Optical Phonons of Yttrium Aluminum Garnet

The dielectric properties and optical phonons of yttrium aluminum garnet (YAG) have been studied by measurements of the infrared (IR) reflection spectra and the Raman effect. A factor group analysis of the YAG space group shows that there are $18 T_{1u}$ IR-active modes, and $3 A_{1g}$, $8 E_g$, and $14 T_{2g}$ Raman-active modes. The IR data have been interpreted by a Kramers-Kronig transform to yield values of the complex dielectric constant and frequencies of the longitudinal and transverse long-wavelength $T_{1u}$ modes. 15 of the 17 predicted $T_{1u}$ optic modes have been observed. All the $A_{1g}$ and $E_g$ modes have been identified, but at least 3 of the $14 T_{2g}$ modes are missing from the observed Raman spectra.