HYDROGEN-BONDING IN MEYERHOFFERITE - AN X-RAY AND STRUCTURE ENERGY STUDY

The crystal structure of meyerhofferite, CaB3O3(OH)5.H2O, triclinic, a 6.632(l), b 8.337(1), c 6.4748(6) angstrom, alpha 90.81(1), beta 101.97(1), gamma86.76(1)degrees, V349.66(8) angstrom3, Z = 2, space group P1BAR, has been refined by full-matrix least-squares methods to an R index of 3.0% and a wR index of 3.8% for 1788 unique observed [I greater-than-or-equal-to 2.5sigma(I)] reflections measured with MoKalpha X-radiation. The H-positions were located on difference-Fourier maps and refined using the ''soft'' constraints that H-O distances are approximately 0.97 angstrom. Meyerhofferite is a complex cycloborate with two Bphi4 tetrahedra (phi: unspecified anion) and one Bphi3 triangle linked by corner-sharing to form rings of [B3O3(OH)5]2- composition. The eight-coordinate Ca polyhedra link by edge-sharing to form chains along [001], with enhanced chain rigidity given by the boron-oxygen rings that link by comer-sharing and edge-sharing to two sides of the Ca polyhedral chains. Bonding between the heteropolyhedral chains is through hydrogen bonding only, with six hydrogen bonds along [100], four along [010] and four between elements of the same heteropolyhedral chain per unit cell. Minimum-energy H-positions were calculated with a directionally isotropic H-O potential function. The calculations successfully predict the positions of all hydrogen atoms and the hydrogen-bonding arrangement in the meyerhofferite structure.