Létoublon Antoine, Paofai Serge, Rufflé Benoît, Bourges Philippe, Hehlen Bernard, Michel Thierry, Ecolivet Claude, Durand Olivier, Cordier Stéphane, Katan Claudine, Even Jacky
UMR FOTON, CNRS, INSA-Rennes, F- 35708 Rennes, France.
Institut des Sciences Chimiques de Rennes, UMR 6226, CNRS, Université de Rennes 1 , F-35042 Rennes, France.
J Phys Chem Lett. 2016 Oct 6;7(19):3776-3784. doi: 10.1021/acs.jpclett.6b01709. Epub 2016 Sep 13.
Low frequency dynamics has been studied in a CHNHPbBr hybrid perovskite single crystal by using four different spectroscopy techniques: coherent inelastic neutron, Raman and Brillouin scatterings, and ultrasound measurements. Sound velocities were measured over five decades in energy to yield the complete set of elastic constants in a hybrid halide perovskite crystal in the pseudocubic plastic phase. The C shear elastic constant is very small, leading to a particularly low resistance to shear stress. Brillouin scattering has been used to study the relaxation dynamics of methylammonium cations and to evidence translation-rotation coupling associated with the cubic to tetragonal phase transition at T ≈ 230 K. Low frequency and highly damped optical phonons observed using both Raman and inelastic neutron below 18 meV, do not present softening close to T. The critical dynamics at T ≈ 230 K is compatible with an order-disorder character, dominated by relaxational motions of the molecules.
相干非弹性中子散射、拉曼散射和布里渊散射以及超声测量，对一种CHNHPbBr混合钙钛矿单晶中的低频动力学进行了研究。在五个数量级的能量范围内测量了声速，以得到混合卤化物钙钛矿晶体在伪立方塑性相中的完整弹性常数集。C剪切弹性常数非常小，导致对剪应力的抵抗力特别低。布里渊散射已被用于研究甲铵阳离子的弛豫动力学，并证明与T≈230K时立方相到四方相转变相关的平移-旋转耦合。在18meV以下使用拉曼散射和非弹性中子散射观察到的低频和高阻尼光学声子，在接近T时没有出现软化现象。T≈230K时的临界动力学与一种有序-无序特征相兼容，主要由分子的弛豫运动主导。
