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 , beta = 108.844(6)°, V = 442.75(16) Å^3, 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 beta 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 beta angle remained almost constant; at T>Ttr the thermal volume expansion is due to dilatation of the structure in the [101] plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of b. 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 Q^2 (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 Q0^2 = 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.
Thermal expansion and high temperature P21/c - C2/c phase transition in clinopyroxene-type LiFeGe2O6 and comparison to NaFe(Si,Ge)2O6
CAMARA ARTIGAS, Fernando;
2010-01-01
Abstract
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 , beta = 108.844(6)°, V = 442.75(16) Å^3, 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 beta 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 beta angle remained almost constant; at T>Ttr the thermal volume expansion is due to dilatation of the structure in the [101] plane, mostly along [0 1 0], and pure shear in the (0 1 0) plane due to the decrease of b. 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 Q^2 (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 Q0^2 = 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
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