Size effects in the crystal field of small dielectric particles of SrCl2:Gd3+

A technique based on evaporation in a viscous inert gas flow has been used to produce Gd3+-doped SrCl2 small particles whose average size varied from 100 to 500 A. 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 Gd3+ impurity ions occupied predominantly cubic-symmetry sites in the SrCl2 small particles. (iii) The ''small-particle'' EPR spectrum of Gd3+ 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 SrCl2 in the solid state. Similarly a power law for the b4 spin-Hamiltonian parameter given by b4 proportional a/sup -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