Mukhopadhyay Sanghamitra, Gutmann Matthias J, Jiménez-Ruiz Mónica, Jochym Dominik B, Wikfeldt Kjartan T, Refson Keith, Fernandez-Alonso Felix
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK.
Phys Chem Chem Phys. 2017 Dec 13;19(48):32216-32225. doi: 10.1039/c7cp06039d.
A detailed study of the thermal behaviour of atomic motions in the organic ferroelectric croconic acid is presented in the temperature range 5-300 K. Using high-resolution inelastic neutron scattering and first-principles electronic-structure calculations within the framework of density functional theory and a quasiharmonic phonon description of the material, we find that the frequencies of the well defined doublet in inelastic neutron scattering spectra associated with out-of-plane motions of hydrogen-bonded protons decrease monotonically with temperature indicating weakening of these bonding motifs and enhancement of proton motions. Theoretical mean-square displacements for these proton motions are within 5% of experimental values. A detailed analysis of this observable shows that it is unlikely that there is a facile proton transfer along the direction of ferroelectric polarization in the absence of an applied electric field. Calculations predict constrained thermal motion of proton along crystallographic lattice direction c retaining the hydrogen bond motif of the crystal at high temperature. Using the Berry-phase method, we have also calculated the spontaneous polarization of temperature dependent cell structures, and find that our computational model provides a satisfactory description of the anomalous and so far unexplained rise in bulk electric polarization with temperature. Correlating the thermal motion induced lattice strain with temperature dependent spontaneous polarizations, we conclude that increasing thermal strain with temperatures combined with constrained thermal motion along the hydrogen bond motif are responsible of this increase in ferroelectricity at high temperature.
本文对有机铁电体巴豆酸中原子运动的热行为进行了详细研究，研究温度范围为5 - 300K。利用高分辨率非弹性中子散射以及在密度泛函理论框架下的第一性原理电子结构计算，并对该材料采用准谐波声子描述，我们发现非弹性中子散射谱中与氢键质子平面外运动相关的明确定义的双峰频率随温度单调降低，这表明这些键合模式减弱且质子运动增强。这些质子运动的理论均方位移在实验值的5%以内。对这一可观测现象的详细分析表明，在没有外加电场的情况下，质子不太可能沿铁电极化方向发生容易的转移。计算预测质子沿晶体学晶格方向c的热运动受到限制，在高温下保持晶体的氢键模式。使用贝里相位方法，我们还计算了温度依赖晶胞结构的自发极化，发现我们的计算模型对体电极化随温度异常且迄今未解释的升高提供了令人满意的描述。将热运动引起的晶格应变与温度依赖的自发极化相关联，我们得出结论，随着温度升高热应变增加以及沿氢键模式的热运动受限是导致高温下铁电性增加的原因。
