Sci Rep. 2012;2:372. doi: 10.1038/srep00372. Epub 2012 Apr 20.
Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the "simple cubic" (SC) unit cell adopted by calcium over a broad pressure ranging from 30-90 GPa and at temperature as low as 40 K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the [110] plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s → d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes.
实验证据表明，在 30-90 GPa 的宽压力范围和低至 40 K 的温度下，钙采用的“简单立方”（SC）单元晶格会出现异常大且各向异性的振动。X 射线衍射图谱显示（110）布拉格反射的优先展宽，表明原子位移不是各向同性的，而是局限于[110]平面。这种不寻常的观察可以用简单的化学观点来解释。由于压力诱导的 s → d 跃迁，处于简单立方结构的八面体环境中的钙原子受到 Jahn-Teller 变形。第一性原理分子动力学计算证实了这一假设，并表明由于立方单元晶格经历大振幅四方波动，变形是动态的。目前的结果表明，即使在极端压缩下，原子构型也具有高度的流动性，因为它在不断变化。
