Structure,superconductivity and magnetism of new rare earth-rhodium silicides RE2Rh3Si5 of U2Co3Si5-type

New ternary silicides RE₂Rh₃Si₅ (RE = Y, La, Nd, Sm, Gd, Tb, Dy, Ho, Er) have been prepared. They crystalize in the U₂Co₃Si₅-type structure. Only the silicides containing the diamagnetic rare earths Y and La show a superconducting transition at T_Cr = 4.4 ± 0.2K and T_Cr = 2.7 ± 0.1K respectively, those with magnetic rare earths order antiferromagnetically at low temperature.     

Magnetic properties of new ternary silicides : Tb2Ir3Si5 and both polymorphic forms of TbIr2Si2

A new ternary silicide Tb₂Ir₃Si₅ of U₂Co₃Si₅-type structure has been prepared as well as two polymorphic forms of TbIr₂Si₂. Both modifications are tetragonal, the low-and high-temperature forms are isostructural with ThCr₂Si₂ and CaBe₂Ge₂ respectively. The three compounds order antiferromagnetically. A high Néel temperature is observed for the low-temperature modification (T_N = 80K, θ_p = 42K). Its magnetic structure is similar to that of TbRh₂Si₂ for which ferromagnetic layers perpendicualr to the c-axis are coupled antiferromagnetically (+?+?). Tb₂Ir₃Si₅ and the high-temperature modification of TbIr₂Si₂ have low T_N and θ_p values. The difference observed in the magnetic behavior of these compounds is likely due to the influence of the environment of the Tb³⁺ ion on the density of states at Fermi level.     

Silicophosphates with an intersecting tunnel structure: AM3P6Si2O25 and AMo3P5.8Si2O25

Fourteen oxides with an original intersecting tunnel structure have been isolated; they correspond to the formula AM₃P₆Si₂O₂₅ and AMo₃P_5.8Si₂O₂₅ with A = K, Rb, T1, Cs and M = Ti, Mo, Sn. These compounds crystallize in the trigonal system, P31c or P3?1c with $\text{a}\text{? 8.3}\text{A}\text{?}$ and $\text{c}\text{? 17.3}\text{A}\text{?}$. The structure of the oxide KMo₃P_5.8Si₂O₂₅ has been established from a single crystal study, and compared to that of the rubidium and thallium compounds: the framework is built up from corner-sharing MoO₆ octahedra, PO₄ tetrahedra and Si₂O₇ groups forming tunnels which run along [100], [010] and [110]. The influence of the nature of the A ions and of the M ions on the structural properties of these phases is discussed.     

Elastic and thermodynamic properties of OsSi, OsSi2 and Os2Si3

All components of the elasticity tensors for OsSi (P2₁3), OsSi₂ (Cmca) and Os₂Si₃ (Pbcn and P-4c2) were computed by means of the stress–strain method in the framework of the density functional theory. The total energies and stress were calculated using density functional theory within the local density and conjugate gradient approximations. From the knowledge of the elastic constants, the Born stability criteria were discussed and the Debye approximation was used to evaluate the enthalpies of formation (ΔH_f) of the different compounds. The ΔH_f calculated at 298 K for OsSi is in correct agreement with the experimental data but for Os₂Si₃(Pbcn) ΔH_f is 30 kJ/mol of atoms more exothermic than the measured one. Moreover, we have found that from 1200 K the P-4c2 phase of Os₂Si₃ is most stable from the energy point of view than the Pbcn structure.     

A new family of superconducting silicides: The ThMxSi2−x compounds (M = Rh or Ir with 0 < x < 1) of α-ThSi2-type structure

New superconducting materials have been prepared by substitution of silicon by Rh or Ir atoms in the α-ThSi₂ disilicide. The ThM_xSi_2?x compounds crystallize in the tetragonal α-ThSi₂-type structure with a large homogeneity range: 0? x ? 0.96 for Rh and 0? x ? 1 for Ir. A sharp increase of the superconducting transition temperature T_cr is observed for x > 0.75 in both cases. Among the superconducting compounds with an α-ThSi₂-type structure, ThRh_0.96Si_1.04 and ThIrSi have the highest T_cr values with respectively 6.45 and 6.5 K about twice that of α-ThSi₂ (T_cr = 3.16 K). Superconductivity enhancement has been attributed to the formation of Rh-Rh or Ir-Ir metallic type bonding.     

