Frantti J, Fujioka Y, Zhang J, Vogel S C, Wang Y, Zhao Y, Nieminen R M
J Phys Chem B. 2009 Jun 11;113(23):7967-72. doi: 10.1021/jp9024987.
The best piezoelectric materials are solid solutions in the vicinity of the steep morphotropic phase boundary (MPB) separating rhombohedral and tetragonal phases in the composition-temperature plane. A classical example is the lead zirconate titanate [Pb(Zr(x)Ti(1-x))O(3), PZT] system, with x approximately 0.52, where the two phases are separated by a boundary extending from the lowest temperatures up to several hundred degrees. The origin of the boundary has been under keen studies for 40 years. Recent interest is largely due to the need to develop new, lead-free piezoelectrics, for which a natural starting point is to understand the properties of the present systems. Here, we demonstrate, through high-pressure (up to 8 GPa) neutron powder diffraction experiments and density functional theory computations on lead titanate (PbTiO(3), PT), that it is the competition between two factors which determines the MPB. The first is the oxygen octahedral tilting, giving advantage for the rhombohedral R3c phase, and the second is the entropy, which in the vicinity of the MPB favors the tetragonal phase above 130 K. If the two factors are in balance over a large temperature range, a steep phase boundary results in the pressure-temperature plane.
最佳的压电材料是在组成-温度平面中分离菱面体相和四方相的陡峭准同型相界(MPB)附近的固溶体。一个经典的例子是锆钛酸铅[Pb(Zr(x)Ti(1-x))O(3),PZT]体系,其中x约为0.52,在该体系中,这两个相由一条从最低温度延伸至几百摄氏度的边界分隔开来。40年来,人们一直在深入研究这个边界的起源。近期的研究兴趣很大程度上源于开发新型无铅压电材料的需求,而了解现有体系的特性是开发新型无铅压电材料的自然起点。在此,我们通过对钛酸铅(PbTiO(3),PT)进行高压(高达8 GPa)中子粉末衍射实验以及密度泛函理论计算表明,决定MPB的是两个因素之间的竞争。第一个因素是氧八面体倾斜,它有利于菱面体R3c相;第二个因素是熵,在MPB附近,熵在130 K以上有利于四方相。如果这两个因素在较大温度范围内保持平衡,在压力-温度平面上就会形成一条陡峭的相界。