Bismuth ferrite (BiFeO3) is a prototypical multiferroic material with coexisting ferroelectric and magnetic orders, and is considered promising for next-generation electronic devices. However, challenges that remain to be overcome limit its commercial application and stimulate the research. This work reports a study replacing the Bi site with multiple rare earth elements for the synthesis of (Bi1_4xLaxSmxNdx Pr-x)FeO3 thin films at compositions 0.00 <= x <= 0.20 to explore possible new properties based on the evolution of their structural, ferroelectric, dielectric, and magnetic properties. With increasing the substitution of rare-earth cations, the room-temperature magnetization increases, whereas the dielectric permittivity and electrical conductivity decrease. Concurrently, the crystalline structure evolves from the rhombohedral phase (space group R3c) in pure BiFeO(3 )to a predominantly orthorhombic phase (space group Pbnm), coexisting with residual traces of a rhombohedral phase in the high-entropy composition (x = 0.20). Remarkably, ferroelectricity is retained in films with Bi content as low as 60 mol% (x = 0.10), while in high-entropy composition, the observed local hysteresis loops and the contrasts in PFM amplitude and phase images suggest the presence of a possible improper ferroelectric state, which must be confirmed or refuted through further experiments.

Bismuth ferrite (BiFeO3) is a promising material for developing the next generation of multifunctional electronic devices. However, the production of high-quality BiFeO3 thin films is compromised by the tendency for structural and electronic defects to form during synthesis, which degrades their functional properties. In this work, BiFeO3 thin films were prepared by chemical solution deposition to determine optimal conditions for minimizing oxygen vacancies and to evaluate the impact of these point defects on their physical properties. The films were pyrolyzed at 300 °C for 60 min and 360 °C for 10 min, and crystallized in air and in an O2 atmosphere, at 600 °C and 640 °C for 40 min. High oxygen vacancies were observed in films prepared at low pyrolysis temperatures and crystallized in air, whereas oxygen vacancies were minimized in the film pyrolyzed and crystallized at high temperatures in an O2 atmosphere. The oxygen vacancies markedly affected the films’ physical properties, leading to increased dielectric loss, dielectric dispersion, dc conductivity, and leakage current, with consequent degradation of photovoltaic and magnetic performance. These findings highlight the critical importance of controlling synthesis parameters to suppress oxygen vacancy formation and achieve high-quality BiFeO3 thin films.

Entropy-stabilized ceramics have attracted researchers significantly due to their potential vast multi-functional applications in various engineering fields. The present study emphasizes the synthesis, sintering temperature, Raman, and electrical behavior of a spinel (CoAlFeNTi)3O4 high entropy oxide (HEO). The HEO is synthesized through the modified solid-state reaction method at two different sintering temperatures (1100 °C and 1250 °C) and characterized further with the XRD, Raman, and SEM for structural and microstructural behavior. XRD analysis confirmed the formation of a single cubic spinel phase with the Fd-3 m space group. In addition, Raman analysis also confirmed that the synthesized HEOs have a spinel structure with an inverse spinel nature. With the enhancement of the sintering temperature, XRD analysis indicates that the crystallinity and crystallite size of the HEO enhanced. The current density (J) versus applied electric field (E) characteristics displayed that both 1100 °C and 1250 °C sintered HEOs possessed leakage current density at zero applied electric field and an ohmic conductance of 4.59× 10^-10 mhos/cm and 3.43× 10^-10 mhos/cm respectively. Moreover, J - E characteristics also showed that the enhancement of sintering temperature enhanced the resistive switching behavior of the different temperature sintered spinel HEOs. This improved resistive switching behavior in the J-E curve indicates that the synthesized spinel HEO can find potential application in resistive switching memory devices.

High entropy is a hot topic in materials research due to several interesting and surprising phenomena, of which one crucial aspect is entropic stabilization. As well-known materials for optoelectronic and electrochemical applications, halide perovskites (HPs) suffer from instability issues and would benefit greatly from increased configurational entropy. Despite that, only a few literature reports have connected HPs with the concept of high-entropy materials. Furthermore, mixing A-site cations, especially organic ones, to achieve maximized configurational entropies has not been explored in detail either in experimental or computational works. Aiming to obtain high-entropy organic A-site HPs, we synthesized and characterized a system of penta-organic A-site cations HP of general formula GAxFAxEAxACxMA1-4xPbI3. Results on the structure and phase transitions show that single-phase solid solutions can be obtained for x values up to almost 0.08, resulting in one of the highest configurational entropies ever reported in A-site-only mixed HPs. The high-entropy HPs also showed band gaps of about 1.5 eV, decreased ionic transport, and remarkable stability compared to the unsubstituted composition. The results consolidate the potential of maximizing the configurational entropy as a design parameter in HPs.

