PbFCl-type Structure Research Articles

Superconductivity in a PbFCl-type pnictide: NbSiAs

We report a new superconductor, NbSiAs, with a PbFCl-type structure, which exhibits bulk superconductivity with a superconducting transition temperature (Tc) of 8.2 K and a shielding volume fraction of 40 vol%. Isothermal magnetization and heat capacity measurements indicate NbSiAs is a BCS-like type II superconductor with a normalized specific heat jump value (Cel/γTc) of ∼1.95. Strongly hybridized bonds between Nb 4d and Si 3p orbitals enhance the Debye temperature, which is favorable for the emergence of superconductivity.

Mechanoluminescence and thermoluminescence of BaFCl:Sm 2+ and BaFBr:Sm 2+ crystals

The alkaline-earth fluorohalide crystals MFX, where M=Ca, Sr, Ba, Pb and X=Cl, Br, I, form an important class of materials crystallizing in the PbFCl-type tetragonal structure which is also called the matlockite structure. These compounds have long been of interest because of the various defect species which can be detected by spin resonance and associated techniques. The crystals were prepared by slow cooling of the melt of a stoichiometric mixture of BaF 2 and the corresponding chloride or bromide under 0.2 bar of ultrapure argon (5N5), often slightly fluorinated. We have studied the mechanoluminescence (ML) of BaFBr:Sm 2+ and BaFCl:Sm 2+ crystals. It is seen that after the impact of a moving piston, initially the ML intensity increases with time, attains a maximum value and then it decreases with time up to a particular minimum value, and then it increases again, attaining a peak value and finally disappears. The first peak lies in the deformation region and the second peak lies in the post-deformation region. The ML intensity of the BaFCl:Sm 2+ crystal is much higher than the ML intensity of the BaFBr:Sm 2+ crystal. For different impact velocities, the ML intensity increases with velocity; and the total ML intensity attains a saturation value for higher impact velocities. The total ML intensity increases with the increase in the applied load. It is suggested that the moving dislocation produced during deformation of crystals captures holes from hole-trapped centers (like H centers), and the subsequent radiative recombination of the dislocation holes with electron gives rise to ML. Thermoluminescence (TL) of BaFBr:Sm 2+ and BaFCl:Sm 2+ crystals was studied after exposure to ultraviolet rays with the help of a TLD reader. The peak of TL for the BaFBr:Sm 2+ crystal is found at ∼247°C and for BaFCl:Sm 2+ crystals at 283°C. The TL intensity initially increases with increase in the UV radiation and then it attains saturation for higher values of UV exposure. The absorption spectrum was recorded with the help of a UV–visible spectrophotometer (Shimadzu). The band found at 275 nm was attributed to H centers.

The β-polymorph of uranium phosphide selenide

β-UPSe was synthesized from the reaction of U2Se3, P and Se in a CsCl flux in a fused-silica tube. It crystallizes with four formula units in the tetra­gonal space group I4/mmm in the UGeTe structure type. The asymmetric unit comprises one U (site symmetry 4mm), one Se (4mm), and one P (mmm.) atom. The U atom is coordinated in a monocapped square-anti­prismatic arrangement, where the square face is formed by P atoms and the other five vertices are Se atoms. The P site is disordered about a mirror plane, showing half-ocupancy for each of the two resulting P atoms. The title structure is related to that of α-UPSe, which crystallizes with two formula units in the tetra­gonal space group P4/nmm in the PbFCl structure type.

Half-Heusler phase related structural perturbations near stoichiometric composition FeZnSb

Half-Heusler phases XYZ (Pearson symbol cF12) are chemically versatile and rich in physical properties. The half-Heusler phase in the Fe–Zn–Sb ternary system was reported in the year 2000. In this work, two new ternary phases are identified in the vicinity of the equiatomic composition FeZnSb in the same system: Fe 1− x ZnSb (tetragonal, space group P4/ nmm, Pearson symbol tP6− δ, Z=2: a=4.1113(6) Å, c=6.0127(12) Å for x=0.08 ( 1), and a=4.1274(6) Å, c=6.0068(12) Å for x=0.12 ( 2)); and Fe 7.87Zn 6.72Sb 8 (Fe 0.98Zn 0.84Sb) ( 3) (cubic, space group Fm-3 m, Pearson symbol cF96− δ, Z=4, a=11.690(13) Å). 1 and 2 crystallize in the PbFCl-type structure, and 3 adopts a unique 2×2×2 supercell of a normal half-Heusler structure. The structures of both the tetragonal and cubic phases can be described as assemblies of half-Heusler structure related subunits. Electrical resistivity measurement on the pure sample of 2 shows it has metallic-like behavior, and its thermal and magnetic properties are also characterized.

