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

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.