Exploring Multi-Anion Chemistry in Yttrium Oxyhydrides: Solid-State NMR Studies and DFT Calculations

Area of Science:

• Materials Science
• Solid-State Chemistry
• Spectroscopy

Background:

• Rare earth oxyhydrides (REOH) with FCC lattices are known for photochromism.
• The relationship between structure, anion composition, and photochromic efficiency is established, but the underlying mechanism remains unclear.

Purpose of the Study:

• To elucidate the photochromism mechanism in rare earth oxyhydrides.
• To investigate local environments, anion oxidation states, and ion dynamics in yttrium oxyhydrides (YOH).

Main Methods:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy (¹H, ²H, ¹⁷O, ⁸⁹Y).
• Density Functional Theory (DFT) calculations for modeling YOH structures and properties.
• 2D NMR correlation experiments to probe sample heterogeneity.

Main Results:

• DFT models correlate well with experimental NMR data for ordered and disordered YOH sublattices.
• NMR reveals sample heterogeneity, with hydride-rich (x ≈ 0.25) and hydride-poor (x ≈ 1) domains.
• Presence of hydroxide ions (OH⁻) and formation of molecular hydrogen (H₂) via proton-hydride reaction, observed as a mobile component in ¹H NMR.

Conclusions:

• The photochromism mechanism involves heterogeneities and the formation of molecular hydrogen.
• Structural and compositional details, including domain formation and H₂ trapping, are crucial for understanding photochromic behavior in rare earth oxyhydrides.