Mechanosynthesis of Nanocrystalline Fully Stabilized bcc γ-phase of Bi₂O₃ without Any Additive: Manifestation of Ferroelasticity in Microstructure, Optical, and Transport Properties

Abstract

In the present work, polymorphic phase transformation of Bi2O3 nanoparticles by the mechanical alloying method has been investigated in detail. X-ray diffraction study reveals that the room temperature stable monoclinic α-phase of Bi2O3 can be stabilized into body-centered cubic γ-phase by applying mechanical energy only to the Bi2O3 powder, without adding any dopant to the Bi2O3 matrix. The crystallite size of the γ-phase is reduced up to ∼24 nm after 18 h of milling the analytical grade α-Bi2O3 powder. Optical band gaps of all synthesized nanocrystalline powders are found to be in the semiconductor region (2.76–3.02 eV). Oxygen ion conductivity increases with the increase of γ-phase and becomes maximum in the 18 h milled sample, in which full phase transformation (α- to γ-phase) has occurred. The metastability of the γ-phase is revealed on further mechanical alloying up to 20 h when milled powder decomposes primarily to α-phase along with other crystalline polymorphic phases which are identified by X-ray diffraction as well as Raman spectra. The temperature dependence of complex conductivity follows the Arrhenius relation and also supports the semiconducting nature of the sample as obtained by optical band gap measurement. The small polaron hopping between localized states is confirmed from the real part of the complex conductivity data. The milling time and measuring temperature independent nature of the conductivity and the relaxation mechanism is confirmed from the master plot of complex impedance and ac conductivity spectra.

Characterization of the Ge/Bi2O3 Interfaces

“…In β-phase the coordination about the Bi atom is four folds and the coordination polyhedron is described by a distorted trigonal bipyramid in which one of the basal corners is occupied by the electron pair 20 . In accordance with the literature data, the monoclinic α-phase exhibits transition to the face centered cubic δ-phase by heating the compound at 730 o C 21 . It is mentioned that owing to the large thermal hysteresis during the natural cooling cycle, δ-phase transform to tetragonal β-phase (usually stabilize at 650 ºC) and to body centered cubic (bcc) γ-phase.…”

“…The bcc phases result from controlled cooling of δ phase. Both of the β-and δ phases convert to the monoclinic phase in the temperature range of 500-650 o C 21 . In our previous investigation 13 we have reported that the Bi 2 O 3 thin films which are deposited onto Yb substrate exhibit body centered cubic γ-phase.…”

Structural and Enhanced Optical Properties of Stabilized γ‒Bi2O3 Nanoparticles: Effect of Oxygen Ion Vacancies

“…Annealing of Bi NPs in the air leads to the formation of bismuth oxides, which can be described as [ 37 , 38 ]. This route is very simple, cost-effective, and also free from any additional chemicals such as surfactants and template agents, which could introduce impurities as a byproduct [ 24 , 33 ]. Therefore, in this study, the synthesis of γ‒Bi 2 O 3 NPs was carried out in two steps.…”

“…method plays a significant role in defining the size and the structural properties [33]. The fitted value of occupancy at the Bi(1), O(1), and O(3) sites remains very close to 1, whereas the occupancies at the Bi(2) and O(2) sites are 0.943 (8) and 0.861 (29), respectively.…”

“…Liu et al reported the synthesis of RT stabilized γ–Bi 2 O 3 (using the solution crystallization method) over a wide temperature range (300 to 700 °C) [ 4 ]. Recently, Bandyopadhyay et al reported the transformation from α–Bi 2 O 3 to RT stabilized γ–Bi 2 O 3 by the mechanical alloying method [ 33 ]. However, as discussed above, the critical factor responsible for RT stabilization of high-temperature metastable γ–Bi 2 O 3 is still unknown.…”