Successive Change of Phase Transition Character in Lead Zinc Niobate Titanate Single Crystals

Abstract

(1 x)Pb(Zn1=3Nb2=3)O3–xPbTiO3 ((1 x)PZN–xPT) single crystal with a composition near the morphotropic phase boundary (MPB, x 1⁄4 0:08{0:105) exhibits excellent electrical property, which makes it a promising candidate for the next generation electromechanical transducer application. (1 x)PZN–xPT is relatively easy to grow in single crystal form by a fluxing method over the whole composition range. However, the fluxing method is unsuitable for growing large crystals due to its poor reproducibility and the occurrence of spontaneous nucleation. By comparison, the Bridgman method can obtain excellent reproducibility and large productivity by introduction a seed crystal into crystal growth. In this note, (1 x)PZN–xPT single crystals with composition x 1⁄4 0:05, 0.09 and 0.15 were grown by a modified Bridgman method using an allomeric Pb[(Mg1=3Nb2=3)0:69Ti0:31]O3 (PMNT 69/31) single crystal as a seed. 7) The phase transition behavior dependence on composition of the as-grown (1 x)PZN–xPT crystals were discussed. Lattice structure of the (1 x)PZN–xPT crystals was investigated by X-ray diffraction measurement (XRD, Rigaku D/MAX-3C) using ground crystals. The obtained crystals are of pure perovskite phase and the crystal structure changes from rhombohedral in PZNT95/5, through coexistence of rhombohedral and tetragonal in PZNT91/9, and to tetragonal in PZNT85/15 with the increase of PbTiO3 content. Table I shows the lattice parameters of the (1 x)PZN–xPT crystals calculated by the least-squares method. The lattice parameters of the PZNT91/9 crystal were calculated on a rhombohedral unit cell due to its slight tetragonal distortion. It can be seen that the PZNT91/9 crystal can be pictured as providing a ‘‘bridge’’ between the rhombohedral and tetragonal structures since its cell is similar to both the rhombohedral and tetragonal lattices. Dielectric property of the (1 x)PZN–xPT crystals was measured by a computer-interfaced analyzer (HP4192A). Figure 1 shows temperature dependence of the dielectric constant of the (1 x)PZN–xPT crystals. There is just one unique anomaly in the PZNT95/5 crystal, where the temperature of dielectric peaks (Tm) appears at around 140 C accompanied with apparent frequency dispersion. Tm increases from 138.1 C at 120Hz to 144.6 C at 100 kHz, whereas the maximum value of dielectric constant (m) decreases from 20921 to 16842. The full-width-at-halfmaximum (FWHM) of the dielectric peaks is around 50 C, which shows the nature of a relaxor. As a comparison, a relatively sharp dielectric anomaly appears in the PZNT85/ 15 crystal at about 200 C, which indicates the crystal changes to normal ferroelectric (FE). The relaxor nature of the (1 x)PZN–xPT crystals can be described by a quadratic law, which was a modified Curie– Weiss law proposed by Uchino et al.: 1= 1⁄4 1=max þ ðT TmaxÞ=C ð1Þ where n is a diffuseness index and C0 is a constant. The diffuseness index can be solved graphically by eq. (1) using a log–log plot as illustrated in Fig. 2. The slope of the curve represents the value of the diffuseness index, while the intercept gives the diffuseness parameter by following equation: 1⁄4 ðe =2maxÞ ð2Þ Table I. Lattice parameters of the (1 x)PZN–xPT single crystals.