Barium tetragonal

In general, the chemistry of inorganic lead compounds is similar to that of the alkaline-earth elements. Thus the carbonate, nitrate, and sulfate of lead are isomorphous with the corresponding compounds of calcium, barium, and strontium. In addition, many inorganic lead compounds possess two or more crystalline forms having different properties. For example, the oxides and the sulfide of bivalent lead are frequendy colored as a result of their state of crystallisation. Pure, tetragonal a-PbO is red pure, orthorhombic P PbO is yeUow and crystals of lead sulfide, PbS, have a black, metallic luster. 

Barium titanate [12047-27-7] has five crystaUine modifications. Of these, the tetragonal form is the most important. The stmcture is based on corner-linked oxygen octahedra, within which are located the Ti" " ions. These can be moved from their central positions either spontaneously or in an apphed electric field. Each TiO 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 appHcation of an electric field, the material is said to be ferroelectric. At ca 130°C, the Curie temperature, the barium titanate stmcture changes to cubic. The dipoles now behave independentiy, and the material is paraelectric (see Ferroelectrics). 

MCrFj (M = K, Rb, or NH ) have been isolated from aqueous media and shown to have tetragonal unit cells and to be paramagnetic with antiferromagnetism at very low temperatures. Sr2Cr"Cr Fj2 and related non-stoicheiometric phases with calcium and barium have been isolated and identified. ... 

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

An insoluble sulphate may be detected by the formation of turpeth mineral on the addition of a 10 per cent, solution of mercuric nitrate in dilute nitric acid (1 in 100) the small yellow tetragonal crystals of the basic sulphate may be identified microscopically. The reaction takes place immediately in the cold with calcium and mercurous sulphates, less readily with strontium and lead sulphates, whilst with barium sulphate boiling is necessary. ... 

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. 

A ferroelectric model material is barium titanate BaTi03. On cooling from high temperatures, the permittivity increases up to values well above 10,000 at the phase transition temperature Tc. The inverse susceptibility as well as the dielectric permittivity follows a Curie-Weiss law x1 f 1 oc (T — O). The appearance of the spontaneous polarization is accompanied with a spontaneous (tetragonal) lattice distortion. 

The phase transition in barium titanate is of first order, and as a result, there is a discontinuity in the polarization, lattice constant, and many other properties, as becomes clear in Figure 1.7. It is also clear in the figure that there are three phase transitions in barium titanate having the following sequence upon cooling rhombohedral, orthorhombic, tetragonal and cubic. 

The optical properties of ferroelectric materials are characterized by birefringence. Barium titanate is isotropic only in the cubic phase. The tetragonal and the rhombohedral phases are... 

Figure 9.1 Tetragonal tungsten-bronze structure of SiiBai-xNboOf, (Strontium barium nio-bate, SBN). Characteristic are the corner-linked niobium-oxygen-octahedra and 3 types of vacancies A,Bi,2,3 and C. Sr2+-ions are embedded in the A- and B2-positions and Ba2+ in the Bi-positions according to their ionic radii. The C-positions and B3-position remain empty.

The substitution of lead by barium enhances the piezoelectric properties which peak when Pb/Ba I. At this and higher barium contents the structure changes to tetragonal with the polar axis parallel to the A-site tunnels. There is a morphotropic boundary, similar to that found in PZT compositions, and peak values of piezoelectric properties are found near the Pb1/2Ba1/2Nb206 composition. d33 rises to 220 pCN-1 and d31 rises to — 90pCN 1, while Qm increases to 300 and the Curie point falls to 250 °C thus most of the features peculiar to... 

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