Au 6 und AlAu

The intermetallic phases Al 11Au 6 and AlAu have been synthesized from the elements by a high pressure—high temperature reaction in a modified Belt-type apparatus. Al 11Au 6, previously called Al 2Au, crystallizes cubic, space group Fm3 m, a = 5.9988(3) A ̊ , Z = 2 3 , in a CaF 2-type structure with a partially occupied aluminium site ( wR = 0.011, 55 independent reflections with I > σ( I)). AlAu crystallizes monoclinic, space group P2 1 m , with a = 6.3998(11) A ̊ , b = 3.3286(4) A ̊ , c = 6.3254(8) A ̊ , β = 93.140(9)° and Z = 4 ( wR = 0.057, 631 independent reflections with I > σ( I)). AlAu must be considered as a monoclinic distorted NiAs-type with increased coordination numbers of 8 and 9 for aluminium and gold respectively.

Phase stability and oxygen transport characteristics of yttria- and niobia-stabilized bismuth oxide

As a part of an overall study to explore the potential application of stabilized Bi2O3 as oxygen separator in various electrochemical systems, an investigation of the stability and transport characteristics of yttria- and niobia-stabilized bismuth oxide was undertaken. Polycrystalline Bi2O3 samples containing 25mol % Y2O3 were fabricated by pressureless sintering powder compacts at 1000‡ C in air. Samples containing 15mol % Nb2O5 were also fabricated by pressureless sintering at 900‡ C in air. The resulting samples were dense and of an equiaxed microstructure with grain size in the range from 28Μm for the yttria-stabilized and 42Μm for the niobia-stabilized materials, respectively. X-ray diffraction of the as-sintered specimens showed them to be single phase with CaF2-type structure. Ionic conductivity was measured by an a.c. technique over a wide range of temperatures. It was observed that the ionic conductivity of the yttria-stabilized bismuth oxide was greater than that of the niobia-stabilized one.

X-ray characterization of tin x films with CaF 2-type structure

A compound with CaF 2-type structure was characterized in TiN x films. This structure was obtained when TiN x films were sputter deposited from plasma containing high levels of nitrogen. The structure was characterized by X-ray diffraction. The differences between the CaF 2-type structure and the normal NaCl-type structure are a higher lattice parameter ( a = 0.43 nm), strong (111) and (220) peaks and the absence of (200) peaks. The color of these films was copper red.

Microstructure evolution and ordering in commercial Mg-PSZ

Sintered commercial ZrO2-9 mol % MgO (PSZ) alloy was heat-treated at different temperatures in the range 900 to 1400 ~ C. The microstructure was studied using transmission electron microscopy (TEM). The as-sintered material was characterized by either fine tetragonal precipitation in cubic matrix grains, or coarser precipitates which had transformed martensitically to the monoclinic symmetry. The diffuse scattering intensity (DSI) was observed to originate from the cubic lattice, and was correlated with the short-range ordering of the oxygen vacancies present in the cubic matrix. However, by annealing at temperatures below 1100 ~ C for relatively short times, long-range ordering occurred in the cubic matrix. The ordered phase was/3-Mg2ZrsOx2 with a rhombohedral symmetry, which belongs to the homologous series of Mo O2n-2 (M TO 12) defect structures derived from the CaF2-type structure. The ordering process is characteristic only for the cubic regions between the fine-tetragonal precipitates. This microstructure is considered to be a pseudo-equilibrium state and is related to the limited extent of diffusion.

Neutron diffraction study of U 0.5Np 0.5O 2

A neutron powder diffraction study of U 0.5Np 0.5O 2 has been performed recently at the D2 spectrometer of the ILL high flux reactor. Neutron diffraction patterns were recorded for the temperature range 3 ≤ T ≤ K. This compound crystallizes in the CaF 2type structure. The lattice parameter has been found to be a = 5.4624(3) Å in the temperature range 3 ≤ T ≤ 67 K. A magnetic ordering was observed below T N≅ 12 K, in agreement with the previous Mössbauer experiments [1]. Surprisingly the observed magnetic order is different from the one previously found in UO 2 [2]. Actually only one weak magnetic reflection quite near to the 1 2 , 1 2 , 1 2 line referred to the nuclear unit cell was observed; the high Miller indice reflections are probably too small due to the magnetic form factor to be seen. The magnetic order is compatible with the ordered moment −0.6μ B per Np atom - observed by Mössbauer resonance. A striking fact is the large width of the magnetic reflection which decreases only slightly with T remaining higher by a factor of about 5 as compared to the width of the 1,1,1 nuclear reflection; this fact is probably to be attributed to a short range of the magnetic order (a correlation length of about 20 Å has been found). Moreover clustering effects of Np and U ions should be excluded.

