Pressure-enhanced ferroelectric polarisation in a polar perovskite-like [C2H5NH3]Na0.5Cr0.5(HCOO)3 metal–organic framework

Abstract

We report the high-pressure structure-related properties of a ferroelectric heterometallic formate perovskite framework templated by the ethylammonium cation (CH₃CH₂NH₃⁺, EtA⁺). High-pressure X-ray diffraction studies show a first-order structural phase transition at 3.6(2) GPa from the polar Pn ambient phase to a centrosymmetric P2₁/n high-pressure phase. A high-pressure Raman scattering experiment indicates the same transition in the 4.0–4.4 GPa range. The mechanism of the phase transition involves strong compression and distortion of the NaO₆ subnetwork and a decrease in the space available for the accommodated EtA⁺ cations, resulting in a change in their configuration within the pores at 3.7 GPa. Using density functional theory the value of the ferroelectric polarisation within the ac plane is calculated to be 0.9 μC cm⁻² at ambient pressure, increasing in magnitude to a value of 1.1 μC cm⁻² at a pressure of 3 GPa before vanishing at the phase transition.