Preparation and electrical properties of Ca5Si3 and Sr5Si3 powders

Single phase Ca₅Si₃ and Sr₅Si₃ powders were prepared, and their electric and thermoelectric properties were investigated. The Ca₅Si₃ and Sr₅Si₃ powders are synthesized by exposure of the Si powders to Ca and Sr fluxes, respectively. It is found that both silicides show a p-type conduction and semiconductor-like behavior. The electronic band structures of the silicides are calculated using the first-principles total-energy calculation program in pseudopotential schemes with plane-wave basis functions. The calculated result predicts the possibility of a semiconductor-like property with a sharp pseudogap at the Fermi level for Ca₅Si₃, as experimentally obtained. The silicide would be expected to be a new semiconductor-like conducting material.     

Magnetisation of bulk Mn11Si19 and Mn4Si7

The purpose of this paper is to determine by experiment whether Mn₁₁Si₁₉ and Mn₄Si₇ in their bulk states have a finite magnetic moment or not. Magnetisation measurements were carried out on these materials using both SQUID system and Kerr rotation system. The high quality samples were grown using the temperature gradient solution growth method. SQUID measurements revealed that Mn₁₁Si₁₉ has finite magnetism while Mn₄Si₇ does not in their bulk states. It was also confirmed that Mn₄Si₇ became magnetic and Mn₁₁Si₁₉ got to exhibit a distinctive hysteresis in their powdery state. The enhancement of magnetism implied that the surface of the samples was to a great extent linked to its magnetism.     

Synthesis and characterization of Mg3Cr2Si3O12-Fe3Cr2Si3O12 garnet solid solutions

Homogeneous garnet Fe₃Cr₂Si₃O₁₂ and its solid solutions with garnet Mg₃Cr₂Si₃O₁₂ (four intermediate samples) were synthesized at high pressure. Garnet Mg₃Cr₂Si₃O₁₂ was not synthesized in pure form. The sample contained traces of the two phases, MgSiO₃ and Cr₂O₃. The products were investigated by means of X-ray powder diffraction and differential scanning calorimetry. Molar volumes of the solid solutions as a function of composition fit linear regression with an accuracy of 0.2%. No difference in heat capacity within the limits of experimental error was detected. In thermodynamic evaluations, the garnet solid solutions Fe₃Cr₂Si₃O₁₂-Mg₃Cr₂Si₃O₁₂ may be treated as ideal solid solutions.     

The structure and magnetocaloric effect of rapidly quenched Gd5Si2Ge2 alloy with low-purity gadolinium

A magnetic refrigerating material Gd₅Si₂Ge₂ was prepared by arc-melting method under argon atmosphere with low-purity commercial gadolinium. The structure and magnetocaloric effect (MCE) of the as-cast and rapidly quenched Gd₅Si₂Ge₂ alloys were investigated by means of X-ray diffraction and magnetic measurements. The as-cast Gd₅Si₂Ge₂ consists of Gd₅Si₂Ge₂-type, Gd₅Si₄-type, Gd₅(Si,Ge)₃ and Gd(Si,Ge) phases from powder XRD results. The alloys quenched at a velocity of 25 m/s or 40 m/s both adopt in a single phase with orthorhombic Gd₅Si₄-type structure. The lattice parameters under different quenched velocities calculated by least-squares method have no significant difference. After being rapidly quenched at 40 m/s, the Curie temperature of Gd₅Si₂Ge₂ is about 303 K and its maximum magnetic entropy change is 7.25 J/kg K (0-5 T).     

Refinement of the crystal structure of CeOs2Si2 (ThCr2Si2-type)

The crystal structure of CeOs₂Si₂ has been determined from single crystal X-ray counter data. CeOs₂Si₂ is tetragonal, I4/mmm, Z = 2; the cell parameters are a = 0.41617(5) and c = 0.98481(28) nm. The final reliability factor $\text{R}\text{=}\text{|Δ}\text{F}\text{|}\text{|}\text{F}{}_{\mathrm{o}}\text{|}$ is 0.027 for 129 observed reflections (|F_o|>26). CeOs₂Si₂ is isotypic with the ordered structure type of ThCr₂Si₂ (BaAl₄-derivative type).     

