A new (Mg0.5Fe0.53+⁠)(Si0.5Al0.53+⁠)O3 LiNbO3-type phase synthesized at lower mantle conditions


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The Fe3+AlO3 component is considered to affect physical properties of bridgmanite such as elastic-wave velocities. Though investigation of the maximum solubility of this component in bridgmanite, it was found that a LiNbO3-structured phase (LN-phase) forms instead of bridgmanite, which is a novel compound. In this article, we reported synthesis of a LN-phase with (Mg0.5Fe3+0.5)(Si0.5Al3+0.5)O3 composition probably through a diffusionless back-transformation from bridgmanite synthesized at a pressure of 27 GPa and a temperature of 2000 K for a duration of 20 hours.

Fig. 1. Characteristics of the  (Mg0.5Fe3+0.5)(Si0.5Al3+0.5)O3 LN–phase: (a) a powder X-ray diffraction profile, (b) a back-scattered electron image of the sample cross section, (c) an optical photograph of selected single crystals, and (d) a Mössbauer spectrum (Black dots are the experimental data, while red curves are the fitting results. Red dots are the difference between calculated and experimental results. Abbreviations: CS: center shift, QS: quadruple splitting, FWHM: Full width at half maximum.)

 

Figure 1 shows results of characterization of the LN-phase. The powder X-ray diffraction pattern (Fig. 1a) indicates that the product consists of one single LN-phase. The back-scattered electron image of the sample cross section (Fig. 1b) shows one single phase without any accessary phase and homogeneity of chemical composition. The Mössbauer spectrum (Fig. 1c) demonstrates that all Fe of this LN-phase is ferric.

Fig. 2 Crystal structure of the LN–phase. The AO6 (A = Fe3+0.5Mg0.5) and BO6 (B = Al3+0.5Si0.5) octahedral are painted by red and blue colors. Yellow spheres are oxygen.

Figure 2 shows the crystal structure of this phase determined by single-crystal X-ray diffraction. It has the space group of R3c and lattice parameters of a = b = 4.8720 (6) Å, c = 12.898 (2) Å, and a unit-cell volume of V = 265.14 (8) Å3. The structure consists of edge-shared  AO6 (A= Fe3+0.5Mg0.5) and BO6 (B = Al3+0.5Si0.5) octahedra. The AO6 octahedra (10.3 Å3) are significantly larger than the BO6 octahedra (8.6 Å3), reflecting the ionic radii of the cations. Although the AO6 octahedron should have formed by distortion of AO12 dodecahedron in the perovskite structure, the distortion of the AO6 octahedra is smaller than that of the BO6 octahedra.


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