Strain-induced suppression of thermochromism in divalent cobalt molybdate thin films

Thermochromic oxides provide a platform for coupling lattice, electronic, and magnetic degrees of freedom, with divalent cobalt molybdate (CoMoO₄) as a prototypical example. Despite extensive studies on powders, its thin-film behaviour - critical for device applications - remains largely unexplored. Here, we present a combined experimental and theoretical investigation of CoMoO₄ thin films, including the first terahertz-to-visible/UV optical spectra of β-CoMoO₄ in thin-film form. In contrast to bulk powders, which undergo a first-order β → α structural transition near 230 K, the thin films retain the high-temperature β-phase across the entire temperature range. We observe that our thin films exhibit an altered magnetic behaviour compared to that of classical powder samples with the same chemical composition. We also show that microstrain fundamentally reshapes the phase landscape, suppressing thermochromism and stabilizing the high-temperature phase. The optical spectra reveal pronounced anomalies in the lattice and electronic responses, including a softening of a low-energy, cation-dominated phonon (42 cm^-1) upon cooling, in contrast to conventional hardening of higher-energy modes. This behaviour indicates incipient atomic displacements analogous to those driving the bulk β → α transition, despite the absence of a structural phase change, and saturates between 200 and 150 K. Temperature-dependent X-ray diffraction confirms persistent β-phase symmetry with increasing microstrain, consistent with strain-induced frustration of the first-order transition. At higher energies (0.12-3.7 eV), the optical response exhibits a blue-shift of Co²⁺ crystal-field transitions and charge-transfer excitations, indicating a strain-enhanced ligand field. Supported by structural refinements and theoretical calculations, we identify crystal field strengthening as the key mechanism stabilising the β-phase. These results establish strain as a thermodynamic lever to control phase stability and functional properties in thermochromic oxides.