[en] Graphical abstract: BICOTIVOX formulated as Bi₂V_0.9Co_0.1−xTi_xO_5.35+x has been synthesized and the complex impedance has been also illustrated in the Nyquist representation. Highlights: • BICOTIVOX formulated as Bi₂V_0.9Co_0.1−xTi_xO_5.35+x has been synthesized. • γ-BICOTIVOX is stabilized in the all investigated compositions. • The AC impedance spectroscopy for Bi₂V_0.9 Co_0.1−xTi_xO_5.35+x system was also studied in this report. • The total ionic conductivity and dielectric permittivity of BICOTIVOX have been also illustrated. -- Abstract: In the present work Bi₂V_0.9Co_0.1−xTi_xO_5.35+X; 0.02 ⩽ x ⩽ 0.08 was synthesized through standard solid state reaction method. X-ray diffraction and differential thermal analysis has been observed for various compositions and it was found that the tetragonal γ-phase stabilized with γ′ ↔ γ phase transition for all the compositions. The ionic conductivity and dielectric permittivity of BICOTIVOX have been investigated by using AC impedance spectroscopy. The values of two components of complex impedance and the relaxation time of oxide ion movement through the grain interior and grain boundary decrease with increase temperature. The thermal dependence of total ionic conductivity revealed maximum (2.64 × 10⁻⁴ S cm⁻¹) for x = 0.04 at 300 °C

[en] Influence of dopant concentration on thermal behaviour of Bi₄Hf_xV_2-xO_11-(x/2)-δ was studied over composition range 0 ≤ x ≤ 0.40 by combination of data obtained from X-ray powder diffraction, differential scanning calorimetric and conductivity measurements. For very low dopant concentrations, the system was found to mimic the parent compound in exhibiting two consecutive transitions,α ↔ β ↔ γ, with slightly different onset temperatures compared to that of parent, whereas the existence of β ↔ γ transition was well confirmed in composition range 0.15 ≤ x ≤ 0.20 and typified in Arrhenius plots to two line regions of different activation energy. For composition range 0.25 ≤ x ≤ 0.40, no significant structural changes associated with γ' ↔ γ transition is visible in X-ray powder diffraction data with variable temperature. Despite this, the existence of γ' ↔ γ transition was evident by some complex incommensurate modulations observed in Arrhenius plots and DSC/DTA thermograms. It was surprising to note the appearance of a new destructive γ ↔ β' transition, resulting from distortion of the tetragonal phase structure at higher temperatures. The relationship between phase stability and ionic conductivity was also rationalized.

[en] A continuous series of Bi₄Ba_xV_2-xO_11-(3x/2)-δ solid solutions are prepared by conventional solid state reactions. The polymorphism and electrical properties of these samples are studied by FT-IR spectroscopy, X-ray diffraction, differential thermal analysis (DTA) and AC impedance. The solid solutions with composition 0.07≤x≤0.13 are isostructural with the monoclinic phase α-Bi₄V₂O₁₁. However, orthorhombic β-phase is observed for x=0.17 and tetragonal γ-phase is stabilized for x≤0.30. X-ray and DTA results reveal the occurrence of α↔γ transition for x≤0.13, β↔γ transition for x=0.17 and γ'→γ transition for x≥0.20. AC impedance plots at 280 deg. C for all compositions show greater contribution of grain to ionic conductivity than grain boundary. The highest ionic conductivity, σ_300^oC=4.456x10^-5 S cm^-1 is observed for x=0.17. The ionic conductivity of the substituted compounds is higher than the parent α-Bi₄V₂O₁₁, due to the increased oxygen ion vacancies generated as a result of barium doping.

[en] Samples of Bi₄Ce_xV_2-xO_11-(x/2)-δ were prepared by the conventional solid-state reactions. The phase transition changes were studied by combining results obtained from high temperature X-ray powder diffraction (HT-XRPD), differential thermal analysis (DTA) and conductivity measurements. For x = 0.15 composition, the phase transitions; α ↔ β and β ↔ γ were clearly evident in the temperature ranges 423-450 and 533-550 deg. C, respectively. Although no subtle change in variation of the unit cell parameters was observed for x = 0.25 composition, the Arrhenius plot and DTA thermogram showed evidence of γ' ↔ γ phase transition in the temperature range 450-500 deg. C

[en] Samples of BINAVOX (Bi₂Na_xV_1-xO_5.5-2x), in the composition range 0 ≤ x ≤ 0.20 were synthesized by sol-gel citrate route. Thermal decomposition and phase structure were characterized using FT-IR, Powder X-ray diffraction and simultaneous thermogravimetric and differential thermal analysis. It has been found that BINAVOX xerogels are completely calcinated above 550 deg. C for 3 h of thermal treatments. Furthermore, the highly conducting γ-phase was stabilized for compositions with x ≥ 0.15. AC impedance spectroscopy revealed that the diffusion of oxide-ion vacancies is thermally activated in both grain interiors and boundaries. In general, the relaxation time of grain boundaries contribution is two orders of magnitude higher than that of grain interiors. The dielectric permittivity showed a general decay with the increase in sodium substitution. Importantly, the composition dependence of electrical conductivity exhibited a maximum for x = 0.13 at lower temperatures (σ₃₀₀ ∼ 1.34 x 10^-5 S cm^-1), which is attributed to the reduction in the activation energy of conduction.

