Ferrous ammonium sulphate is a double salt of ferrous sulphate and ammonium sulphate. It has the formula FeSO4.(NH4)2SO4.6H2O. It contains two different cations Fe2+ and NH4+. It is prepared by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water containing a little of sulphuric acid, and then subjecting the resulting solution to crystallization when pale green crystals of ferrous ammonium sulphate separate out.
This video explains how to prepare pure sample of ferrous ammonium sulphate (Mohr’s salt).