(K_(0.5)Na_(0.5))Nb_(1-x)Ta_(x)O_(3)铁电性的第一性原理研究

First-principles study of the ferroelectric properties of (K_(0.5)Na_(0.5))Nb_(1-x)Ta_(x)O_(3)

随着铅基陶瓷的大量使用,造成了严重的环境和健康问题,为了能够找寻替代铅基陶瓷的材料,人们开始了对无铅陶瓷材料的探索.(K_(0.5)Na_(0.5))NbO_(3)材料由于其优异的铁电压电性能,成为了无铅陶瓷材料的代表,但是碱性组分在高温下的高挥发性使其应用范围受到了极大的限制.基于这样的考虑,本文基于密度泛函理论的第一性原理方法,探究了(K_(0.5)Na_(0.5))Nb_(1-x)TaxO_(3)(x=0.125,0.25,0.375,0.5,0.625,0.75,0.875,1)九种体系的铁电性能,结果表明:随着Ta浓度的增加,晶格常数发生了突变,由此猜测KNN材料随着Ta浓度的增加,发生了正交-单斜的相变;Ta的掺入能够降低KNN材料的烧结难度,且随着Ta浓度的增加,形成难度逐渐降低;掺杂体系中,Ta-4f轨道与Na-3p,K-4p以及O-2s轨道电子发生了杂化,使导带底Nb-4d与Na-3p电子的态密度峰向高能区移动,禁带宽度呈现增加的趋势;在分析KNN材料铁电性作用机理时发现KNN的铁电极化类型是离子位移型极化,且在[001]方向上不具有极化能力.分析掺杂后各个体系的极化强度,发现随着Ta浓度的增加,极化强度逐渐降低,但是在低浓度掺杂时极化有向上增加的趋势,这可能是体系在低浓存在着正交与单斜的准同型相界(Morphotropic phase boundary,MPB)导致的,由此可以判断,低浓度掺杂有利于提高KNN材料的铁电性能.

With the large-scale use of lead-based ceramics,serious environmental and health problems have arisen,in order to be able to find alternative materials for lead-based ceramics,people have begun to explore lead-free ceramic materials.(K_(0.5)Na 0.5)NbO_(3) materials have become representative of lead-free ceramic materials due to their excellent ferroelectric properties,but the temperature sensitivity of KNN ceramics makes its application range extremely limited.Based on this consideration,in this paper,we use the density functional theory of first principles method to explore the ferroelectric properties of nine systems(K_(0.5)Na_(0.5))Nb_(1-x)Ta_(x)O_(3)(KNN)(x=0.125,0.25,0.375,0.5,0.625,0.75,0.875,1).The results show that with the increase of Ta concentration,the lattice constant undergoes a sudden change,so it is speculated that the orthogonal-monoclinic phase transition of KNN material occurs with the increase of Ta concentration;the incorporation of Ta can reduce the sintering difficulty of KNN material,and with the increase of Ta concentration,the formation difficulty gradually decreases;in the doping system,the Ta-4f orbital and Na-3p,K-4p and O-2s orbital electrons are hybridized,so that the density of states peaks of Nb-4d and Na-3p electrons at the conduction band bottom move towards the high-energy region.The gap width shows an increasing trend.When analyzing the ferroelectric mechanism of KNN material,it was found that the iron polarization type of KNN is ion displacement polarization,and does not have polarization ability in the[001]direction.Analysis of the polarization intensity of each system after doping,it is found that with the increase of Ta concentration,the polarization intensity gradually decreases,but the polarization has an upward trend when doping at low concentration,which may be caused by the presence of orthogonal and monoclinic Morphotropic phase boundary(MPB)in low concentration,from which it can be judged that low concentration doping is conducive to improving the ferroelectric properties of KNN materials.