High pressure magnetic, structural, and electronic transitions in multiferroic Ba3NbFe3Si2O14

The magnetic, structural, and electronic properties of multiferroic Ba3NbFe3Si2O14 from the langasite family were investigated by several methods including synchrotron X-ray diffraction (XRD) and synchrotron Mössbauer source (SMS) technique at high hydrostatic pressures (up to 60 GPa) created in diamond anvil cells. A cascade of structural phase transitions at pressures of about 3.5, 17.5, and 41 GPa was revealed by structural XRD, magnetic SMS, and electronic methods, including Raman, Mössbauer, and optical absorption. The transition at 17.5 GPa leads to strong changes in unit cell parameters and a sharp drop in the cell volume by about 7%. This drastically changes the magnetic properties of the crystal, which separates into two magnetic phases. The Néel temperature TN in one of the phases is increased up to 100 K, which is more than three times higher than the TN value of the parent compound (∼27 K). Above 41 GPa, the optical gap decreases to about 0.7 eV, implying a transition from dielectric to a semiconducting state, while the TN value of the magnetic phase decreases from 100 to 58 K. The magnetic transitions are explained by the redistribution of Fe ions between tetrahedral and possibly octahedral (or/and interstitial) sites resulting in enhancement of exchange interactions. The appearance of a low spin state of Fe3+ ions in octahedral sites is also discussed.