[Back]

F. Klimashin, M. Arndt, P. Polcik, N. Koutná, H. Riedl, C.M. Koller, H. Euchner, D. Holec, P.H. Mayrhofer:
"The Power Of Vacancies In Transition Metal Nitrides";
Talk: VEIT 2017, Sosopol, Bulgarien (invited); 2017-09-25 - 2017-09-29; in: "Twentieth International Summer School On Vacuum, Electron And Ion Technologies", (2017), 53.

A variety of material properties (e.g. electrical, magnetic, and mechanical) is vastly influenced by the structural defects. These defects, and vacancies in particular, are, according to the principles of thermodynamics, unavoidably present in any crystalline solid as they increase the entropy of the system. A considerably higher concentration of structural defects (e.g. several orders of magnitude higher concentration of vacancies than their equilibrium concentration) can be inherent in solids when using non-equilibrium synthesis techniques, like PVD. In interstitial compounds of transition metals (TM), like borides, carbides, nitrides and oxides, the quenched-in vacancies prevalently form on both sublattices, so that ideal stoichiometry is particularly complex to achieve. Many of these compounds are berthollides with large homogeneity range.

Considering these aspects, the excellent agreement between experimental findings within PVD Ti-Al-N and earlier DFT-based calculations of defect-free Ti1−xAlxN crystals was perplexing. In an analogous manner (partly for the sake of simplicity), similar defect-free structures were taken as the basis for the calculations of many other TM-based interstitial compounds. However, Ti-Al-N turned out to be a "fortunate case" with in fact marginal impact of vacancies on its phase stability (but not elastic constants). For other material systems, the impact of structural defects (and in particular structural and constitutional vacancies) is much stronger and has to be taken into account. Our research activities of the recent years were dedicated to investigating - computationally and experimentally - the role of vacancies in phase stability, structural ordering, and mechanical properties of PVD TM and TM-based interstitial compounds.