  We performed Raman studies and a dielectric characterization of the
pseudo-kagome Cu3Bi(SeO3)2O2X (X = Cl, Br). These compounds share competing
nearest-neighbour ferromagnetic exchange and frustrating next-nearest-neighbour
antiferromagnetic exchange as well as highly noncollinear magnetic ground
state. However, at low temperature they differ with respect to the existence of
inversion symmetry. For both compounds there exists a pronounced interplay of
polar phonon modes with quantum magnetic fluctuations. A novel Raman mode
appears for temperatures below the Neel temperature with a Fano lineshape and
an enormous intensity that exceeds most of the phonon lines. We discuss a
possible contribution of longitudinal magnons to this signal. In contrast, one
magnon scattering based on linear transvers magnons is excluded based on a
symmetry analysis of spin wave representations and Raman tensor calculations.
There exists evidence that in these pseudo-kagome compounds magnetic quantum
fluctuations carry an electric dipole moment. Our data as well as a comparison
with previous far-infrared spectra allow us to conclude that Cu3Bi(SeO3)2O2Cl
changes its symmetry most likely from Pmmn to P21mn with a second order
structural phase transition at T*=120 K and becomes multiferroic.
Cu3Bi(SeO3)2O2Br represents an interesting counter part as it does not show
this instability and stays inversion symmetric down to lowest temperatures,
investigated.
