A-site Doping Research Articles

Effective Ca2+-doping in Sr1-xCaxFeO3-δ oxygen carriers for chemical looping air separation: A theoretical and experimental investigation

Perovskite iron oxides are promising oxygen carrying materials due to their effectiveness and the low cost of iron. The effect of Ca2+ doping on oxygen ion diffusion in Sr1-xCaxFeO3-δ (x = 0, 0.125, 0.25, 0.375, 0.5) is investigated by combining density functional theory (DFT) calculations and experimental measurements. The oxygen ion diffusion is determined by two key factors of oxygen vacancy formation and migration. The DFT results show that the oxygen vacancy formation energies greatly decrease as Ca2+ content reaches x = 0.125, then gradually decrease with Ca2+ contents up to 0.375, and finally increase as the Ca content reaches x = 0.5. A combination of experimental thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results corroborate this trend for Ca2+ contents between 0 and 0.4. The Fe-O bonding dominates the effect of Ca2+ doping on the oxygen vacancy formation. Shortened Fe-O bonds cause the decrease in the formation energy at lower Ca2+ contents, while the lengthened bonds by FeO6 octahedron distortion cause the increase in the formation energy at higher Ca2+ contents. Kinetically, the oxygen migration barrier is lowered upon Ca2+ doping through the increasing lattice spacing for oxygen diffusion. Therefore, an appropriate Ca2+ doping of x = 0.125–0.375 promotes the oxygen ion diffusion in Sr1-xCaxFeO3-δ. Our findings provide the effective Ca2+ doping value for Sr1-xCaxFeO3-δ and a material design clue for the isovalent A-site doping system of oxygen carriers.

Sr-doping effects on conductivity, charge transport, and ferroelectricity of Ba0.7La0.3TiO3 epitaxial thin films* *Project supported by the National Natural Science Foundation of China (Grant No. 51872099), the Science and Technology Program of Guangzhou, China (Grant No. 2019050001), the Fund from the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, China (Grant No. 2017B030301007), and the 111 Project, China.

Sr-doped Ba0.7La0.3TiO3 (BSLTO) thin films are deposited by pulsed laser deposition, and their microstructure, conductivity, carrier transport mechanism, and ferroelectricity are systematically investigated. The x-ray diffraction measurements demonstrate that Sr-doping reduces the lattice constant of BSLTO thin films, resulting in the enhanced phonon energy in the films as evidenced by the Raman measurements. Resistivity-temperature and Hall effect measurements demonstrate that Sr can gradually reduce electrical resistivity while the electron concentration remains almost unchanged at high temperatures. For the films with semiconducting behavior, the charge transport model transforms from variable range hopping to small polaron hopping as the measurement temperature increases. The metalic conductive behaviors in the films with Sr = 0.30, 0.40 conform to thermal phonon scattering mode. The difference in charge transport behavior dependent on the A-site cation doping, is clarified. It is revealed that the increasing of phonon energy by Sr doping is responsible for lower activation energy of small polaron hopping, higher carrier mobility, and lower electrical resistivity. Interestingly, the piezoelectric force microscopy (PFM) results demonstrate that all the BSLTO films can exhibit ferroelectricity, especially for the room temperature metallic conduction film with Sr = 0.40. These results imply that Sr-doping could be a potential way to explore ferroelectric metal materials for other perovskite oxides.

