The local state of hydrogen atoms and proton transfer in the crystal structure of natural berborite, Be2(BO3)(OH)‧H2O: Low-temperature single crystal X-ray analysis, IR and 1H NMR spectroscopy, and crystal chemistry and structural complexity of beryllium borates

Abstract

Beryllium borates are mostly water-free, with the natural mineral berborite, Be2(BO3)(OH)‧H2O, being the only known exception. A sample of berborite from the Vevja quarry, Tvedalen, Larvik, Norway was studied by single-crystal X-ray diffraction, variable temperature 1H nuclear magnetic resonance (NMR), and infrared (IR) spectroscopy. The crystal structure is the trigonal 1T-polytype with space group P321 and is based on electroneutral [BeØ2(BΔO3)]-layers formed by BeO3Ø-tetrahedra and BΔO3-triangles. Due to statistical occupancy of the Ø-ligand by hydroxyl groups (50%) and water molecules (50%), berborite-1T and its related polytypes are characterized by complex systems of hydrogen bonds. Accurate analysis shows that in the structure of berborite there are two independent systems of hydrogen bonds involving mobile protons. Lowering of temperature results in the complete stabilization of protons at the critical “freezing-point” of 243 K. The crystal chemistry and complexity of layered beryllium borates are discussed. Despite the broad structural diversity of these compounds, their crystal structures are classified as simple (20–100 bits per unit cell) or intermediately complex (100–500 bits per unit cell). The complexity of berborite polytypes increases in accordance with the quantity of layers from 29.038 (1T) to 78.077 (2H), which is in good agreement with the negative contribution of complexity to the configurational entropy.