Structural phase transition in NH₄F under extreme pressure conditions

Umbertoluca Ranieri, Christophe Bellin, Lewis J. Conway, Richard Gaal, John S. Loveday, Andreas Hermann, Abhay Shukla*, Livia E. Bove*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

Ammonium fluoride (NH₄F) exhibits a variety of crystalline phases depending on temperature and pressure. By employing Raman spectroscopy and synchrotron X-ray diffraction beyond megabar pressures (up to 140 GPa), we have here observed a novel dense solid phase of NH₄F, characterised by the tetragonal P4/nmm structure also observed in other ammonium halides under less extreme pressure conditions, typically a few GPa. Using detailed ab-initio calculations and reevaluating earlier theoretical models pertaining to other ammonium halides, we examine the microscopic mechanisms underlying the transition from the low-pressure cubic phase (P-43m) to the newly identified high-pressure tetragonal phase (P4/nmm). Notably, NH₄F exhibits distinctive properties compared to its counterparts, resulting in a significantly broader pressure range over which this transition unfolds, facilitating the identification of its various stages. Our analysis points to a synergistic interplay driving the transition to the P4/nmm phase, which we name phase VIII. At intermediate pressures (around 40 GPa), a displacive transition of fluorine ions initiates a tetragonal distortion of the cubic phase. Subsequently, at higher pressures (around 115 GPa), every second ammonium ion undergoes a rotational shift, adopting an anti-tetrahedral arrangement. This coupled effect orchestrates the transition process, leading to the formation of the tetragonal phase.
Original languageEnglish
Article number220
Pages (from-to)1-8
Number of pages8
JournalCommunications Chemistry
Volume7
Issue number1
DOIs
Publication statusPublished - 30 Sept 2024

Fingerprint

Dive into the research topics of 'Structural phase transition in NH₄F under extreme pressure conditions'. Together they form a unique fingerprint.

Cite this