The ferroelastic domain structure of NdGaO_{3} is analyzed by three theories: group symmetry, tensor method and twinning by pseudosymmetry. The theories provide a coherent description of domain pairs and a determination of the position of W and S walls.
Absorption spectra of SrLaAlO_{4} and SrLaGaO_{4} single crystals are investigated as well as their changes after irradiation by γ-quanta (^{60}Co source). Both crystals reveal a similar behaviour that is caused, as it seems, by the same nature of colour centres in them. The conclusion that oxygen defects as well as Fe impurities play a significant role in the colour centres formation in these crystals.
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