NaBaR(BO₃)₂ (R = Dy, Ho, Er and Tm): Structurally perfect triangular lattice materials with two rare earth layers

We report the crystal growth of NaBaR(BO₃)₂ (R = Dy, Ho, Er and Tm) by the flux method. In the rhombohedral crystal structures, [RO₆] octahedra and planar [BO₃] units connect by bridging O atoms, forming triangular layers in the ab plane that are evenly spaced along c. Two perfectly triangular rare earth layers of identical dimension are found. Anisotropic magnetic susceptibility is observed for all; NaBaEr(BO₃)₂ is nearly isotropic while NaBaTm(BO₃)₂ is the most anisotropic. Heat capacity measurements reveal no long-range magnetic ordering down to 0.35 K for R = Dy or Er, while the Ho material displays a significant entropy loss. All the magnetic entropy appears to be absent in the Tm case by low temperature, suggesting that a singlet ground state may have formed.