A-site Substitution Research Articles

Static disorder in a perovskite mixed-valence metal–organic framework

Effects of A-site and M-site substitutions on the structural properties of perovskite dimethylammonium iron formate.

Elucidation of the role of guanidinium incorporation in single-crystalline MAPbI3 perovskite on ion migration and activation energy.

Ion migration plays a significant role in the overall stability and power conversion efficiency of perovskite solar cells (PSCs). This process was found to be influenced by the compositional engineering of the A-site cation in the perovskite crystal structure. However, the effect of partial A-site cation substitution in a methylammonium lead iodide (MAPbI3) perovskite on the ion migration process and its activation energy is not fully understood. Here we study the effect of a guanidinium (GUA) cation on the ion transport dynamics in the single crystalline GUAxMA1-xPbI3 perovskite composition using temperature-dependent electrochemical impedance spectroscopy (EIS). We find that the small substitution of MA with GUA decreases the activation energy for iodide ion migration in comparison to pristine MAPbI3. The presence of a large GUA cation in the 3D perovskite structure induces lattice enlargement, which perturbs the atomic interactions within the perovskite lattice. Consequently, the GUAxMA1-xPbI3 crystal exhibits a higher degree of hysteresis during current-voltage (J-V) measurements than the single-crystalline MAPbI3 counterpart. Our results provide the fundamental understanding of hysteresis, which is commonly observed in GUA-based PSCs and a general protocol for in-depth electrical characterization of perovskite single crystals.

Structural evolution, electrochemical kinetic properties, and stability of A-site doped perovskite Sr1−xYbxCoO3−δ

Structural evolution occurs upon A-site partial substitution of Sr by Yb in SrCoO3−δ, leading to enhanced performance and stability.

A Highly Enhanced Photoluminescence of Eu3+-Activated CaTiO3 Phosphors via Selective A-Site and B-Site Cation Substitutions (Sr2+ and Sn4+)

Sr2+ and Sn4+ doped CaTiO3:Eu3+ phosphors were prepared by a high temperature solid-state method. X-ray diffraction characterization shows that the sample calcined at 1200°C is pure and has an orthorhombic crystal lattice. Photoluminescence (PL) measurement shows that CaTiO3:Eu3+ has five excitation peaks at 363 nm, 381 nm, 398 nm, 418 nm, and 466 nm, respectively corresponding to the transitions 7F0 → 5D4, 7F0 → 5L7, 7F0 → 5L6, 7F0 → 5D3 and 7F0 → 5D2 of Eu3+. The main emission peaks of CaTiO3:Eu3+ at 592 nm and 613 nm are ascribed to the 5D0 → 7FJ (J = 1, 2) transitions of Eu3+. In addition, PL emissions at 592 nm and 613 nm of CaTiO3:Eu3+ phosphors are enhanced remarkably through A-site substitution of Ca2+ with Sr2+ or B-site substitution of Ti4+ with Sn4+. Noticeably, the emission intensity of Ca(Ti,Sn)O3:Eu3+ is nearly three times higher than that of CaTiO3:Eu3+. The reason is that substitution of Ti4+ with Sn4+ induces a large lattice distortion, which promotes the 5D0 → 7F2 transition probability and thus enhances the emissions at 613 nm. It is also found that Sr2+ substitution narrows the optical bandgaps of CaTiO3:Eu3+ , while Sn4+ substitution widens them. In addition, chromatic purity of the phosphors shows a remarkable dependence on the asymmetric ratio.

Effect of A-site substitution by Ca or Sr on the structure and electrochemical performance of LaMnO3 perovskite

The effect of A-site substitution by Ca or Sr on the crystal structure and electrochemical performance of LaMnO3 (LMO) perovskite have been investigated. La0.85Ca0.15MnO3 (LCM) keeps the same orthorhombic structure with the parent LMO composition, while La0·85Sr0·15MnO3 (LSM) transforms to rhombohedral structure. The A-site substituted compositions with more oxygen vacancies are more beneficial to the oxygen intercalation than the parent LMO sample according to the modified anion-intercalation mechanism, and the electrochemical performance is significantly optimized. LCM and LSM electrodes exhibit a specific capacitance of 140.5 mF cm−2 and 129.0 mF cm−2 at 0.5 mA cm−2 in an aqueous electrolyte, respectively, which are four times larger than that of LMO. LCM electrode yields the highest capacitance behavior because of the finer morphology, lower ion diffusion resistance, higher concentration of oxygen vacancy and fuller utilization of the perovskite bulk structure. In addition, LCM//LCM symmetric supercapacitor exhibits an energy density of 29.7 μWh.cm−2 at a powder density of 500 μW cm−2. Our work indicates that A-site substituted compositions by Ca or Sr exhibit great potential in the supercapacitor applications, and further optimization is expected.

