High-pressure crystal chemistry of coesite-I and its transition to coesite-II

The high-pressure crystal chemistry of coesite was studied by means of single crystal X-ray diffraction in the pressure interval similar to 2-34 GPa and at ambient temperature. We compressed the samples using diamond-anvil cells loaded with neon as pressure-transmitting medium and collected X-ray diffraction data using synchrotron radiation. The thermodynamically stable coesite - coesite-I - was observed up to similar to 20 GPa, with the following unit-cell parameters: a = 6.6533(12) angstrom, b = 11.9018(10) angstrom, c = 6.9336(10) angstrom, beta = 121.250(20)degrees and V = 469.38(15) angstrom(3). The volume-pressure data of coesite-I are described by means of a third-order Birch-Murnhagan EoS with parameters V-0 = 547.26(66) angstrom(3), K-T0 = 96(4) GPa, K-T0' = 4.1(4). Above such pressure we witness the formation of a well crystallized coesite-II, previously observed only by spectroscopic studies. The structure of the novel high-P polymorph was determined and refined at similar to 28 and similar to 31 GPa with final R indices of 8% and 12%, respectively. Coesite-II has P2(1)/n symmetry and a unit cell that is doubled along the b-axis with respect to that of the initial coesite-I: a = 6.5591(10) angstrom, b = 23.2276(14) angstrom, c = 6.7953(9) angstrom, beta = 121.062(19)degrees and V = 886.84(19) angstrom(3) at similar to 28 GPa. All Si atoms are in tetrahedral coordination. The displacive phase transition I->II is likely driven by the extreme shortening (0.05 angstrom or 3.2%) of the shortest and the most compressible Si1-O1 bond, related to the stiff 180 degrees Si1-O1-Si1 angle. Under compression the linear angle bends, resulting in two independent angles, one of which, however, retains almost linear geometry (similar to 178 degrees). The requirement of this angle to be close to linear likely causes further Si-O compression down to an extremely short distance of similar to 1.52 angstrom which prompts subsequent structural changes, with the formation of a triclinic phase at similar to 31 GPa, coesite-III.