Metal-Doping of La_5.4MoO_11.1 Proton Conductors: Impact on the Structure and Electrical Properties

Abstract

La5.4MoO11.1 proton conductors with different metal doping (Ca2+, Sr2+, Ba2+, Ti4+, Zr4+, and Nb5+ ) have been prepared and structurally and electrically characterized. Different polymorphs are stabilized depending on the doping and cooling rate used during the synthesis process. The most interesting results are obtained for Nb-doping, La5.4Mo1–x Nb x O11.1–x/2, where single compounds are obtained in the compositional range 0 ≤ x ≤ 0.2. These materials are fully characterized by structural techniques such as X-ray and neutron powder diffraction and transmission electron microscopy, which independently confirm the changes of polymorphism. Scanning electron microscopy and impedance spectroscopy measurements in dry/wet gases (N2, O2, and 5% H2–Ar) showed an enhancement of the sinterability and electrical properties of the materials after Nb-doping. Conductivity measurements under very reducing conditions revealed that these materials are mixed ionic-electronic conductors, making them potential candidates for hydrogen separation membranes.

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

“…However, when the cooling rate is set to a slower pace, the molybdenum ordering becomes quite evident. For R1, which can be obtained for La 5.4 MoO 11.1 at a cooling rate of 50 °C·min –1 , or for cation-doped samples, La 5.4 Mo 0.9 Nb 0.1 O 11.05 and La 5.4 Mo 0.9 Ti 0.1 O 11 , at 5 °C·min –1 , all from 1500 °C, molybdenum/dopant is located in three different positions: the initial one, M1 (0,0,0) and two new ones, M5 ( , , 0) and M8 ( , , 0) (Table S5). The slowest cooling rate, 0.5 °C·min –1 , favors the most thermodynamically stable structural arrangement, where one molybdenum remains in the M1 position and the other two converge to the new and final M2 position ( , , ) (Table S6).…”

“…The symmetry of these compounds can also be tailored by modifying the cationic and anionic frameworks. 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 .…”

“…05 , respectively, at 700 °C in a dry N 2 atmosphere, because of the generation of additional oxygen vacancies. 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.…”

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.…”