Insulating perovskites

Insulating perovskites are generally described as dielectrics (a term used to characterise a polarisable insulator), piezoelectrics, pyroelectrics and ferroelec-trics. These names (which are general and not confined to perovskites) describe the response of the material, which is always an electric polarisation, to an applied external stimulus. In a dielectric, for example, the stimulus is an applied electric field, and the response is an electric polarisation of the material. Both the response and the stimulus must be described with respect to the crystal structure of the material, as vectors. Thus, for a dielectric, the response of the material, the electric polarisation, needs to be characterised by a vector P, and the stimulus, the applied electric field, needs to be specified as a vector E, both described with respect to the crystal structure of the perovskite. In a cubic crystal P is proportional and parallel to E, but for most crystals this is not true and the relationship between these two vectors needs to be described in tensor notation. 

There is a well-established hierarchy of relationships between these insulating properties and the crystal structure of the phase. All insulating perovskites 

Perovskites Structure-Property Relationships, First Edition. Richard J. D. Tilley. 2016 John Wiley Sons, Ltd. Published 2016 by John WUey Sons, Ltd. 

10 point groups with no unique polar axis  

P Polarization E Applied electric field (T Applied mechanical stress Pg Spontaneous polarization  

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Another field of intensive research is the insulating perovskite alloys with exceptional dielectric and piezoelectric properties [74], like the so-called relaxor ferroelectric alloys PZT (PbZrxTii-xOs), PZN-PT (Pb(Zni/3Nb2/3)03-... 

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. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

The most important of these are perovskite structure solids with a formula A2+b4+o3 that can be typified by BaCeC>3 and BaZrCV The way in which defects play a part in H+ conductivity can be illustrated by reference to BaCeCV BaCeC>3 is an insulating oxide when prepared in air. This is converted to an oxygen-deficient phase by doping the Ce4+ sites with trivalent M3+ ions (Sections 8.2 and 8.6). The addition of the lower valence ions is balanced by a population of vacancies. A simple substitution reaction might be formulated ... 

The compound NCu02 (N = Ca0 86Sr014) is an insulator, but its structure, which is a simple defect perovskite made of layers (CuOa) sandwiched between layers (N), can be considered as the parent structure of a large family of superconductors. The sequence. ..(Cu0 )OiC(N)e o(Cu02)OjC. .. is in fact one of the building blocks of many compounds considered in this review. The refined parameters for NCu02 are given in Table 3. 

Fig. 13.3. The phase diagram of Ao.33A o.67Mn03 (A = divalent cation, A = rare earth) as a function of temperature and the global instability index of the idealized perovskite structure. The points show the observed transition temperatures in various compounds. FMM = ferromagnetic metal, PMI = paramagnetic insulator, FMI = ferromagnetic insulator (from Rao et al. 1998).

Our work has applied these techniques to the study of the binary insulating materials including the fluorites, alkali halides, alkaline earth oxides, and perovskites. Many of these are simple materials that are commonly used as models for all solid state defect equilibria. Our work has had the goal of determining at the microscopic level the defect equilibria and dynamics that are important in understanding solid state chemistry as well as developing new tools for the studies of solid materials. 

The extensive variety of properties that these compounds show is derived from the fact that around 90% of the metallic natural elements of the periodic table are known to be stable in a perovskite-type oxide structure [74], Besides, the possibility of synthesizing multicomponent perovskites by partial substitution of cations in positions A and B gives rise to substituted compounds with a formula A, A B,. B 03 ft. The resulting materials can be catalysts, insulators, semiconductors, superconductors, or ionic conductors. 

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