The vibration properties of a single crystal of yttrium iron garnet (Y₃Fe₅O₁₂) were studied at high quasi-hydrostatic pressure by Raman spectroscopy. Raman spectra were measured with diamond anvil cells (DAC) in the pressure range of 0-72 GPa at room temperature. In the pressure region of ~ 50 GPa, a radical change in the spectra was found, indicating a phase transition. This correlates with the transition from the crystalline to the amorphous state, which was previously detected by the X-ray method, as well as with the metallization effect established from the optical absorption spectra. At this transition a spin crossover also undergoes in iron ions Fe³⁺, which transit from a high-spin state (HS, 3d⁵, S = 5/2) to a low-spin state (LS, 3d⁵, S = 1/2). In this work, the pressure dependences of the phonon modes in Y₃Fe₅O₁₂ from ambient pressure to the critical pressure of the phase transition are documented in detail. To further study the unique electronic properties of Y₃Fe₅O₁₂ garnet at pressures in the phase transition region, it is necessary to measure electrical resistance at high pressures and cryogenic temperatures.

The results of this study are very important, both for the physics of systems with strong electron correlations, and for geophysics, where various iron oxides are considered as one of the constituents of the Earth's mantle

Figure 1. (a) Photo of a Y₃Fe₅O₁₂ crystal ~ 10 μm thick in a DAC cell in an experiment with an NH₃BH₃ medium. (b) Raman spectrum of a Y₃Fe₅O₁₂ crystal in different frequency ranges at ambient pressure and room temperature. (c) Evolution of the Raman spectra of the Y₃Fe₅O₁₂ crystal with increasing pressure in the quasi-hydrostatic NH₃BH₃ medium, and (d) the dependence of the Raman frequencies on the pressure. The shaded area indicates the pressure range of the proposed dielectric-to-metal transition. At a pressure of ~ 47 GPa, the shape of the spectrum changes dramatically, indicating the onset of the phase transition, which ends after 54 GPa. The Raman spectra were excited using a COBOLT DPSS laser with a wavelength of 660 nm.

Aksenov S.N., Mironovich A.A., Lyubutin I.S., Troyan I.A., Sadykov R.A., Siddharth S. Saxena (Montu), Gavriliuk A.G.
JETP Letters 114, issue 12 (2021)