A-cation Size Research Articles

Elucidating the Role of Alkali Metal Carbonates in Impact on Oxygen Vacancies for Efficient and Stable Perovskite Solar Cells.

Effectively suppressing nonradiative recombination at the SnO2/perovskite interface is imperative for perovskite solar cells. Although the capabilities of alkali salts at the SnO2/perovskite interface have been acknowledged, the effects and optimal selection of alkali metal cations remain poorly understood. Herein, a novel approach for obtaining the optimal alkali metal cation (A-cation) at the interface is investigated by comparatively analyzing different alkali carbonates (A2CO3; Li2CO3, Na2CO3, K2CO3, Rb2CO3, and Cs2CO3). Theoretical calculations demonstrate that A2CO3 coordinates with undercoordinated Sn and O on the surface, effectively mitigating oxygen vacancy (VO) defects with increasing A-cation size, whereas Cs2CO3 exhibits diminished preferability owing to enhanced steric hindrance. The experimental results highlight the crucial role of Rb2CO3 in actively passivating VO defects, forming a robust bond with SnO2, and facilitating Rb+ diffusion into the perovskite layer, thereby enhancing charge extraction, alleviating deep-level trap states and structural distortion in the perovskite film, and significantly suppressing nonradiative recombination. X-ray absorption spectroscopy analyses further reveal the effect of Rb2CO3 on the local structure of the perovskite film. Consequently, a Rb2CO3-treated device with aperture area of 0.14 cm2 achieves a notable efficiency of 22.10%, showing improved stability compared to the 20.11% achieved for the control device.

Specific Heat Capacity of A<sub>2</sub>FeCoO<sub>6-<i>δ</i></sub> (A = Ca or Sr)

A2FeCoO6-δ (A = Ca or Sr) is synthesized by the solid-state synthesis method and their specific heat capacities are evaluated at 40˚C using a heat flow meter. The effect of the A-cation size on the specific heat capacity of these compounds is observed. The specific heat capacity of Sr2FeCoO6-δ is found to be the highest, and that of Ca2FeCoO6-δ is the lowest while CaSrFeCoO6-δ shows the intermediate value. The specific heat capacity decreases with the decrease of the average A-site ionic radius, demonstrating the relationship between heat capacity and A-site ionic radius. The relationship between specific heat capacity and molar mass is also confirmed as the δ value decreases or molar mass increases from Ca2FeCoO6-δ to CaSrFeCoO6-δ to Sr2FeCoO6-δ.

Influence of the Thermal Processing and Doping on LaMnO3 and La0.8A0.2MnO3 (A = Ca, Sr, Ba) Perovskites Prepared by Auto-Combustion for Removal of VOCs

Single-phase oxygen stoichiometric LaMnO3 and doped La0.8A0.2MnO3 (A = Ca, Sr, Ba) perovskites have been prepared by a simple one-step auto-combustion method. Cation-deficient LaMnO3+δ and La0.8A0.2MnO3+δ were obtained by calcination of the former samples in air at 750 °C. The samples were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, temperature-programmed reduction, temperature-programmed oxygen desorption, and N2 physisorption in order to apply them as catalysts in the complete catalytic oxidation of acetone as a model volatile organic compound. The studied phases show the expected orthorhombic and rhombohedral perovskite crystal structures. Catalytic experiments performed with all the samples show measurable activity already at 100 °C. At 200 °C, doped La0.8A0.2MnO3 samples show higher activity than undoped LaMnO3, with increasing conversion with larger A-cation size. Calcined samples also show higher activity than as-prepared ones making La0.8Ba0.2MnO3+δ the best catalyst at this temperature. All doped samples show >95% acetone conversion at T ≥ 250 °C with a weak dependence on the sample processing or A cation doping. The collected evidence confirms that the most important factors for the catalytic activity of these oxides are the Mn4+/Mn3+ molar ratio on the surface of the samples and the cation-deficiency of the bulk perovskite structure. In addition, increasing the symmetry of the bulk crystal structure appears to have an additional favourable effect. Despite the observation of the presence of surface carbonates, we show that it is possible to use the as-prepared samples without further thermal treatment with good results in the oxidation of acetone.

