Effects of pressure on the structure of bikitaite

Abstract

The structural behaviour of bikitaite, Li 2 (Al 2 Si 4 O 12 ). 2H 2 O, was investigated under hydrostatic pressure using X-ray single-crystal diffraction data. A Merrill-Bassett diamond anvil cell was mounted with glycerol, as non penetrating pressure-transmitting medium, ruby chips and a small crystal of quartz as the calibrant. A strong anisotropic compression was observed by linear regressions of lattice parameters against P, bikitaite being softer along the c axis (βc = 9.3(1) 10 -3 GPa -1 ), than along b (β b = 6.6(1) 10 -3 GPa -1 ) and a (β a = 2.4(1) 10 -3 GPa -1 ) (β a : β b : β c = 1 : 2.75 : 3.9). Fitting the cell-volume — pressure data to a second order Birch-Murnaghan equation of state, as indicated by the finite strain-stress plot, yielded K 0 = 44.2(4) GPa, with K’ = 4 and V 0 = 295.58(2) A 3 . The evolution of the bikitaite structure with P was studied by comparing the results of refinements with data collected at room conditions, at 3.2 GPa and after decompression. The structure can be described as sheets of six-membered rings parallel to (001), connected by pyroxene-like chains. 8-ring and 5-ring channels run along [0 10] and inside the 8-ring channel there is a onedimensional chain of water molecules, which is linked to the framework through the extra-framework Li atoms. Under pressure, the kinking of the pyroxene-like chain decreased the free diameters of the 5-ring channels, strongly reducing the distance between the ab planes. On the contrary, the tridymite-like planes with 6-membered rings were more rigid. The positions of the extra-framework cations and water were maintained at HP even though the configuration of the water chains changed slightly: the distances between the water molecules decreased, whereas the kinking angle of the chain increased.