Negative linear compressibility and strong enhancement of emission in Eu[Ag(CN)₂]₃·3H₂O under pressure

The framework material Eu[Ag(CN)₂]₃·3H₂O exhibits a negative linear compressibility (NLC) of -4.2(1) TPa^-1 over the largest pressure range yet observed (0-8.2 GPa). High-pressure single-crystal X-ray diffraction data show that the rapid contraction of the Kagome silver layers under compression causes the wine-rack lattice to expand along the c-axis. The hydrogen bonds between the water molecules and the main frameworks constrain the structural deformation under pressure and eventually a weak NLC effect generated. Furthermore, we found that the pressure-induced emission intensity increases almost 800-fold at 4.0 GPa, followed by a gradual decrease and disappearance at 8.1 GPa. Under compression, high pressure significantly tunes the triplet level positions near the Eu³⁺ ions, and horizontal displacement between a quenching excited state and the excited levels of Eu³⁺ facilitates the energy transfer process to the 5D₀ excited state and limits the nonradiative corssover at elevated pressures, thus increasing the emission intensity. In addition, we observe a gradual band gap reduction with increasing pressure, and the sample could not be returned to the initial state after the pressure was completely released. By controlling the structural flexibility, we observe a coupled NLC and pressure-induced strong enhancement of the emission properties of Eu[Ag(CN)₂]₃·3H₂O, which provides a new route for the design of new optical devices with intriguing luminescence properties under extreme environments.