Magneto-structural transformations via a solid-state nudged elastic band method: Application to iron under pressure

Abstract

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter, in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volumecollapse during the pressure-induced transformation in iron-from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom. We extend the solid-state nudged elastic band method to handle a non-conserved order parameter, in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron-from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom. C 2015 AIP Publishing LLC. [http://dx

Reliable thermodynamic estimators for screening caloric materials

“…102 So, the electronic transformation is accompanied by discontinuity in pressure that drives the volume change. 103 The possible causes for electronic transitions include initial structure change or application of an external field. In particular, as is well established, the magnetostructural transformation of FeRh can be caused by application of an external magnetic field and/or stress, strain, or thermal expansion.…”

Two-dimensional polar metal of a PbTe monolayer by electrostatic doping

“…[10][11][12][13][14][15][16] They are endowed with a plethora of novel properties and physical phenomenon, and show promising potential in applications of nanoscale electronic and optoelectronic devices, and energy storage and conversion technologies. [17][18][19][20] Very recently, a few 2D materials have been experimentally demonstrated or theoretically proposed to have non-trivial intrinsic ferroelectricity [21][22][23][24][25][26][27][28][29], although the depolarization field is significantly enhanced at nanoscale and is believed to suppress electric polarization in common ferroelectrics. Despite the tremendous progresses, however, metallic ferroelectricity in atomistic 2D materials is rarely explored, since both conducting electrons and reduced dimensions are supposed to quench ferroelectricity.…”

“…Polarization switching in this ferroelectric monolayer can be achieved by folding the direction of Pb-Te bonds. We further employed the solid-state nudge elastic band (SSNEB) method [22,23] to estimate the minimum energy pathway for polarization conversion. The results are presented in Figure 2(b), in which the initial and final states are set as F and F phases, respectively.…”

Viable Materials with a Giant Magnetocaloric Effect

“…In general, transition temperature T c of a magnetostructural [329] or magneto-volume phase transition depends on external magnetic field, stress, and strain. Response of a caloric material to one physical field does not disallow response to others.…”