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The family of solid electrolytes having the general formula Bi 4 V 2 -х М х O 11 -δ , referred to as BIMEVOX in scientific literature, features rather high values of oxy gen ion conductivity within a moderate temperature range of 550-950 K. Studies of Bi 4 V 2 -х Fe х O 11 -δ (BIFEVOX) solid solutions established the concentra tion bounds and structural features of BIFEVOX poly morphs and ion transport characteristics depending on composition and temperature, and provided optimal conditions and methods for manufacturing ultrafine BIFEVOX powders [1][2][3][4][5].Regardless of the preparation method, various researchers reported that solid solutions in the concen tration range of 0.3 ≤ х ≤ 0.5 have a tetragonal structure (space group I4/mmm). For low dopant concentrations (х ≤ 0.2) [2], conductivity versus temperature curves feature typical consecutive phase transitions [γ → β (850 K) and β → α (725 K)]; these transitions corre spond to a change in structure (I4/mmm → Amam → P 1) and are accompanied with a change in activation energy of conduction in the solid solutions. For γ phases of the solid solutions (space group I4/mmm), the curves are linear plots, and the activation energies at high tem peratures have values typical of BIMEVOX, namely, 0.2-0.4 eV. Transition to the ordered γ' phase upon lowering temperature is accompanied by an insignif icant change in the slope of the logσ-10 3 /T curve and an increase in activation energy to 0.5-0.7 eV. All transitions observed in the conductivity curves correlate with changes in linear thermal expansion curves. In samples prepared by solution technologies for BIFEVOX, conductivity is always higher than in samples prepared by a solid phase process.Pena et al. [3] discovered that constant current con ductivity decreases at relatively low temperatures (356-433 K) where the iron content increases, whereas the activation energy increases from 0.2 to 0.97 eV; these trends can serve as evidence for a significant contribu tion of oxygen ion conductivity to the overall conduc tivity of Bi 4 V 2 -x Fe x O 11 -y (0 ≤ x ≤ 0.9, 0 ≤ y ≤ 1).A structural study of tetragonal Bi 4 V 2 -x Fe x O 11 -δ as a function of temperature and partial oxygen pressure discovered that this polymorph is stable over a wide range of thermodynamic parameters (T = 298-1073 K, logP O 2 [atm] = -0.68 to -18.0) [4,5]. Under heating, a sample experiences only thermal expansion. As the oxygen partial pressure changes from 0.21 to 10 -18 atm, the tetragonal BIFEVOX structure transforms to orthorhombic in a low oxygen medium (logp O 2 [atm] < -14.0) at temperatures above 773 K. However, a sample does not undergo decomposition. The structural and thermal stability of BIFEVOX in air during long expo sures (of at least two weeks at one temperature) over a temperature range of 723-1083 K with 50 K steps was evaluated earlier [6]. Structural alterations or the appearance of additional phases have not been found for γ BIFEVOX phases over the entire temperature range of thermocycling.Complex oxides of the BIMEVOX fami...

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