Materials with layered structures XIII: electrical and optical properties of layered chalcogenides in the system CoIn2S4-CoIn2Se4

The system CoIn₂S_4xSe_4(1−x) has been investigated by X-ray powder methods on samples quenched at 700 °C. The spinel type phase has a phase width of 1≥x>0.9. A new layered compound is formed for 0.9>x>0.45 which crystallizes with the α-FeGa₂S₄-type with a=392.6 pm and c=1270.3 pm (x=0.5) for the hexagonal cell. Platelike crystals of the layered phase are obtained by transport reactions with iodine in a temperature gradient 750→700 °C. The band gaps of these crystals measured by optical absorption vary from 1.2 to 1.4 eV. The electrical conductivities of the crystals are found in the order of 10⁻⁵ Ω⁻¹ cm⁻¹.     

Polymorphism of Bi1−xLnxO1.5 phases (0<x<0.40): Characterization of a new compound Bi4Ln2O9 (x=0.33; Ln=La, Pr, Nd)

The Bi_1−xLn_xO_1.5 solid solutions (Ln=La, Pr, Nd), of the β₂/β₁/ε (Bi-Sr-O) structural type, have been investigated in their Ln-rich domains. For Ln=La, Pr, and Nd, the upper limits are 0.35, 0.35 and 0.33, respectively. The Bi₄Ln₂O₉ ε phase (x=0.33) appears to be the single definite compound. For Bi₄La₂O₉, Bi₄Pr₂O₉ and Bi₄Nd₂O₉, the ε-type cells are respectively: a=9.484(4) Å, b=3.982(2) Å, c=7.030(3) Å, β=104.75(3)°; a=9.470(5) Å, b=3.945(2) Å, c=6.968(4) Å, β=104.73(3)° and a=9.439(3) Å, b=3.944(2) Å, c=6.923(2) Å, β=105.03(3)°. Upon heating, each monoclinic (ε) compound transforms successively into rhombohedral phases (β₂/β₁) and finally into a cubic fluorite-type phase. For La- and Pr-based compounds, all transitions are reversible; for Nd, depending on the thermal treatment, the reversibility of ε→β₂ can be incomplete. These transformations are characterized using X-ray thermodiffractometry, differential thermal analysis, dilatometry and impedance spectroscopy versus temperature. Examination of Bi₄(Ln, Ln′)₂O₉ samples allows to correlate the evolution of the thermal behavior and of the unit cell parameters, to the lanthanide size. A partial plot of the (Bi₂O₃)_1−x-(La₂O₃)_x phase diagram (0≤x≤0.40) is proposed.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

New substitutions and novel derivatives of the Aurivillius phases, Bi5TiNbWO15 and Bi4Ti3O12

We investigated isomorphous substitution of several metal atoms in the Aurivillius structures, Bi₅TiNbWO₁₅ and Bi₄Ti₃O₁₂, in an effort to understand structure-property correlations. Our investigations have led to the synthesis of new derivatives, Bi₄LnTiMWO₁₅ (Ln = La, Pr; M = Nb, Ta), as well as Bi₄PbNb₂WO₁₅ and Bi₃LaPbNb₂WO₁₅, that largely retain the Aurivillius (n = 1) + (n = 2) intergrowth structure of the parent oxide Bi₅TiNbWO₁₅, but characteristically tend toward a centrosymmetric/tetragonal structure for the Ln-substituted derivatives. On the other hand, coupled substitution, 2Ti^IV → M^V + Fe^III in Bi₄Ti₃O₁₂, yields new Aurivillius phases, Bi₄Ti_3−2xNb_xFe_xO₁₂ (x = 0.25, 0.50) and Bi₄Ti_3−2xTa_xFe_xO₁₂ (x = 0.25) that retain the orthorhombic noncentrosymmetric structure of the parent Bi₄Ti₃O₁₂. Two new members of this family, Bi₂Sr₂Nb₂RuO₁₂ and Bi₂SrNaNb₂RuO₁₂ that are analogous to Bi₂Sr₂Nb₂TiO₁₂, possessing tetragonal (I4/mmm) Aurivillius structure have also been synthesized.     

