10 featured Publications

336. Thermal Expansion Anomaly Regulated by Entropy

Thermal expansion, defined as the temperature dependence of volume under constant pressure, is a common phenomenon in nature and originates from anharmonic lattice dynamics. However, it has been poorly understood how thermal expansion can show anomalies such as colossal positive, zero, or negative thermal expansion (CPTE, ZTE, or NTE), especially in quantitative terms. Here we show that changes in configurational entropy due to metastable micro(scopic)states can lead to quantitative prediction of these anomalies. We integrate the Maxwell relation, statistic mechanics, and first-principles calculations to demonstrate that when the entropy is increased by pressure, NTE occurs such as in Invar alloy (Fe3Pt, for example), silicon, ice, and water, and when the entropy is decreased dramatically by pressure, CPTE is expected such as in anti-Invar cerium, ice and water. Our findings provide a theoretic framework to understand and predict a broad range of anomalies in nature in addition to thermal expansion, which may include gigantic electrocaloric and electromechanical responses, anomalously reduced thermal conductivity, and spin distributions.

320. Perspective on Materials Genome®

The author’s perspective on Materials Genome® is presented in this paper through several related projects. Current thermodynamic and kinetic databases of multicomponent materials consist of Gibbs energy functions and atomic mobility of individual phases as functions of temperature, composition, and sometimes pressure, i.e., with the individual phases based on crystal structures as the genome (building blocks) of materials. It is articulated that if an individual phase has its internal configurations, such as magnetic spin configurations and ferroelectric polarization, change significantly with respect to temperature, stress, and magnetic and electric fields, then those individual configurations instead should be considered as the genome of the individual phase. The “mutation” of an individual phase is governed by the entropy of mixing among the individual stable and metastable configurations, named as microstate configurational entropy, and responsible to anomalies in individual phases. Our ability to tailor the properties of those individual configurations as a function of compositions is the key for the design of materials.

230. Thermodynamic fluctuations in magnetic states: Fe3Pt as a prototype

To understand the Invar anomalies, such as negative thermal expansion and spontaneous magnetization, we have applied our recently developed thermodynamic framework for a system with itinerant-electron magnetism to the ordered Fe3Pt. The framework has coherently predicted the finite temperature intermixing between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs). We have also discovered a tri-critical point at which a high-temperature second-order phase transition, between the fully ordered FM phase and the paramagnetic phase which is disordered due to SFCs, becomes first order at low temperatures.