A-site occupancy in the lead-free (Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 piezoceramic: Combining first-principles study and TEM

The crystal structure of the lead-free piezoelectric ceramic (Bi1/2Na1/2TiO3)0.94–(BaTiO3)0.06 was investigated by first-principles calculations and high-resolution transmission electron microscopy (HRTEM) imaging. Structures with different A-site occupation were relaxed by total energy calculations within density functional theory and then used for simulating the corresponding HRTEM images. Simulated and experimental HRTEM images were compared and the closest match selected for structure interpretation. By combining these techniques, we have identified the Bi(Ba)/Na distribution on the A-site to be homogeneous. We exclude the possibility that regions visible in HRTEM images within one grain can be attributed to different ordering but to a slight tilting of the structure with respect to the electron beam

Crystallographic and Magnetic Structure of the Perovskite-Type Compound BaFeO_2.5: Unrivaled Complexity in Oxygen Vacancy Ordering

We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO_2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group <i>P</i>2₁/<i>c</i> [<i>a</i> = 6.9753(1) Å, <i>b</i> = 11.7281(2) Å, <i>c</i> = 23.4507(4) Å, β = 98.813(1)°, and <i>Z</i> = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba₇(FeO_4/2)₁(FeO_3/2O_1/1)₃(FeO_5/2)₂(FeO_6/2)₁ = Ba₇Fe^[6]₁Fe^[5]₂Fe^[4]₄O_17.5 and is in agreement with the ⁵⁷Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO_2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX_2.5 units per unit cell, and number of different crystallographic sites). The magnetic structure was determined from the powder neutron diffraction data and can be understood in terms of “G-type” antiferromagnetic ordering between connected iron-containing polyhedra, in agreement with field-sweep and zero-field-cooled/field-cooled measurements

Crystallographic and Magnetic Structure of the Perovskite-Type Compound BaFeO_2.5: Unrivaled Complexity in Oxygen Vacancy Ordering

We report here on the characterization of the vacancy-ordered perovskite-type structure of BaFeO_2.5 by means of combined Rietveld analysis of powder X-ray and neutron diffraction data. The compound crystallizes in the monoclinic space group <i>P</i>2₁/<i>c</i> [<i>a</i> = 6.9753(1) Å, <i>b</i> = 11.7281(2) Å, <i>c</i> = 23.4507(4) Å, β = 98.813(1)°, and <i>Z</i> = 28] containing seven crystallographically different iron atoms. The coordination scheme is determined to be Ba₇(FeO_4/2)₁(FeO_3/2O_1/1)₃(FeO_5/2)₂(FeO_6/2)₁ = Ba₇Fe^[6]₁Fe^[5]₂Fe^[4]₄O_17.5 and is in agreement with the ⁵⁷Fe Mössbauer spectra and density functional theory based calculations. To our knowledge, the structure of BaFeO_2.5 is the most complicated perovskite-type superstructure reported so far (largest primitive cell, number of ABX_2.5 units per unit cell, and number of different crystallographic sites). The magnetic structure was determined from the powder neutron diffraction data and can be understood in terms of “G-type” antiferromagnetic ordering between connected iron-containing polyhedra, in agreement with field-sweep and zero-field-cooled/field-cooled measurements