Study of A-site disorder dependent structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3

This paper reports the studies on the effect of A-site substitution by strontium on the structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3 (x = 0.1, 0.2 and 0.3). The investigated samples are prepared by conventional solid-state reaction technique. XRD analysis at room temperature has confirmed orthorhombic structure of the sample with space group Pnma. The dependence of structural parameter, Curie temperature and coercivity on Sr doping content has been thoroughly investigated. It is observed that substitution of Sr2+ for Ca2+ increases lattice parameter, tolerance factor and the Curie temperature. However, the coercivity (Hc) decreases with increasing Sr content while the charge ordering process is weakened with increasing Sr content. Field cooled (FC) and zero-field cooled (ZFC) dc magnetizations measurements at low field and low temperature indicate that there is a spin-glass (SG) like state occurred. Temperature dependent ac susceptibility at different frequency indicates a spin-glass-like transition of the sample.

Local piezoresponse in BiFeO3–HoFeO3 ceramics across morphotropic phase boundary

This study highlights structural evolution, nanoscale polarization switching and electromechanical mechanisms in lead-free perovskite (1-x)BiFeO3–xHoFeO3 ceramics in the vicinity of the morphotropic phase boundary (MPB). A phase crossover from ferroelectric rhombohedral R3c phase to nonpolar orthorhombic Pnma phase is identified as the system crosses the MPB. Local electric-field switched piezoresponse microscopies indicate a decreasing switchable-polarization percentage near the MPB. Ferroelectric-relaxor phase crossover takes place within the grains, accompanied by a V-shape hysteresis loop of electromechanical strain vs. bias voltage in the vicinity of the MPB. The oxygen K-edge synchrotron X-ray absorption suggests a decreased Bi 6s2 lone pair as the system approaches the MPB, which plays an important role in modification of local ferroelectric polarization. The enhanced ferromagnetic behavior may originate from collective effects, including structural distortion, exchange interactions, suppression of antiferromagnetic spin structure, and fewer oxygen vacancies, caused by the A-site Ho substitution.

Room-Temperature Ferromagnetism in Mn-Doped CuCrO<sub>2</sub> Nanopowders

(Cu1-xMnx)CrO2 (0≤ x ≤6 at%) and Cu (Cr1-yMny)O2 (0≤ y ≤6 at%) nanopowders were prepared by combining solid-state reaction and ball milling. The Mn concentration dependences of microstructure, morphology and magnetic properties were investigated. It is found that all the samples have a pure 3R-CuCrO2 delafossite structure. The lattice expansion supports the Mn entrance into the Cu and Cr sublattices, respectively, in (Cu1-xMnx)CrO2 and Cu (Cr1-yMny)O2, which is further proved by X-ray photoelectron spectroscopy to some degree. A-site Mn substitution brings about paramagnetic behavior. However, room-temperature ferromagnetism is achieved in B-site Mn-doped samples, originating from the hole-mediated Cr3+–Mn3+ double-exchange interaction. The saturation magnetization of this CuMO2 delafossite (M=Cr, Mn) is about an order of magnitude higher than literature values, and gradually decreased with the Mn addition due to the combined influence of the number of the M–M pairs, the M–M distances and the hole density.

Phase Regulation Strategy of Perovskite Nanocrystals from 1D Orthomorphic NH4 PbI3 to 3D Cubic (NH4 )0.5 Cs0.5 Pb(I0.5 Br0.5 )3 Phase Enhances Photoluminescence.