Rietveld refinement of the semiconducting system Bi2−xFexTe3 from X-ray powder diffraction

The semiconducting system Bi_2−xFe_xTe₃ (x = 0.0, 0.02, 0.04 and 0.08) was synthesized at 1000 °C for 30 h. The scanning electron microscope (SEM) image reveals the tendency of the Bi_2−xFe_xTe₃ system to form a sheet structure with more pronounced alignment and to enhance the formation of some microstructure tubes. The structure of the system under study was refined on the basis of X-ray powder diffraction data using the Rietveld method. The analysis revealed the complete miscibility of Fe in the Bi₂Te₃ matrix and hence the formation of single phase. The system crystallizes in the space group R-3m [1 6 6]. The lattice parameters and the unit cell size slightly change by the incorporation of Fe. The refinement of instrumental and structural parameters led to reliable values for the R_B, R_F and Chi².     

Study on sulfur diffusion in CuInSe2 thin films using two thermal profiles

An insurmountable disadvantage of CuInSe₂ is the low band gap, which limits the open-circuit voltage to value well below 500 mV in solar cells. The incorporation of sulfur into CuInSe₂ thin film was investigated to establish a scientific basis for the graded band gap CuIn(Se_1 − x,S_x)₂ thin films. CuIn(Se_1 − x,S_x)₂ thin films were obtained by reactive annealing of Cu₁₁In₉ precursors in a mixture of sulfur and selenium atmosphere while post-sulfurization of single phase CuInSe₂ did not result in CuIn(Se_1 − x,S_x)₂ thin films. A band gap of 1.36 eV, was obtained for the prepared CuIn(Se_1 − x,S_x)₂.     

Crystal structure and Eu/Tb doped luminescent properties of a new borate Ba3BiB9O18

Undoped and doped either by Eu³⁺ or Tb³⁺ bismuth borate Ba₃BiB₉O₁₈ was structurally characterized and analyzed by fluorescence spectroscopy. Belonging to synthetic borate member of the family Ba₃XB₉O₁₈, layers of planar triangular B₃O₆ groups connecting with deformed BaO₆ hexagons are interleaved by 9-coordinate Ba atoms, and 6-coordinate Bi atoms. Its crystal structure was determined and refined from powder X-ray diffraction data by the Rietveld method and the results showed that Ba₃BiB₉O₁₈ belongs to space group P6₃/m with unit cell dimensions of a = 7.1999(2) Å, c = 17.3567(6) Å, and z = 2. Curves of differential thermal analysis and thermogravimetric analysis showed that Ba₃BiB₉O₁₈ is a congruent melting compound and chemically stable above 728 °C. Ba₃Bi_1−xEu_xB₉O₁₈ and Ba₃Bi_1−xTb_xB₉O₁₈ form a continuous solid solution from x = 0.01 to x = 0.9. The ultraviolet excited photoluminescence intensity increased with both Eu³⁺ and Tb³⁺ concentration in the matrix of Ba₃BiB₉O₁₈. There may be an interesting correlation between spectroscopic properties and lattice structural features of doped Ba₃BiB₉O₁₈.     

Novel composition above the limit of Bi:Zr solid solution; synthesis and physical properties of Bi1.33Zr0.67O3+δ

This paper presents an increase to x = 0.67 of the zirconium content in the conductive Bi_2−xZr_xO_3+δ solid solution. Complete incorporation of Zr in the β_III-Bi₂O₃ structure, confirmed by X-ray powder diffraction, has produced a phase with a lower volume and superior conductivity than those predicted by an earlier study. The observed β_III-δ Bi_2−xZr_xO_3+δ phase transition around 730 °C has been characterised for the first time and shows a segregation of a mixture of predominantly γ-Bi₂O₃ and approximately 30% of the ZrO₂, before total reincorporation of the Zr in the high temperature δ-phase.     

