High-pressure elastic behaviour and structural re-arrangement in monazite-type minerals

ATO4 compounds crystallize with different structural topologies (e.g. zircon, monazite, scheelite, barite) as a function of the crystal radii of the A and T cations as well as of temperature and pressure. Monazite-type compounds include, among the others, phosphates, arsenates, chromates and the most common mineral is the LREE-bearing monazite-(Ce), ideally CePO4. These compounds share a monoclinic P21/n structure with T cations in tetrahedral coordination and A cations showing a nine-fold coordination with the surrounding oxygens. In Earth Sciences, monazite, that is a common accessory mineral in different kinds of rocks, is of interest for petrological and geochronological implications. In materials science, monazite-type compounds find several technological applications, including phosphors, lasers, ionic conductors, and radioactive waste management [1]. Such a multidisciplinary interest has led to extensive studies on the behaviour of these isostructural compounds at non-ambient conditions [2], resulting in a deep characterization of the elastic and structural response to compressional, thermal and chemical stimuli. However, the adoption of up-to-date synchrotron facilities and crystallographic methods can still disclose undiscovered features of such a behaviour. In this contribution we present the compressional and combined high-P and high-T behaviour of natural monazite-(Ce) and gasparite-(Ce) (ideally CeAsO4, [3]). In situ isothermal high-P single-crystal experiments have been performed at the beamlines ID15B of ESRF (Grenoble, France) and P02.2 of Petra-III (Hamburg, Germany). Combined HPHT in situ single-crystal experiments on monazite-(Ce) have been performed at the P02.2 beamline of Petra-III making use of the available graphite resistive heated DAC [4]. The analysis of the unit-cell evolution with pressure of both the minerals allowed to describe a slight change in the elastic behaviour at around 15-18 GPa, magnified by the compressional trend of the monoclinic β angle. The refined structural models allowed to track for both monazite-(Ce) and gasparite-(Ce) the P-driven approach towards the REE site of a tenth oxygen atom, which enters the coordination sphere of the A (REE) cation at a pressure corresponding to the change in the elastic behaviour. Independently from the different crystal chemistry, both the monazite-type minerals showed P-induced structural rearrangements with an increase in the coordination number of the REE site from 9 to 10, occurring before the known phase transition to a post-barite type polymorph [2]. References: [1] Clavier L et al. (2011) J Eur Ceram Soc 31: 941. [2] Errandonea D (2017) Phys Status Solidi B 254: 1700016 [3] Pagliaro et al. (2022) Mineral Mag 86: 150-167 [4] Hwang et al. (2023) Rev Sci Instrum 94: 083903