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.     

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.     

Ferroelectric relaxor behavior and dielectric spectroscopic study of 0.99(Bi0.5Na0.5TiO3)-0.01(SrNb2O6) solid solution

0.99(Bi_0.5Na_0.5TiO₃)-0.01(SrNb₂O₆) was prepared by simple solid state reaction route. Material stabilized in rhombohedral perovskite phase with lattice constants a = 3.9060 Å, α = 89.86° and a_h = 5.4852 Å, c_h = 6.7335 Å for hexagonal unit cells. Density of material was found 5.52 g_m/cm³ (92.9% of theoretical one) in the sample sintered at 950 °C. The temperature dependent dielectric constant exhibits a broad peak at 538 K (?_m = 2270) at 1 kHz that shows frequency dependent shifts toward higher temperature - typical relaxor behavior. Modified Curie-Weiss law was used to fit the dielectric data that exhibits almost complete diffuse phase transition characteristics. The dielectric relaxation obeys the Vogel-Fulcher relationship with the freezing temperature 412.4 K. Significant dielectric dispersion is observed in low frequency regime in both components of dielectric response and a small dielectric relaxation peak is observed. Cole-Cole plots indicate polydispersive nature of the dielectric relaxation; the relaxation distribution increases with increase in temperature.     

CuIn1−xAlxS2 thin films prepared by sulfurization of metallic precursors

CuIn_1−xAl_xS₂ thin films (x = 0, 0.09, 0.27, 0.46, 0.64, 0.82 and 1) with thicknesses of approximately 1 μm were formed by the sulfurization of DC sputtered Cu-In-Al precursors. All samples were sulfurized in a graphite container for 90 min at 650 °C in a 150 kPa Ar + S atmosphere. Final films were studied via X-ray diffraction (XRD), scanning electron microscopy (SEM) and micro-Raman spectroscopy. It was found that all samples were polycrystalline in nature and their lattice parameters varied slightly nonlinearly from {a = 5.49 Å, c = 11.02 Å} for CuInS₂ to {a = 5.30 Å, c = 10.36 Å} for CuAlS₂. No unwanted phases such as Cu_2−xS or others were observed. Raman were recorded at a room temperature and the most intensive and dominant A₁ phonon frequency varied nonlinearly from 294 cm⁻¹ (CuInS₂) to 314 cm⁻¹ (CuAlS₂).     

Synthesis and structure determination of In3TeO3F7: An indium oxyfluorotellurate IV derived from W bronze structure with Te in hexagonal tunnels

After InTeO₃F and InTe₂O₅F recently described, a new compound In₃TeO₃F₇ is characterized in the In-Te^IV-O-F system. The crystal structure was determined by single X-ray diffraction and refined to R₁ = 0.028. In₃TeO₃F₇ crystallizes in orthorhombic space group Cmmm, a = 7.850(2) Å, b = 27.637(6) Å, c = 4.098(1) Å, V = 889.1(4) Å³ and Z = 4. Its structure consists of the stacking, via vertices, of identical layers composed of InF₆ and InO₂F₄ octahedra sharing corners and of InO₄F₃ pentagonal bipyramids sharing edges and vertices. The Te cations statistically occupy one or the other of two close sites located inside tunnels delimited by the In polyhedra and are bonded to F anions located in the same tunnels.The structure can be considered as an intergrowth of parallel strips of MIn₃F₁₀ and hexagonal tungsten bronze (c)-types. It is compared to other structures such as the bronze Sb_0.157WO₃, TeMo₅O₁₆ and Sb₂Mo₁₀O₃₁, phases also comprising Te⁴⁺ or Sb³⁺ inside hexagonal tunnels. The electronic lone pair of Te⁴⁺ is stereochemically active and a perfect O/F ordering occurs on the anionic sites.     

On the sol-gel synthesis and structure, optical, magnetic and impedance behaviour of strontium cobaltite powder

An attempt has been made to synthesize strontium cobaltite powder by a sol-gel process using strontium and cobalt nitrates as precursors and characterize that with regard to formation, optical absorption, magnetic behaviour, and impedance characteristics. The synthesis process involves gel formation, digestion for 4 h, drying at 150 °C for 24 h, and calcination at 1000 °C for 10 h in air. The product is shown to contain dumbbell shape particles and correspond to a rhombohedral phase that matches well with the composition Sr₆Co₅O₁₅, lattice parameters a = 6.869 Å, α = 87.543°, Z = 1, and space group R32 with ∼7 wt% of a secondary phase of spinel cubic Co₃O₄ (a = 8.0862 Å). The optical absorption exhibits peaks at 4.12, 1.90, 1.58, 1.23 and 0.87 eV which are attributed to charge transfer transitions involving oxygen and/or cobalt ions and Co³⁺ d-d transitions. Further, the sample appears to be weakly ferromagnetic with two Curie temperatures (T_C) as 750 and 790 K, coercivity of 70 Oe and remanent magnetization ∼9 × 10⁻⁴ emu/g. Impedance characteristics over a wide frequency range of 20 Hz-2 MHz at 131-350 K reveal contributions from two parallel ‘RC’ circuits belonging to bulk and grain boundaries with the later displaying significant space charge polarization, the relaxation time of which decreases with increase of temperature (range being 5-32 μs).     

