ESR of Fe(III) in ferrous ammonium sulphate hexahydrate, Fe(NH4)(SO4)2·6H2O: A case of strong tetragonal crystal field

An unusual radiation dose dependent EPR line atgeff= 2.54 in feldspars: possible evidence of Fe3+O2−↔ Fe2+O−and exchange coupled Fe3+–Fe2+–nO−

“…The intensity of the g = 4.3 EPR line of Fe 3+ in feldspars is widely used to assess the positional order in feldspar samples. Due to an interesting coincidence of admixture of Kramers' doublets of the 6 S state of Fe 3+ in a strong crystal field, a slightly anisotropic line around g = 4.3 is reported in a number of widely different matrices such as borosilicate glasses and biological systems [14][15][16][17]. The implication of this observation is that this line is not unique to Fe 3+ at tetrahedral sites, and definitive conclusions on the site symmetry of ferric iron cannot be drawn using powder samples.…”

“…In feldspars, the origin of the g = 4.3 line of Fe 3+ is better understood, and has been used to elucidate (Al 3+ , Si 4+ ) order-disorder over the four distinct tetrahedral positions T1(O), T1(m), T2(O), and T2(m) [5,6]. In rhombic systems, occurrence of the g = 4.3 line is widespread amongst the high spin ferric compounds, including biological crystals such as transferrins and lactoferrins, and is a consequence of symmetry [15][16][17]. In the tetragonal limit, an isotropic g = 4.3 line arises from the relation between fourth degree crystal field parameters (as seen in ferrous tutton salt and ferrous ammonium sulfate), through radiation induced conversion of ferrous to ferric [17].…”

“…In rhombic systems, occurrence of the g = 4.3 line is widespread amongst the high spin ferric compounds, including biological crystals such as transferrins and lactoferrins, and is a consequence of symmetry [15][16][17]. In the tetragonal limit, an isotropic g = 4.3 line arises from the relation between fourth degree crystal field parameters (as seen in ferrous tutton salt and ferrous ammonium sulfate), through radiation induced conversion of ferrous to ferric [17]. The iron site in ferrous ammonium sulfate has sixfold coordination with water.…”