NMR study of phase transitions in dicalcium barium propionate and dicalcium lead propionate

Abstract

Dicalcium Barium Propionate (DBP) undergoes a first order phase transition at 267 K and a second order one at 204 K. Dicalcium lead propionate (DLP) also shows two phase transitions—a second-order one at 333 K and a first-order one at 191.5 K. The proton second moment $(M_2)$ in the temperature range 160-300 K and the proton spin-lattice relaxation time $(T_1)$ in the temperature range 77-350 K have been measured to study the motion of the propionate ion in relation to the phase transition. In DBF, the $M_2$ variation with temperature shows two transitions corresponding to the two phase transitions reported. On the other hand the $T_1$ measurements yield only a single minimum and a slope change at the second-order phase transition temperature. In DLP, the $M_2$ study yields a transition in the region 180-190 K, near the first-order phase transition, but no $M_2$ transition at the second-order phase transition, $(T_1)$ measurements show a single minimum, as well as a slope change at the second-order phase transition. At the first-order phase transition, $T_1$ changes smoothly. These observations are discussed in relation to propionate ion dynamics.