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

Reacting RE 2 O 3 and H 3 BO 3 in an ultra‐alkaline NaOH hydroflux at about 250 °C yielded pure, crystalline samples of Na 2 [ RE (BO 3 )(OH) 2 ] ( RE =Y, Gd−Er). The compounds dehydrate to Na 3 RE (BO 3 ) 2 upon heating in air to about 500 °C. Na 2 [ RE (BO 3 )(OH) 2 ] ( RE =Tb−Er) are photoluminescent under UV radiation. Their UV‐Vis spectra show the typical absorptions associated with 4 f –4 f transitions and absorption edges in the UV (band gaps ≥5.7 eV). The RE 3+ 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 (BO 3 ) 3− groups connect the chains into layers. The crystal structure of Na 2 [Ho(BO 3 )(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 C 2/ 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.