In a growing number of materials, phonons have been found to generate a
thermal Hall effect, but the underlying mechanism remains unclear. Inspired by
previous studies that revealed the importance of Tb3+ ions in generating
the thermal Hall effect of Tb2Ti2O7, we investigated the role of
Tm3+ ions in TmVO4, a paramagnetic insulator with a different crystal
structure. We observe a negative thermal Hall conductivity in TmVO4 with a
magnitude such that the Hall angle, ∣κxy/κxx∣, is
approximately 1 x 10−3 at H = 15 T and T = 20 K, typical for a
phonon-generated thermal Hall effect. In contrast to the negligible
κxy found in Y2Ti2O7, we observe a negative
κxy in YVO4 with a Hall angle of magnitude comparable to that of
TmVO4. This shows that the Tm3+ ions are not essential for the
thermal Hall effect in this family of materials. Interestingly, at an
intermediate Y concentration of 30 % in Tm1−xYxVO4,
κxy was found to have a positive sign, pointing to the possible
importance of impurities in the thermal Hall effect of phonons