Conference Agenda

The physical state, temperature, and composition of the Earth's interior can be constrained by analysing how seismic waves propagate through the Earth's mantle and core. The speeds of seismic waves are determined by the elastic properties of the materials in the Earth's interior. The rocks of the Earth's mantle are polycrystalline aggregates of mineral grains with anisotropic elastic properties. The effective elastic response of a rock depends on how mineral grains interact with each other and whether stresses between grains can be relaxed.

We measured the speed of sound waves in sintered polycrystals of stishovite at pressures of the lower mantle by Brillouin spectroscopy and found that the speed of shear waves remains close to the value implied by negligible relaxation of stresses. In particular, we did not observe a strong reduction of shear wave speeds in response to the second-order phase transition from stishovite to CaCl₂-type SiO₂.

To directly observe how elastic strains and stresses are distributed between the grains of a polycrystalline material, we subjected magnesium oxide powder to loading cycles at seismic frequencies and at pressures of the lower mantle. Our analysis of time-resolved X-ray diffraction patterns that were recorded simultaneously to loading cycles shows that polycrystalline MgO responded with uniform elastic shear strain to loading cycles with small strain amplitudes. This behaviour again indicates minimal relaxation of stresses between grains. The results of our cyclic loading experiments will help to improve mineral-physical models for the Earth's mantle and for the analysis of seismic wave propagation.

Alkali halides and alkaline-earth metal oxides are model ionic solids for studying how crystal structure and chemical composition affect refractive index and electronic polarizability. We present high-pressure refractive index for B1 and B2 phases of NaCl (up to 100 GPa), KCl (up to 100 GPa), CaO (up to 120 GPa), and MgO (up to 140 GPa; B1 data from [1]). To our knowledge, the refractive index of NaCl-B2, KCl-B2, and CaO-B1 and -B2 has not been reported at these pressures before. Measurements were performed using an all-optical method in diamond-anvil cell experiments [2].

We determine Cl⁻ and O²⁻ electronic polarizabilities in the B1 and B2 structures as functions of cation–anion distance, coordination number, and cation chemistry. Polarizability increases with increasing interionic distance and decreasing coordination number. The cation introduces an approximately constant shift, with larger cations producing a stronger reduction in anion polarizability. The obtained dependences are in agreement with previous ab initio results.

The refractive index is described using a density-based model derived from the Lorentz–Lorenz relation, expressed via the constant Mueller parameter (Λ). We obtain: NaCl-B1 Λ = 0.56, NaCl-B2 Λ = 0.60, KCl-B1 Λ = 0.46, KCl-B2 Λ = 0.42, CaO-B1 Λ = 0.87, CaO-B2 Λ = 0.69, MgO-B1 Λ = 1.14. The model tends to underestimate refractive index at high compression, especially for low-bulk-modulus compounds, due to nonmonotonic polarizability.

[1] L. Schifferle, S. Speziale, and S. S. Lobanov, J. Appl. Phys. 132, 12 (2022).

[2] S. S. Lobanov et al., Phys. Rev. Lett. 128, 077403 (2022).

Simultaneous measurements of acoustic wave velocities and densities of minerals relevant to the Earth’s and other planetary interiors are essential for interpreting seismic observations in terms of possible mineral compositions present at depth. Such combined measurements provide internally consistent data that are independent of external pressure calibrations and therefore can be better extrapolated at conditions not yet reachable in the laboratory. Such measurements at elevate pressures/temperatures conditions, however, are still challenging especially when using in house facilities. Here we present acoustic wave velocities collected for garnet and ringwoodite single-crystals with compositions relevant for the Martian mantle using a Brillouin scattering system coupled with an X-ray diffractometer and a CO₂ laser-heating setup.

Perovskites like MgSiO₃ are major constituents of the Earth’s lower mantle. However, the extreme conditions there make it very challenging to study their elastic properties experimentally [1]. Therefore, investigating structure-property correlations and structural instabilities in synthetic perovskites, which are stable at ambient conditions and thus readily accessible, can be helpful.

In addition, synthetic perovskites also play an important role in technological applications. For example, crystals from the (SrAl_0.5Ta_0.5O₃)_1-x(LaAlO₃)_x (short LSAT) mixed solid solution series are promising substrate materials for epitaxial growth of high-temperature superconductors [2], manganese perovskites exhibiting colossal magnetoresistance [3] and GaN as a light source material for light emitting diodes and blue lasers [4].

Two Czochralski grown crystals of the LSAT solid solution series were investigated for thermoelastic properties and thermal expansion in the temperature range between 103 K and 1673 K. The coefficients of thermal expansion were obtained by high-resolution dilatometry and the elastic stiffnesses by resonant ultrasound spectroscopy. We show that the elastic anisotropy of A^[12]B^[6]O₃ perovskites is primarily controlled by the formal charge of the cations on the cuboctahedral and octahedral sites. At ambient and higher temperatures, the cubic LSAT samples fit well to this concept. However, pronounced elastic softening below ambient temperatures indicate precursor effects of a phase transition that occurs just below 100 K.

[1] Sinogeikin et al., Geophys. Res. Letters 31 (2004) L06620, 5 pages.

[2] Mateika et al., J. Cryst. Growth 109 (1991) 447–456.

[3] Berkowski et al., J. Cryst. Growth 257 (2003) 146–152.

[4] Ito et al., J. Cryst. Growth 235 (2002) 277–282.

Cold compression of fayalite, Fe₂SiO₄, may either lead to amorphization or the formation of metastable phases, while the transition to the thermodynamically stable spinel-type polymorph ahrensite or the dissociation into FeO and SiO₂ oxides is kinetically hindered at low temperatures. The structures and physical properties of these metastable phases remained unknown until recently. They have been constrained in a comprehensive study by our experimental group members. To place the structural observations into a broader context, we use electronic structure calculations within density-functional theory (DFT) to complement the experimental findings. We have used the DFT+U approach, which includes the Hubbard-U correction to the electronic interactions, and the PBEsol exchange-correlation functional. First, we performed simulations of the structural, thermodynamic and elastic properties of the known stable phases up to 40 GPa. The predicted properties agree well with previous experimental and simulation studies. Then, we simulated the two newly identified phases, named Fay-II and Fay-II’. They are structurally similar to the forsterite-II structure of Mg₂SiO₄ with coexisting 4- and 6-fold coordinated Si. They both crystallize in the triclinic space group P-1 with 8 or 16 formula units per unit cell, respectively. The DFT simulations confirm the metastable nature of Fay-II and Fay-II’. Finally, the physical and vibrational properties of the metastable phases predicted by the simulations are presented and discussed. Completing the knowledge of metastable states in the two endmembers of the olivine solid-solution series is important not only for understanding these particular minerals, but also for the broader field of mineral physics.