Effect of Ta seed layer on crystalline structure and magnetic properties in an exchange-biased Co/IrMn system

Ta-seeded and un-seeded layers of a top-configuration Co/IrMn system were deposited onto glass substrate by DC sputtering. Three sets of deposition conditions for Co(50 Å)/IrMn(t_IrMn Å) and Co(t_Co Å)/IrMn(90 Å), where t_IrMn = 15, 30, 60, 90, 110, and 150 Å, and t_Co = 15, 25, 50, 75, 100, 125, and 150  Å, were: condition (a) substrate temperature (T_s) was kept at room temperature (RT). Condition (b) T_s set to RT, with in-plane magnetic field, H = 500 Oe. In condition (c), condition (b) was followed by post-deposition annealing in the magnetic field at T_A = 250 °C for 1 h, then field cooled to RT. X-ray diffraction (XRD) patterns and grazing incidence scans revealed maximum IrMn (1 1 1) texture to occur for post-deposition annealed Ta seed layer samples. The IrMn (1 1 1) texture-effect significantly influences magnetic properties, including exchange-biasing field (H_ex), interfacial energy (J_k), and coercivity (H_c). The Ta seed layer also significantly influences magnetic properties. Adding a Ta seed layer to the Co/IrMn system increases H_ex, because of the IrMn (1 1 1) texture. For Ta-seeded Co/IrMn under condition (c), H_ex tended to saturate for t_IrMn ≥ 90 Å. Under conditions (a) and (b), H_ex decreased with increasing t_IrMn for t_IrMn ≥ 90 Å. H_ex values for all un-seeded Co/IrMn systems increased with t_IrMn. J_k versus t_IrMn plot is proportional only to H_ex in the Ta-seeded and un-seeded layers of a top-configuration Co/IrMn system, due to the interfacial energy formula, t_Co is fixed, and saturation magnetization (M_s) of the Co layer is constant. Results for the Ta-seeded system showed a strong relationship between H_c and t_IrMn, due to coupling-decoupling interactions between Co spin, and IrMn layers close to the Co/IrMn interface. The H_ex versus t_Co result shows that the H_ex is proportional to (1/t_Co). The H_ex values with the Ta seed layer are almost slightly larger than those without a Ta seed-layer. The dependence of J_k on t_Co is similar to the trend in M_s on t_Co, J_k tends to saturate slowly as t_Co increases. Surface pinning occurred in all systems, revealing an inverse relation between H_c and t_Co. Removing the Ta seed-layer weakens IrMn (1 1 1) texturing, reducing H_ex. The maximum observed H_ex and J_k values were 205 Oe and 0.11 erg/cm², respectively.     

Transport mechanism in a ceramic system: LiCo3/5Fe1/5Cu1/5VO4

The electrical impedance and modulus properties of a LiCo_3/5Fe_1/5Cu_1/5VO₄ ceramic system were measured by impedance spectroscopy method in the frequency range 10²-10⁶ Hz and temperature range 22-250 °C. X-ray diffraction study reveals formation of the compound in a cubic crystal system with lattice parameters a = 8.2756 (3) Å. Field emission scanning electron microscopy is used to investigate the grain morphology of the material. Nyquist plots confirm the existence of bulk and grain boundary effects at 22 °C ≤ T ≤ 200 °C, and bulk, grain boundary and polarization effects at T ≥ 225 °C. Electrical modulus study indicates a non-Debye behavior of the material. A detailed study of bulk conductivity shows electric conduction in the material as a thermally activated process.     

