Impact of the electric field and initial dipole configuration on the polarization and electrocaloric properties of (Ba0.50Sr0.50)TiO3 thin films

A first-principles-derived scheme combined with an efficient Monte Carlo technique is used to investigate the ferroelectric properties and phase transition characteristics of stress-free (Ba 0.50 ,Sr 0.50 )TiO 3 (BST50) thin films under different electric field conditions. BST films exhibit a paraelectric-to-tetragonal ferroelectric phase at zero electric field, followed by a transition from the tetragonal phase to the monoclinic phase as the temperature decreases. Our results reveal that the presence of an electric field along the z-axis shifts the transition temperatures and improves the out-of-plane polarization, whereas the applied electric field along the in-plane direction induces multiple phases at T c =580 K and E x , E y =54 MV/m, confirming the significant effect of the magnitude and direction of the electric field on the sequence of phase transitions. Interestingly, and as a previously unreported phenomenon, our calculations reveal the suppression of polarization and a reduction in the paraelectric-to-tetragonal transition temperature at low values of the electric field applied along the growth direction. This phenomenon was also observed with various electrical boundary conditions and film thickness. In contrast, the polarization and transition temperatures in the Ba 0.50 Sr 0.50 TiO 3 bulk and BTO films are expected to increase, indicating that the polarization and transition temperature consistently increase as the applied electric field increases. This result can be attributed to the competition between the initial dipole configuration and the strength of the applied electric field, as revealed by our analysis of the polarization as a function of temperature at the layer level. Our calculations highlight the remarkable tunability of the electrocaloric effect in (Ba 0 . 50 Sr 0 . 50 )TiO 3 thin films. The maximum electrocaloric response predicted by our calculations is 140 mK·cm/kV at an applied field of E z = 10 MV/m. These findings highlight the strong field-induced increase of the ECE in BST thin films and provide a practical direction for improving BST-based devices for tunable ferroelectric and electrocaloric applications.