  A recent experimental study of TbB4 at a low temperature using resonant x-ray
Bragg diffraction implies a magnetic symmetry not found in any other rare-earth
tetraboride. The evidence for this assertion is a change in the intensity of a
TbB4 Bragg spot on reversing the handedness (chirality) of the primary x-ray
beam [Misawa et al., Phys. Rev. B 108, 134433 (2023)]. It reveals a magnetic
chiral signature in TbB4 that is forbidden in phases of rare-earth tetraborides
known to date. For, the previous magnetic symmetries are parity-time
(PT)-symmetric with anti-inversion present in the magnetic crystal class.
Misawa et al. appeal to a (PT)-symmetric diffraction pattern to interpret their
interesting experimental results. In addition to the use of symmetry that does
not permit a chiral signature, calculated patterns impose cylindrical symmetry
on Tb sites with no justification. We review magnetic symmetries for TbB4
consistent with a published neutron powder diffraction pattern and
susceptibility measurements. The investigated symmetry templates are
non-centrosymmetric, non-collinear antiferromagnetic constructions with
propagation vector k = (0, 0, 0). An inferred chiral signature for a
parity-even absorption event has an interesting composition. There is the
anticipated product of Tb axial dipoles and charge-like quadrupoles (from
Templeton-Templeton scattering). Beyond this contribution, though, symmetry
allows a product of dipoles in the chiral signature. A predicted change in the
intensity of a Bragg spot with rotation of the crystal about the reflection
vector (an azimuthal angle scan) can be tested in future experiments. Likewise,
contributions to Bragg diffraction patterns from Tb anapoles and higher-order
Dirac multipoles.