The present study reports the synthesis of a novel (Al0.2Co0.2Fe0.2Ni0.2Ti0.2)(3)O-4 high entropy oxide (HEO) through the solid-state reaction method and its structural, dielectric, electric, and magnetic properties. For the first time, a cumulative study of dielectric, electric, and magnetic properties of the spinel HEO has been investigated in detail. The obtained HEO has a single-phase spinel structure with Fd-3m space group, confirmed through X-ray diffraction and Raman spectroscopy techniques. Moreover, Raman spectroscopic analysis also confirms that the synthesized spinel HEO is an inverse spinel. The dielectric and magnetic characterizations reveal a better dielectric permittivity of epsilon' = 35 at 1 MHz and high saturation magnetization Ms = 8.58 emu/g with low magnetic coercivity. The leakage current characteristics studied in terms of J-E curves indicate an ohmic conduction mechanism at a low electric field. Moreover, the high dielectric permittivity with resistive switching behavior (observed in the J-E curve) indicates its potential application in resistive switching memory devices, which have better functionality and enhanced scalability.

Thermal degradation experiments in GAxFAyMA1−x−yPbI3 perovskites show that mixed compositions are generally more stable, but not always. Large GA+ cations may decrease the stability and even change the order of the degradation kinetics.

Halide perovskites (HPs) are a well-known class of mixed electronic and ionic conductors with diverse applications in optoelectronic devices. The simultaneous transport of ionic and electronic carriers has beneficial and detrimental effects depending on the intended applications. There is an extensive understanding of the charge transport characteristics in HPs since the phenomenon is of applied relevance. However, considering that several applications use compositions containing mixed cations, a deeper understanding of how the degree of substitution and the characteristics of the substituent cations affect the charge transport characteristics is needed. To this end, we experimentally studied the ionic conductivity (σion), current–voltage hysteresis (J–E hysteresis), mobility (μe) and density (ne) of electronic carriers, and bandgap energies (Eg) of up to 24 compositions of methylammonium lead iodide partially substituted with guanidinium and formamidinium. The results indicate that σion, J–E hysteresis, and μe decrease with the degree of substitution, with the J–E hysteresis being smaller the larger size of the substituent cation. At the same time, σion appears to be lower in compositions with equimolar substituents, in which the entropy of mixing is maximum. On the other hand, a slight increase in ne was observed with the substitution degree, showing highest values for FA+-rich compositions, where Eg is the lowest. The results advance the understanding of how it is possible to customize charge transport properties through the rational design of compositions in HPs.

Halide perovskites are a class of materials of consolidated optoelectronic and electrochemical applications, reaching efficiencies compared to established materials in respective fields. In this scenario, the design and understanding of composition-structure-property relations is imperative. In solid solutions containing mixed cations, some direct relations between the sizes of the substituents and the properties of perovskites are generally observed. However, in several cases, these relations are not observed, implying that other characteristics of these cations play a major role. Despite its importance, this understanding has not been comprehensively deepened. To address this issue, we synthesized and characterized the structure, electrical behavior, and stability of methylammonium lead iodide-based perovskites with equal amounts of the substituents guanidinium, ethylammonium, and acetamidinium. These three large organic cations have essentially equal sizes but other remarkably different characteristics, such as the number of N-H bonds, intrinsic dipole moment, and order of C-N bonds. Herein, we show that these cations have dramatically different effects over important fundamental and applied properties of resulting perovskites, including the orthorhombic-to-tetragonal and tetragonal-to-cubic phase transitions, microstructural development, ionic conductivity, I-V hysteresis, electronic carrier mobility, and stability against light-induced degradation. These effects are correlated with the characteristics of the large substituent cations and help pave the way for a better rational chemical design of halide perovskites.

