Size effects in the crystal field of small dielectric particles of SrCl_{2}: Gd^{3+}

Abstract

A technique based on evaporation in a viscous inert gas flow has been used to produce ${\mathrm{Gd}}^{3+}$-doped Sr${\mathrm{Cl}}_{2}$ small particles whose average size varied from 100 to 500 \AA{}. Electron paramagnetic resonance (EPR) investigations of these samples have shown that: (i) No observable phase separation occurred during the evaporation of Gd-doped crystals. (ii) The ${\mathrm{Gd}}^{3+}$ impurity ions occupied predominantly cubic-symmetry sites in the Sr${\mathrm{Cl}}_{2}$ small particles. (iii) The "small-particle" EPR spectrum of ${\mathrm{Gd}}^{3+}$ is characterized by significant differences relative to the spectrum observed in either a single crystal or a "large-grain" powder specimen. Such differences take the form of changes in the relative positions and widths of the EPR transitions. For the small-particle samples, the shifts and linewidths of the $〈100〉$ shoulders in the EPR powder pattern were measured as a function of the average particle size. The observed shifts in the EPR lines are interpreted as arising from a contraction in the crystal lattice whose magnitude decreases with increasing particle size. This interpretation was confirmed by high-resolution electron-diffraction measurements. The heterogeneous broadening of the EPR lines was attributed to the effect of the particle size distribution as well as to internal strains and surface-related defects. By comparing the EPR and electron-diffraction results and using a simple "drop" model, it was possible to deduce a value of 0.435 N/m for the surface tension of Sr${\mathrm{Cl}}_{2}$ in the solid state. Similarly a power law for the ${b}_{4}$ spin-Hamiltonian parameter given by ${b}_{4}\ensuremath{\propto}{a}^{\ensuremath{-}n}$ with $n=18.7$ was determined. This exponent is about three times larger than the expected value. This work represents the first observation by EPR of crystallographic size effects in small dielectric particles.