Study on the crystal structure of the rare earth oxyborate Yb26B12O57 from powder X-ray and neutron diffraction

Yb₂₆B₁₂O₅₇, a rare earth oxyborate previously assigned as Yb₃BO₆, has been studied by various techniques including neutron and X-ray diffraction, ¹¹B NMR spectroscopy, electron diffraction and high angle angular dark field-scanning transmission electron microscopy (HADDF-STEM). It crystallizes in the space group C2/m with cell parameters of a = 24.5780(4) Å, b = 3.58372(5) Å, c = 14.3128(3) Å and β = 115.079(1)°. The structure consists of slabs of rare earth sesquioxide and borate groups. The sesquioxide part is identified from the structural refinement, and is observed from HADDF-STEM image, while elucidation of borate groups is not straightforward. An additional oxygen atom (O31), which links two B₂O₅ groups into a B₄O₁₁ polyanion, is identified from the analysis of neutron diffraction data. The occupancy of this oxygen site is only quarter, which results in a random distribution of B₄O₁₁ and B₂O₅ groups along the b-direction. The chemical formula of ytterbium oxyborate is Yb₂₆(BO₃)₄(B₂O₅)₂(B₄O₁₁)O_24, instead of the simple stoichiometric formula Yb₃BO₆. This compound is paramagnetic but its susceptibility deviated from the Curie-Weiss law at low temperature.     

Synthesis, crystal structures, and optical properties of NaCdPnS3 (Pn = As, Sb)

Two compounds NaCdPnS₃ (Pn = As (1), Sb (2)) were synthesized as transparent yellow and red platelets by reacting cadmium powder with the molten mixtures of Na₂S/As₂S₃/S and Na₂S/Sb/S at 500 °C. Both compounds are plagued with crystal twinning and acceptable crystal structure refinement could only be obtained by identifying the type of the twinning laws. NaCdAsS₃ (1) crystallizes in the monoclinic space group P2₁/n (no. 14) with a = 5.6561(8), b = 16.5487(15), c = 5.6954(8) Å, β = 90.315(11)°, and Z = 4. NaCdSbS₃ (2) crystallizes in the monoclinic space group C2/c (no. 15) with a = 8.1329(6), b = 8.1296(4), c = 17.2600(13) Å, β = 103.499(6)°, and Z = 8. The structures of both compounds feature a ²_∞[CdPnS₃]⁻ layer composed of [CdS₄] tetrahedra and [AsS₃] or [SbS₃] pyramidal units. Between the ²_∞[CdPnS₃]⁻ (Pn = As, Sb) layers the Na⁺ cations reside in a distorted octahedral environment of S atoms. Compound 1 is characterized with UV/Vis diffuse reflectance spectroscopy and IR and Raman spectra.     

Thermoelectric properties of Bi2SexTe3−x prepared by Bridgman method

Bi₂Se_xTe_3−x crystals with various x values were grown by Bridgman method. The electrical conductivity, σ, was found to decrease with increasing Se content. The highest σ of 1.6 × 10⁵ S m⁻¹ at room temperature was reached at x = 0.12 with a growth rate of 0.8 mm h⁻¹. The Seebeck coefficient, S, was less dependent on Se content, all with positive values showing p-type characteristics, and the highest S was measured to be 240 μV K⁻¹ at x = 0.24. The lowest thermal conductivity, κ, was 0.7 W m⁻¹ K⁻¹ at x = 0.36. The electronic part of κ, κ_el, showed a decrease with increasing Se content, which implies that the hole concentration as the main carriers was reduced by the addition of Se. The highest dimensionless figure of merit, ZT, at room temperature was 1.2 at x = 0.36, which is attributed to the combination of a rather high electrical conductivity and Seebeck coefficient and low thermal conductivity.     

Growth, crystal structure and optical properties of layered dibarium cadmium diborate, Ba2Cd(BO3)2

A novel dibarium cadmium diborate, Ba₂Cd(BO₃)₂, has been successfully synthesized by standard solid-state reaction. Large sheet-like crystal with size up to 20 mm × 15 mm × 0.7 mm has been obtained using top-seed solution growth method. Ba₂Cd(BO₃)₂ crystallizes in the monoclinic space group C2/m with a = 9.6305(4) Å, b = 5.3626(3) Å, c = 6.5236(2) Å, β = 118.079(3)°, Z = 2. The crystal structure is composed of isolated [BO₃] triangles, [CdO₆] octahedra and [BaO₉] polyhedra. CdO₆ are vertex-connected with six BO₃ to form infinite _∞[Cd(BO₃)₂] layers extending in (0 0 1) plane, and two rows of Ba atoms closely occupy two side of _∞[Cd(BO₃)₂] layers to forming stoichiometric sheets. IR and transmittance spectrum of Ba₂Cd(BO₃)₂ were reported.     

