Direct Observation of Amorphous to Amorphous Apparently First-Order Phase Transition in Fused Quartz

Abstract

We report for the first time the direct observation of an apparently first-order phase transition in amorphous silica from the low density amorphous phase to the high density amorphous phase with a discontinuous volume change of about 20% from the dilatometric measurements using the piston cylinder apparatus at 36 kbar and 680 degrees C. Our Raman spectroscopy measurements on the retrieved samples quenched from high pressure and high temperature experiments do not show any Raman peaks indicating a large modification in the intermediate range order in the structure of amorphous silica.

The pressure-amorphized state in zirconium tungstate: a precursor to decomposition

“…We obtain this from the ratio of the size L of the unannealed (amorphous) sample to that, L 0 , after annealing to 1000 K. From the data ( figure 1) the linear compression L/L 0 turns out to be 0.906 (5). Considering the amorphous phase to be isotropic, the volume compression V /V 0 then becomes 0.743 (12), where V and V 0 are the volumes in the amorphous and the cubic phases respectively.…”

“…Theoretical phase diagrams based on total energy calculations [25], not available for ZrW 2 O 8 so far, are also desirable as these would throw further light on the phenomena of pressure-induced amorphization and decomposition in this system. Compounds that undergo irreversible amorphization have often exhibited polyamorphism [26], i.e., the existence of low-and high-density amorphous phases, such as those in quartz [5] and ice [27]. It would be interesting to investigate whether this compound also exhibits polyamorphism.…”

“…At high pressure many materials exhibit structural phase transitions, while some become amorphous [1][2][3][4][5][6][7]. Although it is widely believed that the phenomenon of pressure-induced amorphization (PIA) arises due to kinetic hindrance of equilibrium phase transitions, the structure of the equilibrium high-pressure phase, which the compound should have ideally evolved to, has remained speculative or unknown in most cases.…”

“…On the other hand, across amorphization a reliable estimate of volume change is difficult to obtain from x-ray diffraction due to the absence of sharp diffraction peaks in the disordered state. Earlier, a large-volume pistoncylinder apparatus was used for obtaining V across amorphization in ice [1] and across the transformation of low-density a-SiO 2 to high-density a-SiO 2 [5]. Here we obtain the volume collapse V across the irreversible pressure-induced amorphization in zirconium tungstate ZrW 2 O 8 , a material with large isotropic negative thermal expansion [11][12][13][14], from the linear dimension of the p-amorphized samples during isochronal annealing leading to the crystallization to the original cubic structure.…”

The model developed for stress-induced structural phase transformations of micro-crystalline silicon films

“…One such prediction was a kinetically hindered first order amorphous to amorphous phase transition in SiO 2 [21]. An experiment has recently confirmed such a first order amorphous to amorphous phase transition [22]. Such calculations require candidate structures which may be obtained from constant pressure simulations and sometimes experiments [23,24].…”