Surface Energetics of Nanoscale LaMnO_(3+δ) Perovskite

摘要

Perovskite type LaMnO_3 and related materials are important compounds with many useful and unique physical and chemical properties. There is a lack of experimental thermochemical data on the energetics of LaMnO_3 nanoparticles. In this work, a series of LaMnO_(3+δ) samples were synthesized via the citrate method and calcined at 700℃-1050℃. All samples displayed rhombohedral structure (X-ray diffraction) with similar oxygen stoichiometry 3+5 = 3.16-3.18 (iodometric titration coupled with gravimetric analysis). The BET surface area varied from null for bulk sample to 6.88 ± 0.08×10~3m~2/mol for the sample calcined at 700℃. The water content varied linearly with the surface area with the highest value being 2.34 wt%. The chemisorbed water adsorption enthalpy was -63.0 ± 4.1 kJ/mol with the chemisorbed water coverage of 8 H_2Om~2. High-temperature oxide melt drop solution calorimetry, performed in sodium molybdate at 702℃, yielded enthalpy of formation from La_2O_3, Mn_2O_3, and O_2 of bulk LaMnO_(3.16) of -77.85 ± 1.94 kJ/mol. After correction of drop solution enthalpies of nanometric samples for water content, the calorimetric data were used to calculate the surface energy of LaMnO_(3+δ). The energy of the anhydrous surface was 2.27 ± 0.29 J/m~2, and that of the hydrous surface was 2.02 ± 0.27 J/m~2. These values are higher than the surface energies of LaMnO_(3.00) predicted elsewhere by theoretical methods, probably due to the different oxygen content and possibly more complex surface structure and exposed surface planes. The measured surface energy of LaMnO_(3+δ) lies between the values reported recently for BaTiO_3 and PbTiO_3 and close to the reported values for MnO_2. This suggests that LaMnO_(3+δ) surface is predominantly MnO_2-terminated, in line with the trends predicted by theoretical calculations.