Hydride-Buffered Absence of Local Distortions in Mixed-Valent EuVO₂H Films Revealed by X-ray Fluorescence Holography

Valence transitions in europium compounds, such as EuO and EuNi₂(Si_1-xGe_x)₂, usually induce substantial structural distortions because the ionic radius difference between Eu²⁺ (1.25 Å) and Eu³⁺ (1.06 Å) exceeds the Hume-Rothery tolerance. Here, we conducted valence-specific structural analysis of strained EuVO₂H thin films using X-ray fluorescence holography (XFH), where intersite charge transfer between Eu²⁺ and V³⁺ converts ∼30% of Eu²⁺ into Eu³⁺ (and V³⁺ into V²⁺). XFH measurements at the Eu L-edge revealed that a 15 nm single-crystalline film maintains remarkable structural coherence, with Eu²⁺ and Eu³⁺ randomly distributed and showing minimal local distortion despite the >15% size mismatch. This unusual stability is attributed to the size flexibility of H^- anions in the EuH layers, which act as structural buffers. The ability to accommodate mixed-valence states without introducing structural disorder likely underlies the enhanced magnetic anisotropy in EuVO₂H.