High-Pressure Synthesis of Light Lanthanide Dodecaborides (PrB 12 and CeB 12 ): Effects of Valence Fluctuation on Volume and Formation Pressure.

Light lanthanide dodecaborides, RB 12 (R = Pr and Ce), were synthesized from a stoichiometric mixture of hexaborides and boron using a laser-heated diamond anvil cell under high-pressure and high-temperature conditions. Contrary to the expectation that lighter lanthanide elements require higher pressure to crystallize RB 12 , in situ X-ray diffraction experiments reveal that cerium dodecaboride crystallizes at 26 GPa, which is significantly lower than that required to form the heavier praseodymium dodecaboride (35 GPa). In addition to the lower formation pressure, an anomalous volume reduction is also observed in CeB 12 , which can be explained by a valence fluctuation between Ce 3+ and Ce 4+ indicated by X-ray absorption near-edge structure measurements. A polyhedral coordination change from a truncated cube in RB 6 to a truncated octahedron in RB 12 and associated shortening of the R-B bond length result in an increase in bulk modulus and hardness.