Equilibrium and metastable phase transitions in silicon nitride at high pressure: A first-principles and experimental study

Abstract

We have combined first-principles calculations and high pressure experiments to study pressure induced phase transitions in silicon nitride (Si3N4). Within the quasi-harmonic approximation, we predict that the α phase is always metastable relative to the β phase over the wide pressuretemperature range. Our lattice vibration calculations indicate that there are two significant and competing phonon-softening mechanisms in the β-Si3N4, while phonon softening in the α-Si3N4 is rather moderate.When the previously observed equilibrium high-pressure and high-temperature β→γ transition is by-passed at room temperatures (RT) due to kinetic reasons, the β phase is predicted to undergo a first-order structural transformation to a denser P6 phase above 39 GPa. The estimated enthalpy barrier height is less than 70 meV/atom, which suggests that the transition is kinetically possible around room temperatures. This predicted new high-pressure metastable phase should be classified as a "post-phenacite" phase. Our high-pressure X-ray diffraction experiment confirm this predicted room-temperature phase transition around 34 GPa. No similar RT phase transition is predicted for α-Si3N4. Furthermore, we discuss the differences in pressure dependencies of phonon modes in the α, β and γ phases and the consequences on their thermal properties. We attribute the phonon modes with negative Grüneisen ratios in the α and β phases as the cause of the predicted negative thermal expansion coefficients (TEC) at low temperatures in these two phases, and predict no negative TEC in the γ phase.

Phase relations in silicon and germanium nitrides up to 98 GPa and 2400°C

“…These results indicate that the β‐γ phase boundary is located at a pressure between 10 and 13 GPa under high temperatures from 1600 to 1800°C. On the other hand, the theoretically predicted β‐γ phase boundaries are inconsistent with each other and they are located at pressures between 6 and 12 GPa. With respect to the postspinel phase boundary, only some theoretical studies have been carried out and they predicted that the transition occurs at a pressure of 150‐210 GPa (ref.…”

Nitrogen-rich transition metal nitrides

“…The condensed nitrogen is introduced either as a cryogenic liquid or as a pressurized gas into DACs and can act as a pressure transmitting medium (PTM). In high-P,T experiments the use of a nitrogen PTM can stabilize nitride materials for studies under extreme conditions, and facilitate the formation of new structures [91][92][93]. It can also act as a reaction component in high-P,T synthesis experiments [17-20, 53, 91, 94].…”

Theoretical calculations of stability, mechanical and thermodynamic properties of IVA group Willemite-II nitrides

“…Among these nitrides, silicon nitride (Si 3 N 4 ) is a useful ceramic with numerous applications because of its high mechanical strength, high hardness, low density, high thermal stability, good oxidization resistance and other desirable hightemperature properties. [1][2][3][4][5][6][7][8] Experimental determinations and theoretical investigations of some C 3 N 4 polymorphs and Ge 3 N 4 were extensively researched. [9][10][11][12][13][14][15][16] The theoretically searched new materials among Willemite-II-C 3 N 4 and Willemite-II-Si 3 N 4 have cubic structure with space group I " 43d.…”