Thermodynamic and Physical Properties of (NH₄)₂MnCl4·2H₂O by Nuclear Magnetic Resonance Relaxation Times

Abstract

The phase transition temperatures and thermodynamic properties of $(NH_4)_2MnCl_4{\cdot}2H_2O$ grown by the slow evaporation method were studied using differential scanning calorimetry and thermogravimetric analysis. A structural phase transition occurred at temperature $T_{C1}$ (=264 K), whereas the changes at $T_{C2}$ (=460 K) and $T_{C3}$ (=475 K) seemed to be chemical changes caused by thermal decomposition. In addition, the chemical shift and the spin-lattice relaxation time $T_{1{\rho}}$ were investigated using $^1H$ magic-angle spinning nuclear magnetic resonance (MAS NMR), in order to understand the role of $NH_4{^+}$ and $H_2O$. The rise in $T_{1{\rho}}$ with temperature was related to variations in the symmetry of the surrounding $H_2O$ and $NH_4{^+}$.

Figure 1. Tetragonal structure of (NH₄)₂MnCl₄∙2H₂O crystals.

Figure 2. Differential scanning calorimetry (DSC) thermogram of (NH₄)₂MnCl₄∙2H₂O upon heating

Figure 2. Thermogravimetric analysis (TGA) of (NH₄)₂MnCl₄∙2H₂O (inset: changes in transparency

Figure 3. Chemical shift in ¹H magic-angle spinning nuclear magnetic resonance (MAS NMR) spectrum of (NH₄)₂MnCl₄∙2H₂O as a function of temperature (inset: ¹H MAS NMR spectrum at 300 K, and spinning sideband is marked with asterisks).

Figure 4. Saturation recovery traces for delay time of ¹H in (NH₄)₂MnCl₄∙2H₂O at 190, 270, 350, and 430 K.

Figure 6. Temperature dependences of the ¹H spin-lattice relaxation time in the rotating frame T_1ρ for H₂O and NH₄⁺ in (NH₄)₂MnCl₄∙2H₂O

Table 1. Phase transition temperatures (T_C), crystal structures, and ¹H spin-lattice relaxation time in the rotating frame (T1ρ) values of (NH₄)₂MnCl₄∙2H₂O and (NH₄)₂CuCl₄∙2H₂O.