ESR Study of a Cr5+ Centre in Ferroelectric Ammonium Sulphate

X‐band electron spin resonance (ESR) studies of (CrO4)2‐ doped, X‐irradiated single crystals of ferroelectric ammonium sulphate ((NH4)2SO4, TC = 223 K) at 300 and 208 K are reported. The paramagnetic centre responsible for the ESR spectrum is identified to be Cr5+. Superhyperfine interaction of the unpaired electron with two equivalent protons is observed. The spin‐Hamiltonian parameters which are nearly axial at 300 K, with g‖ < g⟂ indicating a d orbital ground state, acquired rhombic character below TC indicating a distortion of the sulphate tetrahedron. An increase in the value of the proton superhyperfine constant in the ferroelectric phase is indicative of stronger hydrogen bonding.