Double salt compounds is a type of molecular compound or higher order compound which produced from first order compounds.
There are many man made as well as natural occurring well known double salt compounds in co-ordination chemistry. Such as Carnallite, potash alum Mohr’s salt etc.
If a solution of ferrous sulfate and ammonium sulfate in their stoichimetric proportions are allowed to evaporate, then a light green color molecular compound, ferrous ammonium sulfate hexahydrate with molecular formula FeSO4, (NH4)2 SO4, 6H2O is obtained.