First-Principles Study On Li/Ta、Ba/Ti Doped KNbO₃ Lead-Free Piezoelectric Ceramics

Piezoelectric ceramics are one of the most important functional materials in today’s era,which has a very wide range of applications in integrated circuit,new energy technology and so on.Although traditional lead-based piezoelectric ceramics PZT has excellent performance,it contains a large number of toxic heavy metal lead elements.Based on the concept of green environmental protection,the development of high-performance lead-free piezoelectric ceramics has become one of the research focuses.In this paper,we aim to obtain a new type of lead-free piezoelectric ceramics with high piezoelectric performance,take potassium niobate KNbO₃（KN）,Li/Ta and Ba/Ti doping systems K_1-xLi_xNb_1-yTa_yO₃ and K_1-xBa_xNb_1-yTi_yO₃ as the research object,based on the first principles method,VASP calculation software is adopted,the structure,electronic,elastic and piezoelectric properties of KN and its doping system are studied,and the microscopic mechanism of atom doping affecting the piezoelectric properties of KN is revealed from the atomic scale.The influence of different atoms and different proportions of doping on the performance of KNbO₃ is explored,which provides a theoretical basis for atom doping modified KNN-based piezoelectric ceramics.The main research contents and results are as follows:（1）The properties of K_1-xLi_xNbO₃ and KNb_1-yTa_yO₃ doped by Li and Ta are calculated.The results show that the doping of Li improves the elasticity of KNbO₃,but the piezoelectric stress tensor decreases slightly with the increase of doping amount.The addition of Ta atom decreases the elasticity of KNbO₃,but increases the piezoelectric stress tensor.Both Li and Ta atom doping systems are direct band gap semiconductors.In Li doped system,K_0.375Li_0.625NbO₃ structure has the largest piezoelectric constant d₃₃（63.4 p C/N）,which is 2.6times of KNbO₃ piezoelectric constant d₃₃（24.4 p C/N）.In the Ta doped system,KNb_0.375Ta_0.625O₃ has the largest piezoelectric constant d₃₃（37.2 p C/N）,which increases the piezoelectric constant of KNbO₃ to 1.5 times.（2）The K_0.875Li_0.125Nb_0.875Ta_0.125O₃ co-doping with Li/Ta system is calculated.The results show that when Li and Ta atoms are co-doped,the elastic properties of KNbO₃ can be improved,and the toughness is better than that of single-atom doped structure,the Born effective charge values of the co-doped structure are higher than KNbO₃.Li and Ta atom co-doped K_0.875Li_0.125Nb_0.875Ta_0.125O₃ structure of the piezoelectric constant d₃₃（95.3 p C/N）is3.9 times that KNbO₃.（3）The properties of K_1-xBa_xNbO₃ and KNb_1-yTi_yO₃ doped with Ba and Ti are calculated.The results show that the addition of Ba increases the elastic constant of KNbO₃,while the addition of Ti decreases the elastic constant.The piezoelectric stress tensor of K_1-xBa_xNbO₃system decreases significantly when x is greater than 0.125.Both doping systems are direct band-gap semiconductors.In Ba doped system,K_0.875Ba_0.125NbO₃ has the largest piezoelectric constant d₃₃（49.8 p C/N）,the largest piezoelectric constant d₃₃（41.3 p C/N）is found in KNb_0.625Ti_0.375O₃ in Ti doped system,compared with KNbO₃,the piezoelectric constant d₃₃（24.4 p C/N）is increased to 2.1 times and 1.7 times respectively.（4）The K_0.875Ba_0.125Nb_0.875Ti_0.125O₃ co-doping with Ba/Ti system are calculated.The results show that when Ba and Ti atoms are co-doped,the elastic properties of KNbO₃ can be improved,with stronger anisotropy of Young’s modulus.The band structure is shown as a direct band gap semiconductor with a narrow band gap of 1.78e V.Ba and Ti atom co-doped K_0.875Ba_0.125Nb_0.875Ti_0.125O₃ structure of the piezoelectric constant d₃₃（61.3 p C/N）is 2.5 times of KNbO₃.