High-temperature enthalpies of binary dodecaborides

The enthalpies of three dodecaborides have been determined from 1300 to 2200°K by a drop calorimetric technique. The enthalpies were fitted with the Shomate function to give Zr0.6Y0.4B12:HT−H298=4.204× 101T+1.260× 10−2T2−(2.528) 106T−1−5.175× 103 cal/mole,ErB12:HT−H298=3.947× 101T+1.357× 10−2T2−(2.706) 106T−1−3.896× 103 cal/mole,UB12:HT−H298=4.232× 101T+1.399× 10−2T2−(2.430) 106T−1−5.713× 103 cal/mole.Minor differences between the enthalpies of these dodecaborides were analyzed and correlated with different electronic conduction contributions, which can be predicted from the valence states of the metal atom. The metal atoms achieve their usual chemical valency by donating two electrons to the boron lattice, and the remainder to a conduction band. The uranium atoms in uranium dodecaboride appear to exist in the +5 and/or +6 states, whereas the +3, +4, and +3 states of erbium, zirconium, and yttrium are most probable.