Enhanced activation of peroxymonosulfate by Sr-doped LaFeO3 perovskite for Orange I degradation in the water

A-site doping of perovskite has been considered to be an effective method to simultaneously boost catalytic performance and limit metal release in heterogeneous catalysis. Herein, various Sr-doped LaFeO3 perovskites (La1-xSrxFeO3-δ) were synthesized and evaluated as a heterogeneous catalyst to activate peroxymonosulfate (PMS) for the degradation of Orange I (OI) in water. Specifically, La0.6Sr0.4FeO3-δ (LSFO-40) exhibited a remarkable improvement in catalytic activity with a k constant of 0.0411 min−1, which was approximately 3 times higher than that of LaFeO3 (0.0142 min−1). The superior stability was obtained with less metal leaching after Sr doping. Such enhancement was mainly attributed to the modified surface properties created by Sr doping, such as a rapid transformation of Fe(II)/Fe(III) redox pair, a large number of oxygen vacancies and surface hydroxyl groups, additionally, the strong interaction between PMS and Fe sites and facilitated electron transfer for PMS activation catalyzed by LSFO-40 also stimulated OI degradation. These also promoted the generation of a considerable amount of superoxide radical (O2−) and singlet oxygen (1O2), which was tested as the main reactive oxygen species (ROSs). The synergistic effect of iron and oxygen species ensured a continuous ROSs generation and excellent catalytic performance. Finally, neither a wide pH range of 5.0–9.0 or co-existing anions (Cl− and HCO3−) showed a negligible inhibitory effect on OI removal, implying a great potential application of the LSFO-40/PMS system in practical wastewater treatment.

Mechanism of enhanced energy storage density in AgNbO3-based lead-free antiferroelectrics

The mechanisms underpinning high energy storage density in lead-free Ag 1–3 x Nd x Ta y Nb 1- y O 3 antiferroelectric (AFE) ceramics have been investigated. Rietveld refinements of in-situ synchrotron X-ray data reveal that the structure remains quadrupled and orthorhombic under electric field ( E ) but adopts a non-centrosymmetric space group, Pmc2 1 , in which the cations exhibit a ferrielectric configuration. Nd and Ta doping both stabilize the AFE structure, thereby increasing the AFE-ferrielectric switching field from 150 to 350 kV cm −1 . Domain size and correlation length of AFE/ferrielectric coupling reduce with Nd doping, leading to slimmer hysteresis loops. The maximum polarization (P max ) is optimized through A-site aliovalent doping which also decreases electrical conductivity, permitting the application of a larger E . These effects combine to enhance energy storage density to give W rec = 6.5 J cm −3 for Ag 0.97 Nd 0.01 Ta 0.20 Nb 0.80 O 3 . Schematic diagram illustrating how energy storage density is optimized through doping in AgNbO 3 and shedding light on the design of novel antiferroelectric (AFE) materials with excellent energy storage density. ● Excellent performance achieved for AgNbO 3 -based antiferroelectric lead-free energy storage ceramics. ● The first observation of a field induced ferrielectric phase. ● 4 principles proposed for the design of high energy density properties of AgNbO 3 -based antiferroelectric ceramics: i) increased resistivity, ii) optimization of maximum polarization, iii) decreased antipolar/polar coupling, and iv) enhanced AFE stability.

Precise Control of Perovskite Crystallization Kinetics via Sequential A-Site Doping.

Two-step-fabricated FAPbI3 -based perovskites have attracted increasing attention because of their excellent film quality and reproducibility. However, the underlying film formation mechanism remains mysterious. Here, the crystallization kinetics of a benchmark FAPbI3 -based perovskite film with sequential A-site doping of Cs+ and GA+ is revealed by in situ X-ray scattering and first-principles calculations. Incorporating Cs+ in the first step induces an alternative pathway from δ-CsPbI3 to perovskite α-phase, which is energetically more favorable than the conventional pathways from PbI2 . However, pinholes are formed due to the nonuniform nucleation with sparse δ-CsPbI3 crystals. Fortunately, incorporating GA+ in the second step can not only promote the phase transition from δ-CsPbI3 to the perovskite α-phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of perovskite solar cells over 23%. This work demonstrates the unprecedented advantage of the two-step process over the one-step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.

