We use first-principles calculations to investigate the stability of biaxially strained Pnma perovskite CaMnO${}_3$ towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain-relaxation mechanism. Our first-principles calculations of oxygen vacancy defect formation energy indeed show a strong strain dependence: We find that tensile strain lowers the formation energy, consistent with the established chemical expansion concept that oxygen deficiency increases the molar volume in oxides. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.

We propose an extension of the van der Waals equation which is designed to incorporate, in an approximate fashion, the effects of the network of hydrogen bonds that exist in liquid water. The resulting model qualitatively predicts the unique thermodynamic properties of water, including those of the deeply supercooled states. It also reconciles two proposals for the phase behavior of supercooled and stretched water and provides a thermodynamic origin for the observed polymorphism of the amorphous solid form of water.

Fluids such as liquids and glasses are not usually thought of as possessing distinct forms or phases. As Poole et al . discuss in their Perspective, recent results show that liquids can exhibit “polymorphism”—different phases characterized by different densities.