Room temperature ferroelectricity in fluoroperovskite thin films

Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: a Raman and first-principles study

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. On this note, we have investigated the ferroelastic structural transition of perovskite-type trigonal Ba₂ZnTeO₆ across T_c ∼ 150 K. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, we report new intriguing observations associated with the phase transition in Ba₂ZnTeO₆. We observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba₂ZnTeO₆ is manifested by a sharp rise in the intensity of the Rayleigh profile concomitant with the huge damping (or softening) of the soft mode (at ∼31 cm^-1) near T_c, shedding light on the lattice dynamics during the phase transition. This is further corroborated by our phonon calculations that show that the soft mode (E_g) in the high-symmetry structure involving TeO₆ octahedral rotation (with Ba and Zn translation) condenses into A_g and B_g modes in the C2/m low-symmetry phase. While most of the phonon bands split below T_c confirming the phase transition, we observed thermal hysteretic behavior of phonon modes, which signifies the first-order nature of the transition and the presence of coexisting phases as corroborated by our temperature-dependent X-ray diffraction and specific heat measurements.

Microwave-assisted synthesis of NaMnF3 particles with tuneable morphologies

Here, the synthesis of sub-micron MMnF₃ (M = Na or K) particles by a rapid microwave-assisted approach is reported. Adjustment of the Na⁺-to-Mn²⁺ ratio in the reaction mixture yielded tuneable morphologies, i.e., rods, ribbons, and plates. Relaxometric results indicated that poly(acrylic acid)-capped MMnF₃ particles exhibited characteristic magnetic properties, which endows them with potential T₁-weighted contrast agent capabilities.

Unveiling the mechanisms behind the ferroelectric response in the Sr(Nb,Ta)O2N oxynitrides

Oxynitride perovskites of the type ABO₂N have attracted considerable attention thanks to their potential ferroelectric behavior and tunable bandgap energy, making them ideal candidates for photocatalysis processes. Therefore, in order to shed light on the origin of their ferroelectric response, here we report a complete analysis of the structural and vibrational properties of SrNbO₂N and SrTaO₂N oxynitrides. By employing first-principles calculations, we analyzed the symmetry in-equivalent structures considering the experimentally reported parent I4/mcm space group (with a phase a⁰a⁰c^- in Glazer's notation). Based on the I4/mcm reference within the 20-atoms unit-cell, we found and studied the ensemble of structures where different octahedral anionic orderings are allowed by symmetry. Thus, by exploring the vibrational landscape of the cis- and trans-type configuration structures and supported by the ionic eigendisplacements and the Born effective charges, we explained the mechanism responsible for the appearance of stable ferroelectric phases in both anionic orderings. The latter goes from covalent-driven in the trans-type ordering to the geometrically-driven in the cis-type configuration. Finally, we found in both cases that the biaxial xy epitaxial strain considerably enhances such ferroelectric response.

Polar Rectification Effect in Electro-Fatigued SrTiO3-Based Junctions

Rectifying semiconductor junctions are crucial to electronic devices. They convert alternating current into a direct one by allowing unidirectional charge flows. Analogous to the current-flow rectification for itinerary electrons, here, a polar rectification that is based on the localized oxygen vacancies (OVs) in a Ti/fatigued-SrTiO₃ (fSTO) Schottky junction is first demonstrated. The fSTO with OVs is produced by an electrodegradation process. The different movabilities of localized OVs and itinerary electrons in the fSTO yield a unidirectional electric polarization at the interface of the junction under the coaction of external and built-in electric fields. Moreover, the fSTO displays a pre-ferroelectric state located between paraelectric and ferroelectric phases. The pre-ferroelectric state has three sub-states and can be easily driven into a ferroelectric state by an external electric field. These observations open up opportunities for potential polar devices and may underpin many useful polar-triggered electronic phenomena.

Direct Magnetization-Polarization Coupling in BaCuF_{4}

Herewith, first-principles calculations based on density functional theory are used to describe the ideal magnetization reversal through polarization switching in BaCuF_{4} which, according to our results, could be accomplished close to room temperature. We also show that this ideal coupling is driven by a single soft mode that combines both polarization, and octahedral rotation. The later being directly coupled to the weak ferromagnetism of BaCuF_{4}. This, added to its strong Jahn-Teller distortion and its orbital ordering, makes this material a very appealing prototype for crystals in the ABX_{4} family for multifunctional applications. The described mechanism behaves ideally as it couples the ferroelectric and the magnetic properties naturally and it has not been reported previously.