High-pressure phase relations in Zn2SiO4 system: A first-principles study

Abstract

Recent experimental studies have shown that phases III and IV of Zn2SiO4 recovered from highpressure experiments are retrograde phases. In order to clarify the phase relation of this system, first-principles density functional theory calculations of 11 Zn2SiO4 phases including phases III and IV were conducted. Phase III, having a tetrahedral olivine structure, exhibited an extraordinarily high compressibility, which is due to large volume reductions in vacant octahedral sites corresponding to M 1 and M 2 sites in olivine structure. Both phases III and IV have much higher enthalpies compared to those of phases II and V up to 10 GPa, and they are not stable high-pressure phases. Instead, Na2CrO4-and Ag2CrO4-structured phases have volumes and enthalpies next to phase V at around 9 GPa, and they are most likely candidate structures for high-pressure phases of III and IV, respectively. In both structures, a half of Zn is occupying a tetrahedral site, and the remaining half is occupying an octahedral site. Compared to those phases, olivine phase has slightly higher volume and enthalpy, being a less likely candidate. "Phase transitions" of phases II, III and IV observed during structural optimization under pressure are also reported. The transition of phase II was discussed in relation to similar transition known for Si3N4.

Formation of Mg-Orthocarbonate through the Reaction MgCO3 + MgO = Mg2CO4 at Earth’s Lower Mantle P–T Conditions

“…19 (numbers in parentheses indicate the number of formula units (f.u.) in the unit cell): ringwodite (Mg 2 SiO 4 -Fd3̅ m (8)), 20 porierite (Mg 2 SiO 4 -Pmma (4)), 21 wadsleyite (Mg 2 SiO 4 -Imma (8)), 22 forsterite (Mg 2 SiO 4 -Pnma (4)), 23 Zn 2 SiO 4 -Pbca (8), 20 Zn 2 SiO 4 -I4̅ 2d (4), 20 Na 2 SO 4 -Fddd (8), 24 Li 2 SO 4 -P2 1 /c (4), 25 and Ca 2 CO 4 -Pnma (4). 15 The calculations with the USPEX method 26−29 have been performed for 1−4 f.u.…”