Why Pb(B1/3B'(2/3)) O3 Perovskites Disorder More Easily than Ba(B1/3B'(2/3)) O3 Perovskites and the Thermodynamics of (1:1)-Type Short-Range Order in PMN
Benjamin P. Burton
Fully relaxed, planewave pseudopotential calculations of formation energies (δE) were performed for ordered supercells in the perovskite based system (1 - X) PbNbO3 - (X).PbMgO3, including six different supercells at the X = 1/3 composition [Pb(Mg 1/3Nb2/3)O3; PMN. Some of the corresponding supercell calculations were also performed for the systems Pb(Zn 1/3Nb2/3)O3 (PZN), Pb(Mg1/3Ta2/3)O3 (PMT) and Ba(Zn1/3Ta2/3)O3 (BZT). A striking difference between the PMN results and those for PZN, PMT and BZT is that all the PMN formation energies are within - 4 kJ/(ABO3 - mole), which indicates a very low energy barrier to disordering. The BZT results by contrast, span a range of at least - 40 kJ/mole, consistent with a very stable ordered ground state (GS) phase. The (δE) results suggest a candidate GS for PMN in which (Nb)-layers alternate with chessboard layers perpendicular to  cubic. Ising type thermodynamic models, that are roughly consistent with the )+ results, were used to simulate the finite temperature behavior of a prototype A(B1/3B'2/3)O3 perovskite that exhibits the transition sequence (1 : 2) -> (1 : 1)-> Disordered. This model reproduces the characteristic microstructure of PMN, and explains it in terms of equilibrium (1 : 1) short range order in the disordered phase.
Why Pb(B<sub>1/3</sub>B'<sub>(2/3)</sub>) O<sub>3</sub> Perovskites Disorder More Easily than Ba(B<sub>1/3</sub>B'<sub>(2/3)</sub>) O<sub>3</sub> Perovskites and the Thermodynamics of (1:1)-Type Short-Range Order in PMN, Journal of Physics and Chemistry of Solids, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=850280
(Accessed May 31, 2023)