Libron-phonon coupling and hydrogen-bond dynamics in the vacancy-ordered perovskite (NH4)2SnCl6: a temperature- and pressure-dependent Raman study
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.