Microwave dielectric properties of La1−xBix(Mg0.5Sn0.5)O3 ceramics

The La_1−xBi_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. An apparent density of 6.50 g cm⁻³, a dielectric constant (?_r) of 20.2, a quality factor (Q × f) of 58,100 GHz and a temperature coefficient of resonant frequency (τ_f) of −84.2 ppm °C⁻¹ were obtained for La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Structural characterization and photocatalytic properties of novel Bi2YVO8

Bi₂YVO₈ was prepared by solid-state reaction for the first time. The structural and photocatalytic properties of Bi₂YVO₈ were studied. The results showed that this compound has the tetragonal crystal system with space group I4/mmm. The band gap of Bi₂YVO₈ was estimated to be about 2.09 eV by plotting (αhν)² versus hν and obtaining the hν axis intercept value according to Tauc's equation. For the photocatalytic water splitting reaction, H₂ or O₂ evolution was observed from pure water with Bi₂YVO₈ as the photocatalyst under ultraviolet light irradiation (wavelength = 390 nm). Degradation of aqueous methylene blue photocatalyzed by this compound was investigated under visible light irradiation. Bi₂YVO₈ showed higher photocatalytic activity compared to Bi₂YTaO₇, Bi₂InTaO₇ or TiO₂ (P-25). Complete removal of aqueous methylene blue was achieved after visible light irradiation for 170 min. The decrease of the total organic carbon and the formation of inorganic products such as SO₄²⁻ and NO₃⁻ revealed the continuous mineralization of aqueous methylene blue during photocatalytic reaction.     

Effects of complex substitution of La and Nd for Bi on the microwave dielectric properties of BiNbO4 ceramics

La₂O₃ and Nd₂O₃ were used to substitute Bi₂O₃ and the effects of complex substitution on the sintering behavior and the microwave dielectric properties of BiNbO₄ ceramics were studied. With 0.5 wt.% CuO-V₂O₅ mixtures addition, all of the Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics could be densified below 920 °C. The triclinic phases are identified in Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics with x=0.01 sintered at 820 °C and the triclinic intensities increase with increasing the x value and sintering temperature. The saturated bulk density slightly decreases from 7.17 to 7.13 g/cm³ and the ε_r value from 44.24 to 42.76 with increasing x from 0 to 0.07 for Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics. The saturated Q×f value is between 10,300 and 12,400 GHz depending on the x value. The τ_f values of dense Bi_1−x(La_0.38Nd_0.62)_xNbO₄ ceramics decrease from 28.32 to 12.79 ppm/°C with x varying from 0 to 0.01 and remain almost unchanged with further increasing x.     

Structure, dielectric and ferroelectric anisotropy of Sr2−xCaxBi4Ti5O18 ceramics

Sr_2−xCa_xBi₄Ti₅O₁₈(x = 0, 0.05) powders synthesized by solid state route were uniaxially pressed and sintered at 1225 °C for 2 h. The obtained dense ceramics exhibited crystallographic anisotropy with a dominant c axis parallel to the uniaxial pressing direction which was quantified in terms of the Lotgering factor. Microstructure anisotropy of both compositions (x = 0, 0.05) consisted of plate like grains exhibiting their larger surfaces mostly perpendicular to the uniaxial pressing direction. Dielectric and ferroelectric properties of Sr_2−xCa_xBi₄Ti₅O₁₈ ceramics were measured under an electric field (E) applied parallel and perpendicularly to uniaxial pressing direction. The obtained dielectric ?_R(T) and polarization (P-E) curves depended strongly on E direction thus denoting a significant effect from microstructure and crystallographic texture. Sr_2−xCa_xBi₄Ti₅O₁₈ properties were also significantly affected by Ca content (x): Curie temperature increased from 280 °C (x = 0) to 310 °C (x = 0.05) while ?_R and remnant polarization decreased for x = 0.05. The present results are discussed within the framework of the processing and crystal structure-properties relationships of Aurivillius oxides ceramics.     

