Structural instabilities and sequence of phase transitions in SrBi2Nb2O9 and SrBi2Ta2O9 from first principles and Monte Carlo simulations
Abstract
Despite their structural similarities, SrBi2Ta2O9 (SBT) and SrBi2Nb2O9 (SBN) undergo a different sequence of phase transitions. The phase diagram of SBT as a function of the temperature includes an intermediate phase between the high-temperature phase and the ferroelectric ground state, while in the niobium compound the intermediate phase is suppressed and a direct transition between the high- and low-temperature structures is observed. We present ab initio calculations that reveal the relevance of a trilinear coupling between three symmetry-adapted modes to stabilize the ground state in both compounds. This coupling is much stronger in SBN than in SBT. Within the framework of the phenomenological Landau theory, it is shown that, by solely increasing the strength of the trilinear coupling, the topology of the phase diagram of SBT can change enough to suppress the intermediate phase. Monte Carlo simulations on an idealized ϕ4 Hamiltonian confirm that the trilinear coupling is the key parameter that determines the sequence of phase transitions, and that for higher dimensionality of the order parameters the stability region of the intermediate phase is narrower.
- Publication:
-
Physical Review B
- Pub Date:
- May 2015
- DOI:
- 10.1103/PhysRevB.91.184106
- Bibcode:
- 2015PhRvB..91r4106P
- Keywords:
-
- 77.80.-e;
- 77.84.-s;
- 63.20.Ry;
- 71.15.Mb;
- Ferroelectricity and antiferroelectricity;
- Dielectric piezoelectric ferroelectric and antiferroelectric materials;
- Anharmonic lattice modes;
- Density functional theory local density approximation gradient and other corrections