Systematic investigation of structural, transport, magnetic and thermodynamic properties of hexagonal R$_2$Pt$_6$Al$_{15}$ (R = Y, La-Nd, Sm-Lu) series.

We have synthesized single crystals of new hexagonal intermetallic series of compounds R$_2$Pt$_6$Al$_{15}$ with R = Y, La-Nd, Sm-Lu. Structural analysis have confirmed hexagonal $P6_3/mmc$ crystal structure with ordered R-site. Magnetic, transport and heat capacity measurements show that most of the members of the series order antiferromagnetically at low temperature with highest $T_N$ = 7.5 K for Gd$_2$Pt$_6$Al$_{15}$ and moments are along c-axis except Dy$_2$Pt$_6$Al$_{15}$ and Ho$_2$Pt$_6$Al$_{15}$. Ce$_2$Pt$_6$Al$_{15}$ does not order, but shows insulating behavior with a strong divergence in heat capacity divided by temperature ($C/T$). Mixed valence state of Eu$^{2+}$/Eu$^{3+}$ state has been observed in Eu$_2$Pt$_6$Al$_{15}$ with antiferromagnetic ordering below 3 K. Magnetic entropy and crystal electric field are analyzed for all the members from the magnetic contribution of heat capacity. This series of compounds serves a unique example of a rare earth series with only one rare earth (R) site having hexagonal point-symmetry.