Bi4LnNb3O15 (Ln = La, Pr, Nd) and Bi4LaTa3O15: New intergrowth Aurivillius related phases

We describe the synthesis and characterization of new intergrowth Aurivillius related phases, Bi₄LnNb₃O₁₅ (Ln = La, Pr, Nd) and Bi₄LaTa₃O₁₅. Both powder X-ray diffraction and electron microscopy investigations show that the compounds adopt orthorhombic structures with the cell parameters a ∼ 5.5 Å, b ∼ 5.5 Å and c ∼ 20.9 Å, suggesting an ordered intergrowth structure that consists of n = 1 [Bi₂NbO₆]⁻ and n = 2 [Bi₂LnNb₂O₉]⁺ Aurivillius fragments which are stacked alternately along the c-axis. The oxides do not show a second harmonic generation (SHG) response toward 1064 nm laser radiation; they do not show a ferroelectric-paraelectric transition either between 30 and 900 °C in dielectric measurements, indicating a centrosymmetric structure. Optical absorption studies show that the intergrowth phases possess considerably smaller band gaps than the parent Nb₂O₅ and Ta₂O₅.     

New quenched-in fluorite-type materials in the Bi2O3-La2O3-PbO system: Synthesis and complex phase behaviour up to 750 °C

New quenched-in fluorite-type materials with composition (BiO_1.5)_0.94−x(LaO_1.5)_0.06(PbO)_x, x = 0.02, 0.03, 0.04 and 0.05, were synthesised by solid state reaction. The new materials undergo a number of phase transformations during heating between room temperature and 750 °C, as indicated by differential thermal analysis. Variable temperature X-ray diffraction performed on the material (BiO_1.5)_0.92(LaO_1.5)_0.06(PbO)_0.02 revealed that the quenched-in fcc fluorite-type material first undergoes a transformation to a β-Bi₂O₃-type tetragonal phase around 400 °C. In the range 450-700 °C, α-Bi₂O₃-type monoclinic, Bi₁₂PbO₁₉-type bcc and β₁/β₂-type rhombohedral phases, and what appeared to be a ?-type monoclinic phase, were observed, before a single-phase fluorite-type material was regained at 750 °C.     

Chemically deposited lead sulfide and bismuth sulfide thin films and Bi2S3/PbS solar cells

Solar cells with a short-circuit current density (J_sc) of 6 mA/cm², an open circuit voltage (V_oc) of 280 mV and a conversion efficiency of 0.5% under a 1000 W/m² solar radiation were prepared by sequential chemical deposition of Bi₂S₂ (160 nm) and PbS (400 nm) thin films. The optical band gap (E_g) of Bi₂S₃ (160 nm) decreased from 1.67 to 1.61 eV upon heating the as-deposited film at 250 °C in air for 15 min to make it crystalline, but also reduced its thickness to 100 nm. Photoconductivity of this film is 0.003 (Ω cm)^− 1. The E_g of PbS film (200 nm) deposited at 25 °C (24 h) is 0.57 eV, and is 0.49 eV for the film deposited at 40 °C. The electrical conductivity of the latter is 0.48 (Ω cm)^− 1. The photo-generated current density for a Bi₂S₃(100 nm)/PbS(300 nm) absorber stack is above 40 mA/cm² under AM 1.5 G (1000 W/m²) solar radiation. However, the optical losses in the cell structure reduces the J_sc. Spectral sensitivity of the external quantum efficiency of the cell establishes the contribution of Bi₂S₃ and PbS to J_sc. The energy level diagram of the cell structure suggests a built-in potential of 470 mV for the present case. Six series-connected cells gave the V_oc of 1.4 V and J_sc of 5 mA/cm².     

