Crystal Growth and Complex Characterization of Novel Gallium- and Germanium-Rich Tourmalines: Refinement of the Crystal Structure, Cation Distribution, and Raman and Mössbauer Spectroscopy

Abstract

The crystal growth and characterization of novel tourmaline compounds are important in fundamental and applied fields as new tourmaline species and promising piezoelectric materials. Single crystals of Ga,Ge-rich tourmalines (up to 12.43 wt % GeO2 and 11.03 wt % Ga2O3) were grown on a seed as a newly formed, up to 1.2 mm thick layer in hydrothermal solution at 650 °C and 100 MPa. Two crystallizing schemes were developed with quartz and corundum nutrients in the condition of a normal temperature solubility factor and with topaz and quartz nutrients using a reverse temperature solubility factor. The chemical composition and crystal structure of grown tourmalines as well as the distribution of gallium and germanium between crystallographic sites were studied by electron microprobe analysis (EMPA), single-crystal X-ray analysis (SCXRD), and Mössbauer and Raman spectroscopy. The unit cell parameters and refined crystal-chemical formulae of two high-quality single-crystal samples are as follows: a = 16.0518(7) Å, c = 7.1874(4) Å, V = 1603.80(13), and X (Na0.66□0.34) Y (Fe1.78 2+Al0.5Fe0.36 3+Ni0.19Ti0.17) Z (Al4.91Ga1.09) T (Si4.78Ge0.94Al0.28O18) (BO3)3 V (OH)3 W [(OH)0.42O0.58]; a = 16.0083(8), c = 7.1508(4) Å, V = 1587.00(14), and X (Na0.17□0.83) Y (Fe1.31 2+Al1.35Fe0.27 3+Ni0.07) Z (Al5.43Ga0.57) T (Si5.15Ge0.55Al0.30O18) (BO3)3 V (OH)3 W [(OH)0.51O0.49]. Based on the Raman spectroscopy data, it is now possible to indicate the predominance of either divalent or trivalent cations in the Y site for tourmalines, whereas the Z site is fully occupied by trivalent cations.