  Thermal Hall effect (THE) in insulator is a remarkable phenomenon that arises
from the motion of chargeless quasi-particles under a magnetic field. While
magnons or exotic spin excitations were considered as the origin of THE in some
magnetic materials, there are more and more evidences suggesting that phonons
play a significant role. However, the mechanism behind phonon THE is still
unknown. Here we report the observation of THE, including planar THE, in a
broad range of non-magnetic insulators and semiconductors: SrTiO3, SiO2
(quartz), MgO, MgAl2O4, Si and Ge. While the presence of antiferrodistortive
domains in SrTiO3 and chiral phonons in SiO2 may complicate the interpretation
of THE, the striking observations of THE in trivial insulators MgO and MgAl2O4,
as well as in high-purity intrinsic semiconductors Si and Ge, demonstrate that
phonon THE is a universal property of crystals. Without other effects on
phonons such as from magnons, this universal phonon THE is characterized by a
scaling law of |\k{appa}_xy| ~ \k{appa}_xx^2. Our results experimentally
discover a fundamental physics of phonons in magnetic field, which should come
from the direct coupling between atom vibrations and the field. Starting from
this universal phonon THE in crystals, all previous interpretations of THE in
magnetic or non-magnetic materials need to be reconsidered.
