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Barium titanate

Barium titanate

Overview
Barium titanate is the inorganic compound
Inorganic compound
Inorganic 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 formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 BaTiO3. Barium titanate is a white powder and transparent as larger crystals. This titanate
Titanate
In 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 ceramic
Ceramic
A 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 effect
Photorefractive effect
The 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.



The solid can exist in five phases, listing from high temperature to low temperature: hexagonal, cubic
Cubic crystal system
In 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....

, tetragonal
Tetragonal crystal system
In 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...

, orthorhombic
Orthorhombic crystal system
In 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 structure
Crystal structure
In 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.
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Encyclopedia
Barium titanate is the inorganic compound
Inorganic compound
Inorganic 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 formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....

 BaTiO3. Barium titanate is a white powder and transparent as larger crystals. This titanate
Titanate
In 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 ceramic
Ceramic
A 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 effect
Photorefractive effect
The 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, cubic
Cubic crystal system
In 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....

, tetragonal
Tetragonal crystal system
In 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...

, orthorhombic
Orthorhombic crystal system
In 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 structure
Crystal structure
In 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 TiO6 centres that define a cube with Ti vertices and Ti-O-Ti edges. In the cubic phase, Ba2+ is located at the center of the cube, with a nominal coordination number
Coordination number
In 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 Ba2+ to off-center position. The remarkable properties of this material arise from the cooperative behavior of the Ba2+ centres.

Production and handling properties


Barium titanate can be manufactured by heating barium carbonate
Barium carbonate
Barium 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 dioxide
Titanium dioxide
Titanium 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 fluoride
Potassium fluoride
Potassium 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 titanate
Strontium titanate
Strontium 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 trichloride
Nitrogen trichloride
Nitrogen 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 permittivity
Permittivity
In 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 tin
Tin
Tin 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 stiffness
Stiffness
Stiffness 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 modulus
Young's modulus
Young'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 oxide
Oxide
An 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 acid
Sulfuric acid
Sulfuric 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 dielectric
Dielectric
A 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 capacitor
Capacitor
A 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 microphone
Microphone
A 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 transducer
Transducer
A 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/m2 at room temperature and its Curie point
Curie point
In 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 titanate
Lead zirconate titanate
Lead 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 coefficient
Temperature coefficient
The 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 thermistor
Thermistor
A 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 optics
Nonlinear optics
Nonlinear 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 mixing
Four-wave mixing
Four-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. iron
Iron
Iron 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 film
Thin film
A 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 modulation
Electro-optic modulator
Electro-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 BaTiO3, found as microinclusions in benitoite
Benitoite
Benitoite 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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