Abstract
AbstractPb-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 SiO4tetrahedra and AlO6octahedra 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.
Highlights
LAWSONITE-TYPE compounds A+2B+23[(OH)2| C2O7]·H2O (A = Ca, Sr, Ba, Pb; B = Al, Mn, Fe; C = Si, Ge) are high density (3.09 g cm−3 for lawsonite, 4.53 g cm−3 for Pb-lawsonite) hydrous disilicates, which are stable over a broad P–T field (Schmidt and Poli, 1994; Daniel et al, 1999; Pawley and Allan, 2001)
The aim of the present paper is to present single-crystal structure data and to trace the temperature-dependent structural transformations of Pb-lawsonite using Raman spectroscopic and powder X-ray diffraction (XRD) measurements, including principal component analysis (PCA) and subsequent cluster analysis on both data sets
The Pb-lawsonite crystals obtained by the piston cylinder experiments were transparent, clear, idiomorphic, mostly cube-shaped, but less commonly prismatic
Summary
LAWSONITE-TYPE compounds A+2B+23[(OH)2| C2O7]·H2O (A = Ca, Sr, Ba, Pb; B = Al, Mn, Fe; C = Si, Ge) are high density (3.09 g cm−3 for lawsonite, 4.53 g cm−3 for Pb-lawsonite) hydrous disilicates (except for Ge), which are stable over a broad P–T field (Schmidt and Poli, 1994; Daniel et al, 1999; Pawley and Allan, 2001). Libowitzky and Armbruster (1995) found that lawsonite shows phase transitions at 273 and 150 K. These transitions mainly involve changes in the positions of the hydrogen atoms and their immediate neighbours together with modifications in the hydrogen-bond scheme. By referring to the smooth changes of stretching and bending frequencies in infrared (IR) spectra across the phase boundaries they suggested a phase transition of a dynamic order-disorder type
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