| Summary: | SrVO<sub>2</sub>H, obtained by a topochemical reaction of SrVO<sub>3</sub> perovskite using CaH<sub>2</sub>, is an anion-ordered phase with hydride anions exclusively at the apical site. In this study, we conducted a CaH<sub>2</sub> reduction of SrVO<sub>3</sub> thin films epitaxially grown on KTaO<sub>3</sub> (KTO) substrates. When reacted at 530 °C for 12 h, we observed an intermediate phase characterized by a smaller tetragonality of <i>c</i>/<i>a</i> = 0.96 (vs. <i>c</i>/<i>a</i> = 0.93 for SrVO<sub>2</sub>H), while a longer reaction of 24 h resulted in the known phase of SrVO<sub>2</sub>H. This fact suggests that the intermediate phase is a metastable state stabilized by applying tensile strain from the KTO substrate (1.4%). In addition, secondary ion mass spectrometry (SIMS) revealed that the intermediate phase has a hydrogen content close to that of SrVO<sub>2</sub>H, suggesting a partially disordered anion arrangement. Such kinetic trapping of an intermediate state by biaxial epitaxial strain not only helps to acquire a new state of matter but also advances our understanding of topochemical reaction processes in extended solids.
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