Dielectric behaviour and ferroelectricity in alums

Rama Rao, A. H. ; Srinivasan, M. R. ; Bhat, H. L. ; Narayanan, P. S. (1978) Dielectric behaviour and ferroelectricity in alums Ferroelectrics, 21 (1). pp. 433-435. ISSN 0015-0193

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Related URL: http://dx.doi.org/10.1080/00150197808237289


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.

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