Structure of cubic BaTiO3. The red spheres are oxide centres, blue are Ti4+ cations, and the green spheres are Ba2+.
The solid exists in one of four polymorphs depending on temperature. From high to low temperature, these crystal symmetries of the four polymorphs are cubic, tetragonal, orthorhombic and rhombohedralcrystal structure. All of these phases exhibit the ferroelectric effect apart from the cubic phase. The high temperature cubic phase is easiest to describe, as it consists of regular corner-sharing octahedral TiO6 units that define a cube with O vertices and Ti-O-Ti edges. In the cubic phase, Ba2+ is located at the center of the cube, with a nominal coordination number of 12. Lower symmetry phases are stabilized at lower temperatures and involve movement of the Ti4+ to off-center positions. The remarkable properties of this material arise from the cooperative behavior of the Ti4+ distortions.[3]
Above the melting point, the liquid has a remarkably different local structure to the solid forms, with the majority of Ti4+ coordinated to four oxygen, in tetrahedral TiO4 units, which coexist with more highly coordinated units.[4]
Production and handling properties
Scanning Electron Microscopy (SEM) images showing particles of BaTiO3. The different morphologies depend on the synthesis conditions (precipitation, hydrothermal and solvothermal synthesis): size and shape can be varied by changing the concentration of precursors, the reaction temperature and the time. Color (if added) helps to emphasize the grayscale levels. In general, the synthesis of Barium titanate by precipitation from aqueous solution allows to produce particles with spherical shape with size that can be tailored from a few nanometers to several hundred nanometers by decreasing the concentration of reactants. At very low concentration the particles have the tendency to develop a dendritic-like morphology, as reported in the images.
Much effort has been spent studying the relationship between particle morphology and its properties. Barium titanate is one of the few ceramic compounds known to exhibit abnormal grain growth, in which large faceted grains grow in a matrix of finer grains, with profound implications on densification and physical properties.[7] Fully dense nanocrystalline barium titanate has 40% higher permittivity than the same material prepared in classic ways.[8] The addition of inclusions of barium titanate to tin has been shown to produce a bulk material with a higher viscoelasticstiffness 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), meaning that when a force acts on the inclusions, there is displacement in the opposite direction, further stiffening the composite.[9]
Like many oxides, barium titanate is insoluble in water but attacked by sulfuric acid. It is also soluble in concentrated hydrochloric acid, and hydrofluoric acid.[10] Its bulk room-temperature bandgap is 3.2 eV, but this increases to ~3.5 eV when the particle size is reduced from about 15 to 7 nm.[1]
Uses
Scanning transmission electron microscopy of the ferroelastic domains that form in BaTiO3 on cooling through the Curie temperature. The vertex point, where domain bundles meet, moves from the center in isometric crystals (top) to off-center in oblongs (bottom).[11]
Barium titanate is a dielectric ceramic used in capacitors, with dielectric constant values as high as 7,000. Over a narrow temperature range, values as high as 15,000 are possible; most common ceramic and polymer materials are less than 10, while others, such as titanium dioxide (TiO2), have values between 20 and 70.[12]
It is a piezoelectric material used in microphones and other transducers. The spontaneous polarization of barium titanate single crystals at room temperature range between 0.15C/m2 in earlier studies,[13] and 0.26C/m2 in more recent publications,[14] and its Curie temperature is between 120 and 130 °C. The differences are related to the growth technique, with earlier flux grown crystals being less pure than current crystals grown with the Czochralski process,[15] which therefore have a larger spontaneous polarization and a higher Curie temperature.
As a piezoelectric material, it has been largely replaced by lead zirconate titanate, also known as PZT. Polycrystalline barium titanate has a positive temperature coefficient of resistance, making it a useful material for thermistors and self-regulating electric heating systems.
Barium titanate crystals find use in nonlinear optics. 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 with milliwatt-range optical power. For photorefractive applications, barium titanate can be doped by various other elements, e.g. iron.[16]
The pyroelectric and ferroelectric properties of barium titanate are used in some types of uncooled sensors for thermal cameras.
Barium titanate is widely used in thermistors and positive temperature coefficient heating elements. For these applications, barium titanate is manufactured with dopants to give the material semiconductor properties. Specific applications include overcurrent protection for motors, ballasts for fluorescent lights, automobile cabin air heaters, and consumer space heaters.[18][19]
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.[20]
^Nyutu, Edward K.; Chen, Chun-Hu; Dutta, Prabir K.; Suib, Steven L. (2008). "Effect of Microwave Frequency on Hydrothermal Synthesis of Nanocrystalline Tetragonal Barium Titanate". The Journal of Physical Chemistry C. 112 (26): 9659. CiteSeerX10.1.1.660.3769. doi:10.1021/jp7112818.
^Shieh, J.; Yeh, J. H.; Shu, Y. C.; Yen, J. H. (2009-04-15). "Hysteresis behaviors of barium titanate single crystals based on the operation of multiple 90° switching systems". Materials Science and Engineering: B. Proceedings of the joint meeting of the 2nd International Conference on the Science and Technology for Advanced Ceramics (STAC-II) and the 1st International Conference on the Science and Technology of Solid Surfaces and Interfaces (STSI-I). 161 (1–3): 50–54. doi:10.1016/j.mseb.2008.11.046. ISSN0921-5107.
^Godefroy, Geneviève (1996). "Ferroélectricité". Techniques de l'Ingénieur Matériaux Pour l'Électronique et Dispositifs Associés (in French). base documentaire : TIB271DUO. (ref. article : e1870).
