Universal elastic-hardening-driven mechanical instability in α-quartz and quartz homeotypes under pressure

As a fundamental property of pressure-induced amorphization (PIA) in ice and ice-like materials (notably α-quartz), the occurrence of mechanical instability can be related to violation of Born criteria for elasticity. The most outstanding elastic feature of α-quartz before PIA has been experimentally reported to be the linear softening of shear modulus C(44), which was proposed to trigger the transition through Born criteria B(3). However, by using density-functional theory, we surprisingly found that both C(44) and C(66) in α-quartz exhibit strong nonlinearity under compression and the Born criteria B(3) vanishes dominated by stiffening of C(14), instead of by decreasing of C(44). Further studies of archetypal quartz homeotypes (GeO(2) and AlPO(4)) repeatedly reproduced the same elastic-hardening-driven mechanical instability, suggesting a universal feature of this family of crystals and challenging the long-standing idea that negative pressure derivatives of individual elastic moduli can be interpreted as the precursor effect to an intrinsic structural instability preceding PIA. The implications of this elastic anomaly in relation to the dispersive softening of the lowest acoustic branch and the possible transformation mechanism were also discussed.