Cation Engineering for Resonant Energy Level Alignment in Two-Dimensional Lead Halide Perovskites.

Low-dimensional metal halide perovskites are being intensively investigated because of their higher stability and chemical versatility in comparison to their 3D counterparts. Unfortunately, this comes at the expense of the electronic and charge transport properties, limited by the reduced perovskite dimensionality. Cation engineering can be envisaged as a solution to tune and possibly further improve the material's optoelectronic properties. In this work, we screen and design new electronically active A-site cations that can promote charge transport across the inorganic layers. We show that hybridization of the valence band electronic states of the perovskite inorganic sublattice and the highest occupied molecular orbitals of the A-site organic cations can be tuned to exhibit a variety of optoelectronic properties. A significant interplay of A-cation size, electronic structure, and steric constraints is revealed, suggesting intriguing means of further tuning the 2D perovskite electronic structure toward achieving stable and efficient solar cell devices.

Optical and dielectric properties of lead perovskite and iodoplumbate complexes: an ab initio study.

Organic-inorganic lead halide perovskites (APbI3) have shown tremendous growth in solar cell efficiencies; however, our atomistic understanding of the intrinsic instability due to A-cation size effects is far from satisfactory. Here, we report a dispersion corrected density functional theory study of the influence of the A-cation size on the lattice structures of perovskite (α-cubic and γ-orthorhombic) and non-perovskite phases (δ-hexagonal and δ-orthorhombic) through isotropic (cesium and ammonium) and anisotropic cations (methylammonium, formamidinium and dabconium). We found that the local coordination environmental changes and noncovalent interactions are crucial in explaining the enthalpy of formation and relative energy stability. The subtler A-cation's influence on electronic polarization of perovskite and non-perovskite structures were estimated through the optical dielectric constant and then compared with experimental values.

Comparative study of La0.6R0.1Ca0.3Mn0.9Cr0.1O3 (R = La, Eu and Ho) nanoparticles: effect of A-cation size and sintering temperature

A systematic study of nanocrystalline La0.6R0.1Ca0.3Mn0.9Cr0.1O3 (R = La, Eu and Ho) manganites has been undertaken to analyse the effect of sintering temperature and A-site cation radius on the structural and magnetic properties. In order to acquire a series of samples with different particle sizes, the samples were prepared by a solution combustion method and were subjected to annealing at four different temperatures. Rietveld analysis of X-ray diffraction (XRD) data confirmed the presence of orthorhombic symmetry with Pbnm space group for all the samples. The grain size retrieved from the TEM analysis is greater than the noted crystallite size calculated from XRD by the Scherrer’s equation. A paramagnetic-ferromagnetic transition is shown by all our investigated compounds at low temperature. With increasing sintering temperature or particle size, the Curie temperature (Tc) was found to increase, which is attributed to the magnetically disordered surface layer. All the phases obey Curie–Weiss law in the high temperature region and the effective magnetic moment calculated from the high temperature paramagnetic region shows an increase with sintering temperature.

Comparative study of La0.5Nd0.2Ca0.3–K MnO3 (x = 0.0 and 0.05) nanoparticles: Effect of A-cation size and calcination temperature

Abstract In order to study the effect of sintering temperature and A-site cation radius on the structural, magnetic and transport properties, a systematic investigation of La0.5Nd0.2Ca0.3–xKxMnO3 (x = 0.0 and 0.05) nanoparticles has been undertaken. Rietveld analysis of X-ray diffraction data confirmed the presence of orthorhombic symmetry with Pbnm space group for all the samples. A paramagnetic-ferromagnetic transition is shown by all our investigated compounds. The materials exhibit semiconducting behavior and the metal-semiconductor transition temperatures (TMS) are found to be close to Curie temperatures (Tc), indicating a strong correlation between their electrical and magnetic properties. Both Tc and TMS of K-doped samples were found to be higher than their undoped counterparts at same calcination temperatures. In the high temperature region, it was observed that the resistivity data was best fitted by SPH model.

