铋层状化合物相竞争关系的第一性原理热力学研究

Phase Competition in Bismuth Layered Structure Based on First Principles Thermodynamics

铋层状材料纯相的合成存在着很多困难,阻碍了此类材料的广泛应用。本工作利用第一性原理热力学方法研究了三个经典同源铋层状化合物系列CaBi2Nb2O9-nNaNbO3、Bi4Ti3O12-nSr TiO3和Bi4Ti3O12-nCaTiO3的不同层数相之间的竞争关系,试图揭示其相稳定性的热力学机制。结合第一性原理计算,建立了相对Gibbs能量与钙钛矿化学势之间的定量关系,并依此来分析不同层数相之间的竞争。分析表明,随着化学势升高,这三个体系中除了两个相外,其他相都能够在某一特定范围内达到最稳定,可以解释实验上观察到的某些样品中的相混合和无序等现象。相竞争演化的温度依赖关系被分成构型熵和振动熵两部分贡献,前者改变了相竞争的关系,而后者仅仅增加低层相的稳定范围。

The difficulties of synthesis of pure Aurivillius phases largely impede them from extensive application. In this work, the first principles thermodynamics approach was applied to investigate the phase competition relation of three ho- mologous Aurivillius series CaBi2Nb2O9-nNaNbO3, Bi4Ti3Ol2-nSrTiO3 and Bi4Ti3O12-nCaTiO3, in order to uncover the ther- modynamic mechanism of the phase stability. The competition among different phases was analyzed by the relative Gibbs energy as a function of chemical potential of perovskite unit. The analysis reveals that most phases are able to overcome oth- ers to be the most stable ones in a certain range as chemical potential increases, which can be applied to interpret the relevant experimental phenomena including the phase mixture and disordered structures. Temperature dependence of phase competi- tion evolutions are also studied based on the configurational and vibrational entropy effect, the former effect changes the competition relations of different phases, while the latter only increases the stable range of the lower phases.