Abstract Relaxor ferroelectrics play an important role for technological applications, mainly for using in the fabrication of actuators, transducers and sensors. Therefore, there has been an increasing interest of the scientific community in the investigation of the physical properties of such system in order to better elucidate the observed intriguing and unsolved phenomena. In this work, relaxor Pb(Mg1/3Nb2/3)O3 (PMN) ceramic system was prepared and the dielectric properties have been studied in details as a function of frequency, temperature and magnitude of the DC-bias electric field. Results reveal that the temperature dependence of the dielectric permittivity can be successfully described in the whole analyzed temperature range (100–400 K) by using a macroscopic statistical model, which describe the dielectric response in relaxor systems with diffuse phase transition (DPT). By using this phenomenological model, the temperature and DC-bias electric field dependences of the dielectric permittivity have been also revisited, from the analysis of the DPT behavior, promoting a careful discussion on the nature of the phase transition in relaxors materials. The calculated sizes of the polar nanoregions (PNRs) suggest the development of a structural disorder in the studied system, which promote the high diffuse ferroelectric transition as the amplitude of the DC-bias electric field increases.

Samarium-substituted bismuth ferrite (Sm x Bi 1− x FeO 3 ) compositions comprise a system of important materials due to their combination of multiferroic properties. Several dielectric and charge transport reports in literature can be found in this system. However, as a typical polycrystalline electroceramic, their grains and grain boundaries (GBs) are expected to possess very different properties. To this date, these distinctions have not been determined for this system. In this work, through measurements via impedance spectroscopy on Sm x Bi 1− x FeO 3 thin films, we show that using a brick layer model allows the separation of the electrical properties of grains and GBs. Results indicate that grains have dielectric permittivity and electrical conductivity much higher than GBs. Their properties mostly control the characteristics observed in the studied thin films. The introduction of samarium reduces the electrical conductivity and increases the activation energies for charge transport in grains and GBs. In turn, dielectric permittivity is reduced in grains and subtly increased in GBs.

Despite the proven enhancement of MAPbI3 optoelectronic properties and stability by guanidinium substitution, divergences persist regarding fundamental knowledge on this system. This work shows that GAxMA1-xPbI3 solid solutions have guanidinium content-dependent phase transitions, dielectric permittivities, ionic conductivities, activation energies, and relaxation times.

Halide perovskites are a well-known class of materials with many interesting applications. Great attention has been devoted to investigating halide perovskites containing triple methylammonium (MA+), formamidinium (FA+), and guanidinium (GA+) cations. Despite presenting very good applied perspectives so far, the lack of fundamental information for this system, such as its structural, thermal, and optoelectronic characteristics, prompts a step back before any technological leap forward. In the present work, we investigate the physical properties of mechanochemically solvent-free synthesized GAxFAxMA1-2xPbI3 halide perovskite powders with compositions of 0.00 ≤ x ≤ 0.15. We demonstrate that the synthesis of the powders can be performed by a simple manual mechanical grinding of the precursors for about 40 minutes, leading to solid solutions with an only minor content of unreacted precursors. X-ray diffraction, differential scanning calorimetry, and infrared spectroscopy techniques were used to investigate the structure, tetragonal-to-cubic phase transition, and vibrational characteristics of the organic cations with increasing GA+ and FA+ contents, respectively. The band gap and Urbach energies, obtained from ultraviolet-visible spectroscopy analyses, ranged from 1.58 to 1.65 eV and 23 to 36 meV, respectively, depending on the composition. These parameters demonstrate a non-random variation with x composition, which offers the possibility of a rational composition design for a given set of desired properties, demonstrating potential for optoelectronic applications. Finally, the system appears to have adequately tolerated heating for 12 hours at 120 °C in an ambient atmosphere, indicating high thermal stability and low ionic conductivity, which are desirable characteristics for solar cell applications.

Using the GAxMA1−xPbI3 system, we show that several characteristics of mixed-cation lead halide perovskites change under aging under mild conditions.

