Study On Composition And Electric Properties Of Relaxor Ferroelectric PMN-PT Single Crystals

Relaxor-based ferroelectric single crystal Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃?PMN-PT?has ultrahigh piezoelectric coefficient and electromechanical coupling factor,which is considered as one of the most promising materials.Bridgman technique is a critical method in the growth of large sized PMN-PT single crystal.However,the crystals grown by this method always exist composition segregation,resulting in poor stability of electrical properties and limiting their application in practice.In order to explore the mechanism of segregation,a single crystal with nominal composition PMN-32PT was grown by the Bridgman technique.The element segregation along the axial and radial directions,the effects of segregation on electrical properties and the phase transition behaviour of crystals were studied systematically.The single crystal with nominal composition PMN-32PT was grown by the Bridgman technique.The size of as-grown crystal is?25 mm×40 mm.The layer growth mechanism is observed from SEM picture.The XRD pattern shows there is no pyrochlore phase or other heterogeneous phase formation,indicating the crystal is pure perovskite structure.The prepared samples were orientated by RO-XRD and the results show the crystal plane perpendicular to the axial direction is?432?.Along the axial direction,the crystal plane belongs to{771}.The element distribution of Nb Mg and Ti along the axial and radial directions were analyzed by ICP-MS.The effects of elemental content on electrical properties were discussed.The intrinsic links between the composition,structure and performance were revealed.It is indicated that the content of Nb and Mg decrease gradually from the bottom to the top along the axial direction of the single crystal,while the PT content increases from 21.49 mol%-29.16mol%.The variation of the curie temperature,piezoelectric coefficient and remnant polarization along the axial direction are consistent with that of the PT content.The Nb and Mg content increase slightly along the radical direction,but the PT content is relatively stable.The variation of Nb/Mg decreases firstly,and then increases.The electrical properties of the samples along the radial direction are mainly determined by the ratio of the Nb/Mg.The phase transition behaviors of PMN-23PT and PMN-24PT single crystals were analyzed through the temperature dependence of dielectric properties,piezoelectric properties,ferroelectric properties and domain morphology.The temperature-dependent dielectric spectrum of poled PMN-23PT single crystal shows two dielectric peaks located at 64.9?and97?,which correspond to the rhombohedral-tetragonal and tetragonal-cubic phase phase,respectively.The remanent polarization and the piezoelectric constant decrease rapidly around62?,which is consistent with the rhombohedral-tetragonal phase transition temperature.The temperature-dependent dielectric spectrum of poled PMN-24PT single crystal shows three dielectric peaks at 62.2?,68.1?and 76.1?,respectively,before the curie point?102??.This result demonstrates the phase transforms from rhombohedral to tetragonal through three different paths.The remanent polarization and the coercive field are varied rapidly at the temperature range of 60?to 80?,which is consistent with the three low temperature dielectric peaks.