Praseodymium dodecaboride is an inorganic compound of praseodymium and boron with the formula PrB12. It is a boron-rich boride with the cubic UB12-type structure. The compound was synthesized under high-pressure and high-temperature conditions after earlier attempts to prepare pure PrB12 had been unsuccessful.
History The stability of cubic rare-earth dodecaborides is strongly dependent on the size of the metal atom. Classical syntheses produced the UB12-type compounds of the smaller rare-earth elements, while larger light-lanthanide ions did not form pure dodecaborides under ambient-pressure conditions. In 2016, Akopov and co-workers stabilized small concentrations of Pr in ZrB12, but found a solubility limit of only about 4 at.% Pr. Pure PrB12 was subsequently obtained by high-pressure synthesis and reported in 2022.
Preparation Praseodymium dodecaboride was synthesized by Yusa, Iga and Fujihisa from a stoichiometric mixture of praseodymium hexaboride and elemental boron using a laser-heated diamond anvil cell.
PrB6 + 6 B → PrB12
In situ X-ray diffraction showed formation of PrB12 at pressures of approximately 35 GPa. The need for such high pressure is attributed to the relatively large size of the Pr ion compared with the metal sites in the dodecaboride structure. Before the synthesis of pure PrB12, Pr could be incorporated into the cubic dodecaboride structure as a solid solution with zirconium dodecaboride. In Zr1−xPrxB12, the solubility of Pr was only about 4 at.%, and attempts to obtain the pure Pr compound had been unsuccessful even under high pressure.
Properties Crystal structure PrB12 crystallizes in the cubic crystal system with the UB12 structure type, space group Fmm (No. 225). Its lattice parameter at ambient pressure is approximately a = 7.58 Å. The structure contains four PrB12 formula units per unit cell. Each praseodymium atom occupies the center of a large B24 truncated-octahedral cage. The boron atoms form a rigid three-dimensional covalent framework built from interconnected B12 cuboctahedra, characteristic of cubic metal dodecaborides. The UB12-type structure can alternatively be regarded as a modified sodium chloride structure in which the two sublattices are occupied by praseodymium atoms and B12 cuboctahedral clusters.
Effect of pressure The high formation pressure of PrB12 is related to the size of the praseodymium ion. Earlier rare-earth dodecaborides were primarily known for the smaller, heavier lanthanides, because their ions fit more readily inside the B24 cages. The minimum observed formation pressure of approximately 35 GPa is substantially higher than that of cerium dodecaboride, which forms at about 26 GPa despite cerium preceding praseodymium in the lanthanide series. This anomalous difference was attributed to partial Ce4+ character in CeB12, which decreases the effective ionic radius of cerium. No comparable valence fluctuation was reported for PrB12.
Mechanical properties Conversion from the PrB6 structure to the PrB12 structure changes the praseodymium coordination environment from a truncated-cubic boron cage to a smaller truncated-octahedral cage. The resulting shortening of the Pr–B distances is associated with a greater bulk modulus and hardness in the dodecaboride than in the corresponding hexaboride. The high rigidity is also characteristic of the UB12 structure generally and results from the strongly covalently bonded three-dimensional boron framework.
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