Mixed Valence-Driven Phase Evolution and Property Tuning in PdSe2

Abstract Layered palladium diselenide (PdSe2) is an emerging functional dichalcogenide whose structural flexibility enables tunable electronic, magnetic, and transport properties. However, controlled access to compositionally substituted and metastable polymorphs remains challenging using conventional synthetic approaches. Here, we report a benchtop solution-phase route for the rapid and scalable synthesis of highly crystalline orthorhombic PdSe2 (o-PdSe2) and its polymorphs induced by aliovalent substitutions. This sustainable approach enables controlled phase evolution and yields materials suited for detailed structural, spectroscopic, magnetic, and transport studies. Using this platform, we achieve substitution at the Pd site with both 3d and 5d transition metals, Mn and Pt, respectively. Approximately 10 at. % Mn substitution preserves the orthorhombic framework while inducing a complex magnetic ground state. In contrast, Pt substitution strongly modifies Se–Se dimer stability and drives sequential structural transformations. Unlike Pd, Pt prefers a 4+ oxidation state, resulting in a partial destabilization of the Se–Se dumbbells and formation of monoclinic I2/a polymorph with mixed Pt2+/Pt4+ centers at low Pt content (∼12 at. %). This structural reorganization induced by minute amounts of Pt drastically alter charge and heat transport properties, including a carrier-type switch from p-type to n-type conduction, a narrowing of the bandgap, and ultralow thermal conductivity of 0.27 Wm1–K–1 at 300 K. At higher Pt content (≥30 at. %), Pd is progressively forced to +4 oxidation state resulting in the complete destabilization of the Se–Se dimers and structural transformation into a trigonal metallic phase with [MSe6] octahedra. These results establish a versatile soft chemical route to compositionally and structurally tunable PdSe2-based materials with emergent magnetic and transport properties, alluding to their promise for energy conversion and storage applications.