We report the temperature-pressure phase diagram of CaKFe4As4 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 CaKFe4As4 is suppressed and then disappears at p≳ 4 GPa. High pressure x-ray data clearly indicate a phase transition to a collapsed tetragonal phase in CaKFe4As4 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≥ 12 GPa. These findings demonstrate a new type of collapsed tetragonal phase in CaKFe4As4: a half-collapsed-tetragonal phase.
@article{arxiv.1702.06622,
title = {Pressure induced half-collapsed-tetragonal phase in CaKFe$_4$As$_4$},
author = {Udhara S. Kaluarachchi and Valentin Taufour and Aashish Sapkota and Vladislav Borisov and Tai Kong and William R. Meier and Karunakar Kothapalli and Benjamin G. Ueland and Andreas Kreyssig and Roser Valentí and Robert J. McQueeney and Alan I. Goldman and Sergey L. Bud'ko and Paul C. Canfield},
journal= {arXiv preprint arXiv:1702.06622},
year = {2017}
}