Lanthanum niobate, also known as lanthanum orthoniobate, is an inorganic compound with the chemical formula LaNbO4. It is a rare-earth niobate containing La3+ and Nb5+. At room temperature it has a monoclinic fergusonite-type structure and undergoes a reversible transition to a tetragonal scheelite-type structure at about 500 °C.
LaNbO4 is ferroelastic and luminescent. Acceptor-doped forms are also proton conductors at elevated temperatures and have been investigated as electrolytes for proton-conducting solid oxide fuel cells.
Preparation Lanthanum niobate can be prepared by a conventional solid-state reaction between lanthanum(III) oxide and niobium(V) oxide:
La2O3 + Nb2O5 → 2 LaNbO4
In solid-state synthesis, La2O3 is commonly preheated to remove absorbed water and carbonate before stoichiometric amounts of the oxides are mixed and calcined at high temperature.
A sol–gel route has also been used. Lanthanum nitrate and a niobium alkoxide precursor are incorporated into a citric-acid/ethylene-glycol gel, which is subsequently calcined to produce crystalline LaNbO4 powder.
Submicrometre LaNbO4 powders have additionally been prepared by spray pyrolysis from solutions containing lanthanum–EDTA and niobium–malate complexes.
Structure and properties At room temperature, LaNbO4 crystallizes in a monoclinic fergusonite-type structure, commonly described in the space-group setting I2/a (No. 15). Representative lattice parameters are a = 5.567 Å, b = 5.204 Å, c = 11.53 Å and β = 94.1°, with four formula units per unit cell.
The room-temperature structure is a distorted derivative of the tetragonal scheelite structure. Lanthanum is surrounded by eight oxygen atoms. Earlier structural descriptions treated Nb5+ as tetrahedrally coordinated, although more recent high-resolution diffraction analysis describes the niobium environment as strongly distorted six-coordinate, with four short Nb–O bonds of about 1.9 Å and two much longer contacts of about 2.5 Å.
On heating, LaNbO4 undergoes a reversible transition to a tetragonal scheelite-type phase in space group I41/a (No. 88). The transition occurs at approximately 480–530 °C, with the exact temperature depending on sample history and experimental method.
The low-temperature phase is ferroelastic. Its characteristic domain microstructure forms during the symmetry-lowering tetragonal-to-monoclinic transformation.
Optical properties LaNbO4 is a wide-band-gap material, with an optical band gap reported at approximately 4.8 eV. It displays intrinsic luminescence associated with the niobate groups.
Under ultraviolet excitation, LaNbO4 shows broad blue emission. Its luminescence depends strongly on crystallinity and heat-treatment temperature; sol–gel-derived material calcined at about 1100 °C showed substantially stronger absorption and fluorescence than samples treated at lower temperatures.
The compound has consequently been investigated as a host and intrinsic phosphor material for optical and luminescent applications.
Proton conductivity Undoped LaNbO4 is not a particularly strong ionic conductor, but substitution of a small fraction of La3+ by lower-valent cations such as Ca2+ creates oxygen vacancies. In humid atmospheres, these vacancies can incorporate water and form mobile protonic defects.
Acceptor-doped rare-earth orthoniobates were identified as high-temperature proton conductors in 2006. Among the compounds studied, Ca-doped LaNbO4 showed the highest proton conductivity, approximately 10−3 S cm−1 at around 900–950 °C in humid atmospheres.
Protons dominate the ionic conductivity in wet atmospheres below about 800 °C. At higher temperature and under oxidizing conditions, electronic hole conduction becomes increasingly important.
The monoclinic-to-tetragonal structural transition influences proton mobility. Neutron-diffraction studies associate the change in proton transport with changes in the distance between neighbouring oxygen sites that form possible proton-hopping pathways.
Applications The chemical stability and proton conductivity of acceptor-doped LaNbO4 have led to its investigation as an electrolyte for protonic ceramic fuel cells, hydrogen sensors and related high-temperature electrochemical devices.
Complete fuel cells using Ca-doped LaNbO4-based electrolytes have been demonstrated experimentally. Such cells show good open-circuit voltages and chemical stability, including operation in carbon-dioxide-containing atmospheres, although their performance is limited by the comparatively modest proton conductivity of the electrolyte.
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