EPR study of Mn2+ in single crystals of lithium ammonium tartrate monohydrate and determination of symmetry and domain structure of ferroelectric phase

Abstract

An EPR study is performed on Mn2+ doped single crystals of lithium ammonium tartrate monohydrate (LAT), both in the paraelectric and ferroelectric phases. The Mn2+ EPR spectra in the paraelectric phase are attributed to Mn2+ substituting for NH+4 (1) and associated with a neighboring Li+ vacancy. The phase transition at Curie temperature (∼98 K) is found to be of the second order. From the fourfold increase in the number of magnetically distinguishable complexes on going from the paraelectric to the ferroelectric phase and subsequent data for the monodomain crystal, the existence of four kinds of domains is confirmed. The present EPR results on Mn2+ are combined with the previously reported EPR results on Cr3+, and the symmetry of the ferroelectric phase which was believed to be monoclinic (P21) until now is concluded to be triclinic. The triclinic symmetry explains the previously reported but unexplained anomalies in the dielectric constant εaa and elastic compliance SE44 at Tc. The triclinic ferroelectric phase allows for four kinds of domains, viz., I, Ia, Ic, and Ib. The domains Ia and Ic are related to domain I by a twofold rotation about the a and c axes, respectively, and the domains I and Ib are related by a twofold rotation about the b axis. The polarization reversal process involves the necessary destruction of one of the two kinds of domains I and Ib (or Ia and Ic).