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This manuscript reports temperature-dependent Raman studies on nanocrystalline lutetium sesquioxide for a temperature range of 80–420 K. Phase stability under pressure was also investigated up to a pressure of about 15.6 GPa, primarily to deduce the mode Grüneisen parameters, which were further used to estimate the anharmonic parameters under the influence of varying sample temperature. The characterization at ambient revealed the cubic phase and nanocrystalline nature of the sample. The variation in the Raman shift and FWHM studied with an increase in temperature revealed that a few new peaks started developing above about 220 K, which were, unexpectedly, found to be due to the cubic phase of the material. On the other hand, it was observed that with an increase in pressure, the cubic phase peaks appear to be destabilized and new broad bands started to develop. However, until the highest studied pressure of 15.6 GPa, a defined structural phase transition was not observed. The estimated anharmonic constants demonstrated the predominance of the three phonon process in phonon decay as a function of temperature.