Structural evolution and bandgap modification of a robust mixed-valence compound Eu ₉ MgS ₂ B ₂₀ O ₄₁ under pressure

Abstract The recently discovered mixed-valence compound Eu 9 MgS 2 B 20 O 41 is composed of triple-kagomé-layers separated by nonmagnetic Mg 2+ ions, and intervalence charge transfer has been observed in the mixed Eu 2+ and Eu 3+ ions within the kagomé layers, exhibiting similar characteristics typical of a quantum spin liquid. In this study, high-pressure in situ x-ray diffraction measurements on Eu 9 MgS 2 B 20 O 41 were conducted within the range of 0.1 MPa to 64.4 GPa. The results revealed that the stabilization of the ambient-pressure phase, with no transition from mixed valence to single valence observed within the studied pressure range. The bulk modulus of the sample was determined to be 167.3(28) GPa and 180.8(17) GPa, for the single-crystal and powder x-ray diffraction data at room temperature, respectively. These values correspond to approximately 40% of the bulk modulus of diamond. Moreover, absorption spectroscopy measurements were carried out up to 37.9 GPa, revealing a ∼20% reduction in the energy band gap, mainly due to the shortened Eu–O bond lengths. The relationship between pressure and band gap demonstrates a nearly linear trend, with a slope of −0.013 eV/GPa. The findings of the present study imply that the studied sample demonstrates considerable robustness under extreme pressures.