Pressure-induced collapsed-tetragonal phase inSrCo2As2

Abstract

We present high-energy x-ray diffraction data under applied pressures up to $p=29\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, neutron diffraction measurements up to $p=1.1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, and electrical resistance measurements up to $p=5.9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, on ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$. Our x-ray diffraction data demonstrate that there is a first-order transition between the tetragonal (T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6 GPa at $T=7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The pressure for the onset of the cT phase and the range of coexistence between the T and cT phases appears to be nearly temperature independent. The compressibility along the $a$ axis is the same for the T and cT phases, whereas, along the $c$ axis, the cT phase is significantly stiffer, which may be due to the formation of an As-As bond in the cT phase. Our resistivity measurements found no evidence of superconductivity in ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$ for $p\ensuremath{\le}5.9$ GPa and $T\ensuremath{\ge}1.8$ K. The resistivity data also show signatures consistent with a pressure-induced phase transition for $p \ensuremath{\gtrsim}5.5$ GPa. Single-crystal neutron diffraction measurements performed up to 1.1 GPa in the T phase found no evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K. Spin-polarized total-energy calculations demonstrate that the cT phase is the stable phase at high pressure with a $\frac{c}{a}$ ratio of 2.54. Furthermore, these calculations indicate that the cT phase of ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$ should manifest either A-type antiferromagnetic or ferromagnetic order.