第一性原理计算钛酸铅A位掺杂对其负热膨胀性的影响

近年来,实验发现钛酸铅基材料具有负热膨胀性,且其热膨胀程度会受到掺杂元素的影响.目前所研究的A位掺杂体系中,仅Cd原子掺杂能使钛酸铅负热膨胀性增强.所以研究A位掺杂钛酸铅,比较Cd原子与其他原子在掺杂钛酸铅时化学键的异同,有助于深刻理解钛酸铅负热膨胀的本质.本文利用第一性原理,分别优化了Sr、Ba、Cd掺杂钛酸铅的晶格常数,计算了它们的态密度和电荷密度.结果表明Cd―O键的共价性强于Pb―O键,而Ba―O键和Sr―O键几乎呈离子性,Ba/Sr对Pb的替代削弱了化合物的共价性,降低了自发极化强度.与实验测量的热膨胀系数对比可以发现,A位原子与氧原子之间的共价性增强,化合物负热膨胀程度升高;若A位原子与氧原子之间的共价性削弱,负热膨胀程度降低.可见A位原子与氧原子之间的共价性影响了钛酸铅基化合物负热膨胀性.

Effects of A-Site Substitutions on Negative Thermal Expansion in PbTiO3 fromFirst-Principles Calculations

Recent experimental results have indicated that the negative thermal expansion is a common phenomenon in PbTiO₃-based materials, and that this expansion is affected by various substitutions. Interestingly, Cd substitution in PbTiO₃ has a unique effect compared with other A-site substitutions, in that it enhances negative thermal expansion. Therefore, studying A-site substitution in PbTiO₃, the role of which still remains unclear, would provide a deeper understanding of the nature of the negative thermal expansion of PbTiO₃-based materials. Herein we report the results of structural calculations, densities of states and the minimumelectron densities of Pb_1-xSr_xTiO₃, Pb_1-xBa_xTiO₃, and Pb_1-xCd_xTiO₃ supercells on the basis of chemical bond first-principles calculations. The results demonstrate that the hybridization between Cd―O orbitals is more pronounced than that between Pb―O orbitals, while the bonding between Ba/Sr and O is almost ionic in nature. Cd substitution was found to have an unusual effect in terms of enhancing the average bulk coefficient of thermal expansion in PbTiO₃. In contrast, Ba and Sr substitutions reduce the coefficient. Thus, the covalency in the bonding between the A- site and O in PbTiO₃- based materials is responsible for the enhanced negative thermal expansion.