Synergistic electron doping and ion conductive phase incorporating of SrCoO3-δ as desirable cathode materials for intermediate-temperature solid oxide fuel cells

Mixed conducting perovskite-structured oxides are desirable catalysts toward the oxygen reduction reaction (ORR) process. Pure cubic perovskite-structured SrCoO3–δ is a promising class of mixed conductors with high ORR activity. However, it suffers from structural instability and thermal incompatibility at elevated temperatures, thereby impeding its practical application in intermediate-temperature solid oxide fuel cells (IT-SOFCs). In this work, A-site electron doping in SrCoO3–δ was conducted to chemically compensate for the loss of lattice oxygen, thus improving the structural stability of SrCoO3–δ. Among the as-prepared Y-doped Sr1−xYxCoO3−δ (x = 0.05–0.20) samples, 10 mol% Y-doping of Sr0·9Y0·1CoO3–δ (SYC10) experimentally exhibits desirable electrical conductivity and lowest area specific resistance (ASR) while maintaining structural stability at elevated temperatures. The results are confirmed by first-principles calculations that involve density of states and oxygen vacancy formation energy. In addition, Gd0.1Ce0·9O1.95 incorporation further improves the thermal compatibility and cell performance, where the measured thermal expansion coefficients between 30 °C and 1000 °C decrease from 23.3 × 10−6 K−1 for SYC10 to 19.0 × 10−6 K−1 for SYC10–40GDC, and the corresponding ASRs with La0·9Sr0.1Ga0.8Mg0·2O3–δ electrolyte decrease from 0.080 Ω cm2 to 0.044 Ω cm2 at 700 °C. Results suggest that SYC10–40GDC is a desirable cathode material for IT-SOFCs.

Optical, magnetic and magneto-transport properties of Nd 1-xAxMn0.5Fe0.5O3-δ (A=Ca, Sr, Ba; x=0, 0.25)

The effect of A-site doping by alkaline earth metals (A = Ca, Sr and Ba) on optical, magnetic, and electrical properties of Nd1-xAxMn0.5Fe0·5O3-δ (x = 0, 0.25) has been investigated. The UV–vis absorption spectra show that the A-site doping decreases the absorption. Two values of optical band-gap energy (Eg) can be estimated for each studied sample: the higher Eg value is associated with the charge transfer involving iron cations and the lower Eg value – with the charge transfer via manganese cations. Partial substitution of neodymium by alkaline earth metals decreases the Néel temperature (TN) and induces significant irreversibility between the zero-field cooled (ZFC) and field-cooled (FC) data below TN. The field-dependent magnetization at 3 K indicates antiferromagnetic ordering with a spin canting. Temperature dependencies of resistivity demonstrate the change from metallic to semiconductor-type conduction (with increasing temperature) at T = Tp < TN. The Tp temperature significantly decreases with doping. The introduction of alkaline earth metals in Nd1-xAxMn0.5Fe0·5O3-δ noticeably reduces the resistivity in the semiconducting region. The small polaron hopping (SPH) mechanism of conduction is suggested at T > TN. Within the range of Tp < T < TN, the resistivity data are interpreted by the variable range hopping (VRH) mechanism. A significant value of magnetoresistance (∼44%) is observed only for Nd0.75Sr0·25Mn0·5Fe0·5O3-δ at 120 K.

Effect of sintering temperature on structural and electrical transport properties of La0.7Ca0.28K0.02MnO3 ceramics

Manganites with large temperature coefficient of resistance (TCR) have been considered as potential candidate for infrared detecting. To date, A-site doping with alkaline-earth metal (+2 oxidation state) has been widely used to improve the TCR. However, doping with alkali ions (+1 oxidation state) was less explored. In this study, 2% K doped La0.7Ca0.28K0.02MnO3 (LCKMO) ceramics were prepared by sol-gel method and sintered within the temperature range of 1000–1300 °C for structural and electrical transport properties investigation. X-ray diffraction (XRD) analysis demonstrated that all samples exhibited an orthorhombic perovskite structure with space group Pnma. Elevated sintering temperature would lead to an increase in grain size, which could account for the concurrent decrease of the resistivity (ρ). By optimizing the sintering process, the magnetoresistance (MR) and TCR reached their maxima of 60.95% and 13.54% K−1, respectively, for sintering temperature of 1200 °C. By fitting the temperature dependence of resistivity (ρ-T), grain boundary scattering and small-polaron hopping (SPH) were found to dominate in the low-temperature ferromagnetic and high-temperature paramagnetic regions, respectively. In addition, the ρ-T in the whole temperature range of 100–300 K can also be explained by the percolation model.

