Infrared properties of T’-phase ${\mathit{R}}_2$${\mathrm{CuO}}_4$ insulating compounds

Reflectance studies on single crystals of a series of rare-earth (R=Pr, Nd, Sm, Gd) copper oxides have been made to determine phonon frequencies as a function of rare-earth mass. The increase in interatomic force constants with increasing rare-earth mass, which leads to a reduction in the lattice parameters, increases the vibrational frequencies. In addition, the change from the T-phase ${\mathrm{La}}_2$${\mathrm{CuO}}_4$ to the T’-phase ${\mathit{R}}_2$${\mathrm{CuO}}_4$ crystal structure causes changes in the phonon frequencies that are due both to the changing lattice parameters and to the relocation of the apical oxygen atoms.