# Thermodynamics of the Flexible Metal-Organic Framework Material MIL-53(Cr) From First Principles

## TL;DR

This study employs first-principles calculations to analyze the thermodynamics and phase transitions of MIL-53(Cr), revealing free energy minima and barriers between pore structures under various conditions.

## Contribution

It provides a detailed first-principles thermodynamic analysis of MIL-53(Cr), including free energy calculations and phase transition barriers, with improved agreement using specific van der Waals parameterizations.

## Key findings

- Two free energy minima corresponding to narrow and large pore structures.
- Identification of critical pressures where phase stability changes.
- Calculated transition barriers of 3 to 6 kJ/mol.

## Abstract

We use first-principles density functional theory total energy and linear response phonon calculations to compute the Helmholtz and Gibbs free energy as a function of temperature, pressure, and cell volume in the flexible metal-organic framework material MIL-53(Cr) within the quasiharmonic approximation. GGA and metaGGA calculations were performed, each including empirical van der Waals (vdW) forces under the D2, D3, or D3(BJ) parameterizations. At all temperatures up to 500 K and pressures from -30 MPa to 30 MPa, two minima in the free energy versus volume are found, corresponding to the narrow pore ($np$) and large pore ($lp$) structures. Critical positive and negative pressures are identified, beyond which there is only one free energy minimum. While all results overestimated the stability of the $np$ phase relative to the $lp$ phase, the best overall agreement with experiment is found for the metaGGA PBEsol+RTPSS+U+J approach with D3 or D3(BJ) vdW forces. For these parameterizations, the calculated free energy barrier for the $np$-$lp$ transition is only 3 to 6 kJ per mole of Cr$_4$(OH)$_4$(C$_8$H$_4$O$_4$)$_4$.