LaAlSiO 5 and apatite-type La 9.71(Si 0.81Al 0.19O 4) 6O 2—the crystal structures of two synthetic lanthanum alumosilicates

Abstract

The crystal structures of two new synthetic lanthanum alumosilicates with composition LaAlSiO 5 and La 9.71(Si 0.81Al 0.19O 4) 6O 2 have been investigated using single crystal diffraction data collected at room conditions. LaAlSiO 5 crystallizes in the noncentrosymmetric orthorhombic space group P2 12 12 1 with twelve formula units per cell ( a=11.0525(7) Å, b=5.2261(3) Å, c=23.7049(21) Å, V=1369.2(3) Å 3, R(| F|)=0.023 for 2875 independent observed reflections) and belongs to the group of mixed anion alumosilicates. Basic building units are isolated [SiO 4] groups as well as tetrahedral hybrid zweier double layers of composition [(Al,Si) 5O 11]. Stacking of the layers parallel to [001] results in the formation of a three-dimensional structure in which lanthanum cations and isolated [SiO 4] groups are incorporated for charge compensation. The three crystallographically independent La atoms are coordinated by 7–8 oxygen ligands. Concerning the connectedness of the tetrahedra in the structure of LaAlSiO 5 one singular (Q 0), one secondary (Q 2) and four quaternary (Q 4) tetrahedral groups can be distinguished. Bond valence calculations were performed to obtain Al/Si distributions for the symmetrically independent T sites. Four out of six tetrahedra show strong preference for either Al or Si, whereas the remaining two tetrahedral centers show a Al:Si ratio close to 1:1. Using the silicate classification procedure introduced by Liebau and neglecting the differences in the Al/Si distributions among the six crystallographically independent T sites, the structural formula of the present compound can be written as La 3{ hB,2 ∞ 2}[(Al,Si) 5O 11][SiO 4]. From a structural point of view LaAlSiO 5 is closely related to the minerals latiumite and tuscanite. La 9.71(Si 0.81Al 0.19O 4) 6O 2, on the other hand, belongs to the large structure family of apatites (space group P6 3/ m, a=9.7532(9) Å, c=7.2371(5) Å, V=596.2(1) Å 3, R(| F|)=0.019 for 542 independent observed reflections). It is the first example of a detailed structurally characterized apatite containing Al 3+ in the tetrahedral sites.