Relaxing under pressure with a rigid niccolite formate framework

Abstract

The rich dielectric properties of hybrid formate frameworks are tightly related to the dipolar dynamics and phase transitions of these compounds. A complete understanding of such phenomena requires experiments performed under non-standard conditions. Herein, we report high-pressure dielectric properties of anisotropic single crystals of a 1,4-diaminobutane zinc-formate framework [NH₃(CH₂)₄NH₃][Zn₂(HCOO)₆]. Experiments performed over wide temperature and pressure ranges allowed us to determine the equation of state of this compound. We reveal that a reversible structural phase transition, associated with a change of the crystal symmetry and partial ordering of the 1,4-diaminobutane cations, is characterized by a linear increase of the transition temperature with increasing pressure (dT₀/dp = 5.8(2) K GPa⁻¹). The X-ray diffraction and high-pressure dielectric experiments show that zinc cations together with formate anions form a rigid cage with crystal axis-dependent response to pressure, which affects the dynamics of 1,4-diaminobutane cations. We also demonstrate that the structural phase transition of this compound occurs at a constant value of the dielectric relaxation time (isochronic conditions) further revealing a tight relation between the framework and molecular cations. Our experimental results are supported by the molecular dynamics simulations.