Equation of state and structural phase transitions in iron-based Ba3TaFe3Si2O14 langasite at high hydrostatic pressures

Growth and X-ray Diffraction Study and Specific Features of Thermal Expansion of Ba3NbFe3Si2O14 Single Crystal from the Langasite Family

“…The ratio of the maximum to the minimum distance becomes closer to unity: 1.0298 → 1.0278; therefore, the 3 f -tetrahedron becomes more regular when cooling. Note that the same effect (symmetrization of iron tetrahedron) was observed in the Mössbauer spectra of BTFS crystal under the influence of external pressure in the range of 5–15 GPa …”

“…Note that the same effect (symmetrization of iron tetrahedron) was observed in the Mossbauer spectra of BTFS crystal under the influence of external pressure in the range of 5−15 GPa. 17 We found that the mobility of atoms in BNFS along the c axis results in an anomaly in the thermal expansion of this crystal. When lowering the temperature from 293 to 90.5 K, the compression coefficient α along the c axis approximately three times exceeds the compression along the a axis (Table 1).…”

“…It should also be noted sharp changes of a and c lattice parameters found in BTFS at the structural phase transition under high pressure of 20 GPa . At this transition, the parameter a increases and parameter c significantly decreases.…”

“…Another possible structural transition P 321 → A 2 ( C 2) is related to the loss of the 3-fold symmetry axis . X-ray diffraction studies of BTFS polycrystal under high pressures revealed a structural phase transition at P = 20 GPa, which is accompanied by a sharp decrease in the unit-cell volume (by 8.6%), but a transition to the monoclinic structure was not recorded. However, more recently the synchrotron X-ray diffraction studies of this compound under high pressure revealed the transformation to the monoclinic structure at this phase transition near 20 GPa …”

“…7 However, the structural studies of Ba 3 TaFe 3 Si 2 O 14 (BTFS) and Ba 3 NbFe 3 Si 2 O 14 (BNFS) polycrystals 7 were insufficiently accurate, which did not allow observation of this transition. Indeed, an analysis of possibilities of phase transitions that may occur in langasite-family crystals 15 diffraction studies of BTFS polycrystal under high pressures 17 revealed a structural phase transition at P = 20 GPa, which is accompanied by a sharp decrease in the unit-cell volume (by 8.6%), but a transition to the monoclinic structure was not recorded. However, more recently the synchrotron X-ray diffraction studies of this compound under high pressure revealed the transformation to the monoclinic structure at this phase transition near 20 GPa.…”