This work reports this first synthesis of 1D orthomorphic NH4 PbI3 perovskite nanocrystals (NCs) considering the role of inorganic ammonium ions at the nanoscale. The addition of bromide ions at the halogen site did not improve the photoluminescence properties. Furthermore, the 3D cubic phase of (NH4 )0.5 Cs0.5 Pb(I0.5 Br0.5 )3 NCs with bright photoluminescence was synthesized by adding Cs ions into the crystal lattice of (NH4 )Pb(I0.5 Br0.5 )3 . Moreover, the photophysical properties of different phase structures were studied using femtosecond transient absorption (FTA) spectroscopy. The ultrafast trap state capture process is a key factor in the change of photoluminescence properties and the cubic phase may be the best structure for photoluminescence. These results suggest that the ammonium ion perovskite (AIP) nanocrystals could be potential materials for optoelectronic applications through A-site cation substitution.

Influence of A-site substitution on dielectric and impedance behavior of Mn3O4 spinels

In this paper, we report the influence of A-site substitution on dielectric and electrical behavior of Mn3O4 based modified spinels at room temperature. All the three spinels substituted with Mn, Zn and Co at A-site showed slight decrease in relative permittivity (ε′) and dissipation factor (tanδ) in the studied frequency range of 40 Hz to 8 MHz. However, the influence of A-site substitution was noticeable because of it dominant influence on relative permittivity and dissipation factor over pure Mn3O4 spinel. The relative permittivity increased from ∼ 16 to 95 after substitution of Zn in A-site whereas the substitution of Co increased the magnitude upto 40. Similar trend was observed with tan δ measurement after A-site substitution. The frequency dependent impedance measurement revealed the better conducting behavior of pure and modified spinels because of its low impedance range and better frequency dependent ac-conductivity.

Magnetic and microwave absorption properties in 2.6–18 GHz of A-site or B-site substituted BaFe12O19 ceramics

The A-site substituted (20 mol% Sr2+ and Ca2+) and B-site substituted (20 mol% Co3+ and Mn3+) M-type BaFe12O19 (BFO) ceramics were synthesized and the magnetic properties and microwave absorption were investigated. The A-site substitution by Ca2+ or B-site substitution by Co3+ can effectively modify the magnetic properties of the M-type BFO. The B-site dopant suppresses the complex permittivity more greatly than A-site ones and substitution of Co3+ at B-site changes the microwave permeability response to a resonate-like manner in 2.6–18 GHz. The A-site substitution is more effective on the enhancement of peak absorption whilst B-site substitution is more effective on the enhancement of absorption bandwidth. A wide bandwidth of reflection loss (RL) value below − 5 dB (> 70% microwave absorption) of the BaFe11.8Mn0.2O19 ceramic is obtained in 3.6–18 GHz and the minimum RL of − 18.5 dB is obtained at 7.2 GHz with a thickness of 2.5 mm.

Enhanced thermoelectric properties of YbZn2Sb2−xBix through a synergistic effect via Bi-doping

Recently, extensive research has been done to explore the potential thermoelectric performance of p-Type Zintl compounds AB2Sb2 (A = Ca, Yb, Eu, Sr, Mg; B = Zn, Mn, Cd, Mg), mainly focus on A-site substitution and/or B-site substitution effect. Herein, we report that both power factors and thermal conductivities are simultaneously optimized to increase ZT values via substitution of Sb atoms with the isoelectronic Bi atoms in YbZn2Sb2−xBix compounds. We found that the power factor could be enhanced by 27%, as a result of improved electrical conductivities and the maintained Seebeck coefficients. The former is obtained from an increase of carrier density through a shift of Fermi level via Bi doping, which is supported by XPS analysis. The Seebeck coefficients are maintained due to the increased effective mass. Moreover, the lowest lattice thermal conductivity (0.47 Wm−1 K−1 at 723K) is below the lattice thermal conductivity limit due to enhanced phonon scattering via Bi-doping, which is analyzed by the Callaway model. Benefiting from the synergistic effect, the ZT of Bi-doping samples are significantly enhanced compared with that of the pristine one. These results probably offer a synergistic strategy to enhance ZT for p-Type Zintl compounds AB2Sb2 and other high-efficiency thermoelectric materials.

