T-induced displacive phase transition of end-member Pb-lawsonite

Abstract

Pb-lawsonite, PbAl2[(OH)2|Si2O7]·H2O, space group Pbnm, was synthesized as crystals up to 15 μm × 5 μm × 5 μm in size by a piston cylinder technique at a pressure of ∼4 GPa and a temperature of 873 ± 10 K. Temperature-dependent powder and single-crystal X-ray diffraction (XRD) analyses partly using synchrotron radiation as well as Raman spectroscopic investigations reveal a phase transition around 445 K resulting in the Cmcm high-temperature structure. The transformation temperature is considerably higher than that of lawsonite around 273 K, which is characterized predominantly by proton order/disorder. The transition is confirmed using principal component analysis and subsequent hierarchical cluster analysis on both the powder XRD patterns and the Raman spectra. Furthermore, a non-uniform change is observed around 355 K, which is not as pronounced as the 445 K transition and apparently comes from enhanced hydrogen bonding, which stops the atom shifts in Pb-lawsonite. These are the same bonds that mainly characterize the phase transition in lawsonite around 273 K. In contrast, the structural transition of Pblawsonite at 445 K seems to originate from the interaction of the SiO4 tetrahedra and AlO6 octahedra framework with the Pb2+ cation. The structural environment of Pb2+ can be described by a 12-fold coordination above 445 K, which changes towards irregular ten-fold coordination below this temperature. An assignment of the O–H stretching Raman bands confirms moderately strong H bonds in Pb-lawsonite, whereas both strong and weak H bonds exist in lawsonite. Therefore, a further phase transition of Pblawsonite, similar to that of lawsonite around 273 K, is not expected.

Evolution of the α-BaMg(CO3)2low-temperature superstructure and the tricritical nature of its α–β phase transition

“…The observed coordination change with temperature around the Ba atom reveals similarities to the Pb atom at the structural transition in PbAl 2 [Si 2 O 7 |(OH) 2 ]ÁH 2 O (Ende et al, 2016). The transformation in PbAl 2 [Si 2 O 7 |(OH) 2 ]ÁH 2 O occurs at a comparable temperature (450 K).…”

Crystal structure and phase transition in noelbensonite: a multi-methodological study

“…On the other hand, synthetic "Pb-lawsonite" behaves, upon heating, differently from natural Ca end-member lawsonite, despite the same cation (Al) in the octahedra. Although a similar phase transition (Pmcn → Cmcm) occurs for "Pblawsonite" at 172 °C, the structural change seems to be due to the interaction of the SiO 4 tetrahedra and AlO 6 octahedra with the Pb 2+ cation, which is significantly larger than Ca 2+ (Ende et al 2016). Synthetic Sr-rich lawsonite is monoclinic (P2 1 /m) at RT when quenched from ca.…”

“…The effects of the incorporation of a larger cation in the cavities of the lawsonite-type structure has already been discussed by several authors (Liebscher et al 2010;Ende et al 2016). As demonstrated for synthetic "Pb-lawsonite" (Ende et al 2016), hennomartinite (Libowitzky and Armbruster 1996) and noelbensonite (this study), the main effect is the shift of the phase transitions toward higher temperatures. In our study the transition temperatures are practically the same as those determined for hennomartinite.…”

P21/c Postorthopyroxene γ-LiScGe2O6, a New Dense High-Pressure Polymorph and Its Direct Transformation from the Pbca Structure

“…This would change the number and intensity of the Raman bands, as observed, e.g., for dynamic disordered H 2 O molecules and OH groups. 64 …”