Infrared vibrations inLaSrGaO4andLaSrAlO4

We have studied the c-axis and the in-plane infrared response of the anisotropic crystals of ${\mathrm{LaSrGaO}}_4$ and ${\mathrm{LaSrAlO}}_4.$ We identify both transverse and longitudinal phonons. The lattice bands show pronounced features of mode coupling, which we attribute to the random distribution of La and Sr atoms in the lattice. We present a scheme to identify and quantify the coupling that induces the asymmetric line shapes. We use the results to discuss infrared-active vibrations of other compounds, including ${\mathrm{La}}_2{\mathrm{CuO}}_4.$