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Lattice dynamics are often regarded as signatures of the underlying crystal structure. Here, a first‐principle‐based effective Hamiltonian method combined with molecular dynamics simulations is used to study dynamical behaviors of CsPbI3 perovskite across temperature and structural phase transitions. A single (short‐range tilting) parameter in this effective Hamiltonian is varied in order to make the temperature range of the intermediate tetragonal P4/mbm phase, existing in‐between the cubic Pm3¯m and orthorhombic Pnma phases, either broader than observed or completely disappearing. Comparing the dynamics of these different cases allows one to conclude that real CsPbI3 perovskite should have i) two iodine‐octahedral‐tilt related modes that differ in frequency but both significantly soften as the temperature decreases within the cubic phase toward the Pm3¯m‐to‐P4/mbm transition; and ii) one mode that maintains a very low frequency (of the order of 1.0 cm−1) in the entire region of P4/mbm stability, as a result of the temporal exploration of various structural states. Such latter sub‐THz mode mixes fluctuations of antiphase iodine tiltings and Cs antipolar motions because of a trilinear energetic coupling. [ABSTRACT FROM AUTHOR]

An atomic interaction is identified in all perovskite compounds, such as ABO3 oxides, that can potentially result in unconventional structures. The term is harmonic in nature and couples the motions of the A cations with the rotations of the oxygen octahedra in the perovskite lattice. When strong enough, this coupling leads to hybrid normal modes that present both (anti)polar and rotational characters, which are keys to understand a variety of exotic phases. For example, we show that not only does this new coupling explain the long-period soft phonons characterizing prototype antiferroelectric PbZrO3, but it also provides us with an unified description of the complex antipolar structures of a variety of perovskites, including the possible occurrence of incommensurate phases. This coupling is further demonstrated to result, in the continuum limit, in an energy invariant adopting an analytical form that has been previously overlooked, to the best of our knowledge. [ABSTRACT FROM AUTHOR]

The archetypical antiferroelectric, PbZrO3, is currently attracting a lot of interest, but no consensus can be clearly established on the nature of its ground state as well as on the influence of external stimuli over its physical properties. Here, the antiferroelectric state of 45-nm-thick epitaxial thin films of PbZrO3 is established by observing the characteristic structural periodicity of antiparallel dipoles at the atomic scale, combined with clear double hysteresis of the polarization-electric field response related to antiferroelectric–to–ferroelectric phase transitions. Surprisingly, while the antiferroelectric state is identified as the ground state, temperature-dependent measurements show that a transition to a ferroelectric-like state appears in a large temperature window (100 K). Atomistic simulations further confirm the existence, and provides the origin, of such ferroelectric state in the films. Electric-field-induced ferroelectric transitions are also detected by the divergence of the piezoresponse force microscopy response. Using this technique, we further reveal the signature of a ferroelectric ground state for 4-nm-thick PbZrO3 films. Compared with bulk crystals, these results suggest a more complex competition between ferroelectric and antiferroelectric phases in epitaxial thin films of PbZrO3. [ABSTRACT FROM AUTHOR]

The Fe-based multicomponent amorphous alloys (also referred to as metallic glasses) are known to exhibit soft magnetic properties and, it makes them important for many technological applications. However, metallic glasses are in a thermodynamically metastable state and in case of high temperature operating conditions, the thermally activated crystallization would be detrimental to their magnetic properties. The study of crystallization kinetics of metallic glasses gives useful insight about its thermal stability. In the present work, crystallization study of Fe67Co18B14Si1 (2605CO) metallic glass has been carried out using differential scanning calorimetry (DSC) technique. Mössbauer study has also been undertaken to know the phases formed during the crystallization process. The alloy shows two-stage crystallization. The activation energy has been derived using the Kissinger method. It is found to be equal to 220 kJ/mol and 349 kJ/mol for the first and second crystallization peaks, respectively. The Mössbauer study indicates the formation of α-(Fe, Co) and (Fe, Co)3B phases in the alloy. [ABSTRACT FROM AUTHOR]