Efficient Ferrite-Based Perovskite Anode for Solid Oxide Fuel Cells with A-Site and B-Site Co-exsolution

Exsolution of A-site and/or B-site dopants has been widely used to improve the electroactivity and coking resistance of the perovskite anode for solid oxide fuel cells (SOFCs). Here, an A-site and B-site co-exsolved Ba0.3Sr0.7Fe0.9Mn0.1O3−δ (BSFM) perovskite is employed as the anode material of SOFCs. X-ray diffraction results confirm that the cubic BSFM perovskite shows reasonable structure stability in hydrogen, although metallic Fe and SrO exsolve from the provskite lattice. Scanning electron microscopy results find that a whisker-like species grows from the perovskite in addition to the exsolution of Fe nanoparticles. It is verified by transmission electron microscopy−energy-dispersive X-ray spectroscopy that the whisker is composed of BaO and SrO. The surface enrichment of Ba and Sr elements is also proven by X-ray photoelectron spectroscopy. The peak power densities of a single cell with the BSFM anode are 538 and 364 mW cm−2 at 800 and 750 °C in hydrogen, respectively, while the polarization resistances of them are 0.18 and 0.28 Ω cm2 at the same conditions, respectively. The cell operates stably for over 60 h in hydrogen with a current density of 0.2 A cm−2 . However, the BSFM anode shows sluggish activity for methane oxidation. With the addition of Ni to the BSFM anode, its activity for methane oxidation is remarkably improved, and the peak power density increases from 84 mW cm−2 for the BSFM anode to 194 mW cm−2 for the Ni−BSFM anode at 800 °C in methane. The Ni−BSFM anode also exhibits excellent stability in methane and works stably for over 70 h with a current density of 0.2 A cm−2.

Acceptor and Donor Dopants in Potassium Sodium Niobate Based Ceramics

B-site doping in potassium sodium niobate (KNN) with Mn2+(〖Mn〗_Nb^) and Ti4+ (〖Ti〗_Nb^) dopants were soluble but prevented KNN from achieving a high relative density while Sn4+(〖Sn〗_Nb^) was not soluble in the structure as evidenced by second phase peaks in XRD traces. However, SnO2 was an effective sintering aid in KNN-50/50. A-site doping with Sr2+(〖Sr〗_((Na,K))^•) up to 1 mol% initially improved density but higher sintering temperatures were required for compositions with > 1 mol% Sr. All Ti-doped and Sr-doped compositions showed an increase in conductivity, manifested as high values of dielectric loss (tanδ). More than 1% acceptor and donor dopants showed the ionic-type conduction mechanism while 1% behaved the electronic mechanism as attributed from the strongly frequency-dependent tanδ . Sample with 5% and 7% of Sr-doping completely shifted the transition of TO-T to below RT and broadened the TC peaks as the relaxor. In result, these samples have the potential to open the new application in the field of electroceramics.

