Study of the symmetry of Fe³⁺ sites in SnO₂ by electron paramagnetic resonance

Abstract

Electron paramagnetic resonance (EPR) of Fe³⁺ in SnO₂ has been realized in a natural single crystal of cassiterite at 9.55 GHz (X-band) and at 34.40 GHz (Q-band). Spectra show the simultaneous presence of four groups of independent signals, each one typical of the immediate environment of a specific paramagnetic iron. Fe³⁺ always substitutes Sn⁴⁺ in an octahedral site. The four paramagnetic centers are due to four different charge compensation mechanisms. The spin Hamiltonian constant values for the SN center and I1 center confirm the former results of the authors about for these two centers. SN and I1 present a weak deviation from axial symmetry. The first preserves the crystallographic local symmetry of the tin site and the second shows a symmetry deviation of 0.6° probably due to the presence of an OH group in the coordination polyhedron. On the other hand, for the Sd1 center and mostly for the Sd2 center, never previously subjected to single crystal EPR measurements, the study of spectra symmetry and the determination of B ⁰₂ and B ²₂ constants produced new data. The Sd1 center could be due to a relaxation of the lattice together with a non local charge compensation mechanism. The Sd2 center presents a strong deviation from axial symmetry with mm local symmetry coordination due to coupling of Fe³⁺ and Nb⁵⁺. This coupling is proven by EPR studies of synthetic cassiterites doped with iron and niobium.