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 $CoMoO_4$ 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 $CoMoO_4$ thin films, including the first THz-VIS-UV spectra of $\beta - CoMoO_4$ in thin-film form. In contrast to bulk powders, which undergo a first order $\beta \rightarrow \alpha$ structural transition near 230K, the thin films retain the high-temperature $\beta-$phase across the entire temperature range. We show that microstrain fundamentally reshapes the phase landscape, suppressing thermochromism and stabilizing the $\beta-$phase. The optical spectra reveal pronounced phonon and electronic anomalies, including a softening of a low-energy, cation-dominated phonon (42cm$^{-1}$) upon cooling, in contrast to conventional mode-hardening. This behaviour indicates incipient atomic displacements analogous to those driving the bulk $\beta \rightarrow \alpha$ transition, despite the absence of a structural phase change, and saturates between 200 and 150K. Temperature-dependent X-ray diffraction confirms persistent \beta-phase symmetry with increasing microstrain, consistent with strain-induced frustration of the first-order transition. At higher energies (0.12-3.7eV), the optical response exhibits a blue-shift of Co$^{2+}$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 stabilizing the $\beta$-phase. These results establish strain as a thermodynamic lever to control phase stability and functional properties in thermochromic oxides.