  We present high-energy x-ray diffraction data under applied pressures up to p
= 29 GPa, neutron diffraction measurements up to p = 1.1 GPa, and electrical
resistance measurements up to p = 5.9 GPa, on SrCo2As2. 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 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 SrCo2As2 for p <= 5.9
GPa and T >= 1.8 K. The resistivity data also show signatures consistent with a
pressure-induced phase transition for p >= 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 c/a ratio of 2.54. Furthermore, these
calculations indicate that the cT phase of SrCo2As2 should manifest either
A-type antiferromagnetic or ferromagnetic order.