Dynamic and structural properties of orthorhombic rare-earth manganites under high pressure

We report a high-pressure study of orthorhombic rare-earth manganites AMnO3 using Raman scattering (for A = Pr, Nd, Sm, Eu, Tb and Dy) and synchrotron X-ray diffraction (for A = Pr, Sm, Eu, and Dy). In all cases, a structural and insulator-to-metal transition was evidenced, with a critical pressure that depends on the A-cation size. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the a-c plane, and the Raman bands associated with out-of-phase MnO6 rotations and in-plane O2 symmetric stretching modes. Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smallest A-cations, the compression is nearly isotropic in the ac plane, with presumably only very slight changes of tilt angles and Jahn-Teller distortion. As the radius of the A-cation increases, the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significant as a compression mechanism, while the pressure-induced bending of octahedra chains becomes conversely less pronounced. We finally discuss our results in the light of the notion of chemical pressure, and show that the analogy with hydrostatic pressure works quite well for manganites with small A-cations but can be misleading with large A-cations.

Effects of the A-site cation number on the properties of Ln5/8M3/8MnO 3 manganites

The properties of manganites can be tuned by changing the doping level x in Ln 1− xM x MnO 3. A second mechanism allows tuning of magnetic and electronic properties, for fixed x values, by varying the average A-cation radius, 〈 r A 〉. Moreover, for fixed x and 〈 r A 〉 values, the changes in the A-cation size variance, σ 2, also modify the ferromagnetic and metal–insulator transition temperatures. Here, we investigate the influence of the number of A-site cations on Ln 5/8 M 3/8MnO 3 manganites, where x, 〈 r A 〉 and σ 2 values are kept constant, and in the absence of phase separation phenomena. We have found that the number of cation species at the A site ( N A ) has a strong influence on the width of the ferromagnetic and metal–insulator transitions, and a small influence on the average transition temperature. This behavior is opposite to that observed for increasing values of the variance σ 2 in manganites, with the same x and 〈 r A 〉 values, where average transition temperatures are strongly reduced.

Electronic, structural and magnetic phase diagram of Sm1-xSrxMnO3 manganites

The electronic, structural and magnetic phase diagram of the colossal magnetoresistive Sm1−xSrxMnO3 (0.16≤х≤0.67) manganites is constructed on the basis of their systematic studies by high-resolution neutron powder diffraction. It is shown the tendency of researched system to formation of the phase-separated states on crystallographic as well as, in the even greater extent, on magnetic level. A clear correlation between fine specific features and temperature evolution of crystal structures, spin ordering of the manganese ions, and the physical properties of the samarium–strontium series of manganites obtained from temperature magnetic and transport measurements is demonstrated. It was found that ground state of Sm–Sr manganites shows very various physical properties depending first of all on the doping level х and, for given х, determined by two distortion parameters, namely, the average A-cation size <rA> and the local A-cation size mismatch σ2.

Giant volume magnetostriction and its connection with colossal magnetoresistance and lattice-softening La 1−xM xMnO 3 (M=Ca, Ag, Ba, Sr)

Giant volume magnetostriction, reached ∼6×10 −4 in magnetic field 8.2 kOe, is found in La 1− x M x MnO 3 (M=Ba, Sr, Ca, Ag) manganites near Curie-point T C. Volume magnetostriction ω and magnetoresistance exhibit similar dependence on temperature and magnetic field in T C region that is explained by the presence in these compounds of magnetic two-phase ferromagnetic–antiferromagnetic state due to strong s−d and/or d−d exchange. Maximum | ω|-value depends from radius of M-cation: it is larger the larger the difference | R A− R La3+| and A-cation size variance σ 2, where A=La 1− x M x .

