First evidence of P21/n to P21/c structural transformation in pyroxene-type LiAlGe2O6 under high-pressure conditions

High-pressure crystallography and compression behavior of the alkali-scandium-germanate end-members LiScGe2O6 and NaScGe2O6

“…Figure 11 shows the results of the corresponding plot, which clearly reveals four trend lines having a very similar slope, each differentiating the subsets of series with M2= Li, Na, Ca and T= Si, Ge. The slopes of the dashed lines drawn in Figure 11 [38]; NaCrSi 2 O 6 [43]; NaVSi 2 O 6 [12]; NaInSi 2 O 6 [36]; NaGaSi 2 O 6 [44]) HP structures, one P-1 HP structure [12] and even the one unique P2 1 /n phase (LiAlGe 2 O 6 [9,10]). For lowpressure (LP) and HP LiFeGe 2 O 6 second-order EoS were fitted using the data given in [41].…”

“…The analogue germanate, LiAlGe 2 O 6 , shows an apparently identical P-induced transformation at RT [9], but the true symmetry of the low-P form was found to be P2 1 /n [10]. Apart from electronic and magnetic effects [11], such as recognized to be responsible for the P-1 symmetry in NaTiSi 2 O 6 [11,12], the interplay of the relative sizes of the atoms at M1, M2 and T sites in M2M1T 2 O 6 compounds appears to determine to a large extent the variations of true space-group symmetries and to control the phase stability and transformations originating from structural instabilities.…”

The (Na,Li)FeGe2O6 clinopyroxene-type series: a temperature-dependent single-crystal X-ray diffraction and 57Fe Mössbauer spectroscopic study

“…This structure is unique as it consists of a single tetrahedral chain-similar to the C2/c structure-but built up by two symmetrically different GeO 4 sites. Recently, a high-pressure study showed that LiAlGe 2 O 6 undergoes a phase transition to P2 1 /c at a critical pressure of 5.23(2) GPa (Nestola et al 2015).…”

P21/c Postorthopyroxene γ-LiScGe2O6, a New Dense High-Pressure Polymorph and Its Direct Transformation from the Pbca Structure

“…Below 9 GPa, the refined values of α, β, and γ for β-LSG correspond to 90°within an uncertainty of 6% and are comparable to many first-order transitions in pyroxene-type structures. 16,23,25,26,[35][36][37]39 The discontinuity can also be seen in the crystallographic axes, with a strong shortening of the lattice periodicity along the c axis (−2.6%), while in the a-and b-axis directions, changes across the transition point yield opposite signs (+1.0%∥a and +0.02%∥b). This behavior can be explained by changes to the tetrahedron chains and is proven for several other pyroxene transformations.…”

“…Regardless of Si or Ge on the T sites, it is the relative size of the individual cations and their interplay at the T, M1, and M2 positions in the M2M1T 2 O 6 compounds that control the structural stabilities and transformations (e.g., ref ). The exceptional P 2 1 / n germanate spodumene homologue, LiAlGe 2 O 6 , was found to transform to LCEn- P 2 1 / c . Hydrostatic compression of the LCEn- P 2 1 / c spodumene analogue α-LiScGe 2 O 6 shows no phase transition up to the hydrostatic limit of the pressure-transmitting medium at ∼10 GPa .…”