W.Kaminsky, S.Haussühl, A.Brandstaedter, C.Balarew: Physical properties and phase transition in tetragonal X2CuT4.2Q2O, X=K,Rb,NH4,ND4, T=Cl,Br, Q=H,D. Z.Kristallogr. 209 (1994) 395-399.

Refractive indices, Faraday effect, dielectric constants, and elastic constants have been studied on crystals grown from aqueous solutions. The physical properties are closely related to the chemical composition of the crystals. In the ammonium salts the first transition cicle of a virgin crystal yields a different behaviour compared to the subsequent cicles. Copper salts and the isotypic manganese compound exhibit transitions of different type. The transition temperure is correlated to the void occupied by the [NH4]+-ions.