Neodymium niobate is an inorganic compound with the chemical formula NdNbO4. It is a rare-earth niobate containing Nd3+ and Nb5+. At room temperature it adopts a monoclinic fergusonite-type structure and undergoes a reversible transition to a tetragonal scheelite-type structure at elevated temperature.
Preparation Neodymium niobate can be prepared by solid-state reaction of neodymium(III) oxide and niobium(V) oxide:
Nd2O3 + Nb2O5 → 2 NdNbO4
The reaction is typically carried out at high temperature, commonly between about 1100 and 1400 °C.
Large single crystals of NdNbO4 have also been grown from the melt by the Czochralski method. Crystals up to about 20 mm in diameter and 120 mm in length were reported in early growth studies.
Properties Crystal structure At room temperature, NdNbO4 has a monoclinic fergusonite-type structure, usually described in space group I2/a, an alternative setting of C2/c (No. 15).
The structure is a monoclinically distorted derivative of the tetragonal scheelite structure. Neodymium is eight-coordinate, while the niobium environment is strongly distorted. Modern diffraction studies describe the Nb site as effectively six-coordinate, with four shorter and two significantly longer Nb–O bonds.
Single crystals are commonly twinned as a consequence of the symmetry-lowering phase transition on cooling from the high-temperature scheelite phase.
Phase transition and ferroelasticity NdNbO4 undergoes a reversible transition from the monoclinic fergusonite phase to a tetragonal scheelite-type phase in space group I41/a on heating.
Recent phase-equilibrium work places this transition at about 650 °C.
The low-temperature monoclinic phase is ferroelastic. Early single-crystal work showed characteristic ferroelastic twin boundaries and suggested the existence of an intermediate state during the monoclinic-to-tetragonal transformation.
The transition can also be tuned chemically. Substitution of V5+ for Nb5+ lowers the fergusonite-to-scheelite transition temperature substantially; for Nd(Nb0.8V0.2)O4, it falls to about 400 °C.
Dielectric properties NdNbO4 is a low-loss microwave dielectric ceramic. Reported properties for dense material include a relative permittivity of approximately 19.6, a quality-factor product Q × f of about 33,000 GHz, and a temperature coefficient of resonant frequency of approximately −24 ppm/°C.
Because of these properties, NdNbO4-based solid solutions are studied as dielectric resonator ceramics for microwave and millimetre-wave applications.
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