Anharmonicity Effects in Nanocrystals Studied by Raman Scattering Spectroscopy

Abstract

Phonon-phonon interactions were investigated in various nanocrystalline powders like anatase TiO 2−δ , pure CeO 2−δ and ceria doped with Nd(Gd) analyzing temperature dependent Raman spectra of these systems. Phonon confinement model based on size, inhomogeneous strain and anharmonic effects was used to properly describe the evident changes present in the Raman spectra of pure and doped ceria nanocrystalline samples. In small particles of pure and doped ceria nanocrystals, when size effects have minor impact on Raman modes, four phonon anharmonic processes prevail under the three-phonon ones. When nanopowdered particles are grown enough size effects provoke changes of the anharmonic interactions when three-phonon coupling prevails over the four-phonon anharmonic processes. In nanocrystalline anatase TiO 2 evident blueshift of the most prominent E g Raman mode probably originates from dominant four-phonon anharmonic interactions.

Investigations of anharmonic effects via phonon mode variations in nanocrystalline Dy2O3, Gd2O3 and Y2O3

“…It is well known that the variations/shifts in the normal modes with temperature at constant pressure arise either from pure volume contribution, i.e from thermal expansion, or from pure‐temperature contribution, i.e. from cubic and quartic anharmonicities, also known as self‐energy shift, i.e. …”

“…It has also been reported that owing to the nanosized particles, the core and shell of the sample may have varying symmetries . It has also been reported that the anharmonic constants related to the peak widths and peak positions are higher in nanocrystals than in the bulk, implying a greater degree of anharmonicity in nanocrystals . Very few experiments have been carried out to investigate the temperature dependence of Raman mode positions and line widths in nanocrystals …”

Raman scattering of rare earth sesquioxide Ho2O3: A pressure and temperature dependent study

“…This shift of frequency with temperature has contribution from the thermal expansion of the lattice and anharmonic phonon-phonon interaction. The phonon frequency shift as a function of temperature can be expressed as, 34,35 xðTÞ ¼ x 0 þ ðDxÞ latt þ ðDxÞ anh ;…”

The role of lattice oxygen in CO oxidation over Ce18O2-based catalysts revealed under operando conditions

“…The amount of water content is significantly higher in case of NCE18 O and NCG18 O compared to the respective bulk samples.Figure 2 A. F2g Raman mode position for Ce 16 O2 with 81 nm (red solid line) and Ce 18 O2 with 81 nm (blue solid line) obtained at ambient temperature with an incident wavelength of λ = 473 nm. from temperature-induced volume expansion and anharmonicity effects are thus minimized 52,53. 2Bas a function of temperature for CeO2 with 81 nm (red line) and nano-CeO2 with 9 nm grain size (blue line).…”