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

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) Å 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) Å 3 ,. 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 P 2 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) Å , b = 23.2276(14) Å , c = 6.7953(9) Å , β = 121.062(19) ° and V = 886.84(19) Å 3 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.

High‐pressure phase transitions of α‐quartz under nonhydrostatic dynamic conditions: A reconnaissance study at PETRA III

“…At quasi‐hydrostatic conditions and room temperature, the high‐pressure polymorphs, coesite I and coesite II, have also been reported (Černok et al. ).…”

“…The thermodynamic stability fields of high-pressure polymorphs of a-quartz have been experimentally and theoretically investigated since 1953 (Coes 1953): aquartz transforms to coesite with fourfold coordinated Si at 2 GPa and to stishovite with sixfold coordinated Si (Stishov and Popova 1961) at 8 GPa (Kirfel et al 2001) and room temperature. At quasi-hydrostatic conditions and room temperature, the high-pressure polymorphs, coesite I and coesite II, have also been reported ( Cernok et al 2014). However, a great mismatch exists when the static stability field of high-pressure SiO 2 polymorphs is compared to the occurrence of these phases during shock metamorphism.…”

Multiple pathways in pressure-induced phase transition of coesite

“…Because of its abundance in nature, coesite eclogite (sometimes with diamond) was suggested as a highly accurate geobarometer (9). Hence, the behavior of coesite under pressure has been intensively studied in geophysics and materials science up to now (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23). Owing to improvements in hydrostatic compression techniques, recent experiments provide brand-new results about transformation of coesite under high pressure.…”

“…Recent experiments have revealed the transformation process of coesite in detail, that is, the intermediate high-pressure phases (20).…”

“…Cernok et al (20) witnessed a transition from C2/c to P2 1 /c phase (coesite-II) with the "doubled" unit cell along the b axis above 20 GPa at ambient temperature by single-crystal XRD measurements. First-principles metadynamics modeling and single-crystal XRD measurements suggested that four triclinic phases formed at higher pressures above 26 GPa and the HPO phase might be P2/c structure (21).…”