Interstitial solid solution Hf5GaxSn3 (x = 0-1)

Formation of an interstitial solid solution Hf₅Ga_xSn₃ (x = 0-1) based on the binary compound Hf₅Sn₃ (structure type Mn₅Si₃, Pearson symbol hP16, space group P6₃/mcm, a = 8.36562(6), c = 5.70775(4) Å from X-ray powder diffraction) was established at 600 °C. The crystal structure (structure type Hf₅CuSn₃, ordered derivative of Ti₅Ga₄, hP18, P6₃/mcm) was refined on X-ray single-crystal diffraction data for three compositions: Hf₅Ga_0.16(3)Sn₃ (a = 8.3288(12), c = 5.6988(11) Å), Hf₅Ga_0.53(2)Sn₃ (a = 8.4205(12), c = 5.7655(12) Å) and Hf₅GaSn₃ (a = 8.5564(12), c = 5.7859(12) Å). The Ga atoms occupy Wyckoff position 2b at the centres of Hf₆ octahedral interstices.     

Dielectric investigation of MM(PO4)2 double orthophosphates (M = Ca, Sr, Ba, Pb; M = Ti, Zr, Hf, Ge, Sn)

M^IIM^IV(PO₄)₂ (M^II = Ca, Sr, Ba, Pb; M^IV = Ti, Zr, Hf, Ge, Sn) ceramics were prepared by solid state reaction method and consolidated by spark plasma sintering in order to study their electrical properties. The dielectric study performed at room temperature was aimed to determine the basic electrical properties of these compounds. Maxwell-Wagner polarization contributions, which are active at low frequencies cause a strong extrinsic increase of both real and imaginary part of permittivity as well of the dc-conductivity for BaZr(PO₄)₂, BaTi(PO₄)₂, BaHf(PO₄)₂, and SrGe(PO₄)₂ double orthophosphates. Moderate real (ε^′r=20-150) and imaginary low-frequency permittivity (ε^′r=7-300) and typical Debye relaxation with relaxation time in the range of ms is typical for: PbHf(PO₄)₂, PbZn(PO₄)₂, and CaGe(PO₄)₂. At high-frequency (f = 10⁹ Hz), the ceramics have permittivities of 2.29÷8.02 and tangent loss of 0.003÷0.153. The compounds SrGe(PO₄)₂, BaGe(PO₄)₂, BaZr(PO₄)₂ and BaSn(PO₄)₂ have excellent high-frequency dielectric characteristics, with losses of 3÷6% and permittivity slightly above 2 and are possible candidate as microwave ceramics.     

Synthesis, structure, and properties of Li2Pb2CuB4O10 with isolated [CuB4O10] units

The copper borate Li₂Pb₂CuB₄O₁₀ has been synthesized in air by the standard solid-state reaction at temperature in the range 550-650 °C and the structure of Li₂Pb₂CuB₄O₁₀ was determined by single-crystal X-ray diffraction. Li₂Pb₂CuB₄O₁₀ crystallizes in the monoclinic space group C2/c (no. 15) with a = 16.8419(12), b = 4.7895(4), c = 13.8976(10) Å, and β = 125.3620(10)°, V = 914.22(12) Å³, and Z = 4, as determined by single-crystal X-ray diffraction. The Li₂Pb₂CuB₄O₁₀ structure exhibits isolated units of stoichiometry [CuB₄O₁₀]⁶⁻ that are built from CuO₄ distorted square planes and triangular BO₃ groups. The IR spectroscopy and thermal analysis investigations of Li₂Pb₂CuB₄O₁₀ are also presented.     

Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

Growth and characterization of tetragonal structure modification of β-Gd2Si2O7:Ce

Growth procedure, crystal structure, and luminescent properties of tetragonal β-Gd₂Si₂O₇:0.5 at.% Ce (a = 6.65740(10) Å, c = 24.2715(3) Å, sp.gr. P4₃) are studied. Tetragonal modification of this compound is obtained for the first time. Essentially it is isostructural to β-Sm₂Si₂O₇ and some other known disilicates (Ca, La, Ce, Pr, Nd). Obtained samples demonstrate high luminescence yield under X-rays and fast decay.     

Nanocrystals of a new complex perovskite dielectric Ba2TmSbO6

Nanocrystals of a new complex perovskites ceramic oxide, barium thulium antimony oxide - Ba₂TmSbO₆, were synthesized using a single step auto-ignition combustion process. The combustion product was single phase and composed of aggregates of nanocrystals of sizes in the range 20-50 nm. Ba₂TmSbO₆ crystallized in cubic perovskite structure with lattice parameter, a = 8.4101 Å. The polycrystalline fluffy combustion product was sintered to high density (∼97%) at ∼1450 °C for 4 h. Resistivity of the sintered specimen was ∼5 MΩ/cm. The Ba₂TmSbO₆ has dielectric constant (?′) and dielectric loss (tan δ) of 17 and ∼10⁻⁴ at 5 MHz; the new material would probably be developed as a low-loss dielectric material.