Competing exchange interactions in lanthanide triangular lattice compounds ln Zn₃P₃(Ln = La-Nd, Sm, Gd)

Stacked triangular lanthanide compounds LnZn₃P₃(Ln = La-Nd, Sm, Gd) are successfully grown as single-crystalline samples. Magnetic-susceptibility and specific-heat measurements reveal that the compounds with Ce, Nd, and Gd show long-range order at 0.78, 0.63, and 4.7 K, respectively, while two successive transitions are found in SmZn₃P₃at 0.34 and 0.82 K. The light lanthanide compounds LnZn₃P₃(Ln = La-Pr) behave as semiconductors in terms of electrical resistivity, while heavier lanthanide compounds (Ln = Nd, Sm, Gd) show metallic conductivity. This family of compounds provides an excellent opportunity to study the effects of conduction electrons in rare-earth triangular-lattice magnets. The ordering temperatures of the metallic compounds are almost proportional to the de-Gennes factor of those trivalent lanthanide ions. In contrast, the semiconductor compound CeZn₃P₃shows a much higher ordering temperature than that estimated from the de-Gennes factor. Our results indicate that even with a low density of conduction electrons, the Ruderman-Kittel-Kasuya-Yosida interaction possibly has a comparable magnitude to the superexchange interaction and shows the potential to remove geometrical frustration.