Morphotropic changes

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... 

Finally, it is clear that many more solid solutions must be examined for minerals and mineral analogs in order to achieve a fundamental understanding of compositionally induced phase transitions. Currently it is not possible to predict how transition temperatures will change when a particular impurity substitutes in a mineral structure, nor can we predict the interaction length for that impurity in the mineral. Landau-Ginzburg analysis provides an ideal framework for comparing the character of phase transitions that are activated by different variables (temperature, pressure, composition), and future studies of this type will lay an empirical foundation from which the detailed character of morphotropic transitions in minerals may be inferred. 

E. Parthe, R.S. Roth, R.S. Berry, R. Metselaar and S.C. Abrahams, Acta Crystallogr., Sect. A, 54, 1028-1033 (1998). Nomenclature of magnetic, incommensurate, composition-changed morphotropic, polytype, transient-structural and quasicrystalline phases undergoing phase transitions, J.-C. Toledano, R.S. Berry, P.J. Brown, A.M. Glazer, R. Metselaar, D. Pandey, J.M. Perez-Mato, R.S. Roth and S.C. Abrahams, Acta Crystallogr., Sect A, 57, 614-626 (2001), and erratum in Acta Crystallogr., Sect. A, 58, 79 (2002). 

The most favourable concentration for the formation of intermetallic compoimds is 33at.% of R (RM2-RGe2), particularly the composition RMGe. The compounds of equatomic composition crystallize in 20 structure types. The morphotropic transformation within the RMGe series occurs both if R is replaced by another R, and when the M component is changed. 

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