Pressure-induced half-collapsed-tetragonal phase in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$

We report the temperature-pressure phase diagram of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ 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 ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ 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 ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ 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 independent of pressure, occurring at 4.0(5) GPa for temperatures below 150 K. Density functional theory calculations also find a sudden transition to a collapsed tetragonal state near 4 GPa, as As-As bonding develops across the Ca layer. Bonding across the K layer only occurs for $p\ge 12$ GPa. These findings demonstrate a different type of collapsed tetragonal phase in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ as compared to ${\mathrm{CaFe}}_2{\mathrm{As}}_2$: a half-collapsed tetragonal phase.

Figure 1

The ZFC $M$($T$) data for (a) 20 Oe and (b) 1000 Oe applied fields. (c) The temperature-dependent resistivity at representative pressures. Arrows for the 5.78 GPa curve indicate $T_{\text{c}}^{\text{onset}}$, $T_{\text{c}}^{\text{offset}}$, and $T_{\text{c}}^{\text{0}}$ criteria used in this Rapid Communication. The left and right insets show the temperature-dependent resistivity under pressure for 0 Oe and 30 kOe applied fields, respectively. (d) Superconducting transition widths, $\mathrm{\Delta }T=T_{\text{c}}^{\text{onset}}-T_{\text{c}}^{\text{offset}}$, for 0 and 30 kOe. The inset shows the difference of transition width, $\mathrm{\Delta }T\left(H\right)-\mathrm{\Delta }T\text{(0)}$, where $\mathrm{\Delta }T\left(H\right)$ is the transition width at 30 kOe

Figure 2

Temperature-pressure phase diagram of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, constructed from magnetization, resistivity, and x-ray-diffraction measurements under pressure. Solid and opened symbols represent $T_{\text{c}}$ for bulk and filamentary superconductivity, respectively. Red square symbols are from Ref. [29]. The green and blue data represent $T_{\text{c}}^{\text{offset}}$ and $T_{\text{c}}^{\text{onset}}$, respectively. The light-red shaded area represents the bulk superconductivity region. Half-filled pentagon and squares represent the data from high-pressure x-ray diffraction and the resistivity anomaly as seen in Figs. 3 and 4.

Figure 3

(a) Pressure-dependent $a$- (left axis) and $c$- (right axis) lattice parameters obtained from the high-pressure x-ray-diffraction study. The solid symbols represent the data measured within the temperature range of 130(25) K and open symbols are for higher or lower temperature than this range. The lattice parameters are obtained after fitting the one-dimensional (0,2,0) and (1,0,1) peaks. The one-dimensional peak is obtained from a two-dimensional pattern after selecting the region of interest around the peak and integrating azimuthally. The dashed-vertical lines mark the bound of the transition and represent the error bar in Fig. 2. (b) Theoretical $a$- (left axis) and $c$- (right axis) lattice parameters represented with the same scale as (a). (c) Change of the As-As distances across the K (left axis) and Ca (right axis) cationic layers under the transition to the collapsed tetragonal phase. Dashed lines are guides for the eye.

Figure 4

(a),(b) Evolution of the temperature-dependent resistivity of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ with pressure for two samples. (c), (d) Pressure dependence of resistivity at fixed temperature for the same two samples. The arrow indicates the observed resistivity anomaly at $p^{*}$.