Mixed Valence of Ce and Its Consequences on the Magnetic State of Ce9Ru4Ga5: Electronic Structure Studies

We report on X-ray photoelectron spectroscopy (XPS) and ab initio electronic structure investigations of a novel intermetallic material Ce 9 Ru 4 Ga 5 . The compound crystallizes with a tetragonal unit cell (space group I4 m m ) that contains three inequivalent Ce atoms sites. The Ce 3 d core level XPS spectra indicated an intermediate valence (IV) of selected Ce ions, in line with the previously reported thermodynamic and spectroscopic data. The ab initio calculations revealed that Ce1 ions located at 2 a Wyckoff positions possess stable trivalent configuration, whereas Ce2 ions that occupy 8 d site are intermediate valent. Moreover, for Ce3 ions, located at different 8 d position, a fractional valence was found. The results are discussed in terms of on-site and intersite hybridization effects.

New quaternary selenides Tl4Sb8Sn5Se24 and Tl5Sb2Sn4Se14−x (x=0.5)

Synthesis, structural characterization and chemical bonding peculiarities of two new quaternary selenides of the Tl4Sb8Sn5Se24 and Tl5Sb2Sn4Se14−x (x = 0.5) are reported. The crystal structures of these compounds were determined by single crystal X-ray diffraction analysis. The Tl4Sb8Sn5Se24 phase is a new structure type and crystallizes in the triclinic P 1̅ space group (a = 7.9061(13) Å, b = 13.9035(16) Å, c = 21.4665(17) Å, α = 80.058(8)°, β = 84.727(10)°, γ = 73.542(12)°, wR2 = 0.0258, 8200 F2 values, 373 variables). The Tl5Sb2Sn4Se14−x (x = 0.5) phase represents a new structure type with a tetragonal unit cell (P 4/mbm, a = 8.1570(10) Å, c = 21.946(3) Å, wR2 = 0.0346, 1953 F2 values, 44 variables). Crystallographic analysis together with linear muffin-tin orbital band structure calculations reveals that both structures we conform to the concept of Zintl-Klemm and exhibit semiconductor properties.

Crystalline structures of Rb2UBr6 ionic conductor determined by neutron diffraction

The neutron powder diffraction technique has been used for structural studies of Rb2UBr6 solid electrolyte as a function of temperature. The low-, room-, and high-temperature structures have been determined. At the temperature range of 4.2–80 K, the compound crystallizes in a monoclinic unit cell in the P21/c space group. At 80 K and 853 K, the compound crystallizes in a tetragonal unit cell in the P4/mnc space group. At 300 K, the lattice constants are a = b = 7.745(1) and c = 11.064(1) Å. At the temperature range of 853–960 K, a trigonal phase is observed in the Pʒ̄ml space group.

Relation between Crystal Structure and Transition Temperature of Superconducting Metals and Alloys

Using the Roeser–Huber equation, which was originally developed for high temperature superconductors (HTSc) (H. Roeser et al., Acta Astronautica 62 (2008) 733), we present a calculation of the superconducting transition temperatures, T c , of some elements with fcc unit cells (Pb, Al), some elements with bcc unit cells (Nb, V), Sn with a tetragonal unit cell and several simple metallic alloys (NbN, NbTi, the A15 compounds and MgB 2 ). All calculations used only the crystallographic information and available data of the electronic configuration of the constituents. The model itself is based on viewing superconductivity as a resonance effect, and the superconducting charge carriers moving through the crystal interact with a typical crystal distance, x. It is found that all calculated T c -data fall within a narrow error margin on a straight line when plotting ( 2 x ) 2 vs. 1 / T c like in the case for HTSc. Furthermore, we discuss the problems when obtaining data for T c from the literature or from experiments, which are needed for comparison with the calculated data. The T c -data presented here agree reasonably well with the literature data.

Towards a revisitation of vesuvianite-group nomenclature: the crystal structure of Ti-rich vesuvianite from Alchuri, Shigar Valley

Vesuvianite containing 5.85 wt% TiO2from an Alpine-cleft-type assemblage outcropped near Alchuri, Shigar Valley, Northern Areas, has been investigated by means of electron microprobe analyses, gas-chromatographic analysis of H2O, X-ray powder diffraction, single-crystal X-ray structure refinement,27Al NMR,57Fe Mössbauer spectroscopy, IR spectroscopy and optical measurements. Tetragonal unit-cell parameters are:a= 15.5326 (2),c= 11.8040 (2) Å, space groupP4/nnc. The structure was refined to finalR1= 0.031,wR2= 0.057 for 11247I> 2σ(I). A general crystal-chemical formula of studied sample can be written as follows (Z= 2):[8–9](Ca17.1Na0.9)[8]Ca1.0[5](Fe2+0.44Fe3+0.34Mg0.22)[6](Al3.59Mg0.41)[6](Al4.03Ti2.20Fe3+1.37Fe2+0.40) (Si18O68) [(OH)5.84O2.83F1.33]. The octahedral siteY2 is Al-dominant and does not contain transition elements. Another octahedral siteY3 is also Al-dominant and contains Fe2+, Fe3+and Ti. The siteY1 is split intoY1aandY1bpredominantly occupied by Fe2+and Fe3+, respectively. The role of theY1 site in the diversity of vesuvianite-group minerals is discussed.

