OH-F disorder in non-centrosymmetric Zn2(BO3)(OH)0.75F0.25:: ab initio structure determination and NMR study;: comparison with tridymite and fluoride borates

The crystal structure of Zn-2(BO3)(OH)(0.75)F-0.25 is determined ab initio from X-ray and neutron powder data. The unit cell is monoclinic, space group P2(l) (no. 4), Z=2, with a=6.8738(4) Angstrom, b=4.9178(2) Angstrom, c=5.7018(3) Angstrom, beta=98.829(3) degrees, V=190.46(3) Angstrom(3) (neutron powder data). Alternating up and down ZnO3(OH,F) tetrahedra and triangular BO33- groups built corrugated layers. Their connection by hydroxyl ions gives a 3D network similar to that of tridymite. The statistical distribution of fluorine and hydroxyl in infinite chains is evidenced by magical angle spinning-NMR spectroscopy. (C) 2000 Elsevier Science S.A. All rights reserved.