Phase transitions beyond post-perovskite in NaMgF ₃ to 160 GPa

Abstract

Neighborite, NaMgF3, is used as a model system for understanding phase transitions in ABX3 systems (e.g., MgSiO3) at high pressures. Here we report diamond anvil cell experiments that identify the following phases in NaMgF3 with compression to 162 GPa: NaMgF3 (perovskite) → NaMgF3 (post-perovskite) → NaMgF3 (Sb2S3-type) → NaF (B2-type) + NaMg2F5 (P21/c) → NaF (B2) + MgF2 (cotunnite-type). Our results demonstrate the existence of an Sb2S3-type post-post-perovskite ABX3 phase. We also experimentally demonstrate the formation of the P21/c AB2X5 phase which has been proposed theoretically to be a common high-pressure phase in ABX3 systems. Our study provides an experimental observation of the full sequence of phase transitions from perovskite to post-perovskite to post-post-perovskite followed by 2-stage breakdown to binary compounds. Notably, a similar sequence of transitions is predicted to occur in MgSiO3 at ultrahigh pressures, where it has implications for the mineralogy and dynamics in the deep interior of large, rocky extrasolar planets.

Fluorides of Silver Under Large Compression**

“…At ambient pressure it is predicted to be dynamically stable (no imaginaryp honon frequencies). The thermodynamic instability of Ag 2 F 3 (= Ag I Ag II F 3 )u nder compressioni sa nalogous with the pressure-induced decomposition of other ABX 3 compounds, such as NaMgF 3 , [49,50] MgGeO 3 , [49] and MgSiO 3 . [51] At high pressure all of these systems adopt the CaIrO 3 -type structure, which upon compression decomposes to:( i) BX 2 and A 2 BX 4 (as for MgGeO 3 ); (ii)AXa nd AB 2 X 5 (as found for NaMgF 3 ), or (iii) both A 2 BX 4 and AB 2 X 5 (as in MgSiO 3 ).…”

Ab initio prediction of an order-disorder transition in Mg$_2$GeO$_4$: implication for the nature of super-Earth's mantles

“…NaMgF 3 PPv was predicted to exhibit the novel P 2 1 /c-type structure of MgSi 2 O 5 under pressure before its full dissociation into NaF and MgF 2 . The predicted phases and transformations were experimentally confirmed [22]. In contrast, MgGeO 3 PPv under pressure was expected to produce the novel I 42dtype Mg 2 SiO 4 structure.…”

“…This progress has been registered in a series of ab initio discoveries concerning the nature of super-Earth's mantleforming phases in the important MgO-SiO 2 system [6,[18][19][20][21]. They have culminated in partial experimental confirmations in a low-pressure analog system, NaF-MgF 2 [22], and detailed modeling of these planets' internal structure and dynamics [11,12]. In particular, a sequence of "post-post-perovksite" (post-PPv) transitions in the MgO-SiO 2 system [6,[18][19][20][21] was predicted to occur up to ∼3 TPa and 10,000 K, starting from MgSiO 3 PPv and ending in its dissociation into the elementary oxides MgO and SiO 2 .…”

High-pressure single-crystal synchrotron X-ray diffraction study of lillianite

“…While, on the one hand, galenobismutite has the calcium-ferrite structure type, like the aluminous phases of the inner Earth, on the other hand, we do not know yet a deep-Earth analogue of β-Pb3Bi2S6. However, recent results of the ultra-high P/T behavior of NaMgF3 (Dutta et al 2019), used as a model system for understanding phase transitions in ABX3 (e.g. MgSiO3) systems, show a Sb2S3-type (stibnite-type) post-post-perovskite ABX3 phase at 58 GPa, which remains stable to over 150 GPa and heating to over 2000 K. The β-Pb3Bi2S6 structure belongs to the meneghinite homologous series (Olsen et al 2008), where also stibnite lies; this increases the interest on the phase transition of lillianite in relation to the deep-Earth crystallography.…”