Crystal Chemistry and Physical Properties of the Nonmagnetic Kondo Compound HfAs1.7Se0.2

Single crystals of HfAs(1.7)Se(0.2) are grown by chemical transport reaction and their chemical composition characterized in detail by various analytical methods. Chemical analyses and crystal structure investigations by single-crystal X-ray diffraction as well as powder diffraction with synchrotron radiation reveal a tetragonal PbFCl structure type with strong disorder caused by a significant arsenic deficiency (As(0.9)) on the 2a site and mixed occupancy of the 2c site (As(0.8)Se(0.2)). HfAs(1.7)Se(0.2) is a diamagnetic metal which transforms into a superconducting state at T(c)=0.52 K. Similar to other PbFCl-type arsenide selenides, the title compound displays a magnetic-field-independent -AT(1/2) term in the low-temperature electrical resistivity. This unusual term presumably originates from the electron scattering of structural two-level systems. According to the experimental results, HfAs1.7Se0.2 appears to be a rare example of a nonmagnetic Kondo material.

Single-crystal structures of uranium and neptunium oxychalcogenides AnO Q ( An=U, Np; Q=S, Se)

The compounds UOS, UOSe, NpOS, and NpOSe have been synthesized and their structures determined by means of single-crystal X-ray diffraction methods. The results provide more detailed crystallographic information, including more precise interatomic distances, than earlier determinations from powder diffraction data. These isostructural compounds adopt the PbFCl structure type. Each An atom is surrounded by four O and five Q atoms in a distorted monocapped square-antiprismatic arrangement.

Ternary arsenides Zr(Si As1−)As with PbCl2-type (0 ≤x≤ 0.4) and PbFCl-type (x= 0.6) structures

The ternary zirconium silicon arsenides Zr(SixAs1−x)As have been prepared through reaction of the elements at 900 °C. Extensive solubility of Si in the parent binary arsenide ZrAs2 was established in the form of the solid solution Zr(SixAs1−x)As (0 ≤ x ≤ 0.4). Single-crystal X-ray structure determination of the end-member Zr(Si0.39(1)As0.61)As (orthorhombic PbCl2-type, space group Pnma, Z = 4, a = 6.6828(8) Å, b = 3.6377(4) Å, c = 9.1131(11) Å, V = 221.54(4) Å3) revealed that the Si atoms prefer to occupy the interior sites of a four-atom-wide polyanionic ribbon. When the Si composition is increased to x = 0.6, the new ternary arsenide Zr(Si0.57(1)As0.43)As was identified (tetragonal PbFCl-type, space group P4/nmm, Z = 2, a = 3.6539(7) Å, c = 8.2399(15) Å, V = 110.01(4) Å3), in which the Si atoms prefer to occupy the sites of a denser polyanionic square net. Core-line X-ray photoelectron spectra measured on metallic, plate-shaped crystals of Zr(Si0.6As0.4)As support the presence of anionic Si and As atoms. Band structure calculations on idealized ordered models of Zr(Si0.5As0.5)As suggest that a competition between Zr-anion and anion–anion bonding interactions takes place in both structures.

Lutetium(III) oxide bromide, LuOBr

Single crystals of lutetium oxide bromide, LuOBr, were obtained accidentally as a by-product of the reaction of lutetium metal, ruthenium powder and lutetium tribromide, LuBr3, in a sealed tantalum container. As is typical for rare-earth oxide halides of the type REOX (RE = rare-earth metal and X = halogen), LuOBr crystallizes in the tetra­gonal PbFCl structure type (matlockite), where Lu, O and Br are situated on positions with 4mm, \overline{4}m2 and 4mm symmetry, respectively.

Thorium tin antimonide, ThSn0.2Sb1.8

ThSn0.2Sb1.8 represents the upper limit of Sn substitution into ThSb2. The Sn atoms enter the Sb site in the UAs2-type or anti-Cu2Sb-type (a binary variant of the PbFCl-type or ZrSiS-type) structure of ThSb2, which is built up of layers of Th-centred monocapped square antiprisms.

Crystallographic disorder and electron scattering on structural two-level systems inZrAs1.4Se0.5

Single crystals of ZrAs1.4Se0.5 (PbFCl-type structure) were grown by chemical vapour transport. While theirthermodynamic and transport properties are typical for ordinary metals, the electricalresistivity exhibits a shallow minimum at low temperatures. Application of strongmagnetic fields does not influence this anomaly. The minimum of the resistivity inZrAs1.4Se0.5 apparently originates from interaction between the conduction electrons and structuraltwo-level systems. Significant disorder in the As–Se substructure is inferred from x-raydiffraction and electron microprobe studies.

