Why I Count Calories for a Living

Calorimetry measures heat effects, so why should one care?  Enthalpies of formation and phase transformation sing about making and breaking chemical bonds. Heat capacities and entropies dance about how atoms and electrons jostle each other, move, and disorder.  Combining thermodynamic and structural studies provides illuminates what materials form in nature, in the lab, and in technology. I illustrate the insights gained from calorimetry by three examples from our work. (1) Zinc sulfide (ZnS) is important as a semiconductor and an ore mineral. It occurs in two polymorphs, sphalerite and wurtzite. Our recent calorimetric studies have highlighted the importance of surface energy in changing the thermodynamic stability of these polymorphs, especially at the nanoscale, explaining their otherwise puzzling occurrences. (2) Pyrochlore is an ordered derivative of the simple fluorite structure and it has been proposed as a nuclear waste form. However, it is subject to radiation damage, leading to disorder and loss of crystallinity.  A combination of neutron diffraction and calorimetry has shown that the nominally disordered or amorphized products of heavy ion irradiation contain persistent nanodomains of ordered weberite structure which strongly affect stability and physical properties, complicating simple pictures of radiation tolerance. (3) Zeolites and metal organic frameworks contain immense structural porosity, increasing their volume many times over that of a dense phase. Calorimetry has shown that the energetic penalty of porosity is amazingly small, enabling the synthesis, stability, and persistence of many new and useful materials. In all these cases, the microscopic structural and macroscopic thermochemical insights work together to understand and predict the  existence and behavior of materials.