High-temperature P21/c – C2/c phase transition of LiFe3+Ge2O6

A synthetic clinopyroxene with composition LiFe3+Ge2O6 , monoclinic s.g. P21/c, a 9.8792(7), b 8.8095(5), c 5.3754(3) Å,  108.844(6)°, V 442.75(16) Å3, has been studied by in situ high-temperature single crystal X-ray diffraction. The evolution of the lattice parameters with temperature has been recorded at steps of ca. 27 °C up to 860 °C. A sharp and strong variation in all the lattice parameters corresponding to a first order phase transition is observed at Ttr = 516 °C, with a corresponding to an sudden volume increase of 1.44 %. This spontaneous dilatation is reversible, shows a limited hysteresis of  10 °C, and correspond to the vanishing of reflections with h + k = 2n +1 (b-type reflections), thus indicating a symmetry increase to space group C2/c. The strong change in volume may lead to the breaking of the crystal into several fragments, as it has been observed at the diffractometer. While below Ttr an expansion is observed for all the cell parameters but , which remains constant, at T > Ttr the volume increase is due to dilatation of the structure along [010] and pure shear in the (101) plane ( decreases of 1° at the transition and the keep decreasing). The a lattice shows no thermal expansion after the transition while expansion along c axis is very faint (<0.003 Å in 325 °C). Scalar spontaneous strain prior to the transition is 0.0314. The same transition was observed qualitatively looking at changes in birefringence using an heating stage under the polarized microscope: at the transition there is a neat increase in retardation and the crystal breaks along easy cleavage planes. A hysteresis is observed as the Ttr is 526 °C when raising T and 509°C when decreasing T. Temperatures are highly reproducible. The evolution of the order parameter (Q) was followed by integration of selected b-type reflections normalized to the intensity of adjacent reflections with h + k = 2n (a-type reflections), corresponding to Q2. The equilibrium evolution of Q in the framework of the Landau theory gives a jump in the value of Q at Ttr of Qo2= 0.538(2) and a Tc of 208(7) °C, with a negative ratio of the Landau coefficients b/a = 2291 °C. The large value of (Ttr-Tc) indicates that the 1st order transition is well far from being tricritical in character. A close related composition (LiFe3+Si2O6) has a tricritical phase transition at -45 °C (Redhammer et al. 2001). Complete data collection of intensity data were performed at 214, 430, 537, 753 and 861 °C. The evolution of the crystal structure with T indicates that the M2 polyhedron shows the major changes with a volume decrease by about 13.3% compared to that of about 1.3% for M1 polyhedron. For such calculation we assumed a 6-fold coordination for the M2 polyhedron, as no change in coordination is observed after the transition: the long O3B oxygen positioned at 3.11 Å from M2 site at 430 °C, below the transition, gets farther with a distance increased to 3.32Å at 535 °C. The tetrahedra behave as rigid units with neither a significant change of volume after the transition (< 1‰), nor a change of tilting of the basal plane. The strong deformation of M2 is accompanied by a strong rotation of 51.4 ° at Ttr of the kinking angle (O3-O3-O3 angle) of the B chain of tetrahedral, which switch form O-rotated to S-rotated (from 143.3(5)° to 194.7(6)°). The A chain, which is S-rotated at T < Ttr (206.8(5)° at 430 °C) extends 12° at the transition. This fairly uncommon behavior in high-T polymorphs of clinopyroxenes mirrors the one observed in spodumene in decompression by Arlt and Angel (2000). Redhammer, G. J.; Roth, G.; Paulus, W.; Andre, G.; Lottermoser, W.; Amthauer, G.; Treutmann, W.; Koppelhuber-Bitschnau, B.: The crystal and magnetic structure of Li-Aegirine LiFe3þSi2O6: a temperature dependent study. Phys. Chem. Minerals 28 (2001) 337–346. Arlt, T.; Angel, R. J.: Displacive phase transitions in C-centred clinopyroxenes: spodumene, LiScSi2O6 and ZnSiO3. Phys. Chem. Minerals 27 (2000) 719–731.