Nuclear Electric Field Gradient at the Iron Sites in Ca2Fe2O5 and Ca2FeAlO5

Mössbauer spectroscopy has been used to determine the nuclear electric‐field‐gradient (efg) tensor parameters at the Fe³⁺ sites in Ca₂Fe₂O₅ and Ca₂FeAlO₅. In all cases to within experimental error, the efg is axially symmetric and the $z$ principal axis of the efg is parallel to the crystallographic $b$ axis. Analytic expressions are developed for the relative transition probability of $\text{M1}$ radiation connecting $\mathrm{I\hspace{0.17em}=\hspace{0.17em}}\frac{3}{2}$ to $\mathrm{I\hspace{0.17em}=\hspace{0.17em}}\frac{1}{2}$ nuclear states for the situation in which the $z$ principal axis of an axially symmetric efg is perpendicular to an internal magnetic field.