On the polymorphism of RbBif 4

Two low-temperature forms of the fast ionic conductor RbBiF 4 have been identified in addition to the already known high temperature form (CaF 2-type structure). The investigations used X-Ray diffraction, D.T.A., microcalorimetry and electrical measurements. The electrical properties of RbBiF 4 and of the formerly studied KBiF 4 are compared.

Conduction mechanisms in fluorides and oxide fluorides with the fluorite structure : ionic conductivity, N.M.R. and neutron diffraction

The aim of the present study is to establish correlations between the electrical and structural properties in non-stoichiometric fluorides and oxide fluorides with the CaF 2-type structure. Long-range diffusion mechanisms are suggested, they arise from neutron diffraction and nuclear magnetic resonance investigations. These two physico-chemical techniques have proved to be very helpful. They have allowed us to obtain consistent results which account for the dependence of ionic conductivity properties on amount of substitution.

A high temperature variety of BiOF

A high temperature form of BiOF has been detected and stabilized by doping with yttrium (Bi0.95Y0.05OF). The structural properties of Bi0.95Y0.05OF are compared with those of oxide halides of PbFCl- or CaF2-type structure. The electrical properties are discussed.

On the short-range order of amorphous Al?Au films

Al100−xAux films (20≦x≦71) are produced by quench condensation. The analysis of electron diffraction patterns as well as resistivity measurements reveal the liquid-like structure of these films. In contrast to amorphous Al−Cu films the amorphous Al−Au filsm exhibit prepeaks in their interference functions. Atomic distances as large as the smallest Au−Au separations existing in the compound Al2Au are responsible for these prepeaks. The CaF2-type structure of this particular compound is due to an ionic bonding contribution in Al2Au. This bonding contribution, probably caused by the large electronegativity difference between Al and Au, seems to be responsible for the pronounced chemical short-range order in amorphous Al−Au films.

Oriented growth and crystal structure of phases precipitated in Eu2+-doped NaCI single crystals

X-ray investigations of Eu2+-rich precipitates in NaCl single crystals have revealed the presence of two phases. Phase A of chemical formula EuCl2 with a CaF2-type structure and lattice parameter a = 6.969 (2) A preserves strictly the orientation of the matrix lattice. Phase B with the lattice parameter a = 7.47 (1) A and tentative formula EuCl2.2NaCl, containing Eu2+ ions and charge-compensating cation vacancies, is localized at subgrain boundaries and is thermally less stable than phase A.

Vibration Spectra of Vanadium Hydride in Three Crystal Phases by Inelastic Neutron Scattering

The vibration spectra of vanadium hydride in three crystal phases have been investigated by the energy-gain scattering of cold neutrons. The measured spectra are generally split into two broad bands above and below about 300 cm− 1, which are primarily associated with optical hydrogen vibrations and metal-atom vibrations, respectively. Pseudofrequency distributions for the hydrogen vibrations were derived from the measured neutron spectra. These indicate broad optical vibration bands peaked at 970 ± 50 and ∼1400 cm− 1 in the α (bcc) phase, 440 ± 20, 970 and ∼1400 cm− 1 in the β (bct) phase, and ∼1300 cm− 1 in the γ (fcc) phase. The width of the 440-cm− 1 band increases with hydrogen concentration, possibly due to hydrogen–hydrogen interactions. The “metal-atom” vibration spectra also show changes in peak positions and widths as the distribution of phases is changed. The changes in the neutron spectra with temperature and as a function of composition between VH0.20 and VH1.71 correlate quite well with the previously measured phase diagram. A comparison of the present results with neutron spectra for hydrides with known hydrogen positions suggests that in the α phase hydrogens only occupy tetrahedral-type sites, while in the β phase both octahedral and tetrahedral sites are occupied. The results for VH1.49 and VH1.71 indicate that in the γ phase the hydrogens occupy regular tetrahedral sites corresponding to the fluroine positions in a CaF2-type structure.