Raman and Mid-IR Spectral Analysis of the Atacamite-Structure Hydroxyl/Deuteroxyl Nickel Chlorides Ni₂(OH/D)₃Cl

Vibrational spectra (Raman 4000–95 cm⁻¹ and mid-IR 4000–400 cm⁻¹) of the atacamite-structure Ni₂(OH)₃Cl, including a rarely reported kind of asymmetric trimetric hydrogen bond, as a member of the geometrically frustrated material series and its deuteride Ni₂(OD)₃Cl are, to the best of our knowledge, reported for the first time and analyzed at room temperature. Through a comparative study of four spectra according to their crystal structural parameters, we assign OH stretching modes v(OH) in a functional group region (3700–3400 cm⁻¹) and their deformation modes δ(NiOH/D) in the correlation peak region (900–600 cm⁻¹) with the corresponding mode frequency ratios ω_v(OD)/ω_v(OH)≈73% and ω_δ(NiOD)/ω_δ(NiOH)≈75%, and further self−consistently suggest Ni-O and Ni-Cl related modes in the fingerprint region (500–200 cm⁻¹ and 200–0 cm⁻¹, respectively) by use of the unified six-ligand NiO₅Cl and NiO₄Cl₂ frames. This report may contribute to the spectral analysis of other hydroxyl transition-metal halides and to the understanding of the fundamental physics of their exotic magnetic geometrical frustration property from the spectral changes around the corresponding low transition temperatures.