Pressure-induced structural instabilities in (NH₄)₂Cu(SO₄)₂(H₂O)₆ Tutton salt: An in situ Raman and DFPT...

The high-pressure behavior of the ammonium copper(II) Tutton salt (NH₄)₂Cu(SO₄)₂(H₂O)₆ (NHCuSO) is reported for the first time, investigated through in situ Raman spectroscopy in a diamond anvil cell up to 6.5 GPa, and supported by periodic density functional perturbation theory (DFPT) calculations. At ambient conditions, powder X-ray diffraction with Rietveld refinement confirmed that NHCuSO crystallized in the monoclinic system (space group P2₁/a, Z = 2), with its structural cohesion governed by a cooperative lattice of OH⋯O and NH⋯O hydrogen-bonds interconnecting the [Cu(H₂O)₆]²⁺, [SO₄]^2-, and [NH₄]⁺ layers, as quantified by Hirshfeld surface analysis. DFPT calculations reproduced the experimental Raman frequencies and provided reliable vibrational assignments across the 70-3580 cm^-1 spectral range. Under compression, the pressure-dependent Raman spectra revealed a sequence of structural instabilities, reflected by mode softening, band disappearances, and the emergence of new phonon branches at critical pressures. Two phase transitions were identified, i.e., a symmetry-lowering transformation at 2.5 GPa, driven by a reorganization of the NH⋯O hydrogen-bond topology, and a first-order transition at 4.8 GPa, yielding a distinct high-pressure polymorph with a well-defined vibrational fingerprint. A precursor regime of local structural destabilization sets in below 1.6 GPa, well before the main transitions. Additionally, the pressure-dependent DFPT optimizations further reveal that these transitions are governed by a two-stage densification mechanism: below ≈2.5 GPa the compression is accommodated by an 83% collapse of the intermolecular voids and a slight relaxation of the Cu²⁺ Jahn-Teller distortion, whereas above 4.8 GPa, once the voids are largely exhausted, the stress is transferred to the [Cu(H₂O)₆]²⁺ octahedron, shortening the equatorial CuO bonds and enhancing the Jahn-Teller elongation. The results of this study highlight the pivotal role of the hydrogen-bond lattice compressibility and the Jahn-Teller distortion of Cu²⁺ in determining the mechanical stability and pressure-phase diagram of this salt, contributing to a deeper understanding of pressure-structure-property relationships in the Tutton salt family.