at temperatures up to 550 K. High-pressure raman spectra reveal distinguishable characteristic spectral differences located in the wave number range of external modes with the occurrence of band splitting and shoulders due to subtle symmetry changes. Constraints from in situ observations suggest a stability field of CaCO 3 -IIIb at relatively low temperatures adjacent to the calcite-II field. Isothermal compression of calcite provides the sequence from I to II, IIIb, and finally, III, with all transformations showing volume discontinuities. re-transformation at decreasing pressure from III oversteps the stability field of IIIb and demonstrates the pathway of pressure changes to determine the transition sequence. Clausius-Clapeyron slopes of the phase boundary lines were determined as: ΔP/ΔT = −2.79 ± 0. 28 × 10 −3 gPa K −1 (I-II); +1.87 ± 0.31 × 10 −3 gPa K −1 (II/III); +4.01 ± 0.5 × 10 −3 gPa K −1 (II/IIIb); −33.9 ± 0.4 × 10 −3 gPa K −1 (IIIb/III). The triple point between phases II, IIIb, and III was determined by intersection and is located at 2.01(7) gPa/338(5) K. The pathway of transition from I over II to IIIb can be interpreted by displacement with small shear involved (by 2.9° on I/II and by 8.2° on II/IIIb). The former triad of calcite-I corresponds to the [20-1] direction in the P2 1 /c unit cell of phase II and to [101] in the pseudomonoclinic C1 setting of phase IIIb. Crystal structure investigations of triclinic CaCO 3 -III at non-ambient pressure-temperature conditions confirm the reported structure, and the small changes associated with the variation in P and T explain the broad stability of this structure with respect to variations in P and T. PVT equation of state parameters was determined from experimental data points in the range of 2.20-6.50 gPa at 298-405 K providing K T 0 = 87.5(5.1) gPa, (δK T /δT) P = −0.21(0.23) gPa K −1 , α 0 = 0.8(21.4) × 10 −5 K −1 , and α 1 = 1.0(3.7) × 10 −7 K −1 using a second-order Birch-Murnaghan equation of state formalism.Abstract High-pressure phase transformations between the polymorphic forms I, II, III, and IIIb of CaCO 3 were investigated by analytical in situ high-pressure high-temperature experiments on oriented single-crystal samples. all experiments at non-ambient conditions were carried out by means of raman scattering, X-ray, and synchrotron diffraction techniques using diamond-anvil cells in the pressure range up to 6.5 gPa. The composite-gasket resistive heating technique was applied for all high-pressure investigations T. Yagi is on sabbatical leave at
The most reliable information about crystal structures and their response to changes in pressure and temperature is obtained from single-crystal diffraction experiments. We have developed a methodology to perform single-crystal X-ray diffraction experiments in laser-heated diamond anvil cells and demonstrate that structural refinements and accurate measurements of the thermal equation of state of metals, oxides and silicates from single-crystal intensity data are possible in pressures ranging up to megabars and temperatures of thousands of degrees. A new methodology was applied to solve the in situ high pressure, high temperature structure of iron oxide and study structural variations of iron and aluminum bearing silicate perovskite at conditions of the Earth's lower mantle.
The crystal structure of synthetic BaMg(CO3)2 whose mineral name is norsethite was re-investigated by single-crystal X-ray diffraction. Complementary in situ high- and low-temperature studies by means of vibrational spectroscopy (Raman, IR), powder X-ray diffraction techniques and thermal analyses were performed. Diffraction images (298 K) revealed weak superstructure reflections caused by the displacement of the O atoms in the earlier considered Rm structure model (a = 5.0212(9), cnew = 2 cold = 33.581(6) Å , Rc, Z = 6, R1 = 0.011, sinθ/λ < 0.99 Å –1). Thermal analyses reveal decarbonatization in two decomposition steps above 750 K, and the heat-flow curves (difference scanning calorimetry) give clear evidence of a weak and reversible endothermal change at 343±1 K. This agrees with a discontinuity in the IR and single-crystal Raman spectra. The changing trend of the c/a ratio supports this discontinuity indicating a temperature-induced structural transition in the range between 343 and 373 K. As the change of the unit-cell volume is almost linear, the character of the transition is apparently second order and matches the mechanism of a subtle displacement of the oxygen atom position. The apparent instability of the Rc structure is also evidenced by the remarkably larger anisotropic displacement of the oxygen atom.
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