Hydroflux Synthesis and Structural Phase Transition of Rare‐Earth Borate Hydroxides Na2[RE(BO3)(OH)2] (RE=Y, Gd−Er)

• Published in: Chemistry : a European journal 2024-12, Vol.30 (68), p.n/a
• Main Authors: Li, Yuxi, Carrillo‐Aravena, Eduardo, Qu, Jiang, Thakur, Gohil S., Ruck, Michael
• Format: Article
• Language: English
• Subjects: Boron compounds; Cations; Crystal structure; Dehydration; Erbium; Gadolinium; Hydroflux synthesis; Hydroxides; Hydroxometalates; Oxygen atoms; Phase transitions; Photoluminescence; Rare earth compounds; Sodium hydroxide; Ultraviolet radiation; Yttrium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202402783

Reacting RE2O3 and H3BO3 in an ultra‐alkaline NaOH hydroflux at about 250 °C yielded pure, crystalline samples of Na2[RE(BO3)(OH)2] (RE=Y, Gd−Er). The compounds dehydrate to Na3RE(BO3)2 upon heating in air to about 500 °C. Na2[RE(BO3)(OH)2] (RE=Tb−Er) are photoluminescent under UV radiation. Their UV‐Vis spectra show the typical absorptions associated with 4f–4f transitions and absorption edges in the UV (band gaps ≥5.7 eV). The RE3+ cation is coordinated by seven oxygen atoms, which define a distorted pentagonal bipyramid. The bipyramids share trans‐edges of their base forming infinite chains. Triangular (BO3)3− groups connect the chains into layers. The crystal structure of Na2[Ho(BO3)(OH)2] was investigated at various temperatures in the range 100 K≤T≤320 K. Above 310(2) K, the compound crystallizes in the orthorhombic space group Cmcm (β‐phase), below, it undergoes a displacive phase transition of second order resulting in a monoclinic structure in space group C2/c (α‐phase). The critical exponents derived from different structural parameters indicate a cooperative distortion of the borate layers, but a rather uncorrelated adaptation of the sodium cations to their local environment. The other compounds of the series also adopt the structure of the α‐phase at 296 K. A new family of rare‐earth borates is crystallized from an ultra‐alkaline NaOH hydroflux at moderate temperature. Pentagonal bipyramids centered by RE3+ cations form chains that are connected into layers by triangular (BO3)3− groups. Slightly above room temperature, the structure undergoes a second‐order phase transition. Some of the compounds are photoluminescent under UV radiation.