  Y-kapellasite [Y3Cu9(OH)19Cl8] is a frustrated antiferromagnetic insulator
which remains paramagnetic down to a remarkably low N\'eel temperature of about
2 K. Having studied this material in the paramagnetic regime, in which phonons
are the only possible heat carriers, we report the observation of a planar
parallel thermal Hall effect coming unambiguously from phonons. This is an
advantage over the Kitaev quantum spin liquid candidates {\alpha}-RuCl3 and
Na2Co2TeO6 where in principle other heat carriers can be involved [1-4]. As it
happens, Y-kapellasite undergoes a structural transition attributed to the
positional freezing of a hydrogen atom below about 33 K. Above this transition,
the global crystal symmetry forbids the existence of a planar parallel signal -
the same situation as in Na2Co2TeO6 and cuprates [3-5]. This points to the
notion of a local symmetry breaking at the root of the phonon Hall effect. In
this context, the advantage of Y-kapellasite over Na2Co2TeO6 (with high levels
of Na disorder and stacking faults) and cuprates (with high levels of disorder
coming from dopants and oxygen vacancies) is its clean structure, where the
only degree of freedom available for local symmetry breaking is this hydrogen
atom randomly distributed over six equivalent positions above 33 K. This
provides a specific and concrete case for the general idea of local symmetry
breaking leading to the phonon Hall effect in a wide range of insulators.