Effect of cation site-disorder on the structure and magneto-transport properties of Ln5/8M3/8MnO 3 manganites

Five members of Ln 5/8 M 3/8MnO 3 series with A-cation size variance ( σ 2 ) ranging between 3×10 −4 and 71×10 −4 Å 2, and the same A-cation size 〈 r A 〉 = 1.2025 Å , have been synthesized by the ceramic method. The five manganites are single phase and they crystallize in the Pnma perovskite superstructure. The five compositions display ferromagnetic–paramagnetic transitions at temperatures ranging between 130 and 270 K, for the highest and lowest variance sample, respectively. The samples with smaller variances show sharp magnetization transitions and the samples with the larger variances display broad transitions. These transitions have also been studied by differential scanning calorimetry, DSC, and some enthalpy changes are reported. The resistivity study indicates that all samples display the expected metal-to-insulator transitions at temperatures ranging between 140 and 270 K. The samples have been analysed at room temperature by ultra-high-resolution synchrotron powder diffraction and the structural and microstructural features are reported. Furthermore, Nd 5/8Sr 0.255Ca 0.12MnO 3 ( σ 2 = 40 × 1 0 - 4 Å 2 ) and Sm 0.225Nd 0.4Sr 0.308Ca 0.067MnO 3 ( σ 2 = 53 × 1 0 - 4 Å 2 ) samples have also been studied by synchrotron powder diffraction at 140 K, below the transition temperatures. Both samples are found to be single phase above and below the transition by ultra-high-resolution synchrotron powder diffraction. The microstructure of the samples has been investigated through Williamson–Hall plots. Sample broadenings are markedly anisotropic and strongly dominated by microstrains with average values of the Δ d/ d term close to 14×10 −4. A direct correlation is found between the microstrain values and the widths of the magnetization transitions.

Reentrant Spin Glass Behaviour and CE-Type AntiferromagneticPhase in Half Doping (La,Pr)1/2Ca1/2 MnO3 Manganites

The reentrant spin glass behaviour was observed in half doping(La,Pr)1/2Ca1/2MnO3 manganites with a CE-typeantiferromagnetic structure. It shows sequential multiple magnetictransitions from the paramagnetic to the ferromagnetic,antiferromagnetic, spin glass (SG) transitions, which reflects thecomplex magnetic interaction in the ground state of manganites. Thiscan be explained by the competition interaction between theferromagnetic and the CE-type antiferromagnetic matrix, and thedisorder due to the tolerance factor t and A-cation size varianceσ2. The results reveals the coexistence of ferromagneticclusters and SG clusters in the background of the antiferromagneticmatrix in the ground state of half doping(Pr,La)1/2Ca1/2MnO3 systems.

Correction to the Metal-Insulator Transition Temperature due toCation Size and Strain Effects for Colossal MagnetoresistancePerovskites

A phenomenological expression of the metal-insulator transition temperature isproposed for AMnO3 manganese perovskites by taking into account thedistortion of the Mn-O-Mn bond due to A-cation size and the strain-dependenteffect due to performed Jahn-Teller distortions, independently. Usingreasonable physical parameters, the calculated results give excellentagreement with experimental data obtained in polycrystalline samples ofLa2/3(Ca1-xBax)1/3MnO3, providing a strongsupport to this approach.

Cation size and strain effects in La 2/3(Ca 1− x′ Sr x′− x″ Ba x″ ) 1/3MnO 3

Based on investigations of metal–insulator transition temperature T m on La 2/3(Ca 1− x′ Sr x′− x″ Ba x″ ) 1/3MnO 3, we point out that considerations based only on the mismatch effect or the strain-field effect cannot provide a reasonable description for the experimental observations. Including two contributions arising from the average A-cation size (〈 r A〉) and the strain-field effect defined by a quantity S=〈( r A 0− r A) 2〉 gives T m varying as T m= T m(〈 r A〉,0)− pS, where T m(〈 r A〉,0) is the transition temperature in the absence of the strain-field effect and the parameter p is estimated to be ∼1.05×10 6 K nm −2. It is shown that the strain-corrected T m shows a behavior expected by the double-exchange mechanism which monotonously increases towards ∼500 K with increasing 〈 r A〉 towards r A 0 (the A-cation size of an ideal cubic perovskite).

