Resonant piezoelectric spectroscopy shows polar resonances in paraelectric SrTiO 3 at temperatures below 80 K. These resonances become strong at T < 40 K. The resonances are induced by weak electric fields and lead to standing mechanical waves in the sample. This piezoelectric response does not exist in paraelastic SrTiO 3 nor at temperatures just below the ferroelastic phase transition. The interpretation of the resonances is related to ferroelastic twin walls which become polar at low temperatures in close analogy with the known behavior of CaTiO 3 . SrTiO 3 is different from CaTiO 3 , however, because the wall polarity is thermally induced; i.e., there exists a small temperature range well below the ferroelastic transition point at 105 K where polarity appears on cooling. As the walls are atomistically thin, this transition has the hallmarks of a two-dimensional phase transition restrained to the twin boundaries rather than a classic bulk phase transition.

A quadrupole coupling induced 47Ti and 49Ti satellite background which transforms into well-defined satellite lines below T(c) in the ferroelectric phase has been observed in the cubic phase of an ultrapure BaTiO3 single crystal. The results demonstrate the coexistence of a displacive and order-disorder component in the phase transition mechanism and tetragonal breaking of the cubic symmetry due to biased Ti motion between off-center sites in the paraelectric phase above T(c).

An overview of recognized structural defects, impurities and related trapping levels and their role in the scintillation mechanism is provided and discussed in single crystal materials belonging to tungstate, Ce‐ or Pr‐doped aluminum perovskite, garnet and finally to Ce‐doped silicate scintillators. New achievements and open problems in deeper understanding of electron and hole self‐trapping phenomena and of the nature of defects in the crystal structure and their ability to localize migrating charge carriers are indicated. Fast optical ceramics and nanocomposite materials are pointed out as possible future advanced scintillators. Such novel technologies can in principle explore materials which are not available in the bulk single crystal form, but their figure‐of‐merit is dramatically dependent on the surface‐interface defect states and related trapping and nonradiative recombination phenomena. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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