Single crystal growth and heat capacity measurements of triangular lattice R₂Pt₆Ga₁₅ (R =rare earth)

We have succeeded in synthesizing the single crystal of R₂Pt₆Ga₁₅ (R=La-Nd, Sm- Lu) with hexagonal Sc_0.67Fe₂Si₅-type structure using Ga self flux method. The crystal structure was confirmed by the powder X-ray method. The unit-cell volume V of R₂Pt₆Ga₁₅ follows the lanthanide concentration except R = Ce, Eu and Yb, indicating that the valences of R = La, Pr, Nd, Sm, Gd-Tm, and Lu ion are trivalent, whereas those of R = Ce, Eu and Yb ion are deviate from trivalent. We have measured the specific heat C(T) of R₂Pt₆Ga₁₅. It is found that the magnetic order takes place in R₂Pt₆Ga₁₅ (R=Pr, Nd, Sm-Tm). Moreover, the multiple phase transitions were observed in R₂Pt₆Ga₁₅ (R = Nd, Eu, Gd and Ho).