The domain switching dynamics in a relaxor ferroelectric lanthanum-modified lead zirconate titanate thin film with 12 mol. % La was investigated by time-resolved x-ray diffraction. While most frequently epitaxial thin films are investigated, the present work reports results on a polycrystalline thin film. Asymmetric butterfly loops of the strain as a function of the applied electric field evidenced a built-in electric field oriented toward the thin film–substrate interface. The piezoelectric coefficient d33 (in the film reference frame) diminishes with the increasing frequency of an applied AC electric field. From the strain transient during the application of positive-up negative-down voltage pulse sequences with frequencies of up to 100 kHz, characteristic times of the order of 100–200 ns were determined for these relaxor ferroelectric thin films. While switching times ranging from the picosecond to the millisecond range are reported in the literature for different materials, these characteristic switching times are comparable to epitaxial lead zirconate titanate thin films of various compositions despite the polycrystallinity of the present thin film. However, the evidenced built-in electric field significantly influences the switching behavior for different polarities.

This work reports dielectric properties of methylammonium lead iodide as a function of frequency, temperature, and dc bias electric field studied in terms of grain and grain boundary contributions. These results were analyzed from the perspective of a relaxor-like ferroelectric nature. The temperature dependence of dielectric permittivity at different frequencies showed a wide dispersion in the vicinity of the tetragonal-cubic phase transition, suggesting a relaxor ferroelectric feature after excluding artifacts as Maxwell-Wagner effects and dc contributions. The results from plots of the Vogel-Fulcher and the modified Curie-Weiss laws indicate the existence of a freezing temperature of the dipoles responsible for the relaxor character of the material (Tf ∼ 270 K) and a diffuse phase transition at high temperatures (γ ∼ 1.52-1.74). Measurements on heating and cooling protocols under a dc bias electric field suggest field-induced polarized domains like relaxor ferroelectric. The observed signatures of transitions between non-ergodic and ergodic relaxors to ferroelectric states were attributed to polar nano regions dynamics, suggesting a critical electric field around 0.03 kV cm−1.

The dynamic dielectric response of Pb(Mg1/3Nb2/3 )O3 ceramic was experimentally studied as a function of the EAC amplitude field. An increase in real dielectric permittivity was obtained by increasing the applied electrical field within the investigated temperature range for frequencies below 10 kHz. The temperature of maximum dielectric permittivity and freezing temperature decreased with an increase in E AC. Nonlinear permittivity was studied and found to behave similarly to freezing temperature. A statistical model was used to fit the dielectric dispersion of real dielectric permittivity with temperature and frequency. The results are discussed in terms of different factors’ contributions to dielectric permittivity under different EAC field conditions.

The crystalline Bi6Te2O15 and Bi6Te2O13 phases with orthorhombic and cubic symmetries, respectively, were synthesized by solid-state reaction synthesis. A study of the thermal stability of both phases was carried out, being verified that the high temperature cubic phase can be kept at room temperature in a metastable condition during long time. The Raman scattering spectrum, optical reflectivity and photoluminescence emission of these phases were measured for the first time. The optical band gaps E-g = 3.29eV, for the orthorhombic phase, and E-g = 3.08eV, for the cubic phase, were calculated from the reflectivity spectra. Using the brick-layer model the electrical conductivity of a dense ceramic of Bi6Te2O15 was measured, taking in account the grain and grain-boundary effects.

Methylammonium lead iodide (MAPbI3) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI3. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance ∼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA+ and I− ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.

BiFeO3 thin films were prepared using the chemical solution route on Pt/TiO2/SiO2/Si(100) substrates under different crystallization kinetics. The crystallization kinetic effects on the dielectric and electrical properties have been investigated. These properties included dielectric permittivity, electric modulus, electrical conductivity measurements as a function of the temperature (300–525 K) and frequency (102–106 Hz), and leakage current measurements electric field range ± 30 kV/cm at room temperature. The differences observed in conductivity and current density of the BiFeO3 films were discussed in terms of possible defects induced by the crystallization kinetic. An anomalous relaxor-like dielectric behavior characterized by a broad maximum in the real dielectric permittivity as a function of temperature and the low-frequency dielectric dispersion has been observed. The nonexpected peaks in the real permittivity were accompanied by increasing at least four orders in the conductivity’s magnitude at high temperatures. The origin of the relaxor-like dielectric anomalies is discussed, suggesting that the dielectric permittivity peaks are artifacts due to carrier migration correlated to the onset of the Maxwell–Wagner effect.