Density Functional Theory Description of Paramagnetic Hexagonal Close-Packed Iron.

Youngwon Choi,Wei Li,Levente Vitos,Raquel Lizárraga,Zhihua Dong,Se-Kyun Kwon

doi:10.3390/ma15041276

Abstract

The hexagonal close-packed (hcp) phase of iron is unstable under ambient conditions. The limited amount of existing experimental data for this system has been obtained by extrapolating the parameters of hcp Fe–Mn alloys to pure Fe. On the theory side, most density functional theory (DFT) studies on hcp Fe have considered non-magnetic or ferromagnetic states, both having limited relevance in view of the current understanding of the system. Here, we investigate the equilibrium properties of paramagnetic hcp Fe using DFT modelling in combination with alloy theory. We show that the theoretical equilibrium and the equation of state of hcp Fe become consistent with the experimental values when the magnetic disorder is properly accounted for. Longitudinal spin fluctuation effects further improve the theoretical description. The present study provides useful data on hcp Fe at ambient and hydrostatic pressure conditions, contributing largely to the development of accurate thermodynamic modelling of Fe-based alloys.

Highlights

Iron is one of the most common elements on Earth, constituting much of Earth’s outer and inner core
Since spin fluctuations make a sizeable effect on the equilibrium c/a, we examined the approximations used in the present spin fluctuations scheme, such as fluctuating medium approximation (FMA), one-shot from static equilibrium approach (OSA) and mean moment approximation [36,44,45,50,55,56]
We show that the prediction of c/a becomes more accurate when assuming a paramagnetic state for hcp Fe with magnetic moments corresponding to about 1 μ B than the values predicted when using non-magnetic or antiferromagnetic states

Summary

Introduction

Iron is one of the most common elements on Earth, constituting much of Earth’s outer and inner core. Iron has two crystal structures; the body-centred cubic (bcc) and the face centred cubic (fcc) [1]. Above ≈10 GPa, iron is observed in the hexagonal close-packed (hcp) structure [2]. The latter phase has been extensively studied due to its relevance to Earth’s inner-core research [3,4,5,6,7]. The properties of the hcp phase of Fe at zero pressure are very important as well. Thermodynamic modelling requires information about hcp Fe to describe the phase diagram of

Methods

Results

Conclusion