[en] Samples of BIPBVOX.x (Bi_2V_1_–_xPb_xO_5_._5_–_x_/_2) in the composition range 0.05 ≤ x ≤ 0.20 were prepared by ethylene glycol– citric acid sol–gel synthesis route. Structural investigations were carried out by X–ray diffraction, DTA. The highly conducting γ′– phase was effectively stabilized at room temperature for compositions with x ≥ 0.17. Cyclic voltammetric measurements showed reversible redox reactions of vanadium and irreversible redox reaction of Bi"3"+ in the BIPBVOX system during the first cathodic and anodic sweep. However, a higher stability against the reduction of Bi"3"+ to metallic bismuth was seen for x=0.20.

[en] In the present study, sol-gel citrate route was utilized to synthesize BIALVOX (Bi₂Al_xV_1-xO_5.5-x-δ) in the composition range 0 ≤ x ≤ 0.20. Powder X-ray diffraction and simultaneous thermogravimetric and differential thermal analyses confirmed that the calcination of BIALVOX xerogels is fully completed at around 500 ^oC after 3 h of thermal treatment. It has been found that the stabilization of highly conducting γ-phase takes place for x ≥ 0.10. AC impedance spectroscopic investigation suggested that the charge accumulation at grain boundaries is more pronounced as compared to that at grain interiors. However, the maximum electrical conductivity (7.73 x 10^-5 S cm^-1) was noticed for BIALVOX.13 at 300 ^oC. The temperature dependence of dielectric permittivity was also reported.

[en] The new BIMEVOX series of Bi₄Hf_xV_2-xO_11-(x/2)-δ; 0 ≤ x ≤ 0.40 was synthesized by solid state reactions. Structural phase transitions as a function of composition were investigated by FT-IR, X-ray powder diffraction, differential scanning calorimetry and Arrhenius plots of conductivity. Two consecutive phase transitions, α ↔ β ↔ γ, were well characterized for x = 0.05 composition. The existence of β ↔ γ transition was clearly evident for relatively higher compositions 0.15 ≤ x ≤ 0.20 which is typified to two line regions of different activation energy on the Arrhenius plots. However, the existence of only γ' ↔ γ transition was clearly confirmed for high compositions 0.25 ≤ x ≤ 0.40. AC impedance analysis also showed that the ionic conductivity is mainly due to the grain contribution which is clearly pronounced by the short-range diffusibility of oxide ion vacancies within the grains. The composition dependence of conductivity exhibited significant maximum in low temperature region (σ₃₄₀ = 7.31 x 10^-5 S cm^-1) for x = 0.25 composition.

[en] BIHFVOX, formulated as Bi₄Hf_xV_2-xO_11-(x/2)-δ, is a new member of BIMEVOX family. The system with various dopant concentrations (0≤x≤0.40) was prepared by the solid state reaction. The phase stability as a function of composition was investigated using FT-IR, X-ray powder diffraction, differential scanning calorimetry and conductivity measurements. For x=0.05, the α-polymorph was noticed at room temperature with clear evidence for two successive transitions; α↔β↔γ at 436 and 561 deg. C, respectively. For relatively higher dopant concentrations, 0.15≤x≤0.20, the β↔γ transition was clearly evident. The variation of unit cell parameters as a function of temperature for x=0.20 exhibited a subtle change in c parameter around 400 deg. C due to the vacancy ordering phenomenon which is associated with occurrence of such transition. However, the existence of order-disorder, γ'↔γ transition was clearly confirmed for x≥0.25. It was also noticed that the increased dopant concentration of highly sized and charged Hf⁴⁺ ion has a significant role in collapsing the fully disordered tetragonal into orthorhombic β'-domain at higher temperatures. AC impedance spectroscopy showed that the ionic conductivity is principally due to the grain contribution as clearly evident in the increased short-range diffusibility of oxide ion vacancy in the grains with increasing temperature. The composition dependence of conductivity exhibited a maximum for x=0.25 at lower temperatures.

[en] Samples of the BICDVOX system, formulated as Bi₄Cd_xV_2−xO_{11−(3x/2)−δ} in the Cd substitution range 0 ≤ x ≤ 0.25 were synthesized using the standard solid state reaction.The correlation between phase stability and oxide ion performance were investigated by variable temperature XRPD, DSC and AC impedance spectroscopy. The substitution of V⁵⁺ by Cd²⁺ exhibited different phase transitions upon varying composition. For compositions with x ≤ 0.05, two successive transitions; α↔β↔γ are evident, while the β↔γ transition exists in the composition range 0.05 < x < 0.175. However, some temperature dependent phenomena confirmed the exixtence of the γ′↔γ transition, coupled with the tetragonal symmetry stabilization for x ≥ 0.175. The maximum oxide ion conductivity at lower temperatures was observed for x = 0.20. It has also been found that the slow V⁴⁺ → V⁵⁺ re–oxidation results in increased defect trapping effects in the system at higher temperatures. -- Highlights: ► γ-Stabilized BICDVOX at lower dopant concentrations. ► Good oxide-ion conductivity at lower temperatures. ► High temperature-vanadium reduction with lower dopant concentrations.