Fe2N-type SiO2 from shocked quartz

High pressure induced phase transformation of SiO2 and GeO2: difference and similarity

“…The presence of a small amount of d-NiAs phase together with other phases (rutile-or CaCl 2 -types and untransformed material) in the case of a-quartz as starting material has been observed in shockwave experiments [31] and DAC experiments after laser heating [20]. According to our data, the d-NiAs is a pre-rutile/CaCl 2 -types phase and can be formed only by heating of disordered structures.…”

“…Temperature induced crystallization of the amorphous phase to the disordered NiAs-type structure was detected for SiO 2 at pressures above , 30 GPa to at least 60 GPa in a temperature interval of 1000 -1300 K. The disordered NiAs-type structure as a possible metastable post stishovite phase was reported on the basis of static and shock-wave experiments on vitreous glass above 30 GPa for SiO 2 and above 25 GPa for GeO 2 [32] [20,31]. However, the high pressure -temperature stability range of the d-NiAs phase and its relation to the rutile phase remains undefined.…”

“…compression rate and pressure gradient. Temperature induced transformation of this partially disordered crystalline structure to rutile-type (or CaCl 2 -type) may be accompanied by formation of d-NiAs structure at temperatures below 1400 K. The shocked quartz actually shows the presence of , 5% glass [31] that can be a source for hexagonal d-NiAs-type phase. Co-existence of both phases, mainly rutile/CaCl 2 -types and much lower percentage of d-NiAs-types in these experiments [20,31], confirms this interpretation.…”

“…Temperature induced transformation of this partially disordered crystalline structure to rutile-type (or CaCl 2 -type) may be accompanied by formation of d-NiAs structure at temperatures below 1400 K. The shocked quartz actually shows the presence of , 5% glass [31] that can be a source for hexagonal d-NiAs-type phase. Co-existence of both phases, mainly rutile/CaCl 2 -types and much lower percentage of d-NiAs-types in these experiments [20,31], confirms this interpretation. Full profile refinement for the d-NiAs-type phase can be obtained only for cations randomly occupying one-half of octahedral site in the ratio of Wyck positions 2a=4e , 0:3 and cannot be fit by any of the CdI 2 structure types that are based on ordered cations in an hcp network of anions.…”

“…The d-NiAs-type structure as a possible metastable postrutile phase of SiO 2 and GeO 2 was reported on the basis of static and shock-wave experiments on vitreous silica glass [20,31]. However, we have determined that the d-NiAs-type structure precedes rutile/CaCl 2 -types and can be synthesized only from disordered materials in the narrow temperature range of 1000 -1300 K and pressures above 30 GPa to at least 60 GPa (the maximum pressure in this study).…”