Phonon vibrational frequencies and elastic properties of solid SrFCl. An ab initio study

The phonon vibrational frequencies, electronic and elastic properties of SrFCl, one of the members of the alkaline-earth fluorohalide family crystallizing with the PbFCl-type structure, have been investigated, for the first time, at the ab initio level, by using the periodic CRYSTAL program. Both Hartree-Fock (HF) and density functional theory (DFT) Hamiltonians have been used, with the latter in its local density, gradient-corrected (PW91), and hybrid (B3LYP) versions. The structural and elastic properties are in good agreement with experiment, with the exception of those calculated within the local density approximation, which were found to be systematically under-estimated (distances) or over-estimated (elastic properties). As regards the phonon frequencies, B3LYP and PW91 provide excellent results, the mean absolute difference with respect to the experimental Raman data being 4.1% and 3.6%, respectively.

Crystal growth, structure and electrical properties of thorium phosphorosulfide

Single crystals of thorium phosphorosulfide have been grown by the chemical vapour transport method. The X-ray diffraction examination showed that the unit cell of the crystals belongs to a tetragonal system of the PbFCl-type structure. The basal plane resistivity ρ(300 K)=64 μΩ cm and thermoelectric power S(300 K)=−7.7 μV/K, examined between 0.4 and 315 K, show metallic behaviour.

Electronic structure of ionic PbFCl-type compounds under pressure

The electronic structures of alkaline-earth fluoro-halides—SrFBr, SrFI, andCaFBr, which crystallize in the PbFCl-type structure—have been studied usingthe tight-binding linear muffin-tin orbital method within the local densityapproximation. The total energies were calculated using the atomic sphereapproximation and were used to determine the ground state properties ofthese systems. The calculated ground state properties agree fairly wellwith the experimental results. These systems were found to be directband gap insulators. The pressure dependence of the band gap was alsostudied. The band gap closes at high pressures leading to band overlap. Apossible reason for the metallization in these compounds is discussed.

Crystallochemistry and Kondo-like behaviour of the thorium and uranium arsenoselenides

We have grown single crystals of uranium and thorium arsenoselenide and arsenosulphide by the chemical vapour transport method. The crystals show a Kondo-like effect and its contribution to the resistivity was estimated by the determination of the reciprocal residual resistivity ratio (RRRR) ρ(4,2)/ ρ(300). The ratio varied from 0.70 (for UAsS) to 2.10 (for ThAsSe). A high resolution transmission electron microscopy study of ThAsSe crystals reveals a perfect ordering of cations. The X-ray diffraction examination showed that the unit cells of the examined crystals belong to a tetragonal system with the PbFCl-type structure (space group P4/nmm (no. 129)). Anomalously large anisotropic displacement factors have been observed for all the atoms composing the compounds examined. An interdependence between the displacement factor and RRRR has allowed to ascribe both phenomena to the same source. It indicates that the two-level system Kondo model is most suitable for the theoretical description of the systems.

Structure and physical properties of CeSbTe

The structure of CeSbTe was determined by the Rietveld refinement method using powder X-ray diffraction ( R p =6.22% and wR p =8.74%). CeSbTe adopts the PbFCl structure type and crystallizes in the space group Pnma (No. 62) of the orthorhombic system with Z=4 and a=19.0404(6) Å, b=4.3520(1) Å, c=4.3660(2) Å, V=361.78(2) Å 3. It can be viewed as a layered compound which contains an Sb −1 layer distorted from a perfect square net into zig-zag chains. Both its crystallographic and electronic structure indicate a 2-D character. Based on transport properties and band structure calculations, CeSbTe is a metal with a low carrier concentration. Additionally, it exhibits paramagnetic behavior typical of Ce 3+.

Crystal Structures of Two New Holmium Polysulfides in the Series of Rare-Earth Polychalcogenides

The crystal structures of two polysulfide phases HoS1.885(5) (I) and HoS1.863(8) (II) were determined; the integer stoichiometric ratio was found to be Ho8S15. The data were collected on an Enraf-Nonius CAD-4 automatic diffractometer using the standard procedure (MoKα, graphite monochromator, an absorption correction applied based on ϕ-scan data). Crystal I: space group P4/nmm, a = 3.820(1), c = 7.840(3) A, V = 114.40(6) A3, Z = 2 for the composition HoS1.885(5), d calc = 6.542 g/cm3, R = 0.0520 for 184 unique reflections with Ihkl > 2 σI; crystal II: space group P21/m, a = 10.961(2), b = 11.465(2), c = 10.984(2) A, β = 91.27(3)°, V = 1380.0(4) A3, Z = 24 for the composition HoS1.863(8), d calc = 6.486 g/cm3, R = 0.0596 for 5354 unique reflections with Ihkl > 2 σI. In both compounds, the Ho atoms are surrounded by 9 (8+1 for three atoms in II) S atoms forming monocapped square antiprisms. The Ho–S distances vary from 2.717 to 3.067 A irrespective of the type of ion [S2– or (S2)2–]; the maximal distance to the atoms completing the coordination is 3.684 A. The compounds have PbFCl type structures composed of ...(S2)2–...Ho3+...S2–...S2–...Ho3+...(S2)2–... layer packets differently oriented in space relative to the unit cell axes. The S2–...S2– and S2–...(S2)2– interlayer distances are mostly shorter than the sum of the ionic radii and vary within the limits of 3.331-3.558 and 3.029-3.784 A for the first and second types, respectively. For I, the calculated site occupancies and densities are given depending on the composition Ho-S2-x (x = 0.25-0); for II, the most probable formulas of rational compositions in the same range of x are presented.

