Brenner Thomas M, Grumet Manuel, Till Paul, Asher Maor, Zeier Wolfgang G, Egger David A, Yaffe Omer
Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot 76100, Israel.
Department of Physics, Technical University of Munich, 85748 Garching, Germany.
J Phys Chem Lett. 2022 Jun 30;13(25):5938-5945. doi: 10.1021/acs.jpclett.2c00904. Epub 2022 Jun 22.
We employ terahertz-range temperature-dependent Raman spectroscopy and first-principles lattice dynamical calculations to show that the undoped sodium ion conductors NaPS and isostructural NaPSe both exhibit anharmonic lattice dynamics. The anharmonic effects in the compounds involve coupled host lattice-Na ion dynamics that drive the tetragonal-to-cubic phase transition in both cases, but with a qualitative difference in the anharmonic character of the transition. NaPSe shows an almost purely displacive character with the soft modes disappearing in the cubic phase as the change in symmetry shifts these modes to the Raman-inactive Brillouin zone boundary. NaPS instead shows an order-disorder character in the cubic phase, with the soft modes persisting through the phase transition and remaining Raman active in the cubic phase, violating Raman selection rules for that phase. Our findings highlight the important role of coupled host lattice-mobile ion dynamics in vibrational instabilities that are coincident with the exceptional conductivity of these Na ion conductors.
我们采用太赫兹波段的温度相关拉曼光谱和第一性原理晶格动力学计算来表明，未掺杂的钠离子导体NaPS和同结构的NaPSe都表现出非谐晶格动力学。化合物中的非谐效应涉及耦合的主体晶格 - 钠离子动力学，这在两种情况下都驱动了四方相向立方相的转变，但转变的非谐特性存在质的差异。NaPSe表现出几乎纯粹的位移特性，随着对称性变化使软模移至拉曼非活性布里渊区边界，立方相中软模消失。相反，NaPS在立方相中表现出有序 - 无序特性，软模在相变过程中持续存在并在立方相中保持拉曼活性，违反了该相的拉曼选择规则。我们的研究结果突出了耦合的主体晶格 - 可移动离子动力学在与这些钠离子导体的优异电导率相一致的振动不稳定性中的重要作用。
