The Raman spectra of tetragonal TbPO4 (zircon-type I41/amd structure, D4h19 space group) have been measured at ambient conditions and under variable pressure up to 15.5 GPa inside a diamond anvil cell (DAC). Assignment of the Raman active modes of the tetragonal phase has been carried out based on polarized measurements from a single oriented crystal of TbPO4 at ambient conditions. The abrupt Raman mode discontinuities and the appearance of numerous new Raman peaks at a pressure Pc≈9.5 GPa have provided strong evidence for a first-order phase transition to a lower crystal symmetry, most likely monoclinic. The high-pressure structure appears to be more compact compared to the tetragonal one, and is retained upon bringing the crystal to ambient pressure.

We report site-selective oxygen isotope ( 16 O/ 18 O) effect ͑OIE͒ measurements on the in-plane penetration depth ab in underdoped Y 0.6 Pr 0.4 Ba 2 Cu 3 O 7Ϫ␦ , using the muon-spin rotation technique. A pronounced OIE on the transition temperature T c as well as on ab Ϫ2 (0) was observed, which mainly arises from the oxygen sites within the superconducting CuO 2 planes ͑100% within error bar͒. The values of the corresponding relative isotope shifts were found to be ⌬T c /T c ϭϪ3.7(4)% and ⌬ ab Ϫ2 (0)/ ab Ϫ2 (0)ϭϪ6.2(1.0)%. Our results imply that in this compound the phonon modes involving the movement of planar oxygen are dominantly coupled to the electrons.

We report low-temperature and hydrostatic pressure Raman measurements in Pr 0.5 Ca 0.5 MnO 3 thin films grown on SrTiO 3 and LaAlO 3 substrates. From the low-temperature dependence of the modes related with the Jahn-Teller distortions of the octahedra, it is found that an increase appears in their relative intensities at temperatures close to the charge-ordering phase transition. Two rotational modes of vibration of the octahedra show an anharmonic behavior. Strains from the substrate do not induce any shift or new modes in the Raman spectra probably due to their relaxation from the interface towards the surface of the films. From the different orientation of the two films, the symmetry of certain modes has been identified.

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