Continuous alloying between rocksalt and half-Heusler structures drives metal-semiconductor transition in ErNi_xSb

XYZ half-Heusler phases are often described as a XZ rock salt sublattice with an interstitial Y atom. However, transport across a solid solution between rock salt and half-Heusler structures has not previously been studied. In this paper, we demonstrate the exceptional tolerance of Ni vacancies in ErNi$_x$Sb, resulting in a complete alloy in the ErSb-ErNiSb space. Thermoelectric characterization demonstrates a continuous electronic transition associated with the gradual collapse of the band gap with Ni removal. The carrier concentration increase by three orders of magnitude and Seebeck coefficient decreases from 260 uV/K to <5 uV/K. Speed of sound measurements indicate that removal of Ni softens the lattice, consistent with the breakdown of the covalent Ni-Sb sublattice. For low Ni content compositions, the combination of low speed of sound coupled with Ni vacancies strongly hamper the propagation of phonons. The ability to control the electronic and thermal transport properties across the rock salt to half-Heusler continuum opens new material design strategies across thermoelectricity, superconductivity, and non-trivial topology