Suppression of magnetism and development of superconductivity within the collapsed tetragonal phase of ${\mathbf{Ca}}_{\mathbf{0}.\mathbf{67}}{\mathbf{Sr}}_{\mathbf{0}.\mathbf{33}}{\mathbf{Fe}}_{\mathbf{2}}{\mathbf{As}}_{\mathbf{2}}$ under pressure

Structural and electronic characterizations of (${{\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2\mathrm{As}}_2$ have been performed as a function of pressure up to 12 GPa using conventional and designer diamond anvil cells. The compound (${{\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2\mathrm{As}}_2$ behaves intermediately between its end members, displaying a suppression of magnetism and the onset of superconductivity. Like other members of the $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ family, (${{\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2\mathrm{As}}_2$ undergoes a pressure-induced isostructural volume collapse, which we associate with the development of As-As bonding across the mirror plane of the structure. This collapsed tetragonal phase abruptly cuts off the magnetic state and supports superconductivity with a maximum $T_c=22.2\mathrm{K}$. The maximum $T_c$ of the superconducting phase is not strongly correlated with any structural parameter, but its proximity to the abrupt suppression of magnetism as well as the volume-collapse transition suggests that magnetic interactions and structural inhomogeneity may play a role in its development.

Figure 1

An example x-ray diffraction pattern acquired at 1.98 GPa in a DAC. The refinement runs through the data points as a red line, and the residual of the refinement is shown below the pattern as a light blue line. Bragg reflections of the (${{\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2\mathrm{As}}_2$ sample are shown as red tick marks, whereas, Bragg peaks from the Cu pressure marker are indicated by the green stars.

Figure 2

Structural parameters extracted from refinements of x-ray diffraction patterns under pressure: (a) tetragonal lattice parameters $c$ (red circles, left axis) and $a$ (blue squares, right axis), (b) unit-cell volume (green diamonds, left axis) and $c/a$ ratio (orange triangles, right axis). The inset shows the refined $z$ coordinate of the As site as a function of pressure. In all cases, lines are guides to the eye.

Figure 3

Normalized electrical resistivity as a function of temperature for selected pressures (denoted in gigapascals unless otherwise specified). The magnetic/structural transition and the onset of superconductivity are visible in (a). The downward arrows ($T_{ct}$) indicate inflection points in the electrical resistivity curves (see text). The evolution of the superconducting transition with pressure is highlighted in (b).

Figure 4

Phase diagram of (${{\mathrm{Ca}}_{0.67}{\mathrm{Sr}}_{0.33}){\mathrm{Fe}}_2\mathrm{As}}_2$ showing the suppression of magnetism ($T_N$—red squares), the development of superconductivity ($T_c$—blue circles), and the progression of the volume-collapse transition ($T_{ct}$—green, crossed squares) with pressure. The room-temperature value of $T_{ct}$ is determined from x-ray diffraction data; all other data points are from electrical transport measurements. The open blue circles at lower pressures represent incomplete superconducting transitions. Lines and shaded regions are guides to the eye.

Figure 5

Pressure dependence of the As-As distance ($d_{\mathrm{As}-\mathrm{As}}$) across the mirror plane of the crystal structure shown in the inset. Lines through the data are guides to the eye.

Figure 6

(a) Schematic density of states of the ${{\mathrm{B}}_2\mathrm{As}}_2^{-2}$ layers for a hypothetical $A{\mathrm{B}}_2{X}_2$ compound. The Fermi level lowers with increasing $d$-band occupancy, depleting the As-As antibonding states, creating an As-As mirror plane bond, and collapsing the structure. (b) Schematic unit-cell energy (adapted from Ref. 42) versus $d_{\mathrm{As}-\mathrm{As}}$ for a hypothetical $A{\mathrm{B}}_2{X}_2$ specimen; pressure or $d$-electron element substitution should push $d_{\mathrm{As}-\mathrm{As}}$ leftward, providing a driving force for the collapsed phase.

Figure 7

Comparison of the pressure dependence of $d_{\mathrm{As}-\mathrm{As}}$ for the alkaline-earth $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ compounds. The horizontal dashed line indicates $d_{\mathrm{As}-\mathrm{As}}=3.0\AA$. Data for ${{\mathrm{CaFe}}_2\mathrm{As}}_2,{{\mathrm{SrFe}}_2\mathrm{As}}_2$, and ${{\mathrm{BaFe}}_2\mathrm{As}}_2$ are from Refs. 22, 25, and 26, respectively. Inset: volume-collapse transition pressure $P_{ct}$ as a function of unit-cell volume at $P=0$. Lines through data points are guides to the eye.

Figure 8

(a) The twofold $\alpha$ and fourfold $\beta$ As-Fe-As bond angles as a function of pressure. The ideal tetrahedral angle (${109.47}^{\circ }$) is marked by the horizontal green dashed line. The inset defines $\alpha$ and $\beta$ within the corrugated FeAs component of the crystal structure. (b) The pressure dependence of $T_c$ on the same pressure axis; symbols identical to Fig. 4. Lines are guides to the eye.

Figure 9

Comparison of the temperature-pressure phase diagrams for $\left(A\right){{\mathrm{Fe}}_2\mathrm{As}}_2$. Closed circles represent $T_N$, open diamonds represent $T_c$, and crossed or diagonal boxes represent $T_{ct}$. The volume-collapse transition as determined by ET and XRD, respectively. Electrical transport data for ${{\mathrm{CaFe}}_2\mathrm{As}}_2,{{\mathrm{SrFe}}_2\mathrm{As}}_2$, and ${{\mathrm{BaFe}}_2\mathrm{As}}_2$ are from Refs. 39, 40, and 47; structural data are from Refs. 22, 25, and 26. Lines and shaded regions are guides to the eye.