Structure and Magnetic Properties of a Nanosized Iron-Doped Bismuth Titanate Pyrochlore.

The nanosized (50-70 nm) pyrochlore Bi1.5Fe0.5Ti2O7-δ was prepared by a coprecipitation technique. Characterization of Bi1.5Fe0.5Ti2O7-δ was carried out by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Raman spectroscopy, Mössbauer spectroscopy, and magnetic susceptibility measurements. The study of Fe doping in Bi2Ti2O7 was performed by means of density functional theory (DFT) calculations. The nanosized Bi1.5Fe0.5Ti2O7-δ sample crystallizes in the structural type of pyrochlore (Fd3̅m). The distribution of Fe atoms over the sites of Bi and Ti was studied from DFT simulations and then confirmed by the XRD analysis and Mössbauer method. The local distribution, electronic structure, and magnetic behavior of nanosized Bi1.5Fe0.5Ti2O7-δ are determined by the local microstructure of the metastable nanosized sample. Based on the examination of the Mössbauer spectrum of the Bi1.5Fe0.5Ti2O7-δ nanopowder, the following states of oxidation were revealed for iron atoms: Fe4+ in the titanium sites with a fraction of ∼5.7% and two states of Fe3+ (in the Bi and Ti sites) with different geometries of the oxygen surrounding. The ratio of Fe3+ distributed over the sites correlates well with the distribution in the ceramic sample. The presence of Fe4+ was found only in the nanosized Bi1.5Fe0.5Ti2O7-δ. The experimental effective magnetic moment of Fe atoms in the nanosized Bi1.5Fe0.5Ti2O7-δ appeared noticeably lower than that in the ceramic sample. The temperature dependence of µeff within the temperature range of 50-300 K is adequately described by the model of coexistence of Fe3+ and Fe4+ and the existence of clusters.

Structural, Optical, Luminescence, and Electrical Properties of Eu/Li- and Eu/Na-Codoped Magnesium Bismuth Niobate Pyrochlores.

Eu-doped bismuth-based Bi1.5M0.4Mg0.5Nb1.5O7-δ (M = Li and Na) pyrochlores were synthesized by the organic-inorganic precursor combustion technique. The study examined the effect of rare earth element Eu3+ doping on the structural, dielectric, optical, and luminescence properties of synthesized materials. The analysis showed that the substitution of Bi3+ cations with Eu3+ leads to dielectric permittivity decreasing due to the structural distortion for the Eu-concentrated compositions and low polarizability of Eu3+. The band gap values predicted by electronic band structure calculation using DFT-HSE03 are in line with the experimental ones and tended to increase with the decrease in the unit cell parameters with Eu concentration changing. By the optical and luminescence measurements, the specific roles of Li- and Na-containing host types, additional phases, and dopant concentration in bismuth niobate pyrochlores are shown concerning the dielectric, structural, and Eu3+ emission properties. All Eu-doped bismuth-based pyrochlore ceramics behave as high-frequency dielectrics up to 200 °C and have mixed conductivity (electronic, proton, and oxygen) at T > 200 °C. The obtained dielectric parameters make them suitable for high-frequency ceramic capacitors.

Photocatalytic Properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) Pyrochlores: Hybrid DFT Calculations and Experimental Study.

