Structural transitions in iron-based Ba3NbFe3Si2O14 langasite at high pressures

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Synchrotron X-ray diffraction studies of the structural characteristics of Ba3NbFe3Si2O14 langasite were performed at high hydrostatic pressures (up to 60 GPa) created in a diamond anvil cell. The Mössbauer absorption spectra of 57Fe nuclei and optical absorption spectra were recorded at pressures up to 10 and 54 GPa, respectively. The cascade of structural phase transitions at pressures of about ∼ 3, ∼ 20, and ∼ 41 GPa was discovered. The first transition corresponds to the displacements of light oxygen atoms and associated with an increase in the local symmetry of oxygen 3f tetrahedra surrounding iron ions. The volume drop at the first transition is about 1.5%. The second transition at ∼ 20 GPa is accompanied by a pronounced change in the lattice parameters a and c, while the unit cell volume undergoes a stepwise drop by 7%. Significant decrease in the parameter c at the structural transition should result in a strong increase of the exchange coupling constants between iron ions in neighboring ab planes, which may lead to a giant increase in the Néel temperature in this crystal at pressures above ∼ 20 GPa. At the third transition at about 38–44 GPa, only a small anomaly in the relation is observed, but this anomaly is accompanied by substantial changes in the optical gap and in the quadruple splitting parameter.