Fabrication, characterisation and application of (8 mol% Y2O3) ZrO2 thin film on Na3Zr2Si2PO12 substrate for sensing CO2 gas

A planar type CO₂ gas sensor employing (8 mol% Y₂O₃) ZrO₂ (YSZ) thin film on Na₃Zr₂Si₂PO₁₂ (Nasicon) substrate with Na₂CO₃ as an auxiliary electrode has been fabricated and tested in laboratory environment between 700–900 K. The YSZ thin film was fabricated on Nasicon and alumina substrate using radio frequency (RF) magnetron sputtering. The film was examined using SEM and X-ray diffraction (XRD) after treating the Nasicon-YSZ bi-layer structure at 1300 K for 2 h. The results indicate that a crack free YSZ film was produced on Nasicon surface that was well bonded to the substrate. The conductivity of sputtered YSZ thin film measured by ac-impedance spectroscopy has been found to be higher than that of YSZ pellet by approximately half an order of magniture. The bi-electrolyte planar sensor displays rapid response (t₉₅ 200 s) to CO₂ compared to the tube type sensor (t₉₅ 700 s) and the measured open circuit voltage of the electrochemical CO₂ sensor has been found to be Nernstian at all temperatures.     

Substitutions in the Ca2Na6Al6Si6O24(SO4)2 haüyne structure

Four new haüynes having the chemical formula Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Se, Te, Mo and W, were synthesized by solid-state reaction. Various substitutions were then attempted for Ca, Na, Al and Si. The replacement of Ca²⁺ by Sr²⁺ or Cd²⁺ was successful in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, when X = S, Cr, Mo or W, except for Cd, when X = Cr. The synthesis of Mn₂Na₆Al₆Si₆O₂₄(XO₄)₂ could be made when X was Mo or W, and, among the Pb substitutions tried, Pb₂Na₆Al₆Si₆O₂₄(SO₄)₂ was successful. The solubility of Li, K and Ag was partial and was different in different haüynes. Maximum solubility of Li for Na was three atoms in Ca₂Na₆Al₆Si₆O₂₄(SO₄)₂ and the minimum was half an atom in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Mo or W. Maximum replacement of K or Ag for Na was two atoms in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Mo or W and the minimum was 0.5 in Ca₂Na₆Al₆Si₆O₂₄(SO₄)₂. The solubility of Li, K and Ag was 1.5 atoms in Ca₂Na₆Al₆Si₆O₂₄(CrO₄)₂. The solubility of Ga³⁺ or Fe³⁺ for Al³⁺ was partial. A maximum of 0.5, 3 and 4 atoms of Ga³⁺ can be substituted for Al³⁺ in sulfate, chromate and molybdate or tungstate haüynes, respectively. The solubility of Fe³⁺ was one atom in all haüynes. The complete replacement of Si⁴⁺ by Ge⁴⁺ was possible in M²⁺₂Na₆Al₆Si₆O₂₄(XO₄)₂, when M = Ca, Cd or Sr, and X = Mo or W.     

Structural, electronic and elastic properties of V5Si3 phases from first-principles calculations

The phase stability, electronic, elastic and thermodynamic properties of V₅Si₃ has been investigated by using first-principles calculations based on the density functional theory (DFT). In the present calculations, three phases of V₅Si₃ (Cr₅B₃-prototype, W₅Si₃-prototype and Mn₅Si₃-prototype) have been taken into account to check the phase stability. The calculated formation enthalpies indicate that the W₅Si₃-prototype is the stable phase which is in agreement with experiments, whereas the Cr₅B₃-prototype is a potential metastable phase. The elastic constants, bulk modulus, shear modulus and Young’s modulus are calculated in the present work, and are very close to that of the Nb₅Si₃. The density of states and bonding charge density of V₅Si₃ within the W₅Si₃-phase are obtained indicating a strong covalent character of the bonds. Finally, using the Debye-model, the Debye temperature, heat capacity, and thermal expansion have also been calculated and are in good agreement with experimental results.     

The electrical resistivity of RFe2Si2 type rare earth silicides

Electrical resistivity of RFe₂Si₂ (R= Nd, Eu, Dy) compounds has been investigated in the temperature range of 100–300°K, and was found to increase with increasing temperature. The specific resistivity of the compounds is in the same order of magnitude as the RSi₂ rare earth silicides.     

Structural, Electronic Properties and Fermi Surface of ThCr2Si2-Type Tetragonal KFe2S2, KFe2Se2, and KFe2Te2 Phases as Parent Systems of New Ternary Iron-Chalcogenide Superconductors

The first principles FLAPW–GGA method was used for the comparative study of the structural and electronic properties of three related tetragonal ThCr₂Si₂-type phases KFe₂ Ch ₂, where Ch are S, Se, and Te. The main trends in electronic bands, densities of states, and Fermi surfaces for AFe₂ Ch ₂ were analyzed in relation to their structural parameters. We found that anion replacements (S↔Se↔Te) produce no critical changes in the electronic structure of KFe₂ Ch ₂ phases. On the other hand, our analysis of structural and electronic parameters for hypothetical KFe₂Te₂ allows us to propose this system as a perspective parent phase for search of new iron-chalcogenide superconducting materials.     