ChemInform Abstract: Cation Disorder in Ga1212.

Substitution of calcium for strontium in LnSr2-xCaxCu2GaO7 (Ln = La, Pr, Nd, Gd, Ho, Er, Tm, and Yb) materials at ambient pressure and 975 degrees C results in complete substitution of calcium for strontium in the lanthanum and praseodymium systems and partial substitution in the other lanthanide systems. The calcium saturation level depends on the size of the Ln cation, and in all cases, a decrease in the lattice parameters with calcium concentration was observed until a common, lower bound, average A-cation size is reached. Site occupancies from X-ray and neutron diffraction experiments for LnSr2-xCaxCu2GaO7 (x = 0 and x = 2) confirm that the A-cations distribute between the two blocking-layer sites and the active-layer site based on size. A quantitative link between cation distribution and relative site-specific cation enthalpy for calcium, strontium, and lanthanum within the gallate structure is derived. The cation distribution in other similar materials can potentially be modeled.

Cation disorder in Ga1212.

Substitution of calcium for strontium in LnSr2-xCaxCu2GaO7 (Ln = La, Pr, Nd, Gd, Ho, Er, Tm, and Yb) materials at ambient pressure and 975 degrees C results in complete substitution of calcium for strontium in the lanthanum and praseodymium systems and partial substitution in the other lanthanide systems. The calcium saturation level depends on the size of the Ln cation, and in all cases, a decrease in the lattice parameters with calcium concentration was observed until a common, lower bound, average A-cation size is reached. Site occupancies from X-ray and neutron diffraction experiments for LnSr2-xCaxCu2GaO7 (x = 0 and x = 2) confirm that the A-cations distribute between the two blocking-layer sites and the active-layer site based on size. A quantitative link between cation distribution and relative site-specific cation enthalpy for calcium, strontium, and lanthanum within the gallate structure is derived. The cation distribution in other similar materials can potentially be modeled.

Atomic and magnetic structure of perovskite manganites: A-cation size and oxygen isotope substitution effects and homogeneity of magnetic state

A series of (La 1− y Pr y ) 0.7Ca 0.3MnO 3 samples with y=0.5, 0.6, 0.7 and 0.75 has been systematically studied by neutron powder diffraction. The analysis shows that their basic structural parameters including the temperature of metal–insulator-phase transition are the linear functions of an average A-cation radius. For composition with y=0.75 the structural analysis was performed on two samples, one containing the natural mixture of oxygen isotopes (99.7% 16 O , metallic and ferromagnetic at T⩽ T FM≈110 K), and the other one enriched by 18 O in 75% (insulating in the whole temperature range and with collinear antiferromagnetic order at T⩽ T AFM≈150 K). These two samples have been found to be identical in crystal and magnetic structure down to transition of the sample with 16 O into the metallic ferromagnetic phase. This means that their quite different transport and magnetic properties at T⩽110 K are driven solely by the different oxygen atoms dynamics. Evidences of low-temperature two-phase state: AFM-dielectric+FM-metallic have been obtained for the samples with y⩾0.6.

Cation Size Variance Effects in High-Tolerance Factor Ln0.7M0.3MnO3 Perovskites

A series of five Ln0.7M0.3MnO3 perovskites (Ln=trivalent lanthanide; M=divalent alkaline earth) has been prepared. These have the same mean A-cation site radius of 1.26 Å (equivalent to a perovskite tolerance factor of 0.98) but varying amounts of A-cation size disparity quantified by the size variance σ2. As this increases, the temperatures of the metal–insulator and Curie transitions show a strong linear decrease but that of the structural transition between rhombohedral R3c and orthorhombic Imma phases increases. Extrapolation from these and previous data indicates that the maximum metal–insulator temperature for an Ln0.7M0.3MnO3 perovskite is ∼550 K. Different methods for estimating the Curie temperature are compared.