The Role of Structural and Compositional Heterogeneities in the Insulator-to-Metal Transition in Hole-Doped APd₃O₄ (A = Ca, Sr)

The cubic semiconducting compounds APd₃O₄ (A = Ca, Sr) can be hole-doped by Na substitution on the A site and driven toward more conducting states. This process has been followed here by a number of experimental techniques to understand the evolution of electronic properties. While an insulator-to-metal transition is observed in Ca_1–xNa_xPd₃O₄ for x ≥ 0.15, bulk metallic behavior is not observed for Sr_1–xNa_xPd₃O₄ up to x = 0.20. Given the very similar crystal and (calculated) electronic structures of the two materials, the distinct behavior is a matter of interest. We present evidence of local disorder in the A = Sr materials through the analysis of the neutron pair distribution function, which is potentially at the heart of the distinct behavior. Solid-state ²³Na nuclear magnetic resonance studies additionally suggest a percolative insulator-to-metal transition mechanism, wherein presumably small regions with a signal resembling metallic NaPd₃O₄ form almost immediately upon Na substitution, and this signal grows monotonically with substitution. Some signatures of increased local disorder and a propensity for Na clustering are seen in the A = Sr compounds.