Dielectric behaviour and ferroelectricity in alums

Abstract

The alum family of double salts with the general formula M¹⁺M³⁺(RO⁴)₂.12H₂O where M¹⁺ is a monovalent ion (M¹⁺ = K, Rb, Cs, Tl, NH₄, CH₃NH₃, NH₃OH₃ NH₃NH₂, etc.) and M³⁺ is a trivalent metal such as Al, Fe, Cr, V, In, Ga, etc. and R is S or Se, form an isomorphous series and their general features indicate a common cubic space group Pa3. Lipson showed subsequently that there exist three different structure types α, β and γ and the structure of a particular alum is dependent on the radius of the monovalent atom. The α structure is typical of medium sized ions, the β of the larger ones and the γ of the small Na atom. Ferroelectricity has been reported only in alums containing NH₄, CH₃NH₃, NH₃NH₃ and NH₃OH. Their hindered rotations as well as the influence of sulphate group disorder on the dielectric behaviour of alums is still not clear. No study of the temperature dependence of the low frequency dielectric constant of some of the alums, particularly those of Cs, Rb and Tl, have been made so The present investigation was undertaken to correlate their dielectric behaviour with their composition and structural differences. Under the same experimental conditions, methyl ammonium and ammonium alums also were studied and compared with the known data.