The limits (temperature-pressure) of the phase transitions of the different rare earth oxide (as R203 and RO2) compounds are still the subject of a large amount of discussion. In the case of thin films, these phenomena are increased overall, when such materials are irradiated by an electron beam or when they are widely contaminated by water vapour. For praseodymium and neodymium the formation of stable B-Pr203 and B-Nd203 crystals throughout an A-R203 matrix and, in like manner, the epitaxy of the B-Tb203 and TbO2 phases, appear as very unexpected phenomena. These facts are tentatively explained as a consequence of acoustic elastic wave (as shock-waves) generation which could be compared to high-pressure phenomena.