# Pressure induced half-collapsed-tetragonal phase in CaKFe$_4$As$_4$

## TL;DR

This study maps the pressure-temperature phase diagram of CaKFe$_4$As$_4$, revealing a pressure-induced transition to a half-collapsed-tetragonal phase that suppresses superconductivity around 4 GPa.

## Contribution

It identifies a novel half-collapsed-tetragonal phase in CaKFe$_4$As$_4$ induced by pressure, combining experimental and theoretical evidence.

## Key findings

- Superconductivity is suppressed and disappears near 4 GPa.
- A phase transition to a half-collapsed-tetragonal structure occurs at ~4 GPa.
- Band structure calculations confirm As-As bonding across Ca-layer at transition.

## Abstract

We report the temperature-pressure phase diagram of CaKFe$_4$As$_4$ established using high pressure electrical resistivity, magnetization and high energy x-ray diffraction measurements up to 6 GPa. With increasing pressure, both resistivity and magnetization data show that the bulk superconducting transition of CaKFe$_4$As$_4$ is suppressed and then disappears at $p$ $\gtrsim$ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe$_4$As$_4$ under pressure that coincides with the abrupt loss of bulk superconductivity near 4 GPa. The x-ray data, combined with resistivity data, indicate that the collapsed tetragonal transition line is essentially vertical, occuring at 4.0(5) GPa for temperatures below 150 K. Band structure calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding takes place across the Ca-layer. Bonding across the K-layer only occurs for $p$ $\geq$ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe$_4$As$_4$: a half-collapsed-tetragonal phase.