First evidence of P 2 1 / n to P 2 1 / c structural transformation in pyroxene-type LiAlGe 2 O 6 under high-pressure conditions

The high-pressure behavior of the pyroxene-type compound LiAlGe2O6, the unique representative of a P21/n-pyroxene, was investigated by in-situ X-ray diffraction and Raman spectroscopy on single-crystal samples hydrostatically pressurized in a diamond-anvil cell. The structure was found to undergo a first-order phase transition on compression, with a critical transition at 5.23±0.02GPa. Together with a strong volume discontinuity of −ΔV/V0=−4.1% the transition shows a remarkable hysteresis loop over at least 0.70GPa pressure interval. The bulk modulus of the low- and high-pressure polymorphs corresponds to K0=114(1) and 123(2)GPa, respectively, as described by a 2nd order Birch–Murnaghan equation of state. Based on the systematic extinctions the transition was identified as a P21/n-to-P21/c transformation. The mechanism of structural transformation was identified to be controlled by the stereochemistry of the Li atoms at the M2 site, which changes its coordination number from 5 to 6. The formation of new Li–O bonds involves the co-operative folding of Ge2O6-chains, which explains the anisotropy of axial elasticities and the spontaneous strain across the transformation. Simultaneously the distortion correction of AlO6 units associated with the transition further explains the preference of the P21/c structure under pressure.