Exotic behavior and crystal structures of calcium under pressure

TL;DR

This study uses ab initio simulations to predict new high-pressure phases of calcium, revealing a different structural sequence and superconducting properties that align well with experimental data.

Contribution

It introduces a novel theoretical phase sequence for calcium under pressure, including the prediction of a 1/amd structure as the ground state and complex host-guest phases at higher pressures.

Findings

1/amd structure predicted as ground state at 33-71 GPa

Superconducting transition temperature reaches ~20 K at 120 GPa

New complex host-guest structure identified above 134 GPa

Abstract

Experimental studies established that calcium undergoes several counterintuitive transitions under pressure: fcc \rightarrow bcc \rightarrow simple cubic \rightarrow Ca-IV \rightarrow Ca-V, and becomes a good superconductor in the simple cubic and higher-pressure phases. Here, using ab initio evolutionary simulations, we explore the behavior of Ca under pressure and find a number of new phases. Our structural sequence differs from the traditional picture for Ca, but is similar to that for Sr. The {\beta}-tin (I41/amd) structure, rather than simple cubic, is predicted to be the theoretical ground state at 0 K and 33-71 GPa. This structure can be represented as a large distortion of the simple cubic structure, just as the higher-pressure phases stable between 71 and 134 GPa. The structure of Ca-V, stable above 134 GPa, is a complex host-guest structure. According to our calculations, the predicted phases are superconductors with Tc increasing under pressure and reaching ~20 K at 120 GPa, in good agreement with experiment.