Low-temperature magnetic behaviour on the triangular lattice in hexagonal Ba${}_3$Tb(BO${}_3$)${}_3$

The hexagonal polymorph of Ba${}_3$Tb(BO${}_3$)${}_3$ contains Tb${}^{3+}$ ions on a quasi-2D triangular lattice, resulting in geometric magnetic frustration. Powder samples of Ba${}_3$Tb(BO${}_3$)${}_3$ have been investigated using specific heat, powder neutron diffraction (PND), inelastic neutron scattering (INS) and muon-spin relaxation spectroscopy ($\mu$SR). No long-range magnetic ordering is observed down to the lowest measured temperatures of 75 mK in PND and specific heat data and 1.5 K in the $\mu$SR data. Modelling the INS spectrum using a point charge model suggests that the ground state is a singlet with a low-lying doublet on each of the two crystallographically independent Tb${}^{3+}$ sites and that both the Tb ions display weak XY single-ion anisotropy.