Synthetic LiAlGe₂O₆: The first pyroxene with P2₁/n symmetry

Abstract

The structure determination of synthetic LiAlGe₂O₆ [Z = 4, space group P2₁/n, a = 9.8892(5), b = 8.3929(5), c = 5.3995(3) Å, b = 110.646(3)°], is a new pyroxene structure-type, and represents the first structural example of P2₁/n pyroxene symmetry. The crystal structure of the Ge-analog phase of spodumene was solved from single-crystal X-ray diffraction data by classical Patterson methods with a subsequent structure refinement converging to R₁ = 0.0169. The new P2₁/n pyroxene polymorph was found to consist of a single S-type rotated tetrahedral chain type, which is-similar to ordered P2₁/n omphacite-composed of alternating Ge1O₄ and Ge2O₄ tetrahedra located at two distinct sites within a single chain. This (Ge1Ge2O₆)_n chain is S-rotated, strongly bent (O4-O2-O4 = 154.8°) compared to that of the C2/c spodumene structure, and assumes angles comparable to those of the two chains of spodumene at 3.3 GPa within the P2₁/c symmetry. As a consequence of the interplay between the M1 and M2 sites, the new polymorph reveals a larger angular distortion for the AlO₆ octahedra, and the Li coordination is reduced from sixfold to fivefold coordination. This establishes Li^[5]Al^[6](Ge1^[4] Ge2^[4]O₆) as the corresponding crystallochemical formula for the new P2₁/n representative within the monoclinic (clino)pyroxene family.