Synergic Effect of Metal and Fluorine Doping on the Structural and Electrical Properties of La_5.4MoO_11.1-Based Materials

Abstract

Cationic and anionic frameworks of La 5.4 MoO 11.1 proton conductors have been modified by means of metal (Ti 4+ , Zr 4+ , and Nb 5+ ) and fluorine (F − ) doping. This synergic effect leads to the stabilization of highsymmetry and single-phase polymorphs. The materials have been fully characterized by structural techniques, such as X-ray and neutron powder diffraction and transmission electron microscopy. The fluorine content was determined by ion chromatography. Impedance spectroscopy analysis under different atmospheres (dry and wet N 2 and O 2 and wet 5% H 2 −Ar) showed an improvement in the electronic conductivity under reducing conditions, making these materials potential candidates for hydrogen separation membranes.

Design of Mixed Ionic-Electronic Materials for Permselective Membranes and Solid Oxide Fuel Cells Based on Their Oxygen and Hydrogen Mobility

“…At high temperatures (above 600 °C) in an oxidizing atmosphere (air), the charge transport is dominated by p -type conduction, whereas under reducing conditions n -type conduction prevails [ 82 , 198 ]. Doping with Ti, Zr, and Nb on the Mo site and with fluorine on the oxygen site was studied using La 5.4 MoO 11.1 as an example, but essentially all of the dopants reduced ionic conductivity of the material [ 197 , 199 , 200 ]. A similar situation was observed in La 6−x WO 12−δ (x = 0.4, 0.5) lanthanum tungstates [ 82 , 191 , 192 , 193 , 194 , 195 ].…”

Unravelling Crystal Superstructures and Transformations in the La6–xMoO12−δ (0.6 ≤ x ≤ 3.0) Series: A System with Tailored Ionic/Electronic Conductivity

“…22 However, modification of the anionic network of La 5.4 MoO 11.1 and La 5.4 Mo 0.9 Nb 0.1 O 11.05 by means of fluorination leads to the stabilization of the cubic polymorph at temperatures as low as 1200 °C, where nonfluorinated samples are a mixture of phases. 23 However, these materials showed a marked decrease of the ionic conductivity under dry/wet N 2 and O 2 atmospheres. For instance, between the nonfluorinated and fluorinated La 5.4 Mo 0.9 Nb 0.1 O 11.05 samples, both with R1 symmetry, the conductivity decreased from 0.21 to 0.02 mS cm −1 at 700 °C in a dry N 2 atmosphere, due to the filling of the oxide vacancies by fluorine.…”

“…For instance, Nb-doping on La 5.4 MoO 11.1 leads to the stabilization of the R1 polymorph regardless of the preparation conditions, in addition to an improvement of the densification and electrical properties, from 0.17 to 0.44 mS cm –1 for La 5.4 MoO 11.1 and La 5.4 Mo 0.9 Nb 0.1 O 11.05 , respectively, at 700 °C in a dry N 2 atmosphere, because of the generation of additional oxygen vacancies . However, modification of the anionic network of La 5.4 MoO 11.1 and La 5.4 Mo 0.9 Nb 0.1 O 11.05 by means of fluorination leads to the stabilization of the cubic polymorph at temperatures as low as 1200 °C, where nonfluorinated samples are a mixture of phases . However, these materials showed a marked decrease of the ionic conductivity under dry/wet N 2 and O 2 atmospheres.…”

Room‐temperature mechanochemical synthesis of RE molybdates: Impact of structural similarity and basicity of oxides

“…[1][2][3][4][5][6][7] Ln 6-x Zr x MoO 12-δ (Ln = La, Nd, Sm, Dy, Ho) Zr-doped molybdates, pure La 5.5 MoO 11.25 , pure and doped La 5.4 MoO 11.1 , and Ln 10 Mo 2 O 21 (Ln = Nd, Gd -Ho) have mixed electron-ion (electron-proton) conductivity and are promising materials for SOFC electrolytes and proton-conducting membranes. [5][6][7][8][9][10][11][12][13][14] Zirconium doping improves the stability of these materials to…”

“…In particular, temperatures well above 1000°C are often needed for the synthesis of lanthanide-based multicomponent oxides, rather promising as luminescent and optical materials [1][2][3][4] and oxygen-ion (proton) conductors. [6][7][8][9][10][11][12] MA and MS allow usually lowering the synthesis temperature for initially activated materials and, in some cases, ensure targeted synthesis of unique powder products at room temperature. For example, having a cubic structure, oxygen-ion conducting pyrochlores rather often result from room-temperature mechanochemical synthesis.…”