Mixed cation effect in xR2O-(40 − x)Na2O-50B2O3-10Bi2O3 (R = Li, K) glasses

Mass density, glass transition temperature and ionic conductivity are measured in xLi₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ and xK₂O-(40 − x)Na₂O-50B₂O₃-10Bi₂O₃ glass systems with 0 ≤ x ≤ 40 mol%. The strength of the mixed alkali effect in T_g, dc electrical conductivity and activation energy has been determined in each glass system. The magnitudes of the mixed alkali effect in T_g for the mixed Li/Na glass system are much smaller than those in the mixed K/Na glasses. The impact of mixed alkali effect on dc electrical conductivity in mixed Li/Na glass system is more pronounced than in the K/Na glass system. The results are explained based on dynamic structure model.     

Tantalum influence on physical properties of (K0.5Na0.5)(Nb1−xTax)O3 ceramics

Lead-free (K_0.5Na_0.5)(Nb_1−xTa_x)O₃ ceramics with x = 0.00-0.30 were prepared by the solid-state reaction technique. The effects of Ta on microstructure, crystallographic structure, phase transition and piezoelectric properties have been investigated. It has been shown that the substitution of Ta decreases Curie temperature T_C and orthorhombic-tetragonal phase transition temperature T_O-T, while increasing the rhombohedral-orthorhombic phase transition temperature T_R-O. In addition, piezoelectric activity is enhanced with the increase of Ta content. The ceramics with x = 0.30 have the high value of piezoelectric coefficient d₃₃ = 205 pC/N. Moreover, k_p shows little temperature dependence between −75° C and 75 °C, and d₃₃ exhibits very good thermal stability over the range from −196 °C to 75 °C in the aging test.     

Synthesis of V-doped TiO2 films by chemical bath deposition and the effect of post-annealing on their properties

Amorphous composite films, composed of a Ti_1 − xV_xO₂ solid-solution phase and a V₂O₅ phase, were produced by chemical bath deposition and subsequently air-annealed at various temperatures up to 550 °C. The microstructure and chemical composition of the as-prepared and annealed films were investigated by a combinatorial experimental approach using Scanning electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. Ultraviolet-Visible Spectrometry was applied to determine the optical band gap of the as-prepared and annealed films. It followed that the incorporation of vanadium in the as-deposited films reduces the optical band gap of TiO₂ from about 3.8 eV to 3.2 eV. Annealing of the films up to 350 °C leads to slight increase of band gap, as attributed to a reduction of the defect density in the initially amorphous oxide films due to the gradual development of long-range order and a concurrent reduction of the V⁴⁺-dopant concentration in the Ti_1 − xV_xO₂ solid-solution phase. The films crystallized upon annealing in air at 550 °C, which resulted in drastic changes of the phase constitution, optical absorbance and surface morphology. Due to the lower solubility of V⁴⁺ in crystalline TiO₂, V⁴⁺ segregates out of the crystallizing Ti_1 − xV_xO₂ solid-solution phase, forming crystalline V₂O₅ at the film surface.     

Effect of substitution in the scheelite-like series, AxBa1−xBi2Mo4O16 (A = Ca, Sr, Pb)

A new series of solid solutions of the type, A_xBa_1−xBi₂Mo₄O₁₆ (A = Ca, Sr, Pb) have been isolated. The domain of the solid solutions is very narrow and is in the range (0.01 ≤ x ≤ 0.10). All the phases were synthesized by the solid-state technique. The crystal structure is similar to that of the monoclinic scheelite type BaBi₂Mo₄O₁₆. The structure consists of layers of [Bi₂O₂] units separated by (Ba/AO)₁₀ units and isolated MoO₄ tetrahedra. Their semiconductor band gaps render them as potential materials for photocatalysis.