Pressure-induced volume-collapsed tetragonal phase of ${\text{CaFe}}_2{\text{As}}_2$ as seen via neutron scattering

Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that ${\text{CaFe}}_2{\text{As}}_2$ undergoes a pressure-induced transition to a nonmagnetic volume “collapsed” tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.

Figure 1

(Color online) Scans through [(a) and (b)] nuclear and (c) magnetic peaks in neutron-diffraction pattern at selected temperatures and pressures. Note that the diffraction peaks change position dramatically due to the significant changes in the lattice parameters. In (c), the magnetic ${\left(121\right)}_{\text{OR},\text{magnetic}}$ diffraction peak for point A is clearly observed above the background taken at B. No new magnetic peaks for B and C were observed. Unlabeled peaks in the pattern arise from phases other than ${\text{CaFe}}_2{\text{As}}_2$, such as minor contamination from the Sn flux or ${\text{SiO}}_2$ (silica wool or pieces of the silica ampoule from the single-crystal growth), or the pressure cell. The subscripts denote the crystal structure used for indexing ($\mathrm{T}=\text{tetragonal}$; $\text{OR}=\text{orthorhombic}$). The offset between every data set is $200\text{counts}/3\times {10}^4$ monitor in (a) and $300\text{counts}/3\times {10}^4$ monitor in (b), respectively.

Figure 2

(Color online) Schematic phase diagram and pressure and temperature dependences of the lattice parameters, unit-cell volume, and As-Fe-As bond angles. (a) (left panel) Lines in the schematic $p-T$ diagram denote the high-temperature and superconductivity phase lines determined in Ref. 17. Points A–D label the pressures and temperatures for the diffraction data shown in Fig. 1. (a) (right panel) The unit cell of the tetragonal phase of ${\text{CaFe}}_2{\text{As}}_2$. (b)–(d) Pressure dependence (left panels) at $T=50\text{K}$ and temperature dependence (right panels) at $p=0.63\text{GPa}$ (b) of the lattice parameters, (c) of the unit-cell volume and $c/a$ ratio, and (d) of the As-Fe-As bond angle.

Figure 3

(Color online) Summary of the results of the neutron-diffraction measurements and total-energy calculations. (a) Spin-polarized (SP) and non-spin-polarized (NSP) total-energy calculations for $\Delta V/V=0\%$ and $\Delta V/V=-5\%$ for the tetragonal $\left(\mathrm{T}\right)$ and orthorhombic (OR) phases (note the different energy scales). The tetragonal notation is used for the $c/a$ ratio. (b) For the collapsed phase, the Fe moment is quenched at the minimum in total energy in the spin-polarized calculation. (c) Schematic $p-T$ diagram based upon the diffraction and transport (Ref. 17) measurements. Dashed lines represent estimates of the phase boundaries. The cross-hatched area indicates a region where precise details of how these transition lines intersect still need to be determined. (d) Correlation between the superconducting transition temperatures and the variance ${\sigma }^2$ of the As-Fe-As bond angle. The data were obtained from Refs. 5 and 25, 26, 27, 28, 29 for the $R\text{FeAs}\left(\text{O}/\text{F}\right)$ compounds and from Ref. 4 for $\left(\text{Ba}/\text{K}\right){\text{Fe}}_2{\text{As}}_2$, respectively. The data for ${\text{CaFe}}_2{\text{As}}_2$ are derived from the present measurements.

Figure 4

(Color online) Rietveld analysis of the neutron-diffraction pattern at $p=0\text{GPa}$ and $T=50\text{K}$. Areas with strong contributions from the pressure cell are excluded.

Figure 5

(Color online) Rietveld analysis of the neutron-diffraction pattern at $p=0.63\text{GPa}$ and $T=50\text{K}$. Areas with strong contributions from the pressure cell are excluded.

Figure 6

(Color online) Rietveld analysis of the neutron-diffraction pattern at $p=0.63\text{GPa}$ and $T=250\text{K}$. Areas with strong contributions from the pressure cell are excluded.