The Fluorite-Like Phase Nd₅Mo₃O_16±δ in the MoO₃–Nd₂O₃ System: Synthesis, Crystal Structure, and Conducting Properties

Abstract

This paper describes a study of the system MoO-NdO using a combination of X-ray powder diffraction (XRD), neutron powder diffraction (NPD), thermogravimetric analysis (TGA), and ac impedance spectroscopy (IS). A phase-pure material is observed at a composition of 45.5 mol % NdO, which corresponds to an ideal stoichiometry of NdMoO. XRD and NPD show that the crystal structure is a superstructure of the fluorite arrangement, with long-range ordering of the two cation species leading to a doubled unit cell parameter. The sample is found to be significantly oxygen deficient, i.e. NdMoO, when it is prepared by a solid-state reaction at 1473 K in air. TGA measurements indicate that the sample loses only minimal mass on heating to 1273 K in O. IS studies of the mean conductivity under different atmospheres show that the sample is a mixed conductor between ambient temperature and 873 K, with a dominant electronic component at higher temperatures, as demonstrated by measurements under inert atmosphere. NPD measurements indicate that the anion vacancies are preferentially located on the O2 sites, while studies of the temperature dependence performed under an O atmosphere to 1273 K show significantly anisotropic thermal parameters of the anions. Together with analysis of the total neutron scattering data, this supports a model of oxygen ions hopping between O2 positions, with a vacancy, rather than interstitial, mechanism for the anion diffusion.

Fluorite‐like LixLn5–xMo3O16.5–1.5xFx (Ln = La, Pr, Nd) compounds isostructural with Nd5Mo3O16

“…At present, there is considerable research effort concentrated on the structure, properties, and potential applications of the compounds of the Nd 5 Mo 3 O 16 family 4‐17 . In earlier structural studies of the Ln 5 Mo 3 O 16 (Ln = La–Tb) compounds, 18‐20 they were synthesized under reducing conditions at temperatures on the order of 1000°C.…”

Effect of Sodium and Fluorine Co‐Doping on the Properties of Fluorite‐Like Rare‐Earth Molybdates of Nd5Mo3O16 Type

“…gr. Pn –3 n ), which possess mixed oxide‐ion/electronic conductivity . The feature of these compounds is that they easily lose oxygen under reducing conditions and their composition, in this case, is defined as Ln 5 Mo 3 O 16 .…”

“…One distinctive feature of the Ln 5 Mo 3 O 16+ δ compounds is that firing in air produces no changes in their structure, which remains fluorite‐like, even though their composition changes to Ln 5 Mo 3 O 16+ δ with δ = 0–0.5. The Ln1O 8 and Ln2O 8 coordination polyhedra in their structure have the form of edge‐sharing distorted and undistorted cubes, whereas the MoO 4 tetrahedra are isolated from each other , . As a result, the structure has rather large voids.…”

“…As a result, the structure has rather large voids. The neodymium and praseodymium compounds can be synthesized in the air and contain excess oxygen in the voids, which is responsible for their interstitial oxide ion conductivity , . The conductivity of these compounds was reported to exceed 10 –2 S/cm at 800 °C .…”

“…The neodymium and praseodymium compounds can be synthesized in the air and contain excess oxygen in the voids, which is responsible for their interstitial oxide ion conductivity , . The conductivity of these compounds was reported to exceed 10 –2 S/cm at 800 °C . In mixed rare‐earth molybdates Nd 4 SmMo 3 O 16 and Nd 4.5 La 0.5 Mo 3 O 16 , an increase in the electrical conductivity to 0.06–0.1 S/cm was observed , …”