Structural evolution of hemimorphite at high pressure up to 4.2 GPa

Abstract

The high-pressure structural evolution of hemimorphite, Zn₄Si₂O₇(OH)₂·H₂O, a = 8.3881(13), b = 10.7179(11), c = 5.1311(9) Å, V = 461.30(12) Å³, space group Imm2, Z = 2, was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to 4.2 GPa. In the pressure range of 0.0001–2.44 GPa, the unit-cell parameters change almost linearly. The phase transition (probably of the second order) with symmetry reduction from Imm2 (hemimorphite-I) to Pnn2 (hemimorphite-II) was found near 2.5 GPa. The structure compressibility increases somewhat above the phase transition. Namely, the initial unit-cell volume decreases by 3.6% at 2.44 GPa and by 7.2% at 4.20 GPa. The hemimorphite framework can be described as built up of secondary building units (SBU) Zn₄Si₂O₇(OH)₂. These blocks are combined to form the rods arranged along the c-axis; these rods are multiplied by basic and I-translations of orthorhombic unit cell. The symmetry reduction is caused by the rotation of the rods along their axis. In hemimorphite-I, the compression affects mainly the SBU dimensions, whereas a rectangular section of the channels having mm2 symmetry remains practically unchanged. An appreciable decrease in this section in hemimorphite-II is determined by its oblique distortion with the loss of m planes. It results from opposite rotation of adjacent SBU, which also leads into the loss of I-translation. In hemimorphite-I, the coordination of H₂O molecules is fourfold planar; the hydrogen-bonded hydroxyls and H₂O molecules form infinite ribbons along the c-axis. In hemimorphite-II, an additional short H₂O–O contact appears as a result of asymmetric deformation of the channels. The appearance of this new contact provides the possibility for re-orientation of hydrogen bonds. The planar coordination of H₂O molecules changes to tetrahedral and the ribbons are transformed to islands (OH)₂–H₂O.