Hydrothermal Growth of Lanthanide Borosilicates: A Useful Approach to New Acentric Crystals Including a Derivative of Cappelenite

The reaction of barium borosilicate
glasses in hydrothermal fluids
with NaOH mineralizer at ca. 600–650 °C in the presence
of various sources of Eu3+ ions leads to a new series of
noncentrosymmetric single crystals characterized by single-crystal
X-ray diffraction. Under three slightly different reaction modifications,
three new Eu3+-containing noncentrosymmetric single crystals
were isolated and characterized. Using a barium borosolicate glass
with Eu2O3 added to the reaction mixture led
to NaBaEuSi3O9 (1), which crystallizes
in the noncentrosymmetric orthorhombic space group P212121 with unit cell parameters a = 5.6373(11) Å, b = 11.483(2) Å,
and c = 12.535(2) Å and consists of Eu3+ ions linked by Si3O96– rings.
Using a similar glass feedstock with Eu3+ included in the
starting glass composition led to the compound NaBa3Eu3Si6O20 (2), which was determined
in the noncentrosymmetric polar space group Ama2
with unit cell parameters a = 14.777(3) Å, b = 23.926(5) Å, and c = 5.5533(11)
Å. It is constructed of individual islands of Si4O1310– and lone Si2O76– building fragments. Finally, use of the Eu-containing
glass along with additional Eu2O3 led to the
compound BaEu6(Si3B6O24)­(OH)2 (3), which crystallizes in the polar
space group P6mm with a = 10.8074(15) Å and c = 4.7296(9) Å.
It is related to the rare and poorly understood mineral cappelenite
Ba­(Y,RE)6(Si3B6O24)­F2 and contains six-membered tetrahedral borate rings linked
by silicate tetrahedra. The high percentage of noncentrosymmetric
crystals obtained from of a simple borosilicate glass starting material,
along with various straightforward chemical additives, suggests strongly
that many more noncentrosymmetric crystals can be isolated from this
general system.