Suppression of Thermal Conductivity via Singlet-Dominated Scattering in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>TmFeO</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml

We measured the thermal conductivity of the rare-earth orthoferrites, RFeO_{3}, where R=Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb from 3 to 300 K and see an anomalous strong suppression for TmFeO_{3} over most of the temperature range. Using a Debye thermal transport model, we demonstrate that this suppression is due to resonant scattering between phonons and the Tm^{3+} 4f singlet crystal field levels. The implications of these results are discussed in the context of thermal conductivity studies in quantum magnets.