Synthetic LiAlGe _2O _6: The first pyroxene with P2 _1 symmetry

摘要

The structure determination of synthetic LiAlGe _2O _6 [Z = 4, space group P21, a = 9.8892(5), b = 8.3929(5), c = 5.3995(3) ?, b = 110.646(3)°], is a new pyroxene structure-type, and represents the first structural example of P21 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 pyroxene polymorph was found to consist of a single S-type rotated tetrahedral chain type, which is - similar to ordered P2 omphacite - composed of alternating Ge1O _4 and Ge _2O _4 tetrahedra located at two distinct sites within a single chain. This (Ge1Ge _2O _6)n chain is S-rotated, strongly bent (O _4-O _2-O _4 = 154.8°) compared to that of the C _2/c spodumene structure, and assumes angles comparable to those of the two chains of spodumene at 3.3 GPa within the P2 _1/c symmetry. As a consequence of the interplay between the M1 and M2 sites, the new polymorph reveals a larger angular distortion for the AlO _6 octahedra, and the Li coordination is reduced from sixfold to fivefold coordination. This establishes Li[5]Al[6](Ge _1[4]Ge _2[4]O _6) as the corresponding crystallochemical formula for the new P21 representative within the monoclinic (clino)pyroxene family.