Thermal expansion and high-temperature P21/c-C2/c phase transition in clinopyroxene-type LiFeGe2O6 and comparison to NaFe(Si,Ge)2O6

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, β = 108. 844(6)°, V = 442.75(16) Å3, has been studied by in situ low- and high-temperature single-crystal X-ray diffraction. The variation of lattice parameters and the intensity of the b-type reflections (h + k = 2n + 1, only present in the P-symmetry) with increasing temperature showed a displacive phase transition from space group P21/c to C2/c at a transition temperature Ttr = 789 K, first order in character, with a sudden volume increase of 1. 6% and a decrease of β by 1° at the transition. This spontaneous dilatation is reversible, shows a limited hysteresis of ±10°C, and corresponds to the vanishing of the b-type reflections, thus indicating a symmetry increase to space group C2/c. Below Ttr an expansion is observed for all the cell parameters, while the β angle remained almost constant; at T > Ttr the thermal volume expansion is due to dilatation of the structure in the (1̄,0,1) plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of β. From comparison with silicate analogues, the germanate clinopyroxenes are more expansible, while the P21/c expands more than the C2/c phase. The evolution of Q2 (calculated as the normalized intensity of b-type reflections) with T in the framework of the Landau theory has been done using a standard expression for a first order phase transition. We observe a jump of Q02 = 0. 538(2) at Ttr, with Tc of 481(7) K, b/a = -2,290 K, and c/a = 3,192 K, and thus far from being tri-critical point. A closely related composition (LiFe3+Si2O6) shows an equivalent phase transition at 228 K, which is very close to the tri-critical point and 561 K cooler. This result indicates that a change in the composition of tetrahedral sites can have dramatic effects on the P21/c ↔ C2/c displacive phase transition in clinopyroxenes. The major changes observed in the evolution of the crystal structure with T are observed in the M2 polyhedron, with a volume decrease by ca. 13. 3%, compared to ca. 1. 3% observed in the M1 polyhedron. The tetrahedra behave as rigid units with neither a significant change of volume at T > Ttr (<1‰), nor a change of tilting of the basal plane. No change in coordination is observed at T > Ttr in the M2 polyhedron, which remains sixfold coordinated although a strong deformation of this polyhedron is observed. This deformation is related to a strong change by 51.4° at Ttr of the kinking angle (O3-O3-O3 angle) of the B-chain of tetrahedra, which switches from O-rotated to S-rotated [from 143.3(5)° to 194.7(6)°]. The A-chain is S-rotated at T <Ttr [206.8(5)° at 703 K] and extends by 12° at the transition.