Solid-solid phase-change materials (SSPCMs) are promising for next generation thermal energy storage applications, as they present several advantages over solid-liquid PCMs including safety (no spillage of hot liquid), lower thermal expansion, lower corrosiveness, and no need for encapsulation. Solid-solid transitions occur from RT ordered phases to orientationally disordered high-temperature phases that lie at the boundary between liquids and solids. Adoption of SSPCMs for practical applications require understanding of their structural stabilities. The proposal will investigate two molybdate salts with high latent heat of phase transitions. Both Na2MoO4 and K2MoO4 have rich phase diagrams, and both undergo 3 reversible phase transitions before melting. Variable-temperature powder x-ray diffraction will be conducted to establish relations between all the phases. Moreover, thermal expansion coefficients of phases, and thermal displacement parameter of individual atoms will be derived.