Magnetic and nonmagnetic Ce in the borocarbides

We have measured the magnetic susceptibility, resistivity, and heat capacity of a series of Ce-based intermetallics CeCo2B2C, CeNi2B2C, CeRh2B2C, CePd2B2C, and CePt1.5Au0.5B2C. The Ce ion changes from magnetic to nonmagnetic behavior with decreasing metal d-ion size. In the large limit, trivalent CePt1.5Au0.5B2C is a single-ion Kondo system with a strongly pressure-dependent coherence temperature, Tcoh∼30 to 100 K. CeCo2B2C and CeRh2B2C are both intermediate valent, the latter with a Sommerfeld constant γ of 75(5) mJ/mole K2. CePd2B2C is primarily nonmagnetic with a small temperature-independent Pauli susceptibility of 5(1)×10-6 emu/mole Ce. Finally, the smallest transition-metal material, nonmagnetic CeNi2B2C2, has properties reminiscent of the non-Ce-based borocarbides.