First-principles thermodynamics at finite temperatures: perspective on ordered and disordered phases

A longstanding issue of first-principles calculations is to predict thermodynamic properties for a disordered phase at finite temperatures. Here, we show that a recent advance for this issue is the partition function approach in terms of microstates, which is the key for both ordered phase with one primary microstate and disordered phase consisting of two and more noticeable microstates. For a given microstate, first-principles phonon calculations in terms of the quasiharmonic approach provide a practical pathway to predict its thermodynamic properties. In the present paper, a summary of properties predicted at finite temperatures is presented, and examples are given for ordered phases of anti-fluorite Li₂S, hcp Mg, and fcc Ni as well as disordered phases of Cu₂ZnSnS₄ (CZTS) and fcc Ce. It is shown that (i) the extension from “phase” to “microstate” opens an avenue to quantitatively tailor anomalous properties such as phase transition and thermal expansion anomaly, and these anomalies are traceable from the microstate configurational entropy, and (ii) these microstates can be considered as the building blocks, i.e., the genome, of materials.