Crystal Chemistry of Rare-Earth Chalcogenides

Crystal structures of Ln–X, Ln–Ln'–X, Ln–M–X, and Ln–X–Z compounds (Ln and Ln' are rare-earth elements; M is subgroup I–VIIIa or II–Vb cation and Z is subgroup V–VIIb anion; X is S, Se, or Te) were systematized and analyzed using data obtained by us and data from the literature. The structures of binary Ln–X chalcogenides can be divided into 4 groups: derivatives of NaCl-type structures, structures with one small parameter b≈ 4 A, structures similar to those of oxides and pniktides, and derivatives of PbFCl-type structures. Triple Ln–Ln'–X chalcogenides have structures similar to those of the binary chalcogenides of the first, second, and third groups. It was shown that the crystal structure of Ln–M–X depends on the size of cation M, its formal charge, electronegativity value, and electronic structure. The crystal structures of the phases in the Ln–M–X and Ln–X–Z compounds were compared with binary chalcogenide structures. Different variants of structure characterization and their potentialities are considered. Special attention is given to the defect-containing and ordered structures and to elucidating the role of the cation and anion vacancies in the phase structures.

Study of the Solid—Liquid Equilibrium in Mixed Alkaline Earth Fluorohalides

We have studied the solid-liquid equilibrium of the system Sr1−yBayFCl1−xBrx using DTA and X-ray diffraction techniques. The entire composition range in this system yields solid solutions which crystallize in the PbFCl (Matlockite) structure type. The melting points of the entire composition range have been parametrized (within 5°C rms error) using a biquadratic fit of the available data obtained by experiment and from the literature.

Structure of dysprosium polysulfide DyS1.83 (Dy6S11) according to x-ray diffraction analysis

The crystal structure of a single crystal of dysprosium polysulfide DyS1.83 = Dy6S11 is determined. The unit cell parameters are similar to those established for DyS1.76 = Dy4S7, but the composition is different. The unit cell is monoclinic, space group P21/m, a = 11.009(2), b = 11.531(2), c = 11.023(1) A, β = 91.152(8)∘, V= 1398.1(4) A3; for Z = 4 this gives the composition Dy6S11, regarding the experimental values of the partial statistic of the S atoms and dcalc = 6.303 g/cm3 (EnrafNonius CAD-4 automatic diffractometer, γMoKα, Raniso = 0.0349 for 6381 unique Ihkl > 2σI of 9804 measured ones). An empirical absorption correction was applied based on transmission curves and habit data. The polysulfide is a PbFCl type structure; the monoclinic unit cell parameters are related to a0 and c0 of the tetragonal cell of PbFCl as [101] ≅ 4a0, [010] ≅3a0, [101] ≅2c0. The crystal is a twin with the ratio of components k2 = 1-k1 = 0.243(2) and a transition matrix 001/010/100. The composition DyS1.83±0.02 agrees with the chemical analysis data and the experimental value dexp = 6.30±0.08 g/cm3. The phase individuality of the polysulfide is confirmed by the form of the P-T projection of the P-T-X diagram of the Dy-S system.

Enthalpy of solution of californium oxychloride; calculation of the standard enthalpy of formation of CfOCl

The enthalpy of solution of CfOCl in 1.022 mol dm −3 HCl was measured on the 0.4–1.5 mg scale, and the standard enthalpy of formation at 298 K for this compound was calculated. The CfOCl was prepared by heating in air at 280–320°C the solid hydrated trichloride, obtained by slow evaporation of Cf(III) in HCl solution. X-ray powder diffraction analysis confirmed the material to be the expected oxychloride, which exhibits the PbFCl-type tetragonal structure. Dissolutions of the oxychloride were made at 25.0°C in an isoperibol solution microcalorimeter. Enthalpy of solution values were calculated according to the equation: CfOCl (cr)+2HCl (sln)→CfCl 3(sln)+H 2O (sln). The Δ f H 0 (CfOCl (cr)) was determined to be −920±7 kJ mol −1. Interpolation within Δ f H 0 (AnOCl (cr)), where An=U, Pu, Am, Cm, and Cf, as a function of An(III) ionic radius yielded Δ f H 0 (BkOCl (cr))=−944±10 kJ mol −1.