Piezoelectric enhancements in K0.5Na0.5NbO3-based ceramics via structural evolutions

Abstract The current work aims to compare the effect of systematic A-site and B-site substitutions on the piezoelectricity of Ka0.5Na0.5NbO3 (KNN)-based perovskite ceramics. The A-site elements was replaced by Li+ while Nb5+ was substituted by Sb5+ to form (K0.4675Na0.4675Li0.065)NbO3 (KNLN) and (K0.4675Na0.4675Li0.065)(Nb0.96Sb0.04)O3 (KNLNS) respectively. The ceramics were prepared using solid-state sintering method. The density of the ceramics steadily improved with the substitutions while the crystal structure evolved from monoclinic (in KNN) to the coexistence of monoclinic and tetragonal (in KNLN) and finally tetragonal in KNLNS. Distinct variations on size and morphology were recorded. Although density, crystal structure and morphology have minor effect on the Ec, they imposed considerable influences on Pr, d33 and kp. Despite relatively lower density, KNLN exhibited the highest Pr, d33 and kp at 9.80 μC/cm2,185 pC/N and 0.43 respectively signifying the positive enhancement brought by the co-existence of monoclinic and tetragonal crystal structures. More importantly, this work systematically proved that the co-existence of both structures signified the morphotropic phase boundary (MPB) composition as the primary factor for the enhancement of KNN piezoelectric properties.

Concurrent anomalies in electric field-temperature dependence of direct/converse piezoelectric response in Bi0.5Na0.5TiO3-BaTiO3

Doping in bismuth sodium titanate (BNT)-based ceramic is often intended to realize excellent direct piezoelectric responses mainly through A-site substitution and generate ultrahigh strain behaviors mainly by B-site doping, which indicate the different effects of the doping on direct and converse piezoelectricity. Here, we reported simultaneous occurrences of anomalies in electric field and temperature dependence of direct/converse piezoelectric responses of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3(BNT-BT6) ceramics by detailed analyses of the weak-field piezoelectric coefficient d33, high-field piezoelectric coefficient d33∗, and small signal bias-field piezoelectric coefficient d33∗(E = 0). The multiple piezoelectric responses presented similar anomalies at critical field Ep = 20 kV/cm and critical temperature Tf = 120 °C. The analogous changes in direct/converse piezoelectric behaviors are highly related to the electric-field/temperature-driven phase transformation, which could boost polarization reorientation and domain switching to a lager extent. Particularly, the phase transition resulted in an obvious increase in high-field piezoelectric coefficient d33∗, and so offer a potential strategy for further development of high-performance lead-free piezoelectric materials.

Tuning Photovoltaic Performance of Perovskite Nickelates Heterostructures by Changing the A-Site Rare-Earth Element.

Perovskite rare-earth nickelates (RNiO3) have attracted much attention because of their exotic physical properties and rich potential applications. Here, we report systematic tuning of the electronic structures of RNiO3 (R = Nd, Sm, Gd, and Lu) by isovalent A-site substitution. By integrating RNiO3 thin films with Nb-doped SrTiO3 (NSTO), p-n heterojunction photovoltaic cells have been prepared and their performance has been investigated. The open-circuit voltage increases monotonically with decreasing A-site cation radius. This change results in a downward shift of the Fermi level and induces an increase in the built-in potential at the RNiO3/NSTO heterojunction, with LuNiO3/NSTO showing the largest open-circuit voltage. At the same time, the short-circuit current initially increases upon changing the A-site element from Nd to Sm. However, the larger bandgaps of GdNiO3 and LuNiO3 reduce light absorption which in turn induces a decrease in the short-circuit current. A power conversion efficiency of 1.13% has been achieved by inserting an ultrathin insulating SrTiO3 layer at the SmNiO3/NSTO interface. Our study illustrates how changing the A-site cation is an effective strategy for tuning photovoltaic performance and sheds light on which A-site element is the best for photovoltaic applications, which can significantly increase the applicability of nickelates in optoelectric devices.

Effect of A-site Fe substitution on the magnetic behavior of Dy2Ti2O7 spin ice

Abstract Geometrically frustrated Dy2Ti2O7 spin ice is known for robust magnetic behavior where structural and dielectric properties are coupled with its magnetism. Herein, effects of Fe substitution, at the magnetic Dy site (0 ≤ x ≤ 0.15) in Dy2Ti2O7 compound have been systematically examined in terms of structural distortions and magnetic perturbations through structural, dielectric, optical and magnetic studies. The experimental findings suggest that Fe substitution does not change the crystal electric field acting on the rare earth ion, though it produces significant structural distortions. These findings suggest that spin dynamics non-monotonically depends on the nature and dynamics of neighboring magnetic ion’s spin (Fe spin in this case), because of existing strong spin correlations. This dependency provides a new possibility to tune or modify the spin dynamics of the correlated magnetic states in these materials via magnetic perturbations, which can be controlled by the externally applied magnetic field.

