Discovery of a low-temperature orthorhombic phase of the Cd_{2}Re_{2}O_{7} superconductor

The favorable combination of Raman spectroscopy of the lattice vibrations with ab initio phonon calculations enables us to pinpoint a low-temperature orthorhombic phase which hosts the cryptic 1 K superconducting phase of the 5d pyrochlore-type superconductor Cd_{2}Re_{2}O_{7}. Raman studies are performed on crystals with the natural isotope abundance and cadmium and oxygen isotope enriched samples. Characteristic splitting and the appearance of sharp phonon modes in the temperature-dependent Raman spectra prove a structural phase below ∼80 K, induced by a soft-mode phonon instability of the intermediate-temperature tetragonal crystal structure. The theory predicts that this low-temperature orthorhombic crystal structure is described by the space group F222; it exhibits no phonon instabilities and has the lowest total energy. Future experiments on the F222 phase should establish whether spin-triplet components contribute to the superconducting phase.