Raman Fingerprint of Pressure-Induced Phase Transition in SnO₂ Nanoparticles: Grüneisen Parameter and Thermal Expansion

Pressure-induced phase transition studies in nanomaterials are important to comprehend thermodynamics at the nanoscale. Raman spectroscopic studies at high pressure in a diamond anvil cell were performed up to 40 GPa on rutile tetragonal phase of SnO 2 nanoparticles (NPs) of sizes ∼2, 4, and 25 nm to investigate their phase stability and phonon anharmonicity. In 25 nm NPs, evidence of phase transitions was observed at ∼11 and ∼24 GPa, 4 nm NPs indicated a cubic phase transition ∼21 GPa, and the 2.4 nm quasi-nanocrystals were found to stable up to 30 GPa. Raman spectra down to 90 K indicated that phonons of 2.4 nm NPs were more anharmonic. The analysis of total Raman intensity with increasing pressure suggested propagation of disorder from the surface to the central core of the NPs under pressure. Pressure-induced effects on 25, 4, and 2.4 nm NPs reduced their average diameters to 6.4 ± 2.6, 4.04 ± 1.36, and 3.85 ± 0.9 nm, respectively. Using Raman mode Grüneisen parameters γ j, the thermal expansion coefficient α of the 25, 4, and 2.4 nm SnO 2 NPs at 300 K was estimated as 1.674 × 10 –6, 1.178 × 10 –6, and 1.690 × 10 –6 K –1, respectively.