Lattice contraction in small particles of SrCl 2: Gd 3+: explicit size effect in EPR and Raman spectroscopy

Abstract

Under hydrostatic pressure, electron paramagnetic resonance (EPR) and Raman spectroscopy experiments have been performed on both the bulk and small particles of SrCl 2: Gd 3+. The measurements of the pressure dependence of the fourth-order crystal-field parameter b 4 in EPR and the Raman frequency ω, up to 6.2 and 2 kbar respectively, allow us to separate the size dependence of b 4 and ω in two contributions: the first, called implicit, is due to the lattice contraction as measured by high-resolution electron diffraction, the second, called explicit, is due to the direct influence of the size at constant volume of the unit cell. Our interpretation shows that the size must be considered as a new thermodynamic variable like pressure and temperature. In both studies, the explicit effect represents a similar fraction of the total size effect (60 and 50% in EPR and Raman spectroscopy respectively). This shows that it must be interpreted in the same way. In view of the observed size dependence on the Debye-Waller coefficient in small metallic particles, an interpretation is proposed in which the vibrational amplitude of the ions is treated as size dependent. The EPR results for a 100 Å particle yield a vibrational amplitude twice that of the bulk. The estimated effect of this increase on the Raman frequency is in good agrement with the explicit size effect deduced from the Raman spectroscopy experiments.