Improved stability of nano-sized La0.6Sr0.4Co0.2Fe0.8O3-δ-Ce0.8Sm0.2O1.9 composite cathodes by substituting Sr2+ with Ca2+

The nano-sized composite cathodes prepared by infiltrating La0.6Sr0. 4Co0.2Fe0.8O3-δ (LSCF) or La0.6Ca0.4Co0.2Fe0.8O3-δ (LCCF) into the Ce0.8Sm0.2O1.9 (SDC) scaffolds exhibit different electro-catalytic activity and microstructure evolution. Compared with LSCF-SDC nano-sized composite cathode, the LCCF-SDC composite cathode shows the higher microstructure stability. There is no observable coarsening or sintering and no diffraction peaks of other impurity phase are detected for both LSCF and LCCF after being aged at 600 °C for 500 h, but the lattice distortion is less if La3+ ions are substituted by Ca2+ ions instead of by Sr2+ in LaCo0.2Fe0.8O3-δ (LCFO) lattice. The oxygen vacancy concentration is also less in LCCF than in LSCF. The less lattice distortion and oxygen vacancy concentration prohibit the Ca2+ ions segregation on the LCCF cathode surface because of less strain in the LCCF lattice and less electrostatic interactions between the negatively charge A-site dopants (Ca′La) and the positively charged oxygen vacancies (Vo··) on LCCF surface. The greater binding energy of Ca–O maybe also hinder the enrichment of Ca2+ ions on the cathode surface. After being aged in air at 600 °C for 500 h, more Sr2+ ions gather on the LSCF cathode surface to form a Sr-rich inert phase, which is detrimental to the oxygen reduction reaction on the cathode surface.

Origin of Structural Change Driven by A-Site Lanthanide Doping in ABO3-Type Perovskite Ferroelectrics

Lanthanide doping is widely employed to tune structural change temperature and electrical properties in ABO3-type perovskite ferroelectric materials. However, the reason that A-site lanthanide doping leads to the decrease of the Curie temperature is still not clear. Based on the reported Curie temperature of lanthanides (Ln) doped in two classic ferroelectrics PbTiO3 and BaTiO3 with A2+B4+O3-type perovskite structure, we discussed the relationship between the decrease rate of Curie temperature (ΔTC) and the bond strength variance of A-site cation (σ). For Nd ion doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (Nd-PMNT) ferroelectric crystal as an example, the internal factors of the dramatic decline of the Curie temperature induced by A-site Nd doping were investigated under a systematic study. The strong covalent bonds of Ln-O play an important role in A-site Ln composition-induced structural change from ferroelectric to paraelectric phase, and it is responsible for the significant decrease in the Curie temperature. It is proposed that the cells become cubic around the Ln ions due to the strong covalent energy of Ln-O bonding in A-site Ln doped A2+B4+O3 perovskite ferroelectrics.

Superior Electrostrictive Effect in Relaxor Potassium Sodium Niobate Based Ferroelectrics.

Electromechanical actuators with toxic lead-based electrostrictive materials dominate the market of high-precision electronics devices, so one of urgent aims becomes how to explore the new generation of ecofriendly electrostrictive materials with superior electrostriction behaviors. Herein, a strategy of modifying the active space for the B-site in lead-free relaxor ferroelectrics arouses the potential capacity of electrostriction, including K0.5Na0.5NbO3 (KNN). Through co-doping of Bi3+ and Ni2+, typical relaxor ferroelectrics (1 - x)K0.5Na0.5NbO3-xBiNi2/3Nb1/3O3 (KNN-xBNN, 0.03 ≤ x ≤ 0.07) are constructed because of the incorporation of dopants ions on the A- or B-site promotes structural disorder. Importantly, a limited active space for the B-site is obtained owing to the contracted oxygen-octahedron with smaller A-site dopants induced by lattice distortion. Benefiting from the design strategy, a remarkable enhancement of electrostrictive coefficient Q33 of ∼0.0456 m4/C2 is achieved, which is higher than those of KNN-based materials and also twice as large as those of lead based materials. The Q33 also exhibits a relatively good electric field and is temperature-independent. A giant Q33 of 0.0512 m4/C2 is gained at a high frequency of 100 Hz, meeting the requirement of some commercial actuators for fuel injectors. Such a strategy may help us design high-performance electrostrictive materials.

