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Register a new userPressure induced bcc to hcp transition in Fe: Magnetism-driven structure transformation
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The pressure induced bcc to hcp transition in Fe has been investigated via ab-initio electronic structure calculations. It is found by the disordered local moment (DLM) calculations that the temperature induced spin fluctuations result in the decrease of the energy of Burgers type lattice distortions and softening of the transverse $N$-point $TA_1$ phonon mode with $[bar{1}10]$ polarization. As a consequence, spin disorder in an system leads to the increase of the amplitude of atomic displacements. On the other hand, the exchange coupling parameters obtained in our calculations strongly decrease at large amplitude of lattice distortions. This results in a mutual interrelation of structural and magnetic degrees of freedom leading to the instability of the bcc structure under pressure at finite temperature.
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Many structural transformations involve a group-nonsubgroup relationship between the initial and transformed phases, and hence are beyond the purview of conventional Landau theory. We utilize a systematic and robust methodology to describe such reconstructive martensitic transformations by coupling group-theoretical arguments to first-principles calculations. In this context we (i) use a symmetry-based algorithm to enumerate transformation paths, (ii) evaluate the energy barriers along these transformation paths using all-electron first principles calculations, (iii) deduce the full set of primary and secondary order parameters for each path to establish the appropriate Ginzburg-Landau free-energy functionals, and (iv) for each path, identify special points of the primary order parameter, as a function of local distortions, corresponding to the end product phase. We apply this method to the study of a pressure driven body-centered cubic (bcc) to hexagonal close-packed (hcp) transformation in titanium. We find a generalization of the Burgers mechanism, and also find that there is no energy barrier to this transformation. In fact, surprisingly, we also find a region of volumes in which the intermediate path becomes more stable than either of the end-points (bcc or hcp). We therefore predict a new orthorhombic phase for Ti between 51 and 62 GPa.
The pressure-induced phase transition of bismuth telluride, Bi2Te3, has been studied by synchrotron x-ray diffraction measurements at room temperature using a diamond-anvil cell (DAC) with loading pressures up to 29.8 GPa. We found a high-pressure body-centered cubic (bcc) phase in Bi2Te3 at 25.2 GPa, which is denoted as phase IV, and this phase apperars above 14.5 GPa. Upon releasing the pressure from 29.8 GPa, the diffraction pattern changes with pressure hysteresis. The original rhombohedral phase is recovered at 2.43 GPa. The bcc structure can explain the phase IV peaks. We assumed that the structural model of phase IV is analogous to a substitutional binary alloy; the Bi and Te atoms are distributed in the bcc-lattice sites with space group Im-3m. The results of Rietveld analysis based on this model agree well with both the experimental data and calculated results. Therefore, the structure of phase IV in Bi2Te3 can be explained by a solid solution with a bcc lattice in the Bi-Te (60 atomic% tellurium) binary system.
We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe.
Recent experiments showed that Co undergoes a phase transition from ferromagnetic hcp phase to non-magnetic fcc one around 100 GPa. Since the transition is of first order, a certain region of co-existence of the two phases is present. By means of textit{ab initio} calculations, we found that the hcp phase itself undergoes a series of electronic topological transitions (ETTs), which affects both elastic and magnetic properties of the material. Most importantly, we propose that the sequence of ETTs lead to the stabilisation of a non-collinear spin arrangement in highly compressed hcp Co. Details of this non-collinear magnetic state and the interatomic exchange parameters that are connected to it, are presented here.
Molecular dynamics simulations have been performed to understand the influence of temperature on the tensile deformation and fracture behavior of $<$111$>$ BCC Fe nanowires. The simulations have been carried out at different temperatures in the range 10-1000 K employing a constant strain rate of $1times$ $10^8$ $s^{-1}$. The results indicate that at low temperatures (10-375 K), the nanowires yield through the nucleation of a sharp crack and fails in brittle manner. On the other hand, nucleation of multiple 1/2$<$111$>$ dislocations at yielding followed by significant plastic deformation leading to ductile failure has been observed at high temperatures in the range 450-1000 K. At the intermediate temperature of 400 K, the nanowire yields through nucleation of crack associated with many mobile 1/2$<$111$>$ and immobile $<$100$>$ dislocations at the crack tip and fails in ductile manner. The ductile-brittle transition observed in $<$111$>$ BCC Fe nanowires is appropriately reflected in the stress-strain behavior and plastic strain at failure. The ductile-brittle transition increases with increasing nanowire size. The change in fracture behavior has been discussed in terms of the relative variations in yield and fracture stresses and change in slip behavior with respect to temperature. Further, the dislocation multiplication mechanism assisted by the kink nucleation from the nanowire surface observed at high temperatures has been presented.
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