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

摘要

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 alpha-phase of Bi2O3 can be stabilized into body-centered cubic gamma-phase by applying mechanical energy only to the Bi2O3 powder, without adding any dopant to the Bi2O3 matrix. The crystallite size of the gamma-phase is reduced up to similar to 24 nm after 18 h of milling the analytical grade alpha-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 gamma-phase and becomes maximum in the 18 h milled sample, in which full phase transformation (alpha- to gamma-phase) has occurred. The metastability of the gamma-phase is revealed on further mechanical alloying up to 20 h when milled powder decomposes primarily to alpha-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.