Atomic-scale origin of ultrahigh piezoelectricity in samarium-doped PMN-PT ceramics

Designing high-performance piezoelectric materials based on atomic-scale calculations is highly desired in recent years, following the understanding of the structure-property relationship of state-of-the-art piezoelectric materials. Previous mesoscale simulations showed that local structural heterogeneity plays an important role in the piezoelectric property of ferroelectrics; that is, larger structural heterogeneity leads to higher piezoelectricity. In this Rapid Communication, by combining first-principles calculations and experimental characterizations, we explored the atomic-scale origin of the high piezoelectricity for samarium-doped $\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ (PMN-PT) ceramics, which possesses the highest piezoelectric ${d}_{33}$ of $\ensuremath{\sim}1500\phantom{\rule{0.16em}{0ex}}\mathrm{pC}\phantom{\rule{0.28em}{0ex}}{\mathrm{N}}^{\ensuremath{-}1}$ among all known piezoelectric ceramics. The impacts of various dopants on local structure and piezoelectric properties of PMN-PT ceramics were investigated in terms of the effective ionic radius and cation valence. Our results show that A-site dopants with a valence of 3+ are more effective to produce local structural heterogeneity in PMN-PT when compared with the A-site dopants with a valence of 2+, and a smaller dopant size leads to a larger variation of local structure. According to this study, the outstanding piezoelectricity in Sm-doped PMN-PT ceramics is attributed to the fact that $\mathrm{S}{\mathrm{m}}^{3+}$ is the smallest ions that can entirely go to the A site of PMN-PT rather than the B site. The present work may benefit the design of high-performance piezoelectric materials based on the concept of local structural engineering.

A new highly active and CO2-stable perovskite-type cathode material for solid oxide fuel cells developed from A- and B-site cation synergy

Improving the activity of a cathode material towards the oxygen reduction reaction (ORR) and its tolerance against CO2 impurities is of prime importance to realizing the application of solid oxide fuel cells that operate under intermediate temperatures (IT-SOFCs). Here, by adopting an A- and B-site co-doping strategy, we report a new perovskite-type oxide, Nd0.2Sr0.8Nb0.1Co0.9O3-δ (N0.2SNC), as an active and CO2-tolerant cathode material for IT-SOFCs. Compared with single-site doped Nd0.2Sr0.8CoO3-δ (N0.2SC, A-site doping) and SrNb0.1Co0.9O3-δ (SNC, B-site doping) and the parent oxide SrCoO3-δ (SC), N0.2SNC shows improved electrocatalytic activity for the ORR and superior resistance towards CO2 poisoning, achieving a low area specific resistance (ASR) of 0.037 Ω cm2 at 650 °C. In addition, the ASR value of the N0.2SNC electrode only increases from 0.08 to 0.20 Ω cm2 after operating for 300 min under an air atmosphere with 5% CO2. A- and B-site cation synergy is obtained through co-doping, leading to the improved performance. A peak power density of 1563 mW cm−2 is achieved at 650 °C. This A- and B-site cation synergy may offer a new avenue for developing high-performance and CO2-stable cathode materials for IT-SOFCs.

Observation of electrical and energy storage properties of lead-free transition metal-doped BaBi2(NbTa)O9 nanoceramics prepared through chemical route

A-site doping of BaBi2NbTaO9 by Ni2+ was synthesized by chemical precursor solution decomposition method. The materials exhibited single-phase tetragonal structure. The crystallite size of the calcined powder exhibited 42 nm with high degree of tetragonality. Curie temperature (Tc) was not found upto 600 °C that indicated higher Tc of the material. Impedance spectroscopy showed non-Debye type of relaxation with semiconducting nature. The closeness of the activation energy for imaginary impedance relaxation and DC conductivity indicated same type of charge carriers. P–E hysteresis loop indicated ferroelectric properties of the material. Energy storage densities were calculated for charge and discharge. The highest energy efficiency was observed at 10 kV/cm electric field.

