Crystal Structure of and Electron Paramagnetic Resonance of Mn2+ in Cd2(NH4)2(SO4)3

Abstract

The room temperature structure of Cd2(NH4)2(SO4)3 (CAS), isotypic with the mineral langbeinite K2Mg2(SO4)3 has been refined by full-matrix least squares methods. The structure is cubic with a = 10.362(7) Å and space group P213. There are two inequivalent sites for the cation Cd2+ and e.p.r. spectra of doped Mn2+ show that Cd2+ was substituted in both sites with preferential occupancy by Mn2+ in a 1:3 ratio. Refinement of X-ray data on a mixed crystal of the stoichiometry Cd0.84Mn1.16(NH4)2(SO4)3 shows that site I contains 82% Mn2+ while site II has 38%. The zero-field splitting parameter D was calculated for Mn2+ in two sites using a point-charge model and confirms that Mn2+ substitutes into site I with greater preference. Although a monoclinic ferroelectric phase of CAS has been suggested below 90 K, no change in space group symmetry could be detected at 80 K. The proton second moment, calculated by the van Vleck's formula, and compared with the experimental values, suggests a tunneling motion of the protons among the four vertices of the ammonium tetrahedron.