Barium titanate structure

Barium titanate [12047-27-7] has five crystalline modifications. Of these, the tetragonal form is the most important. The structure is based on comer-linked oxygen octahedra, within which are located the Ti4+ ions. These can be moved from their central positions either spontaneously or in an applied electric field. Each TiO8- octahedron may then be regarded as an electric dipole. If dipoles within a local region, ie, a domain, are oriented parallel to one another and the orientation of all the dipoles within a domain can be changed by the application of an electric field, the material is said to be ferroelectric. At ca 130°C, the Curie temperature, the barium titanate structure changes to cubic. The dipoles now behave independently, and the material is paraelectric (see Ferroelectrics). 

Structural binder A wide range of applications in electronics makes use of the plastics as a structural binder to hold active materials. For example, a plastic such as polyvinylidene fluoride is filled with an electroluminescent phosphor to form the dielectric element in electroluminescent lamps. Plastics are loaded with barium titanate and other high dielectric powders to make slugs for high K capacitors. The cores in high frequency transformers are made using iron and iron oxide powders bonded with a plastic and molded to form the magnetic core. 

Since BaTi40g and Na2TieOi3 have a pentagonal prism tunnel and a rectangular tunnel structure, respectively, we have paid attention to the role of the tunnel structures in photocatalysis. A barium titanate, Ba4Tii303o, in a series of Ba-Ti titanates was chosen as a representative with non-tunnel structure, and we have compared the photocatalytic activity and the ability for the production of surface radical species with uv irradiation between the tunnel and the non-tunnel titanates. 

FIGURE 9.16 Distortions of TiOe octahedra in (a) the tetragonal structure, (b) the orthorhombic, and (c) the rhombohedral structures of barium titanate. 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

The effect of k on d is most clearly demonstrated in the experiment by Fukada and Date (1970) on the polyester resin film, filled with powdered barium titanate and polarized under a d.c. field. The strong piezoelectricity, as shown in Fig. 29, is ascribed to the polarization charge of the ceramic filler and heterogeneous strain due to the composite structure. The real part d exhibits a maximum at 90° C and d" has a peak and a succeeding dip at this temperature where the primary relaxation of polyester resin occurs. The behavior of d and d" is quite similar to that of k and k" in Fig. 16, respectively, in which decreasing X = an corresponds to increasing temperature. 

Barium titanate, BaTi03, is probably the most widely studied ferroelectric oxide. Extensive studies were conducted on this compound during World War II in the United States, England, Russia, and Japan, but the results were not revealed until after the war. Barium titanium(IV) oxide was found to be a ferroelectric up to a temperature of 120°C., which is its Curie point. Above 120°C., barium titanium(IV) oxide has the cubic perovskite structure, and below this temperature the oxygen and titanium ions are shifted and result in a tetragonal structure with the c axis approximately 1% longer than the a axis. Below 0°C., the symmetry of barium titanate becomes orthorhombic, and below —90°C., it becomes trigonal. 

The dielectric constant of barium titanate, along [001] is about 200 and along [100] it is 4000 at room temperature.3 The spontaneous polarization at room temperature is 26 X 10-6 C./cm.2, and the value of the coercive field has been found to vary from 500 to 2000 volts/cm. The crystal structure of barium titanate at room temperature can be represented by a tetragonal unit cell with size of a0 = 3.992 A., and c0 = 4.036 A., but the symmetry becomes cubic above 120°C., at which temperature the crystals no longer exhibit ferroelectric properties. 

The number of course programmes is directly proportional to the demand made by trade and industry. Many factors have been of influence on this instruction, among others the Gibbs phase rule (see the chapter on Phase rule), X-ray diffraction to clarify the structure of solids and the development of synthetic barium titanate and other ceramic materials whose properties could be influenced by controlling composition and process conditions. As early as 1900 it became clear that the study of ceramics required much knowledge of other subjects, as appears from the Ohio State University s course programme of that year. 

In different temperature ranges, barium titanate BaTi03 exists in several stable phases. Table 10.4.2 lists the crystal data and properties of the polymorphic forms 0fBaTiO3, and Fig. 10.4.2 shows their structures. 

Barium titanate, the first ceramic material in which ferroelectric behaviour was observed, is the ideal model for a discussion of the phenomenon from the point of view of crystal structure and microstructure see also [10] and [11]. 

Multilayer capacitors A critical step in the manufacture of multilayer capacitors is, of course, the barium titanate-based starting powders, and the various routes for producing these are described in Section 3.4. The multilayer capacitor structure (Fig. 5.11) enables the maximum capacitance available from a thin dielectric to be packed into the minimum space in a mechanically robust form. 

Alkaline-Earth Titanates. Some physical properties of representative alkaline-earth titanates are listed in Table 15. The most important applications of these titanates are in the manufacture of electronic components (109). The most important member of the class is barium titanate, BaTi03, which owes its significance to its exceptionally high dielectric constant and its piezoelectric and ferroelectric properties. Further, because barium titanate easily forms solid solutions with strontium titanate, lead titanate, zirconium oxide, and tin oxide, the electrical properties can be modified within wide limits. Barium titanate may be made by, eg, cocalcination of barium carbonate and titanium dioxide at ca 1200°C. With the exception of Ba iO barium orthotitanate, titanates do not contain discrete TiO4 ions but are mixed oxides. Ba2Ti04 has the p-I SC structure in which distorted tetrahedral TiO4 ions occur. 

The vast growth in electronic equipment owes to capacitors which are essential in almost all the devices. Barium titanate forms the heart of the capacitors. The perovskite structure contains a small ion of high charge at the centre of an MC>6 octahedron. The high polarizability is the basis of a high dielectric constant in the capacitor. The addition of Nd to the mixed titanate gives a stable capacitance over a wide temperature range. 

Robins LH, Kaiser DL, Rotter LD, Schenck PK, Stauf GT, Rytz D (1994) Investigation of the structure of barium titanate thin films by Raman spectroscopy. J Appl Phys 76 7487... 

Barium titanate, which has many novel properties, is a mixed oxide ceramic. It has the same structure as the mineral perovskite, CaTiOs (Fig. 22.13), except, of course, that Ba replaces Ca. Perovskites typically have two metal atoms for every three O atoms, giving them the general formula ABO3, where A stands for a metal atom at the center of the unit cube and B stands for an atom of a different metal at the cube corners. 

FIG. 13.1. The crystal structures of barium titanate, BaTiOa (a) hexagonal, (b) cubic. 

The complex oxide BaTiOs ( barium titanate ) is remarkable in having five crystalline forms, of which three are ferroelectric. The structure of the high-temperature hexagonal form, stable from 1460°C to the melting point (1612°C), has already been described. The other forms are ... 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

Although correlation between parameters is a function of the data structure and has nothing to do with deficiencies in the model, it has implications for both the choice of the model and the design of the experiment. EVANS described his experiences with the determination of the crystal structure of tetragonal barium titanate (BaTiOa). The problem was ample in that it involved only three atomic positional parameters (one for Ti and two for 0), plus nine thermal parameters. There was considerable interest in the details of the structure because of the ferroelectric properties of the material. The proposed model was essentially a simple cubic arrangement of atoms, but with Ti displaced slightly from the center of an octahedron. By ordinary x-ray standards, this distortion (which was expected to be on the order of 0.15 A) could be measured with a standard error of 0.01-0.02 A if... 

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