Experimental investigations on the extreme, pressure-induced stiffening of smart and phase change materials

某些材料的硬度随压力增加的速率可以超过常规晶体的十倍以上。一个例子是氢化磷酸锌的体积模量随压力的准线性增加，从环境条件下的30 GPa增加到5 GPa时的280 GPa。能够以这种或相关的方式调整其对外部刺激的反应功能的材料被称为“智能”。在目前的情况下，它们在用作摩擦表面的抗磨损涂层时被证明是有用的。然而，有时材料在压缩下会变软，如CrN，在致密相中其体积模量会降低25%。到目前为止，还没有通用的指导方针，因为大多数研究都是针对材料的，所以什么因素会影响材料的刚度随压力变化的方式。在我们的研究中，我们想探索这样一个假设，即对称性的变化驱动刚度的变化，特别是高对称性意味着与密度无关的刚度。这种简单的行为通常会被应变和相关阶参量之间的双线性耦合所抑制。了解对称性和刚度之间的联系将使我们能够确定新涂层材料所需的成分。我们研究的一个特点是，我们不仅要研究应变与位移模态的耦合，还要研究应变与电子结构的耦合。这种分析使我们能够探索设计材料的可能性，这种材料可以通过应力在半导体相和半金属相之间可逆地切换。为了验证我们的研究假设并构建特定材料的朗道理论，我们计划进行高压实验，包括对各种化合物的原位结构分析，包括金属磷酸盐以及由15族元素形成的掺杂和氧化固体。

The rate at which the stiffness of some materials increases with pressure can exceed that of regular crystals by more than a factor of ten. An example is the quasi-linear increase of the bulk modulus of hydrogenated zinc phosphates with pressure from 30 GPa at ambient conditions to 280 GPa at 5 GPa. Materials with the ability to adjust their response functions to an external stimulus in this or related ways have been coined “smart”. In the present context they prove useful when used as anti-wear coatings on rubbing surfaces. Sometimes, however, materials soften under compression, such as CrN, which reduces its bulk modulus by 25% in the denser phase. As of now, no general guidelines are known what factors influence the way in which the stiffness of a material changes in response to pressure, because most investigations are material specific. In our research we would like to explore the hypothesis that a change in symmetry drives the stiffness change, specifically that high symmetry implies stiff irrespective of the density. This simple behavior may often be suppressed by a bi-linear coupling between strain and the relevant order parameter. Understanding the connection between symmetry and stiffness will allow us to identify desired ingredients for new coating materials. A peculiarity of our research will be that we will investigate the coupling of the strain not only to displacive modes but also to the electronic structure. This analysis allows us to explore the possibility of designing materials that can be switched reversibly by stress between a semi-conducting and a semi-metallic phase. In order to validate our research hypothesis and to construct Landau theories for specific materials, we plan on conducting high-pressure experiments incl. in situ structure analysis of a large variety of compounds including metal phosphates as well as doped and oxidized solids formed by group 15 elements.