The photocatalytic properties of Bi2-xTi2O7-1.5x (x = 0, 0.5) pyrochlores are examined via ab initio calculations and experiments. A coprecipitation method is applied for the synthesis of nanopowder pyrochlores. The pyrochlore phase formation starts at 500 °C (Bi2Ti2O7) and 550 °C (Bi1.5Ti2O6.25). Nanopowders are found to be a metastable character of pyrochlore phases. The presence of bismuth and oxygen vacancies enhances the thermal stability of the Bi1.5Ti2O6.25 phase in comparison with the Bi2Ti2O7 phase. The estimated crystallite size is 30-40 nm with noticeable agglomerates of about 100-300 nm according to scanning electron microscopy (SEM) and with the formation of particles (510-580 nm) in the aqueous medium. The isoelectric points of the nanopowders seem to be shifted to the strongly acidic region, resulting in the formation of negative surface particle charges of -33 mV (Bi2Ti2O7) and -27 mV (Bi1.5Ti2O6.25) at pH 5.88 in distilled water. The specific surface area is 11.5 m2/g (Bi2Ti2O7) and 12.00 m2/g (Bi1.5Ti2O6.25). The use of the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional allows achieving an excellent agreement between theoretical and experimental structural parameters. The screened Coulomb hybrid HSE03 functional is the most appropriate for describing the optoelectronic properties. Bismuth titanate pyrochlores are wide-gap semiconductors with strong abilities to be active photocatalysts under visible irradiation. The optical Eg values for direct/indirect transition according to the experiment, 3.19/2.94 eV (x = 0) and 3.24/3.03 eV (x = 0.5), and the DFT/HSE03 calculations, 2.92/2.87 (x = 0) and 3.42/- eV (x = 0.5), are in the visible light region and are close. The calculated low effective masses of the charge carriers and suitable band edge positions confirm the ability of the pyrochlores to act as photocatalysts. The photocatalytic activity has been evaluated through the decomposition of rhodamine B under visible irradiation. Bi2Ti2O7 shows the highest activity in comparison with Bi1.5Ti2O6.25, which is in good agreement with theoretically predicted and experimentally revealed characteristics.

Ab Initio and Experimental Insights on Structural, Electronic, Optical, and Magnetic Properties of Cr-Doped Bi2Ti2O7.

Combined ab initio and experimental study of Cr doping into bismuth titanate pyrochlore was carried out for the first time. Accurate first-principles density functional theory calculations were performed considering a Hubbard U correction (DFT+U) to account for on-site Coulomb interactions of the Cr 3d states. The possibility to synthesize a novel pyrochlore-type compound with a high dopant content Bi1.5Cr0.5Ti2O7 (Bi site doping) in a fine powder state was shown via coprecipitation method, while the single-phase Bi2Ti1.5Cr0.5O7 (Ti site doping) could not be obtained. Detailed descriptions of thermostability, structural, optoelectronic, and magnetic properties of Cr-doped pyrochlores in the fine (particles size 100-300 nm) and "bulk" (1-50 µm) powder states are presented based on the well-matched results of theoretical and experimental investigations. According to the Rietveld refinement of the X-ray diffraction data, Bi1.5Cr0.5Ti2O7 compound is an A-site deficient pyrochlore (Bi1.38Cr0.30)(Ti1.84Cr0.16)O6.44 with chromium distribution between both cationic sites. Metastability of fine powder Cr-containing pyrochlore phase was revealed during long-thermal annealing, while the bulk powder sample was stable up to its melting point 1230 °C. According to the study of electronic structure and optical properties, Cr-doped pyrochlores are wide-band semiconductors with light absorption in the range of 300-500 nm and perspective as photocatalytic active materials under visible light irradiation. Paramagnetic behavior with effective magnetic moment 3.92 µB (Bi1.6Cr0.1Ti2O7-δ) and 3.01 µB (Bi1.5Cr0.5Ti2O7) was experimentally observed. All chromium in magnetically diluted pyrochlore Bi1.6Cr0.1Ti2O7-δ exists in the form of Cr3+ monomers, whereas in the more concentrated magnetic Bi1.5Cr0.5Ti2O7 composition Cr3+-O-Cr3+ dimers may also be present, with a fraction equal to 0.39. This investigation constitutes the first approach to the electronic, structural, optical, and magnetic properties of d-elements doped bismuth titanate pyrochlores from experimental and theoretical viewpoints, emphasizing the power of DFT+U to provide insights and to complement the experimental characterization of these new compounds.