THE HEAT CAPACITY AND THERMODYNAMIC PROPERTIES OF THE THERMAL TRANSITIONS OF STRONTIUM HEXAAMMINE

The heat capacity, transition temperatures and the corresponding enthalpy changes of a 6.51 MPM Sr(NH3)6/NH3 system have been measured from 80 K to 190 K by means of adiabatic calorimetry. From 85 K to 145 K the heat capacity of the solid ammine is approximately linear and displays a modest pretransition increase from [MATH]145 K to 175.74 K where a solid-solid transition occurs for which [MATH]H = 1181 ± 10 cal mole-1 of Sr(NH3)6 and [MATH]Str = 7.61 cal mole-1 K-1 after correction for a pretransition effect of 0.89 cal mole-1 K-1. From [MATH]80 K to [MATH]140 K our Cp data are about midway between others' results reported earlier for two different samples of the hexaammine which differed [MATH]40% at 100 K. The enthalpy change we have observed is [MATH] double that reported by others using differential thermal analysis. For the eutectic transition, we estimate [MATH]Str [MATH] 5.28 kcal mole-1 K-1 of hexaammine melting. An exploratory investigation of the effect of ND3 substitution for NH3 in the hexaammine indicated that the S-S transition was elevated 6.34 K from 175.74 K to 182.08 K ; and that the eutectic transition was increased by 5.17 K from 185.37 K to 190.54 K. The [MATH]Str for the S-S transition, which we take to be an order-disorder transition, is in line with [MATH]Str for similar transitions that occur in the M(NH3)6I2 salts in which "revised" [MATH]Str's are [MATH] Rln 32 which also approximates the experimental [MATH]S for Ca(NH3)6I2 as well as for Sr(NH3)6 - both of which contain comparably large M+2