发表机构
Universidade Federal do Ceará; Instituto de Física da Universidade de São Paulo（塞阿拉联邦大学; 圣保罗大学物理研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
该研究通过拉曼光谱结合X射线衍射，揭示空位有序钙钛矿(NH4)2SnCl6中NH4+子系统与八面体骨架解耦，其librational运动可通过对[SnCl6]2-模式的重整化被拉曼探测，明确了温度和压力下的耦合规律。
AI 中文摘要
空位有序钙钛矿R2MX6结合了刚性无机骨架与分子A位阳离子，但阳离子动力学如何在外部扰动下与晶格声子耦合，在很大程度上仍未被探索。(NH4)2SnCl6中NH4+的 librational 运动因对称性严格属于拉曼禁阻模式，因此几乎仅通过中子散射、NMR和NQR进行探测。我们证明，该运动仍可通过其对允许的[SnCl6]2-模式的重整化作用被拉曼光谱探测到。结合10至300 K及最高10.1 GPa下的单晶X射线衍射与拉曼散射，我们发现平均立方结构随温度平滑变化，无异常；而在~100 K以下，[SnCl6]2-模式出现 libron-声子重整化（E_eff ~ 4.7 meV）和对称性选择性线不对称性，且N-H伸缩振动在120 K附近出现最小值（E_eff ~ 9.3 meV），两者均跟踪铵转子从经典到量子的交叉行为。这两个有效能量低于中子测量得到的裸 librational 跃迁能量13.4 meV，符合自能标度的预期。相比之下，铵的线宽由纯退相而非非谐衰减决定，且无交叉特征。在压力下，八面体模式平滑硬化，而空腔出现两次响应：NH4+的平动F2g模式在1.3 GPa以上获得拉曼强度，且N-H伸缩振动在1.7 GPa附近反转其压力斜率，空间群无变化且卸压后完全恢复。因此，刚性八面体骨架与动态活跃的NH4+子系统基本解耦，该子系统承载对两种扰动的响应，为独立于八面体网络调节阳离子-声子耦合提供了途径。
英文摘要
Vacancy-ordered perovskites R2MX6 combine a rigid inorganic framework with a molecular A-site cation, but how the cation dynamics couples to the lattice phonons under external perturbation remains largely unexplored. The librational motion of NH4+ in (NH4)2SnCl6 is strictly Raman-silent by symmetry, which is why it has been probed almost exclusively by neutron scattering, NMR and NQR. We show that it is nevertheless accessible to Raman spectroscopy, through the renormalization it imposes on the allowed [SnCl6]2- modes. Combining single-crystal X-ray diffraction with Raman scattering between 10 and 300 K and up to 10.1 GPa, we find that the average cubic structure varies smoothly with no anomaly, while below ~100 K the [SnCl6]2- modes acquire a libron-phonon renormalization (E_eff ~ 4.7 meV) and a symmetry-selective line asymmetry, and the N-H stretch passes through a minimum near 120 K (E_eff ~ 9.3 meV); both track the classical-to-quantum crossover of the ammonium rotor. The two effective energies lie below the bare librational transition of 13.4 meV measured by neutrons, as expected for a self-energy scale. The ammonium linewidths, by contrast, are governed by pure dephasing rather than by anharmonic decay and carry no crossover signature. Under pressure the octahedral modes stiffen smoothly, whereas the cavity responds twice: the translational F2g mode of NH4+ gains Raman intensity above 1.3 GPa, and the N-H stretch inverts its pressure slope near 1.7 GPa, with no change of space group and full recovery on decompression. The rigid octahedral framework is thus essentially decoupled from a dynamically active NH4+ subsystem that carries the response to both perturbations, offering a route to tune cation-phonon coupling independently of the octahedral network.
