Single-crystal neutron diffraction and Raman spectroscopic study of hydroxylherderite, CaBePO4(OH,F)

The crystal structure, H bonding and chemical composition of hydroxylherderite from the Bennett pegmatite, Buckfield, Oxford County, Maine, USA were investigated by single-crystal X-ray diffraction and neutron Laue diffraction, electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma-atomic emission spectrometry and polarized Raman spectroscopy [(Ca)(Na0.01Ca1.01)Sigma(Be)(1.02)(Be0.98Li0.01)Sigma(P)(0.99)(Si0.03P0.98)Sigma O-1.01(4)(OH0.67F0.33)Sigma 1, Z = 4, a = 9.7856(5), b = 7.6607(5), c = 4.8025(3) angstrom, beta = 90.02(3)degrees, V= 360.02(4) angstrom(3), space group P2(1)/a]. The neutron-structure refinement converged with fully anisotropic displacement parameters to give a final agreement index R-1 = 0.0363 for 85 refined parameters and 1614 unique reflections with F-o > 4 sigma(F-o). The structure refinement was used to determine the H position and geometry of H bonding in the structure. One H site was found on the O5 anion with an O-H interatomic distance, corrected for "riding motion'', of 0.996(2) angstrom. The H bond of hydroxylherderite is bifurcated with O2 and O4 acceptors forming H bonds with O5...O2 = 3.163(1) angstrom, H...O2 = 2.544(2) angstrom and O5-H ... O2 = 121.8(1)degrees; O5...O4 = 3.081(1)angstrom, H...O4 = 2.428(2) angstrom and O5-H...O4 = 124.4(1)degrees. The highly non-linear O-H...O hydrogen bonds in hydroxylherderite and in the isotypic datolite [ideally CaBSiO4(OH)] are constrained by the tetrahedral network topology. Two main O-H stretching modes were observed in the Raman spectra at 3565 and 3620 cm(-1), which are attributed to the bifurcated H bond. Two additional weak bands at 3575 and 3610 cm(-1) are attributed to Si-P disorder in the tetrahedral sites. Results of this study will contribute to the correlation of H-bonding geometry and O-H stretching frequencies of highly non-linear H bonds.