Electronic structure and elastic properties ofYn+1Co3n+5B2n ()

Abstract

We have studied the electronic structure and elastic properties ofYn+1Co3n+5B2n (spacegroup P6/mmm),where n = 1, 2, 3 and , using ab initio calculations. These ternary borides exhibit abulk-modulus-to-C44 ratio from 1.6 to 1.9, which is rather unusual for ceramics. This may beunderstood on the basis of the electronic structure: predominantly covalent–ionicYCo3B2 layers are interleaved with predominantly metallicYCo5 layers. Covalent–ionic bonding between B and Co may giverise to a large bulk modulus, while weak coupling between theYCo3B2 andYCo5 layers may beresponsible for the low C44 value. On the basis of the similarity in electronic structure and elasticity data, it is reasonable to assumethat the Yn+1Co3n+5B2n compounds investigated here may exhibit similar properties to the so-called MAX phases(Barsoum 2000 Prog. Solid State Chem. 28 201).