Barium titanate is the
inorganic compoundInorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the
chemical formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
BaTiO
3. Barium titanate is a white powder and transparent as larger crystals. This
titanateIn chemistry, titanate usually refers to inorganic compounds composed of titanium oxides. In some cases, the term is used more generally for any titanium-containing anion, e.g. [TiCl6]2- and [Ti7]2-. This article focuses on the oxides....
is a ferroelectric
ceramicA ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
material, with a
photorefractive effectThe photorefractive effect is a nonlinear optical effect seen in certain crystals and other materials that respond to light by altering their refractive index....
and piezoelectric properties.
Structure
The solid can exist in five phases, listing from high temperature to low temperature: hexagonal,
cubicIn crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
,
tetragonalIn crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
,
orthorhombicIn crystallography, the orthorhombic crystal system is one of the seven lattice point groups. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a rectangular base and height , such that a,...
, and rhombohedral
crystal structureIn mineralogy and crystallography, crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a pattern, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
. All of the phases exhibit the ferroelectric effect except the cubic phase. The high temperature cubic phase is easiest to describe, consisting of octahedral TiO
6 centres that define a cube with Ti vertices and Ti-O-Ti edges. In the cubic phase, Ba
2+ is located at the center of the cube, with a nominal
coordination numberIn chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
of 12. Lower symmetry phases are stabilized at lower temperatures, associated with the movement of the Ba
2+ to off-center position. The remarkable properties of this material arise from the cooperative behavior of the Ba
2+ centres.
Production and handling properties
Barium titanate can be manufactured by heating
barium carbonateBarium carbonate , also known as witherite, is a chemical compound used in rat poison, bricks, ceramic glazes and cement.Witherite crystallizes in the orthorhombic system...
and
titanium dioxideTitanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
. The reaction proceeds via liquid phase sintering. Single crystals can be grown around 1100 °C from molten
potassium fluoridePotassium fluoride is the chemical compound with the formula KF. After hydrogen fluoride, KF is the primary source of the fluoride ion for applications in manufacturing and in chemistry. It is an alkali metal halide and occurs naturally as the rare mineral carobbiite...
. Other materials are often added for doping, e.g. to give solid solutions with
strontium titanateStrontium titanate is an oxide of strontium and titanium with the chemical formula SrTiO3. At room temperature, it is a centrosymmetric paraelectric material with a perovskite structure...
. Reacts with
nitrogen trichlorideNitrogen trichloride, also known as trichloramine, is the chemical compound with the formula NCl3. This yellow, oily, pungent-smelling liquid is most commonly encountered as a byproduct of chemical reactions between ammonia-derivatives and chlorine .In pure form, NCl3 is highly reactive...
and produces a greenish or grey mixture, the ferroelectric properties of the mixture are still present in this form.
Much work has been dedicated to its morphology. Fully dense nanocrystalline barium titanate has 40% higher
permittivityIn electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
than the same material prepared in classic ways. The addition of inclusions of barium titanate to
tinTin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
has been shown to produce a bulk material with a higher viscoelastic
stiffnessStiffness is the resistance of an elastic body to deformation by an applied force along a given degree of freedom when a set of loading points and boundary conditions are prescribed on the elastic body.-Calculations:...
than that of diamonds. Barium titanate goes through two phase transitions that change the crystal shape and volume. This phase change leads to composites where the barium titanates have a negative bulk modulus (
Young's modulusYoung's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
), meaning that when a force acts on the inclusions, there is displacement in the opposite direction, further stiffening the composite.
Like many
oxideAn oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s, barium titanate is insoluble in water but attacked by
sulfuric acidSulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
. Its bulk room-temperature bandgap is 3.2 eV, but it increases to ~3.5 eV when the particle size is reduced from about 15 to 7 nm.
Uses
Barium titanate is a
dielectricA dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
ceramic used for
capacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s. It is a piezoelectric material for
microphoneA microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s and other
transducerA transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s. The spontaneous polarization of barium titanate is about 0.15 C/m
2 at room temperature and its
Curie pointIn physics and materials science, the Curie temperature , or Curie point, is the temperature at which a ferromagnetic or a ferrimagnetic material becomes paramagnetic on heating; the effect is reversible. A magnet will lose its magnetism if heated above the Curie temperature...
is 120 °C. As a piezoelectric material, it was largely replaced by
lead zirconate titanateLead zirconate titanate , also called PZT, is a ceramic perovskite material that shows a marked piezoelectric effect. PZT-based compounds are composed of the chemical elements lead and zirconium and the chemical compound titanate which are combined under extremely high temperatures. A filter is...
, also known as PZT. Polycrystalline barium titanate displays positive
temperature coefficientThe temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
, making it a useful material for
thermistorA thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
s and self-regulating electric heating systems.
Barium titanate crystals find use in
nonlinear opticsNonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
. The material has high beam-coupling gain, and can be operated at visible and near-infrared wavelengths. It has the highest reflectivity of the materials used for self-pumped phase conjugation (SPPC) applications. It can be used for continuous-wave
four-wave mixingFour-wave mixing is an intermodulation phenomenon in optical systems, whereby interactions between 3 wavelengths produce a 4th wavelength in the signal. It is similar to the third-order intercept point in electrical systems...
with milliwatt-range optical power. For photorefractive applications, barium titanate can be doped by various other elements, e.g.
ironIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
.
Thin filmA thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
s of barium titanate display
electrooptic modulationElectro-optic modulator is an optical device in which a signal-controlled element displaying electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the modulated beam...
to frequencies over 40 GHz.
The pyroelectric and ferroelectric properties of barium titanate are used in some types of uncooled sensors for thermal cameras.
High purity barium titanate powder is reported to be a key component of new barium titanate capacitor energy storage systems for use in electric vehicles.
Natural occurrence
Barioperovskite is a very rare natural analogue of BaTiO
3, found as microinclusions in
benitoiteBenitoite is a rare blue barium titanium silicate mineral, found in hydrothermally altered serpentinite. Benitoite fluoresces under short wave ultraviolet light, appearing bright blue to bluish white in color. The more rarely seen clear to white benitoite crystals fluoresce red under long-wave UV...
.
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