Phonon dispersion relation in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

The phonon spectrum in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ is calculated in the orthorhombic structure using an unscreened rigid-ion model as a first approximation. The calculated values of the ‘‘neutron-weighted’’ phonon density of states, the optical frequencies, the Debye temperature, and the mean-squared vibrational amplitudes are in fair agreement with the values reported from various experiments.