Nb-Si based in-situ composites, which have been extensively studied in the past decade, are still attracting much interest due to their possible applications as the next generation superalloys. Numerous efforts have been made to achieve the best compromise between room temperature toughness and high temperature strength by microstructure control during material preparation. A prominent eutectic solidification from melt to Nb and Nb3Si phases and a later eutectoid decomposition of Nb3Si to Nb and Nb5Si3 at lower temperature can be observed in the Nb-Si system. It has been argued that these two invariant reactions have great significance on microstructure control, in particular, the stability of Nb3Si phase strongly affects the final microstructure of the material.
A systematic ab-initio study of the energetic influences of various dopants on Nb3Si eutectoid decomposition is of particular importance, since it not only provides a deep insight into the current experimental observations, but also sheds light on the microstructure control and design of Nb-Si superalloy.
We have calculated the heats of formation of Nb, Nb5Si3 and Nb3Si phases with different dopants using ab-initio methods. By careful comparison of our theoretical results with experimental observations, we rationalize the site preferences of these dopants in Nb5Si3 and Nb3Si intermetallic compounds, and their phase partitionings between the ground state Nb and Nb5Si3 phases in Nb-Si alloy. We predict the influences of these dopants on the eutectoid decomposition of Nb3Si.
PbTe has garnered significant interest as a thermoelectric material. It was found that the undistorted ground state structure of PbTe transforms to a paraelectric phase at elevated temperatures, which contradicts the conventional picture of ferroelectric-paraelectric transformation. In addition, the phonon dispersions in PbTe were recently measured using INS at different temperatures. It was found that the phonon scattering in PbTe is strongly anharmonic and displays anomalous behavior at elevated temperatures, i.e. the 'waterfall' effect of the transverse optical (TO) phonon mode at Γ point and the avoided-crossing behavior between the longitude acoustic and TO branches in the Γ→X direction. More importantly, a new spectral feature emerges at the zone center at finite temperature, which is a clear signature of strong interactions and requires a theoretical explanation.
We have performed molecular dynamic simulations using a Taylor expansion interactomic potential obtained from first-principles calculations. The temperature dependent phonon spectra is computed and compared to experimental measurements, yielding insight on the origin of the observed anomalies.
Experimental determination of the crystal structures of materials becomes difficult under extremely high pressure. Fortunately, recent developments in the methods for theoretical predictions of crystal structures provide an effective way in search of novel high-pressure phases. A number of successful applications of these methods which include the ab initio random structure searching and evolutionary approaches have been reported recently. The successful applications of the structure searching approaches show their capabilities of predicting complex configurations given only the compositions of the system.
A large number of elements and compounds undergo superconducting transitions under pressure, in particular, the critical temperatures Tc may increase with hydrostatic pressures. Therefore, the predictions of the crystal structures of high-pressure phases become invaluable in searching for high Tc superconductors.