Synthesis and characterization of framework-substituted Cs8Na16Cu5Ge131

We report the synthesis and temperature-dependent transport properties of a new type II germanium clathrate, Cs₈Na₁₆Cu₅Ge₁₃₁ (space group ; a = 15.42000(9) Å). In comparison to the parent compound Cs₈Na₁₆Ge₁₃₆, Cu substitution for Ge on the clathrate framework was found to increase in magnitude the electrical resistivity and Seebeck coefficient, while reducing the thermal conductivity. The results are consistent with partial charge compensation due to Cu substitution for Ge. In addition to presenting possibilities for new compositions, framework substitution in type II clathrates may offer a route to control the transport properties of these materials, and could offer potential candidates for thermoelectric applications.     

Sn Mössbauer spectroscopy study of the magnetic properties of the HfFe6Ge6-type compounds: SmMn6Sn4Ge2 and GdMn6Sn4Ge2

The HfFe₆Ge₆-type compound SmMn₆Sn₄Ge₂ has been studied by single-crystal magnetisation and ¹¹⁹Sn Mössbauer spectroscopy. The compound orders ferromagnetically at T_c = 420 K and displays an easy-axis anisotropy from T_c to T_SR = 130 K. Below T_SR, both magnetisation and ¹¹⁹Sn Mössbauer spectroscopy measurements indicate a deviation from the [0 0 1] direction and the presence of easy-cone anisotropy. The angle of the moments with respect to the [0 0 1] direction is estimated to 26-31° from Mössbauer spectroscopy results, in good accordance with the magnetisation results. The isotypic compounds GdMn₆Sn₄Ge₂ and GdMn₆Sn₆ studied by ¹¹⁹Sn Mössbauer spectroscopy display easy plane anisotropy in the whole temperature range 300-4.2 K. The anisotropy behaviours of the LMn₆Sn₄Ge₂ compounds are discussed and the coexistence of easy cone anisotropy for both the SmMn₆Sn₄Ge₂ and HoMn₆Sn₄Ge₂ suggests the play of a positive second-order anisotropy constant of the Mn sublattice.     

Determination of the structure of a new tetragonal U2FeAl20 phase

The atomic structure of a new ternary U₂FeAl₂₀ phase appearing in the Al-rich corner of a U–Fe–Al system was solved using electron crystallography and X-ray powder diffraction techniques (XRD). The positions of U atoms were determined from crystallographically processed high-resolution electron microscopy (HRTEM) images. These positions were used as a starting set for determining the coordinates of Fe and Al atoms by difference-Fourier synthesis technique. The U₂FeAl₂₀ phase is tetragonal and belongs to the space group. Its unit cell contains 80 Al, 4 Fe, and 8 U atoms. The lattice parameters obtained after Rietveld refinement are: a = 12.4138 Å, c = 10.3014 Å. The reliability factors characterizing the Rietveld refinement procedure are: R_p = 8.65%, R_wp = 11.2% and R_b = 5.93%.     

Crystal structures and magnetic properties of iron (III)-based phosphates: Na4NiFe(PO4)3 and Na2Ni2Fe(PO4)3

Crystal structures from two new phosphates Na₄NiFe(PO₄)₃ (I) and Na₂Ni₂Fe(PO₄)₃ (II) have been determined by single crystal X-ray diffraction analysis. Compound (I) crystallizes in a rhombohedral system (S. G: R-3c, Z = 6, a = 8.7350(9) Å, c = 21.643(4) Å, R₁ = 0.041, wR₂=0.120). Compound (II) crystallizes in a monoclinic system (S. G: C2/c, Z = 4, a = 11.729(7) Å, b = 12.433(5) Å, c = 6.431(2) Å, β = 113.66(4)°, R₁ = 0.043, wR₂=0.111). The three-dimensional structure of (I) is closely related to the Nasicon structural type, consisting of corner sharing [(Ni/Fe)O₆] octahedra and [PO₄] tetrahedra forming [NiFe(PO₄)₃]⁴⁺ units which align in chains along the c-axis. The Na⁺ cations fill up trigonal antiprismatic sites within these chains. The crystal structure of (II) belongs to the alluaudite type. Its open framework results from [Ni₂O₁₀] units of edge-sharing [NiO₆] octahedra, which alternate with [FeO₆] octahedra that form infinite chains. Coordination of these chains yields two distinct tunnels in which site Na⁺.The magnetization data of compound (I) reveal antiferromagnetic (AFM) interactions by the onset of deviations from a Curie-Weiss behaviour at low temperature as confirmed by Mössbauer measurements performed at 4.2 K. The corresponding temperature dependence of the reciprocal susceptibility χ⁻¹ follows a typical Curie-Weiss behaviour for T > 105 K. A canted AFM state is proposed for compound (II) below 46 K with a field-induced magnetic transition at H ≈ 19 kOe, revealed in the hysteresis loop measured at 5 K. This transition is most probably associated with a spin-flop transition.     

Synthesis, crystal and band structures, and optical properties of a new framework mercury pnictides: [Hg4As2](InBr3.5As0.5) with tridymite topology

A new framework compound, [Hg₄As₂](InBr_3.5As_0.5) (1), has been prepared by the solid-state reaction of Hg₂Br₂ with elemental In and As at 450 °C. Compound 1 crystallizes in the space group P₆₃/mmc of the Hexagonal system with two formula units in a cell: a = b = 7.7408(6) Å, c = 12.5350(19) Å, V = 650.47(12) Å³. The crystal structure of 1 features a novel 3D framework, [Hg₄As₂]²⁺ with tridymite topology. The optical properties were investigated in terms of the diffuse reflectance and infrared spectra. The electronic band structure along with density of states (DOS) calculated by DFT method indicates that the present compound is semiconductor, and the optical absorption is mainly originated from the charge transitions from Br-4p and As-4p states to Hg-6s and In-5p states.