第一性原理计算钛酸铅A位掺杂对其负热膨胀性的影响

Effects of A-Site Substitutions on Negative Thermal Expansion in PbTiO-3 from First-Principles Calculations

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.

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 minimum electron densities of Pb1-xSrxTiO3, Pb1-xBaxTiO3, and Pb1-xCdxTiO3 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—Oorbitals, 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 Asite and O in PbTiO3-based materials is responsible for the enhanced negative thermal expansion.