Lithium niobate

Lithium niobate (LiNbO₃) is a synthetic salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linear and non-linear optical applications. Lithium niobate is sometimes referred to by the brand name linobate.

Lithium niobate is a colorless solid, and it is insoluble in water. It has a trigonal crystal system, which lacks inversion symmetry and displays ferroelectricity, the Pockels effect, the piezoelectric effect, photoelasticity and nonlinear optical polarizability. Lithium niobate has negative uniaxial birefringence which depends slightly on the stoichiometry of the crystal and on temperature. It is transparent for wavelengths between 350 and 5200 nanometers.

Lithium niobate can be doped with magnesium oxide, which increases its resistance to optical damage (also known as photorefractive damage). Other available dopants are iron, zinc, hafnium, copper, gadolinium, erbium, yttrium, manganese and boron.

Single crystals of lithium niobate can be grown using the Czochralski process.

After a crystal is grown, it is sliced into wafers of different orientation. Common orientations are Z-cut, X-cut, Y-cut, and cuts with rotated angles of the previous axes.

Thin-film lithium niobate (e.g. for optical wave guides) can be transferred to or grown on sapphire and other substrates, using the smart cut (ion slicing) process or MOCVD process. The technology is known as lithium niobate on insulator (LNOI).

Nanoparticles of lithium niobate and niobium pentoxide can be produced at low temperature. The complete protocol implies a LiH induced reduction of NbCl₅ followed by in situ spontaneous oxidation into low-valence niobium nano-oxides. These niobium oxides are exposed to air atmosphere resulting in pure Nb₂O₅. Finally, the stable Nb₂O₅ is converted into lithium niobate LiNbO₃ nanoparticles during the controlled hydrolysis of the LiH excess. Spherical nanoparticles of lithium niobate with a diameter of approximately 10 nm can be prepared by impregnating a mesoporous silica matrix with a mixture of an aqueous solution of LiNO₃ and NH₄NbO(C₂O₄)₂ followed by 10 min heating in an infrared furnace.

Lithium niobate is used extensively in the telecommunications market, e.g. in mobile telephones and optical modulators. Due to its large electro-mechanical coupling, it is the material of choice for surface acoustic wave (SAW) devices. For some uses it can be replaced by lithium tantalate (LiTaO₃). Other uses are in laser frequency doubling, nonlinear optics, Pockels cells, optical parametric oscillators, Q-switching devices for lasers, other acousto-optic devices, optical switches for gigahertz frequencies, etc. It is an excellent material for manufacture of optical waveguides. It's also used in the making of optical spatial low-pass (anti-aliasing) filters. Additionally, it is used in pyroelectric infrared (IR) detectors, where it detects temperature changes by generating electric charges.