Exploring anharmonic lattice dynamics and dielectric relations in niobate perovskites from first-principles self-consistent phonon calculations

Kwangrae Kim, Woohyun Hwang, Seung-Hyun Victor Oh and Aloysius Soon

npj Comput Mater 9, 154 (2023)

doi.org/10.1038/s41524-023-01110-8

Published: 24 August, 2023

编辑概述

铌酸钙钛矿的非谐晶格动力学

结构相变在许多晶体系统（包括钙钛矿结构）中都有观察到，这些相变受到各种晶体振动模式的驱动。特别是氧化物的典范代表—钙钛矿（ABO₃）系统，它们由于第一布里渊区高对称点处不同声子模式的软化行为，展现了不同的BO₆八面体畸变，表现出多种结构相变。在各种已知的钙钛矿中，第一主族铌酸盐钙钛矿家族，特别是钾铌酸钙（KNbO₃）和钠铌酸钙（NaNbO₃），是一种很有前景的无铅钙钛矿，展现出各种相变特性和相关属性，具有众多技术应用，而无毒性问题。当前使用密度泛函理论（DFT）计算来预测材料在高温下的相稳定性并不是一件容易的事，因为这些计算通常是在静态几何结构下进行的，只能在0K时提供合理的结果。与此同时，晶体的介电常数也与晶格振动和相关的结构相变密切相关。在本研究中，来自韩国延世大学材料科学与工程系的Aloysius Soon教授课题组，通过将自洽声子（SCP）理论与准粒子（QP）近似相结合，仔细研究了立方KNbO₃和NaNbO₃的非谐晶格动力学。他们发现，热膨胀效应和非谐自能引起的声子频率偏移都被明确地包括在内，以解释声子软化行为。他们通过Lyddane-Sachs-Teller（LST）关系量化地预测了两种铌酸盐的相变温度（即居里温度）和温度依赖的介电常数，发现与以前的实验结果一致。这项研究对这些钙钛矿中复杂相变提供了定量的理解。该论文近期发表于npj Comput Mater 9, 154 (2023).

Editorial Summary

Anharmonic Lattice Dynamics in niobate perovskites

Structural phase transitions are observed in many crystal systems (including perovskites) and intrinsically driven by various lattice vibrational modes. In particular, the archetype of oxides – the perovskite (ABO₃) system expresses diverse structural phase transitions due to the different phonon mode softening behaviors at high-symmetry points of the first Brillouin zone, displaying different BO₆ octahedral distortions. Among the various known perovskites, the Group I niobate perovskite family – especially, potassium niobate (KNbO₃) and sodium niobate (NaNbO₃) – is one of the promising lead-free perovskites actively studied since they show a variety of phase transition characteristics and related properties with numerous technological applications without toxicity concerns. Using standard density-functional theory (DFT) calculations to predict the phase stability of materials at elevated temperatures is not trivial since these calculations are typically performed using a static geometry, providing only reasonable results at 0 K temperatures. Meanwhile, the dielectric constant of a crystal is also closely related to the lattice vibrations and associated structural phase transitions. In this study, a group led by Prof. Aloysius Soon from the Department of Materials Science & Engineering, Yonsei University, carefully investigated the anharmonic lattice dynamics of cubic KNbO₃ and NaNbO₃ using the self-consistent phonon (SCP) theory coupled to the quasiparticle (QP) approximation. They found that both the thermal expansion effects and phonon frequency shifts due to the anharmonic self-energies are explicitly included to address the phonon softening behaviors. They quantitatively predicted the phase transition temperature (i.e., the Curie temperature) and temperature-dependent dielectric constant for both niobates via the Lyddane-Sachs-Teller (LST) relation. The theoretical results align with previous experimental reports. This study provides a more quantitative understanding of the complex phase transitions in these perovskites.