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

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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.