Efficient and Hole-Transporting-Layer-Free CsPbI2 Br Planar Heterojunction Perovskite Solar Cells through Rubidium Passivation.

Recently, inorganic perovskite CsPbI2 Br has gained much attention for photovoltaic applications owing to its excellent thermal stability. However, low device performance and high open-voltage loss, which are the result of its intrinsic trap states, are hindering its progress. Herein, planar CsPbI2 Br solar cells with enhanced performance and stability were demonstrated by incorporating rubidium (Rb) cations. The Rb-doped CsPbI2 Br film exhibited excellent crystallinity, pinhole-free surface morphology, and enhanced optical absorbance. By using low-cost carbon electrodes to replace the organic hole-transportation layer and metal electrode, an excellent efficiency of 12 % was achieved with a stabilized efficiency of over 11 % owing to the suppressed trap states and recombination in the CsPbI2 Br film. Additionally, the annealing temperature for the Rb-doped CsPbI2 Br film could be as low as 150 °C with a comparable high efficiency over 11 %, which is one of the best efficiencies reported for hole-transporting-layer-free all-inorganic perovskite solar cells. These results could provide new opportunities for high-performance and stable inorganic CsPbI2 Br solar cells by employing A-site cation substitution.

Comparative Study on Removal of NOx and Soot with A-site Substituted La2NiO4 Perovskite-like by Different Valence Cation

La1.8M0.2NiO4 (M = Na+, Sr2+, Ce3+) perovskite-like catalysts were prepared by citric acid complexation method. XRD, BET, FT-IR, SEM, XPS, H2-TPR, O2-TPD, MS-Soot-TPR, MS-NO-TPD and catalytic activity measurements were carried out to investigate the effect of A-site substitution on structure and catalytic performance for simultaneous removal soot and NOx. The characterization results show that La1.8M0.2NiO4 catalyst has high concentration of oxygen vacancies, more surface active oxygen, more trivalent nickel ions and better reducibility, which determines its better catalytic performance. The introduction of low valence cations at the A site significantly reduces the characteristic combustion temperature of soot and effectively promotes the reduction of NOx by soot. La1.8Sr0.2NiO4 catalyst exhibited the best soot removal performance with Ti 331 °C and Tm 473 °C, while La1.8Na0.2NiO4 catalyst showed the highest NOx conversion of 90%. Based on in situ DRIFTS and other characterization results, a possible mechanism for simultaneous removal of NOx and soot was proposed.

Stabilizing RbPbBr3 Perovskite Nanocrystals through Cs+ Substitution.

ABX3 -type halide perovskite nanocrystals (NCs) have been a hot topic recently due to their fascinating optoelectronic properties. It has been demonstrated that A-site ions have an impact on their photophysical and chemical properties, such as the optical band gap and chemical stability. The pursuit of halide perovskite materials with diverse A-site species would deepen the understanding of the structure-property relationship of the perovskite family. In this work we have attempted to synthesize rubidium-based perovskite NCs. We have discovered that the partial substitution of Rb+ by Cs+ help to stabilize the orthorhombic RbPbBr3 NCs at low temperature, which otherwise can only be obtained at high temperature. The inclusion of Cs+ into the RbPbBr3 lattice results in highly photoluminescent Rb1-x Csx PbBr3 NCs. With increasing amounts of Cs+ , the band gaps of the Rb1-x Csx PbBr3 NCs decrease, leading to a redshift of the photoluminescence peak. Also, the Rb1-x Csx PbBr3 NCs (x=0.4) show good stability under ambient conditions. This work demonstrates the high structural flexibility and tunability of halide perovskite materials through an A-site cation substitution strategy and sheds light on the optimization of perovskite materials for application in high-performance optoelectronic devices.

Effect of A-site substitution on the simultaneous catalytic removal of NOx and soot by LaMnO3 perovskites

The simultaneous catalytic removal of NOx and soot by La1−xMxMnO3 (M = K, Sr, Pr; x = 0, 0.1, 0.2, 0.3) perovskites was investigated.