Crystallographic aspects of unit cell distortion in Pb(Zr, Ti)O3-Pb(Co, Nb)O3 solid solutions

Abstract

The structure and properties of piezoelectric materials of composition Pb(Zr, Ti)O3 near the morphotropic phase boundary between tetragonal and rhombohedral phases are very controversial for interpretation, since the boundary composition always results in the coexistence of tetragonal and rhombohedral structures. Under this category the ternary solid solution of Pb(A,/3, Bz/3)O3-PbTiO3-PbZrO3 (A is a divalent ion and B a pentavalent ion) has been extensively studied [1-31. According to the work of Bokov and Myl'nikova [4], Pb(Co, Nb)O3 (PCN) has a perovskite structure and a very low Curie temperature of 7 0 ° C. Since the lattice parameters of PCN are very near those of Pb(Zr, Ti)O3; (PZT), it is expected that PCN and PZT will readily form a solid solution. Kudo and Yazaki [5] found that the PCN-PZT solid solution contains rhombohedral, tetragonal and pseudocubic crystal phases. Hanh and Uchino [6] showed that {200} reflections of the PbK(Zr, Nb)O3-PbZrO3-PbTiO3 'system were split into three lines, where two were due to the tetragonal structure and the central one was due to the rhombohedral structure. In the present work, the crystallographic phases and dielectric properties of PCN-PZT solid-solution ceramics were investigated with respect to the degree of solid solution. The starting materials were PbO (99%); ZrO2 (99.9%), TiO2 (SG), Nb205 (99.9%) and CoO (EP). These oxides were thoroughly mixed with distilled water in a zirconia ball-mill for 15 h. The mixtures were calcined at 830°C for 2h. With the addition of 0.5 % PVA, the mixtures were cold-pressed into discs 10mm in diameter at 1000kgcm 2. The green discs were sintered at given temperatures for 45 min by using a double crucible. The sintered discs were lapped down to 1.30mm in thickness and electroded by silver paste and poled by applying 2 kV mm' at 100 ° C. Phases were identified and their lattice parameters were determined by X-ray powder diffraction method using CuKe, a nickel filter and 30 kV-15 mA output. Dielectric properties were measured according to the IRE standard [7]. The calcined powders are rounded and narrowly distributed in size around 1/~m. A perovskite phase identical to PZT is detected as the monocrystalline phase. Therefore the synthesis of good perovskite ceramics from these compositions is very feasible. In the X-ray powder diffraction pattern there will be more peaks in the case of a low symmetry such as triclinic than in the case of a high symmetry such as cubic. On this concept, the unit cell distortion was observed with respect to the degree of solid solution in the system of (1-x)PZT-xPCN as follows [8]. {2 0 0} reflections of PZT-PCN solid solutions sintered at various temperatures are shown in Fig. 1. At each sintering temperature the symmetry is increased as the amount of PCN is increased, that is, from four-fold symmetry for tetragonal structure to threefold symmetry for rhombohedral or pseudocubic structure. These trends are significant as the sintering temperatures are increased. The higher the sintering temperature is, the earlier these trends occur. For each sintering temperature, the lattice parameters as a