Archetypal perovskite

Another important group of oxide materials with a very low electrical conductivity is the oxide dielectrics. A number of these are based upon the perovskites, MXO3 or M0 X02. The archetype of these materials is BaTiC>3, which has a high dielectric constant, or relative permittivity to vacuum, the value at room temperature being 1600, and commercial use is made of the isostructural PbTi(>3 and ZrTi03 which form solid solutions, the PZT dielectrics. These materials lose their dielectric properties as the temperature... 

The geometric relaxation described in Section 12.3.1 occurs by redistributing the bond valence between the bonds until GII and BSI both have acceptable values, but in some cases this relaxation is restricted by symmetry. In the case of per-ovskite, the cubic symmetry of the archetypal ABO3 structure (Fig. 10.4) does not allow any of the bonds to relax unless the symmetry is lowered. Thus true cubic perovskites are rare since they can only exist if the A and B ions are exactly the right size. Most perovskites have a reduced symmetry that allows the bonds to relax. For compounds in which the A-O bonds are stretched, the relaxation takes the form of a rotation of the BOg octahedra and results in a reduction of the coordination number of A. The various relaxed structures based on different expected coordination numbers were modelled in Section 11.2.2.4. 

The structure of perovskite type is the archetype of numerous synthetic and natural compounds with stoichiometry ABX and their derivative structures. The latter are formally obtained from the archetype through mechanisms such as anion or... 

Fig. 2.2 Archetypical structures of the cubic perovskite, ABO3, lejf) and K2Nip4, A2BO4, (right). Green balls large A cations, blue octahedra of small B cation in the center surrounded by six oxide ions...

The simplest conceptual way in which an insulating perovskite such as the archetypal alkaline earth titanates CaTiOj, SrTiOj and BaTiOj can be turned into a conductor is to introduce electrons into the empty 3d tj band formed from the orbitals on the TF" cations. This will occur if the compound can be appropriately doped with aliovalent ions or alternatively reduced chemically. 

SrTiOs is an archetype of the cubic form. A layered structure is obtained for the composition Sr3Ti207, which is built upon blocks of double perovskite slabs shifted to make SrO layers in between [7]. Another structure, Sr4Ti30io, exists with three perovskite slabs [8]. In general, these Ruddlesden-Popper phases can be described with the general formula A2[A iB 03 +i], where n is the thickness of the perovskite slabs (Figure 8.4). The = 1 member also refers to the K2Nip4 structure. 

In order to better understand the influence of chemical variations on the perovskite lattice, the unit cell parameters of the three orthorhombic perovskite series are compared by means of the cell distortion factor d, a useful tool for estimating the departure from the ideal cubic perovskite archetype... 

Perovskite oxide materials possess the general stoichiometry ABO3. Conventionally, the A cation is larger than the B cation. In the archetype, the A cation has an oxidation state of -I- 2 and the B cation has the oxidation state -1- 4. These materials comprise three different ionic species, each with its own equilibrium defect concentration due to three different activation energies for defect formation, which, combined with the constraint of electroneutrality, make for diverse and potentially useful defect chemistry, particularly when considering electronic, hole, and ionic conduction under atmospheres of different oxygen partial pressures [13]. 

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