Nanoparticle Precursor into Polycrystalline Bi₂Fe₄O₉: An Evolutionary Investigation of Structural, Morphological, Optical, and Vibrational Properties

Mullite-type Bi₂Fe₄O₉ was synthesized using a polyol-mediated method. X-ray powder diffraction (XRD) revealed that the as-synthesized sample is nanocrystalline. It transformed into a rhombohedral perovskite-type BiFeO₃ followed by a second transformation into mullite-type Bi₂Fe₄O₉ during heating. Each structural feature, from as-synthesized into crystalline phase, was monitored through temperature-dependent XRD in situ. The locally resolved high resolution transmission electron micrographs revealed that the surface of some heated samples is covered by 4–13 nm sized particles which were identified from the lattice fringes as crystalline Bi₂Fe₄O₉. XRD and Raman spectra demonstrate that the nucleation of both BiFeO₃ and Bi₂Fe₄O₉ might simultaneously commence; however, their growth and ratios are dependent on temperature. The diffuse UV/vis reflectance spectra showed fundamental absorption edges between 1.80(1) and 2.75(3) eV. A comparative study between the “derivation of absorption spectrum fitting method” (DASF) and the Tauc method suggests Bi₂Fe₄O₉ to be a direct band gap semiconductor

Morphotropy and Temperature-Driven Polymorphism in A₂Th(AsO₄)₂ (A = Li, Na, K, Rb, Cs) Series

A new alkaline thorium arsenate family was obtained and systematically investigated. The structures of A₂Th­(AsO₄)₂ (A = Li, Na, K, Rb, Cs) were determined from single crystal X-ray diffraction data. Li₂Th­(AsO₄)₂ and either isostructural K₂Th­(AsO₄)₂ and Rb₂Th­(AsO₄)₂ crystallize in the monoclinic crystal system. Na₂Th­(AsO₄)₂ and Cs₂Th­(AsO₄)₂ crystallize in the orthorhombic and tetragonal crystal systems, respectively. Li₂Th­(AsO₄)₂ consists of [Th­(AsO₄)₂]^2– layers with Li atoms in the interlayer space. The rest of the compounds are based on 3D frameworks. Differences in local environments of ThO₈ coordination polyhedra are described in relation to the symmetry. Despite different local environments of ThO₈ coordination polyhedra and different structural symmetry, underlying nets of A₂Th­(AsO₄)₂ (A = Na, K, Rb, Cs) were shown to be the same. Single-crystal and powder Raman spectra were measured, and bands are assigned. DSC measurements showed phase transitions in K₂Th­(AsO₄)₂ and Rb₂Th­(AsO₄)₂, which were studied using high-temperature powder X-ray diffraction (HT-PXRD). The data of HT-PXRD demonstrates two high-temperature polymorphic modification of K₂Th­(AsO₄)₂ and only one for the isotypic Rb₂Th­(AsO₄)₂. The phase transitions in both K and Rb phases are reversible