Dong Juncai, Zhu Hailiang, Chen Dongliang
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China.
Sci Rep. 2015 Jun 23;5:10810. doi: 10.1038/srep10810.
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 C44, which was proposed to trigger the transition through Born criteria B3. However, by using density-functional theory, we surprisingly found that both C44 and C66 in α-quartz exhibit strong nonlinearity under compression and the Born criteria B3 vanishes dominated by stiffening of C14, instead of by decreasing of C44. Further studies of archetypal quartz homeotypes (GeO2 and AlPO4) 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.
作为冰和类冰材料（特别是α-石英）中压力诱导非晶化（PIA）的一个基本特性，机械不稳定性的出现可能与违反弹性的玻恩准则有关。实验表明，PIA之前α-石英最显著的弹性特征是剪切模量C44的线性软化，有人提出这会通过玻恩准则B3引发转变。然而，通过使用密度泛函理论，我们惊讶地发现，α-石英中的C44和C66在压缩下都表现出强烈的非线性，并且玻恩准则B3消失，这主要是由C14的硬化主导，而不是C44的减小。对典型石英同质异构体（GeO2和AlPO4）的进一步研究反复再现了相同的弹性硬化驱动的机械不稳定性，这表明该晶体家族具有普遍特征，并挑战了长期以来的观点，即单个弹性模量的负压导数可被解释为PIA之前内在结构不稳定性的前驱效应。还讨论了这种弹性异常与最低声学支的色散软化以及可能的转变机制的关系。
