Polymorphism and NIR Photoluminescence of Anhydrous Ytterbium(III) Acetate: Reversible Transformations of Three Polar Structures in the Temperature Range 100–593 K

Anhydrous ytterbium(III) acetate Yb(OAc) 3 was obtained by the solvothermal reaction between Yb(OH) 3 and glacial acetic acid in MeOH and subjected to a comprehensive structural investigation in the temperature range 100–573 K. This study revealed a new polymorphic low‐temperature modification γ‐Yb(OAc) 3 crystallizing in the polar space group Pna 2 1 (No. 33). The γ‐phase exists in the temperature range 100–295 K and undergoes a reversible solid–solid phase transition to α‐Yb(OAc) 3 at 300 K, which crystallizes in the klassengleiche supergroup Cmc 2 1 (No. 36) being polar as well. When the temperature reaches 524 K, the α‐phase reversibly transforms to the β‐phase crystallizing in another polar space group P 6 3 cm (No. 185), which is stable up to 593 K, whereupon decomposition occurs. Differential scanning calorimetry reveals a small endo‐effect (Δ H γ ⇌ α = 1.5 kJ/mol) of the transition γ ⇌ α at 300 K and a larger endo‐effect (Δ H α ⇌ β = 9.1 kJ/mol) of the transition α ⇌ β at 524 K. Analysis of temperature‐dependent powder diffraction data revealed anisotropic expansion of the unit cell in all polymorphs except for the polar axis c in α‐Yb(OAc) 3 , which showed small negative thermal expansion. Additionally, Yb(OAc) 3 exhibits photoluminescence by NIR emission, which was studied in the temperature range 258–348 K, spanning the γ‐ and α‐phases. These investigations showed an unexpected increase in the intensity of the Yb 3+ transition 2 F 5/2 → 2 F 7/2 upon heating the sample.