Unravelling the mechanism of a pressure induced polyamorphic transition in an inorganic glass

B. Kalkan, G. Okay, B.G. Aitken, S.M. Clark and S. Sen, Scientific Reports 10:5208 https://doi.org/10.1038/s41598-020-61997-x (2020).

The atomic structure of a germanium doped phosphorous selenide glass of composition Ge2.8P57.7Se39.5 is determined as a function of pressure from ambient to 24 GPa using Monte-Carlo simulations constrained by high energy x-ray scattering data. The ambient pressure structure consists primarily of P4Se3 molecules and planar edge shared phosphorus rings, reminiscent of those found in red phosphorous as well as a small fraction of locally clustered corner-sharing GeSe4 tetrahedra. This low-density amorphous phase transforms into a high-density amorphous phase at ~ 6.5 GPa. The high-pressure phase is characterized by an extended network structure. The polyamorphic transformation between these two phases involves opening of the P3 ring at the base of the P4Se3 molecules and subsequent reaction with red phosphorus type moieties to produce a cross linked structure. The compression mechanism of the low-density phase involves increased molecular packing, whereas that of the high pressure phase involves an increase in the nearest-neighbor coordination number while the bond angle distributions broaden and shift to smaller angles. The entropy and volume changes associated with this polyamorphic transformation are positive and negative, respectively, and consequently the corresponding Clapeyron slope for this transition would be negative. This result has far reaching implications in our current understanding of temperature induced strong-to-fragile transitions in glass-forming liquids.

Fig 1. | X-ray scattering data. (a) Experimental and EPSR simulated total structure factors S(Q) at ambient pressure and temperature. (b) Evolution of S(Q) during compression. Data are vertically offset to enhance clarity and corresponding pressures in GPa are given alongside each pattern. (c) Pressure dependence of the position of the FSDP. (d) Pressure dependence of the position of the PP in the S(Q). Solid lines through the data points in (c) and (d) are guides to the eye.

Fig 2. | Structure of GPS glass in real space at ambient condition. (a) The radial distribution functions g(r) of GPS glass calculated from the EPSR simulations (solid black lines) compared with g(r) obtained by Fourier transforming the experimental S(Q) data (blue dots). (b) Ge-Se, P-P, P-Se, and Se-Se PDFs, as obtained from Fourier transformation of the corresponding partial structure factors.

Fig 3. | Pressure dependence of RDF. (a) Pressure dependence of g(r) during compression of the GPS glass. Data are vertically offset for clarity and corresponding pressures in GPa are given alongside each pattern. Pressure dependence of the first (b), second (c), third and fourth (d) peak positions in total g(r) and in the P-P, P-Se, Ge-Se, and Se-Se PDFs.

Fig 4. | Coordination environments of Ge, P, and Se atoms. (a) Nearest-neighbor coordination numbers at ambient pressure and temperature as obtained by integrating the area of the corresponding peaks in the PDFs. (b) Pressure dependence of the 1st shell coordination numbers, nGeSe, nPP, and nPSe. (c) The average and the 1st shell total coordination numbers around P and Se atoms as a function of pressure. The lines and curves through the data points in (b) and (c) are guides to the eye only. (d) The ambient BAD functions obtained from the EPSR simulations.

Fig 5. | Bond angle distributions. Pressure dependence of P−P−P (1^st shell), P−Se−P (1^st shell), Se-P-Se (1^st shell) and Se-Se-Se (2^nd shell) bond angle distributions obtained from the EPSR simulations. Data are vertically offset for clarity and corresponding pressures in GPa are given alongside each pattern for the Se-P-Se BAD. Curves for other distributions are plotted in the same sequence.