A Chemical Bonding Interpretation of Unusual Compressibility Trends in Hydrated Magnesium Sulfates.

Hydrated magnesium sulfates (MgSO4·nH2O) are known to form multiple hydration states (n = 0-11) and are essential in planetary science and thermochemical energy storage. Despite their significance in detecting extraterrestrial water reservoirs or in mineral (de)hydration cycles, it is still necessary to understand how the structure-property relationships of these materials evolve at different hydration levels when pressure is applied. Through a systematic first-principles computational investigation, we elucidate the key structural factors governing the densification mechanism under hydrostatic pressure of MgSO4·nH2O crystals. At zero pressure, we propose a useful and transferable rule of thumb that allows for straightforward evaluation of the crystal volume at any hydration level. At increasing pressure, our polyhedral and chemical bonding analyses reveal that the presence in the structure of coordinated and/or interstitial water molecules is the main factor determining the compressibility of these hydrated salts. These findings provide useful insights into the role of hydration in controlling the stability and mechanical properties of hydrated materials under extreme conditions.