Specific features of phase formation and properties of compounds La2W1+xO6+3x (x~ 0.11 − 0.22)
The hexagonal solid solution La2W1 + xO6 + 3x (x ~ 0.11) and the rhombohedral low-temperature phase β-La2WO6 (La2W1 + xO6 + 3x (x = 0)) are synthesized by the methods of preliminary mechanical activation of oxides followed by high-temperature synthesis at 1400°С, 4 h. In addition, by using the method of crystallization from solution, a single crystal of the La2W1 + xO6 + 3x (x ~ 0.22) composition isostructural to La2W1 + xO6 + 3x (x ~ 0.11) is grown. Both solid solutions and the single crystal are studied by the methods of Raman spectroscopy, thermal analysis, and thermogravimetry. Their conductivity is studied by impedance spectroscopy in dry and humid air. The hexagonal single crystal La2W1 + xO6 + 3x (x ~ 0.22) demonstrates strong luminescence in the IR region, in contrast to the hexagonal ceramics La2W1 + xO6 + 3x (x ~ 0.11) and the β-La2WO6 ceramics with the rhombohedral structure. The polycrystalline La2W1 + xO6+3x (x ~ 0.11) ceramics is found to be more stable under redox conditions as compared with the single crystal. The conductivity of the hexagonal single crystal La2W1 + xO6 + 3x (x ~ 0.22) is of the oxygen-ionic nature and lower than the conductivity of ceramics La2W1 + xO6 + 3x (x ~ 0.11) due to its perfect structure. The contribution of the protonic component of conductivity is absent for both the hexagonal solid solution La2W1 + xO6 + 3x (x~ 0.11) and the single crystal La2W1 + xO6 + 3x (x ~ 0.22), their conductivity being of the purely ionic nature with the close values of activation energy (0.89 and 1.08 eV, respectively). The synthesized β-La2WO6 ceramics demonstrates a small contribution of the protonic conductivity in humid air equal to ~1 × 10–6 S/cm at 600°С, which is close to the conductivity of the earlier studied strontium-doped solid solution La1.96Sr0.04WO6 – δ based on β-La2WO6.