Synthesis, Structural Characterization, and Ab Initio Study of Cu5+δIn2+xSb2–x: A New B8-Related Structure Type

A new ternary orthorhombic compound with the formula Cu5+δIn2+xSb2–x, crystallizing in the space group Cmc21 with 36 atoms per unit cell [a = 10.1813(4) Å, b = 8.4562(4) Å, c = 7.3774(2) Å, Z = 4], has been synthesized by conventional high-temperature methods. The structure is based on the B8 archetype (NiAs/Ni2In) and features In/Sb ordering as well as ordering of interstitial copper. Details of the experimental study and the structural parameters of this compound are reported in the first part of the work. In the second part, ab initio calculations based on the density functional theory and the projector augmented-wave method are used to characterize the structural, thermodynamic, and phase-stability properties of the new ternary phase. The present calculations include the lattice parameters, molar volume, bulk modulus and its pressure derivative, the energy of formation from the elements, and the electronic density of states. Moreover, the present ab initio method is used to investigate the thermodynamic properties of the anti-structure Cu5Sb2In2 compound obtained by exchanging the In and Sb Wyckoff symmetric positions.