Polymorphism, Phase Transformations, and Oxide Ion Conductivity in Bi_1.56U_0.22La_0.22O_3.33

Abstract

This paper deals with the polymorphism, phase transitions, and the oxide ion conduction of Bi1.56U0.22La0.22O3.33. Thermal treatment at 950 °C of a mixture of α-Bi2O3 and LaUO4+ x , followed by slow cooling to room-temperature, yields a hexagonal phase of that composition with cell parameters a H = 4.0066(7) and c H = 9.543(2) Å. Quenching of the reaction mixture from 950 °C leads to the formation of a cubic fluorite-type phase with a c = 5.6273(8) Å. Annealing of both cubic and hexagonal phases at 600 °C for 500 h yields a new polymorph that is indexed with a monoclinic lattice, the cell parameters being a M = 7.778(3), b M = 7.834(4), c M = 5.763(3) Å, and β = 89.71(2)°. Phase transitions experienced by each polymorph with temperature are followed by high-temperature X-ray powder diffraction. The three phases are transformed into a new C* cubic phase at temperatures above 820 °C. The oxide ion conduction exhibited by each Bi1.56U0.22La0.22O3.33 polymorph is rather different. At 300 °C the cubic phase, which is the best conducting one, shows a value of σ = 7.2 × 10-5 S cm-1, the conductivity of the hexagonal phase is 2.5 × 10-5 S cm-1, and the monoclinic phase shows the lowest conductivity, 6.6 × 10-7 S cm-1. The plot of conductivity vs inverse temperature shows a linear dependence for each phase. The nonlinear dependencies also observed in the conductivity plots are related to structural transformations taking place during thermal treatments.

Structural study of the trigonal Bi2.34U0.33La0.33O5 oxide ion conductor: Rietveld refinement of X-ray and neutron powder diffraction data

“…8 A simple operation transforms a f.c.c.-type cell into a hexagonal cell: We have previously studied the electrical properties of trigonal Bi 2.34 U 0.33 La 0.33 O 5 . 7 The conductivity at 300 ЊC (σ 300 ЊC = 2.5 10 Ϫ5 S cm Ϫ1 ) shows that this material is a good solid ionic conductor. The conductivity is assigned to the O 2Ϫ motion through the anion vacancies in the oxygen network.…”

“…8 The oxide-ion conductivity shown by the several Bi 1.56 -U 0.22 La 0.22 O 3.33 polymorphs is rather different. 7 At 300 ЊC the monoclinic phase shows the lowest conductivity, σ = 6.6 × 10 Ϫ7 S cm Ϫ1 . For the trigonal polymorph a remarkable increase of conductivity by two orders of magnitude is observed, σ = 2.5 × 10 Ϫ5 S cm Ϫ1 .…”

The cubic BiULaO mixed oxide: Synthesis, structural characterization, thermal stability and electrical properties

“…Thus, in similar Bibased oxides it has been reported that the corresponding cubic phase is transformed into a phase of lower symmetry Table 1 Activation energy (E) and pre-exponential factor (r 0 ) obtained from the fitting of the two straight lines of Fig. 5 Temperature range (-C) E (eV) r 0 (S cm À 1 ) 120 -500 0.96 T 0.01 (1.2 T 0.1) Â 10 5 600 -900 0.42 T 0.02 60 T 10 (hexagonal or monoclinic phase) after heating for a long period at 600 -C (aging) [29,30].…”

Similar and Dissimilar Properties of Polymorphic Phases of NdIr3

“…Polymorphic phases of a material give an excellent opportunity to study the inter-relations of physical properties with different arrangements of the same set of atoms. − The interatomic distances between the atoms, that is, the types of atomic bonds or the types of orbital overlapping, play a key role in controlling the physical parameters. A material, subjected to a particular environment or condition, prefers to form in a crystal structure having minimum formation energy.…”