Studies Of The Mechanism And The Tailoring Of Negative Thermal Expansion In Several Ferroelectrics

As one of the most important functional materials,ferroelectric owns kinds of useful properties such as ferroelectricity,piezoelectricity,dielectric and thermaloelectric performance.Nowadays,the ferroelectrics have been widely employed in electronic technology and industries.Moreover,few ferroelectrics display unusual negative thermal expansion(NTE)behavior.In the past decade,the NTE in PbTiO3-based compounds have been intensely investigated by our group and the mechanism of spontaneous volume ferroelectrostriction(SVFS)was proposed.The Pb 6s-O 2p interaction has been confirmed to be crucial to the lattice distortion,spontaneous polarization(Ps)and the NTE in these PbTiO3-based compounds.Afterwards,the Pb-O thermal vibration was suggested to account for the NTE of PbTiO3 by First-principle calculation.Based on the above results,this thesis concerns on the several basic questions of the NTE in ferroelectrics to get further insight of this unusual behavior.First the possibility of applying the Gruneisen relation to reproduce the NTE of PbTiO3 was explored.Though the main optical modes including soft mode of PbTiO3 own significant negative Gruneisen parameter,it was confirmed that this relation failed to stand well in PbTiO3,mainly due to the strong anharmonicity in PbTiO3 instead of the quasi-anharmonicity.After analyse the PbTiO3-based compound showing enhanced NTE,we found that the intrinsic anharmonicity shows positive correlation to the intensity of NTE in ferroelectrics.Then the oxygen isotope 18O substitution in PbTiO3 was adopted to study the role of atomic thermal vibration in the NTE.With high-resolution synchrotron X-ray diffraction,we determined the 180 induced slightly shorted α-axis but elongated c-axis(both by approximately 1/104).Note the modification to the lattice constants by 180 agrees well with its anisotropic axial thermal expansion of PbTiO3.Combining the studies of Raman and EXAFS of Pb L3 edge,we confirmed that the Pb-O thermal vibration indeed contributes to the axial NTE of PbTiO3,then the volumetric NTE.Besides,we studied a large NTE of-4.7×10-5/K(234～338 K)in the lead-free and non-perovskite compounds Sn2P2S6.With structure analyses and the First-principle calculations,the Sn 5s-S 3p interaction was found to play a key role in the NTE,which is similar with the case in the PbTiO3.Thus,we speculated the s-p interaction was necessary to the ferroelectric NTE.Furthermore,the Pb2+/Ge2+cations with similar ns2 valence electrons but different interaction ability to S 3p electrons were adopted to partially replace the Sn2+ to study the effects of varied s-p interaction on the NTE of Sn2P2S6.Additionally,the ferroelectric SbSI owning s-p interaction was studied and the detected NTE behavior confirmed the above point.Finally,it failed to induce NTE in the BaTiO3 though Ca-doping could significantly reduce the volumetric thermal expansion by 60%.This also indicates that the necessary s-p interaction to the ferroelectric NTE.This thesis provides new insights to the NTE in ferroelectrics.The s-p interaction and strong intrinsic anhannonicity of the soft mode in ferroelectrics could be the essential.The large thermal vibration of relevant atoms due to soft mode would destroy the s-p interaction and then reduce the Ps.Due to the coupling of the Ps and the lattice distortion,the polar axis gets contracted rapidly and brings the volumetric NTE.While without the s-p interaction,it seems impossible to obtain NTE in ferroelectrics.It could explain why there are so few ferroelectrics displaying volumetric NTE.Thus,the above findings benefit the design and the tailoring of ferroelectric NTE in the future.