Calorimetry of Phase, Schottky and other Transitions

Abstract

With an eye toward direct estimation of thermophysical properties we consider the morphology of heat-capacity curves and the major contributions thereto (i.e., lattice-vibration, magnetic, and electronic state contibutions) as well as first and second order transitions, order-disorder transitions, etc. Then we note that the besetting problem of resolution of excess heat capacities of many types of transitions—whether structural or electronic, or otherwise—requires a reliable estimate of the lattice contribution. We see that corresponding state theories have been less than satisfactory; but Lindemann schemes are suggestive. The Latimer scheme has been widely used. For chemical thermodynamic purposes use of a volume-weighted scheme over the range where entropy development largely occurs is demonstrated to provide resolution of Schottky contributions for Ln(OH)3 and LnCl3 systems and has been shown to be superior to other approaches. The lanthanide sesquioxides, sesquisulfides, halides, and especially the trihydroxides provide exemplary models in which the Schottky contributions (occasioned by the crystal-field splitting of the ground state manifold) provide an excellent basis for testing the resolution of the lattice heat-capacity contribution, since the Schottky contribution can be evaluated both by spectroscopy and by thermophysical data. Agreement of the excess contribution then provides basis for judging the quality of the lattice contribution resolution.