Seven T1u infrared active modes account for the pyrochlore structure. Decreasing A3+ cation size in the rare-earth zirconates increases disorder among the cations and the tendency to form the fluorite structure. There is a general trend of increasing absorption frequencies with smaller cell sizes for the higher-wave-number bands, whereas for the lower-wave-number bands there is a mass dependency. New assignments of lattice modes are made for the fifth and sixth highest bands.
Ronald A. McCauley, "Infrared-absorption characteristics of the pyrochlore structure," J. Opt. Soc. Am. 63, 721-725 (1973)