Thermodynamic study on the magnetic transition and structural phase transition in [(CH3)(2)NH2][Na0.5Fe0.5(HCOO)(3)] by using the Landau phenomenological model

Abstract

In this study, we apply the Landau phenomenological model to describe magnetic transition and structural phase transition in a metal formate framework (MOF) of the ferromagnetic [(CH3)(2)NH2][Na0:5Fe0:5(HCOO)(3)], namely, DMNaFe. By using the observed data from the literature for the magnetization, the excess heat capacity (Delta CP), and the dielectric constant (e), we predict some thermodynamic quantities as a function of temperature close to the magnetic transition (T-M 1/4 8:5K) and structural (T-c 1/4 170:8 K) phase transition in [(CH3)(2)NH2][Na0:5Fe0:5(HCOO)(3)]. The thermodynamic quantities predicted from the Landau mean field model indicate that the multiferroics and ferromagnetic [(CH3)(2)NH2][Na0:5Fe0:5(HCOO)(3)]) undergo a weakly first order (or nearly second order) magnetic transition. Structural phase transition in [(CH3)2NH2][Na0:5Fe0:5(HCOO)(3)] is of a first order, as observed experimentally. The thermodynamic model studied here describes the observed behavior in most cases of [(CH3)(2)NH2][Na0:5Fe0:5(HCOO)(3)], and it can be applied to some other MOFs.