A-site bismuth doping, a new strategy to improve the electrocatalytic performances of lanthanum chromate anodes for solid oxide fuel cells

A new strategy is proposed to improve the electrocatalytic performances of LaCrO3-based perovskites. A-site doping bismuth promotes the anode performances even higher than those reported for B-site doping Pd and Ru. For example, at 800 °C, 10 % bismuth doping reduces interfacial polarization resistance from 0.80 to 0.32 Ω cm2 and increases peak power density by ∼40 % to 0.55 W cm–2 in hydrogen. In addition, bismuth doping decreases the interfacial polarization resistance by 74 % in air. Consequently, bismuth doping improves peak power density by up to 80 % at 800 °C for symmetrical structured single cells. The doped material is stable in both air and reducing atmospheres such as in hydrogen at 850 °C for 100 h. The doped bismuth presents a sub-oxidation state, which elevates the average valence states of Cr and Fe, and increases the electronic conductivities and concentrations of surface oxygen species in both air and hydrogen atmospheres.

Oxygen Nonstoichiometry and Defect Equilibria of Yttrium Manganite Perovskites with Strontium A-Site and Aluminum B-Site Doping

In this work, we investigate YMnO3 perovskites with Sr2+ and Al3+ doping, Y1–xSrxMn1–yAlyO3, for use in thermochemical H2O or CO2 splitting for the first time. Oxygen nonstoichiometries (δ) with x ...

Doping and ion substitution in colloidal metal halide perovskite nanocrystals.

The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX3, where A is a monovalent cation (which can be either organic (e.g., CH3NH3+ (MA), CH(NH2)2+ (FA)) or inorganic (e.g., Cs+)), B is a divalent metal cation (usually Pb2+), and X is a halogen anion (Cl-, Br-, I-). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties (e.g., absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A', B', or X' site ions into the A, B, or X sites of ABX3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed.

Preferences of the end members of the lanthanide series for A and B sites in BiFeO3

The preferences of the end members of the lanthanide series (Er3+, Tm3+, Yb3+ and Lu3+) for Bi and Fe sites in BiFeO3 were investigated in this paper using X-ray powder diffraction. It was shown that: (1) the crystal structure of the BiFeO3-derived solid solutions remains rhombohedral in the Ln3+ concentration range from 0.01 to 0.1; (2) for the Er3+-, Tm3+- and Yb3+-doped materials, the unit cell parameters of solid solutions (Bi1-xLnxFeO3 or BiFe1-xLnxO3, x = 0–0.1) decrease with increasing x regardless of the selected stoichiometry; (3) for the materials with the same x, the unit cell parameters increase across the Er3+ - Tm3+ - Yb3+ series despite the decrease in ionic radius that occurs across the lanthanide series; and (4) for the Lu3+-doped materials, the unit cell parameters show only a minor increase. These results suggest that with the decreasing ionic radius of the lanthanides, simultaneous doping on the A and B sites corresponding to the formula Bi1-x+zLnх-zFe1-zLnzO3 (Tm3+, Yb3+) gradually replaces dominant A-site doping (Er3+) and eventually finishes with B-site doping (Lu3+). Variations in the magnetic properties of the doped samples indirectly confirm systematic changes in the preferences of the end members of the lanthanide series for Fe sites in BiFeO3: the values of Ms and Mr decrease drastically with the decreasing size of the Ln3+ cation to the extent that the hysteresis loops for the Lu3+-containing samples have a narrow and lenticular shape, which may be due to the dilution of the Fe-sublattice by Lu3+ cations. The samples show a piezoelectric coefficient d33 of 15 pC/N on average and up to 30 pC/N in individual specimens. Despite belonging to a class of multiferroics, Ln3+-containing solid solutions of BiFeO3 show no magnetoelectric or magnetodielectric response due to the weak coupling between piezoelectric and magnetic subsystems.