A quantitative descriptor of lattice anharmonicity in crystals

Abstract

The anharmonic effect is often one of the physical causes of some special material properties, such as soft mode phase transition, negative thermal expansion, multiferroicity, ultra-low thermal conductivity and so on. However, the existing methods for quantifying the anharmonicity of materials do not propose a clear and accurate anharmonicity descriptor. The calculation of the anharmonic effect requires extremely time-consuming molecular dynamics simulation, the process is complex and the calculation cost is high. Researchers urgently need a quantitative descriptor that can be quickly calculated to understand, evaluate, design, and screen functional materials with strong anharmonicity.<br/>In this paper, we propose a measure to quantify the anharmonicity of materials by only phonon spectrum and static self-consistent calculation through the calculation and analysis of materials composed of germanium and its surrounding elements. In this method, the lattice anharmonicity is decomposed into the anharmonic contribution of independent phonon vibration modes, and the quantitative anharmonicity descriptor <i>σ</i>_(<i>q,j</i>)^<i>A</i> of phonons is proposed. Combining it with the Bose-Einstein distribution, the quantitative descriptor A_ph (T) of temperature-dependent material anharmonicity is proposed. We calculate the bulk modulus and lattice thermal conductivity at 300 K of nine widely representative materials. There is a clear linear trend between them and the quantitative descriptor <i>A</i>_ph(<i>T</i>) proposed by us, which verifies the accuracy of our proposed descriptor. The results show that the descriptor : (i) can systematically and quantitatively classify materials according to the strength of anharmonicity; (ii) intuitively shows the distribution of the anharmonic effect of the material on the phonon spectrum, and realizes the separate analysis of the phonon anharmonicity that affects the specific properties of the material; (iii) saves the expensive first-principles molecular dynamics calculation cost and lays a foundation for screening and designing materials based on anharmonicity.<br/>This study provides an example for the high-throughput study of functional materials driven by anharmonic effect in the future, and opens up new possibilities for material design and application. In addition, for strongly anharmonic materials such as CsPbI3, the equilibrium position of the atoms is not fixed at high temperatures, resulting in a decrease in the accuracy of quantifying anharmonicity using our proposed descriptor. In order to solve this limitation, our future research will focus on the distribution of atomic equilibrium positions in strongly anharmonic materials at high temperatures, so as to propose a more accurate theoretical method to quantify the anharmonicity in strongly anharmonic materials.