A yellow-emitting Ca3Si2O7: Eu phosphor for white LEDs

A series of yellow-emitting phosphors based on a silicate host matrix, Ca_3 − xSi₂O₇: xEu²⁺, was prepared by solid-state reaction method. The structure and photoluminescent properties of the phosphors were investigated. The XRD results show that the Eu²⁺ substitution of Ca²⁺ does not change the structure of Ca₃Si₂O₇ host and there is no impurity phase for x < 0.12. The SEM images display that phosphors aggregate obviously and the shape of the phosphor particle is irregular. The EDX results reveal that the phosphors consist of Ca, Si, O, Eu and the concentration of these elements is close to the stoichiometric composition. The Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be excited at a wavelength of 300-490 nm, which is suitable for the emission band of near ultraviolet or blue light-emitting-diode (LED) chips. The phosphors exhibit a broad emission region from 520 to 650 nm and the emission peak centered at 568 nm. In addition, the shape and the position of the emission peak are not influenced by the Eu²⁺ concentration and excitation wavelength. The phosphor for x = 0.045 has the strongest excitation and emission intensity, and the Ca_3 − xSi₂O₇: xEu²⁺ phosphors can be used as candidates for the white LEDs.     

Synthesis of the intermetallic clathrate Na2Ba6Si46 by oxidation of Na2BaSi4 with HCl

A new preparation route to the intermetallic clathrate-I compound Na₂Ba₆Si₄₆ is introduced, which allows one to make large amounts of product with standard laboratory equipment. The precursor Na₂BaSi₄ is oxidized with gaseous HCl at 673 K to Na₂Ba₆Si₄₆, NaCl and BaCl₂. Full-profile refinement of the crystal structure from the X-ray powder diffraction data revealed a composition close to Na₂Ba₆Si₄₆ (Na_1.94(1)Ba_6.06(1)Si₄₆, space group , a=10.281(1) Å). Differential scanning calorimetry showed an exothermic effect at 874 K, indicating that Na₂Ba₆Si₄₆ is metastable. The product was additionally characterized by scanning electron microscopy. The electronic structure of Na₂Ba₆Si₄₆ was investigated by a first-principles, all-electron full-potential method, predicting metallic conductivity. Na₂Ba₆Si₄₆ obtained by oxidation with HCl shows Pauli paramagnetism; no bulk superconductivity was found down to 1.8 K in a magnetic field of 20 Oe.     

Refinement of the crystal structure of HoRu2Si2 (ThCr2Si2-type)

The crystal structure of HoRu₂Si₂ has been determined from single crystal X-ray counter data obtained from a single crystal specimen which was prepared from an arc melted sample heat-treated at 500° C for 4 weeks and quenched. HoRu₂Si₂ is tetragonal with space group I4/mmm, and cell parameters a=4.1552(9) and $\text{c}\text{=9.5178(68)}\text{A}\text{?}$; Z = 2. The final reliability factor $\text{R}\text{=}\text{solΣΔ}\text{F}\text{|}\text{Σ|}\text{F}{}_{\mathrm{o}}\text{|}$ is 0.052 for 97 independent reflections; $\text{|}\text{F}{}_{\mathrm{o}}\text{| > 2 б}$. HoRu₂Si₂ is isotypic with the ordered structure type of ThCr₂Si₂ (BaAl₄-derivative type).     

Electronic structure and photoluminescence properties of Eu-activated Ca4Si2O7F2

The blue-emitting phosphors Ca_(4−x)Eu_xSi₂O₇F₂ (0 < x ? 0.05) have been prepared by solid-state reaction and the photoluminescence properties have been studied systematically. The electronic structure of calcium fluoride silicate Ca₄Si₂O₇F₂ was calculated using the CASTEP code. The calculation results of electronic structure show that Ca₄Si₂O₇F₂ has an indirect band gap with 5 eV. The top of the valence band is dominated by O 2p and Si 3p states, while the bottom of the conduction band is mainly composed of Ca 3d states. Under the 350 nm excitation, the obtained sample shows a broad emission band in the wavelength range of 400-500 nm with peaks of 413 nm and 460 nm from two different luminescence centers, respectively. The relative intensity of the two peaks changes with the alteration of the Eu²⁺ concentration. The strong excitation bands of the powder in the wavelength range of 200-420 nm are favorable properties for the application as lighting-emitting-diode conversion phosphor.