  Frustration, spin correlations and interplay between competing degrees of
freedom are some of the key ingredients that underlie exotic states with
fractional excitations in quantum materials. Rare-earth based two dimensional
magnetic lattice wherein crystal electric field, spin-orbit coupling,
anisotropy and electron correlation between rare-earth moments offer a new
paradigm in this context. Herein, we present crystal structure, magnetic
susceptibility and specific heat accompanied by crystal electric field
calculations on the polycrystalline sample of Ba3RB9O18 (R = Yb, Er) in which
R3+ ions form a perfect triangular lattice without anti-site disorder. The
localized R3+ spins show neither long-range order nor spin-glass state down to
1.9 K in Ba3RB9O18. Magnetization data reveal a pseudospin Jeff = 1/2 ( Yb3+)
in the Kramers doublet state and a weak antiferromagnetic interaction between
Jeff = 1/2 moments in the Yb variant. On the other hand, the effective moment
{\mu}eff = 8.8 {\mu}B was obtained from the Curie-Weiss fit of the
low-temperature susceptibility data of Er variant suggests the admixture of
higher crystal electric field states with the ground state. The Curie-Weiss fit
of low-temperature susceptibility data for Er system unveils the presence of a
relatively strong antiferromagnetic interaction between Er3+ moments compared
to its Yb3+ analog. Ba3ErB9O18 does not show long-range magnetic ordering down
to 500 mK. Furthermore, our crystal electric field calculations based on
magnetization data of Ba3ErB9O18 suggest the presence of a small gap between
the ground and first excited Kramers doublets. The broad maximum around 4 K in
magnetic specific heat in zero-field is attributed to the thermal population of
the first CEF excited state in Ba3ErB9O18, which is consistent with our CEF
calculations.
