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

Abstract

Abstract The high-pressure crystal chemistry of coesite was studied by means of single crystal X-ray diffraction in the pressure interval ∼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 ∼20 GPa, with the following unit-cell parameters: a = 6.6533(12) Å, b = 11.9018(10) Å, c = 6.9336(10) Å, β = 121.250(20)° and V = 469.38(15) Å3. 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) Å3, K T0 = 96(4) GPa, K ′ T o ${K'_{To}}$ = 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 ∼28 and ∼31 GPa with final R indices of 8% and 12%, respectively. Coesite-II has P21/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) Å, b = 23.2276(14) Å, c = 6.7953(9) Å, β = 121.062(19)° and V = 886.84(19) Å3 at ∼28 GPa. All Si atoms are in tetrahedral coordination. The displacive phase transition I->II is likely driven by the extreme shortening (0.05 Å or 3.2%) of the shortest and the most compressible Si1-O1 bond, related to the stiff 180° Si1-O1-Si1 angle. Under compression the linear angle bends, resulting in two independent angles, one of which, however, retains almost linear geometry (∼178°). The requirement of this angle to be close to linear likely causes further Si-O compression down to an extremely short distance of ∼1.52 Å which prompts subsequent structural changes, with the formation of a triclinic phase at ∼31 GPa, coesite-III.