CH3NH3PbI3: precise structural consequences of water absorption at ambient conditions

The crystal structure of the pristine (I) and aged (II) crystals of CH3NH3PbI3(hereafter MAPbI3) hybrid organic–inorganic lead iodide has been studied at 293 K with high-precision single-crystal X-ray diffraction using a synchrotron light source. We show that (I) and (II) are characterized by an identical tetragonal unit cell but different space groups:I422 for (I) andP42212 for (II). Both space groups are subgroups ofI4/mcm, which is widely used for MAPbI3. The main difference between (I) and (II) comes from the difference in hydrogen bonds between the MA+cation and the PbI3framework which is the direct consequence of H2O insertion in the aged crystal (II).

La3Si6N11

Colorless transparent single crystals of trilanthanum hexasilicon undecanitrogen, La3Si6N11, were prepared at 0.85 MPa of N2and 2273 K. The title compound is isotypic with Sm3Si6N11. Silicon-centered nitrogen tetrahedra form a three-dimensional network structure by sharing their corners. Layers of one type of SiN4tetrahedra and slabs composed of the two different La3+cations and the other type of SiN4tetrahedra are alternately stacked along thecaxis of the tetragonal unit cell. The site symmetries of the two La3+cations are are ..mand 4.., respectively.

Different Structural Models of YB2C2 and GdB2C2 on the Basis of Single-Crystal X-Ray Data

Samples of YB2C2 and GdB2C2 were synthesized by arc-melting and subsequent annealing of the products. Single crystals of the title compounds were examined with single-crystal X-ray methods. Five different tetragonal unit cell settings from the literature (obtained on the basis of symmetry considerations) were used for the refinement of the crystal structure of YB2C2, but a converging refinement was only achieved for the structural model in the space group P4=mbm (no. 127, Z = 2) with a = 533:27(3) and c = 354:58(3) pm, without obvious inconsistencies. Nevertheless, the results for the five different unit cell settings are compared and discussed. A refinement of the crystal structure of the isotypic compound GdB2C2 was also performed with the lattice parameters a = 537:46(6) and c = 364:98(11) pm. Neither EuB2C2 nor YbB2C2 were obtained by melting coldpressed pellets with the nominal composition RE1.5B2C2 in an arc-furnace or an induction furnace, or by just heating the pellets in sealed Ta ampoules

Electrical Properties of Fresnoite Ba2TiSi2O8 Using Impedance Spectroscopy

Fresnoite with composition Ba2TiSi2O8 (B2TS2) was first found in 1965, adopting a non-centrosymmetric structure. It also reported to crystallize in a tetragonal unit cell with a=8.52Å and c=5.210Å leading to some possible application as hydrophone, transducer and second harmonic generation and low temperature co-fired ceramics (LTCC). B2TS2 were synthesized by conventional solid state reaction. Phase pure B2TS2 was obtained after heating the pellets at a final sintering temperature of 1230 °C in air at 92 h. Study found that Fresnoite B2TS2 is a type of materials which are not ferroelectric and instead show perfect dielectric insulator behaviour with resistance >106Ωcm at temperatures below 750°C and also shows nonideal debye respone. The activation energy for conduction of B2TS2 samples is very high, indicating that these materials are highly insulating.

An unexpected tetragonal unit cell forN-(L-2-aminobutyryl)-L-serine

The title dipeptide {systematic name: (S)-2-[(S)-2-azaniumylbutanamido]-3-hydroxypropanoate}, C7H14N2O4, was synthesized in the anticipation that it would form nanoporous crystals with hexagonal symmetry. Single-crystal X-ray diffraction analysis showed that it had instead adopted a unit cell in the space groupI4, similar to L-alanyl-L-alanine [Fletterick, Tsai & Hughes (1970).J. Phys. Chem.75, 918–922]. The resulting packing arrangement has a high density for a peptide (1.462 Mg m−3), which is rendered possible by extensive disorder over two positions for the ethyl side chain of the 2-aminobutyric acid fragment and over three positions for the serine side chain.<!?tpb=17.5pt>