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

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