Temperature-dependent single-crystal neutron diffraction study of natural chondrodite and clinohumites

The crystal structures of natural F-bearing chondrodite [Mg4.64Fe0.28Mn0.01 4Ti0.023(Si1.01O4)(2) F1.02OH0.97] from the Tilley Foster mine (Brewster, N ew York), F-bearing titanian clinohumite [Mg8.805Fe0.006Ti0.214(Si0.993O4)( 4)F0.484OH0.516] from Kukh-i-Lal (Tadjikistan) and F-free titanian hydroxy[ clinohumite [Mg7.378Fe1.12Mn0.052Ni0.014Ti0.453(Si0.996O4)(4)OH1.0] from Va l Malenco (Italy) were refined in space group P2(1)/b (unique axis a) from single-crystal neutron diffraction data, collected on a four-circle diffrac tometer at the High Flux Isotope Reactor at Oak Ridge National Laboratory. Accurate H atom positions were determined at 295 K, 100 K, and 20 (10) K. O nly one H position of approximately 50% occupancy was observed for each str ucture, which confirms a disordered H model. Time-of-flight single-crystal neutron data were also collected at 295 K and 20 K for the Val Malenco clin ohumite as an additional check on space group symmetry. The crystal structu re of the Kukh-i-Lal clinohumite was further investigated by X-ray single-c rystal refinement at 295 K and by piezoelectric measurements. A few, very w eak, symmetry-forbidden reflections were observed for each crystal at both ambient and lower temperatures. The same reflections were observed by all m ethods used. No temperature dependence is indicated, as no additional peaks appear at low temperature, and the intensity of the reflections are sample dependent. It appears that the real structure is made up of P2(1) and Pb d omains so that violations are due to ordering of both H and Ti. No distinct piezoelectric effect was observed that would indicate the absence of a cen ter of symmetry. This points to the simultaneous presence of various enanti omorphic domains, which cancels the piezoelectric effect of individual doma ins. The decrease in unit-cell volume with F substitution in clinohumites c an be explained by the higher concentration of H-site vacancies and the cou pled cationic and anionic substitution on the M3 and O/F site.