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Pressure-controlled polytypism in hydrous layered materials

¹ Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, D.C. 20015, U.S.A.
² Department of Earth and Planetary Sciences, Johns Hopkins University, Olin Hall, 3400 North Charles Street, Baltimore, Maryland 21218, U.S.A.
³ Geology and Geochemistry, MS D469, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A.
⁴ Birck Nanotechnology Center, 915 W. State Street, Purdue University, West Lafayette, Indiana 47907-2054, U.S.A.

Correspondence: ^* E-mail: pdera{at}gl.ciw.edu

An isosymmetric displacive structural transformation in the hydrous layer silicate dickite [Al₂Si₂O₅(OH)₄, monoclinic Cc, a = 5.161(3), b = 8.960(6), c = 14.459(10) Å, ß = 96.77(1)° ], occurring under hydrostatic compression above 2.0 GPa, has been studied using single-crystal X-ray diffraction and diamond-anvil cell techniques. The structure of the high-pressure phase, determined in situ, is monoclinic with space group Cc with unit-cell parameters a = 5.082(3), b = 8.757(6), c = 13.771(9) Å, and ß = 89.60(2)° at 4.1 GPa. The positions of all hydrogen atoms at both ambient and high pressure have been determined by a combination of simulated annealing and energy minimization. The mechanism of the transformation, which may be general for other hydrous layered materials, involves a shift of the 1:1 layers with respect to each other by the vector [1/6, 1/6, 0] and is accompanied by the formation of new hydrogen bonds.

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