Ionic Mobility and Phase Transitions in Perovskite Oxides for Energy Application

Abstract

Perovskite oxides find applications or are studied in many fields related to energy production, accumulation and saving. The most obvious application is oxygen or proton conductors in fuel cells (SOFCs), but the (anti)ferroelectric compositions may find application in high energy capacitors for energy storage, efficient electrocaloric cooling, and electromechanical energy harvesting. In SOFCs, the diffusion of O vacancies and other mobile ionic species, such as H + , are at the base of the functioning of the device, while in the other cases they constitute unwanted defects that reduce the performance and life-time of the device. Similarly, the (anti)ferroelectric phase transitions are a requisite for the use of some types of devices, but the accompanying domain walls can generate extended defects detrimental to the life of the material, and structural phase transformations should be avoided in SOFCs. All these phenomena can be studied by mechanical spectroscopy, the measurement of the complex elastic compliance as a function of temperature and frequency, which is the mechanical analogue of the dielectric susceptibility, but probes the elastic response and elastic dipoles instead of the dielectric response and electric dipoles. The two techniques can be combined to provide a comprehensive picture of the material properties. Examples are shown of the study of structural transitions and hopping and tunneling processes of O vacancies and H in the ion conductor BaCe 1−x Y x O 3−x and in SrTiO 3−x , and of the aging and fatigue effects found in PZT at compositions where the ferro-and antiferroelectric states coexist.

Influence of Temperature, Pressure, and Humidity on the Stabilities and Transition Kinetics of the Various Polymorphs of FAPbI3

“…When t < 1, the BX 6 octahedra are too large to fit the A−X bonds, and therefore, if the B−X bonds are more rigid than the A−X ones, they tilt if an external pressure is applied or when the temperature is lowered and the thermal shrinking of the more anharmonic A−X bonds exerts enough pressure. 57 On the opposite instance of t > 1, the polymorphs with octahedra-sharing faces or edges become favored over cubic perovskite, as explained above. A particularity of FAPI is that at room temperature, it exhibits both the phenomena typical of perovskites with large t, namely, the formation of the hexagonal phases of face-sharing octahedra, and small t, namely, tilting of the octahedra just below room temperature.…”

“…In this context, it is usual to introduce the concept of tolerance factor defined in eq . When t < 1, the BX 6 octahedra are too large to fit the A–X bonds, and therefore, if the B–X bonds are more rigid than the A–X ones, they tilt if an external pressure is applied or when the temperature is lowered and the thermal shrinking of the more anharmonic A–X bonds exerts enough pressure . On the opposite instance of t > 1, the polymorphs with octahedra-sharing faces or edges become favored over cubic perovskite, as explained above.…”

Growth of SrTiO3 Single Crystals with a Diameter of about 30 mm by the Verneuil Method

“…The SrTiO 3 (STO) crystals are commercially produced by the Verneuil process (flame fusion method − ). Substrates for thin films growth are the main application of the STO crystals, although in the last few years STO has been explored as an active material for novel devices in the power, energy, and microelectronics fields. − At present, the STO substrates obtained by the flame fusion method are in the range of 10–20 mm in diameter. Attempts to grow STO crystals of high quality by other methods were undertaken by different groups. − Some of these methods are not convenient for commercial production due to small sizes, small growth rates, or low yield.…”

Structural Transitions and Stability of FAPbI3 and MAPbI3: The Role of Interstitial Water

“…When < 1, the BX octahedra are too large to fit the A–X bonds and therefore, if they are also more rigid than the A–X bonds, below a critical value they tilt without shrinking. This generally occurs during cooling, when the weaker and therefore more anharmonic A–X bonds shrink more than the B–X ones, bringing t below the critical value [ 51 ]. On the opposite side, above a critical value , the A cation cannot fit in the cavities between the BX octahedra and a series of structures with different octahedral connectivity is favoured [ 8 , 52 , 53 ].…”