Structure and magnetism of the triangular lattice material YbBO₃

YbBO₃ is a member of the orthoborate family of materials which contains a triangular arrangement of Yb³⁺ ions. Here we study the physical properties of YbBO₃ with neutron diffraction, inelastic neutron scattering, specific heat, and ac susceptibility measurements. The neutron diffraction measurements confirm that our samples of YbBO₃ crystallize in the monoclinic space group C 2 / c (#15) which contains two crystallographically distinct Yb³⁺ sites decorating a slightly distorted triangular lattice. Heat capacity and ac susceptibility measurements indicate a potential transition to magnetic order at 0.4 K. In agreement with these observations, neutron diffraction measurements at 0.044 K observe magnetic Bragg peaks which can be indexed by a propagation vector of (0 0 1). Although determining a unique spin configuration corresponding to the observed magnetic Bragg peaks is not possible, model refinements indicate that the ordered moments are likely in the range of 0.4-0.9 μ B and, notably, require moments on both Yb sites. In addition to the magnetic Bragg peaks, diffuse scattering at low Q is observed indicating that the transition does not correspond to complete long range magnetic order. The two-site picture for YbBO₃ is further evidenced by the number of crystal field excitations observed by inelastic neutron scattering measurements. Together these results show that YbBO₃ is a two-site triangular lattice material with signatures of long-range order as well as shorter ranged spin correlations.