Mössbauer Hyperfine Interactions in Tetrahedral Fe(III) Ions

Abstract

The ⁵⁷Fe Mössbauer spectra have been measured for a series of compounds containing tetrahedral FeX₄⁻ anions, where X was Cl⁻, Br⁻, and NCO⁻. Two of them became antiferromagnets at low temperatures: NEt₄FeCl₄ ${\mathrm{(T}}_N\text{\hspace{0.17em}=\hspace{0.17em}3.0°}\mathrm{K})$ and NEt₄FeBr₄ ${\mathrm{(T}}_N\text{\hspace{0.17em}=\hspace{0.17em}3.9°}\mathrm{K})$ . In an external magnetic field the latter compound showed a spin flop. The remainder were paramagnetic and magnetic hyperfine interactions were observed by applying a large magnetic field at low temperatures. The hyperfine field, ${\mathrm{|\hspace{0.17em}H}}_n\mathrm{\hspace{0.17em}|}$ , was approximately independent of the cation R⁺ for FeCl₄⁻ and was about 470 kG. For the other ligands ${\mathrm{|\hspace{0.17em}H}}_n\mathrm{\hspace{0.17em}|}$ decreased with increasing covalency of the Fe–X bond, being 420 and 394 kG for X = Br⁻ and NCO⁻. The values of the isomer shift (approximately 0.3 mm/sec at 77°K relative to Fe) and ${\mathrm{|\hspace{0.17em}H}}_n\mathrm{\hspace{0.17em}|}$ were lower than those of corresponding octahedral high‐spin Fe(III) salts. Salts with large cations (AsPh₄FeCl₄) or large anions [NEt₄Fe(NCO)₄] showed a quadrupole splitting, and the sign of the electric field gradient (efg) in NEt₄Fe(NCO)₄ was positive.