Alkaline-earth Perovskites Research Articles

Optoelectronic Properties Prediction of Lead-Free Methylammonium Alkaline-Earth Perovskite Based on DFT Calculations.

Dynamical stability plays an essential role in phase transition and structure, and it could be a fundamental method of discovering new lead-free perovskite materials. The perovskite materials are well-known for their excellent optoelectronic properties, but the lead element inside could be a hindrance to future development. This research is trying to predict the promising cation candidates in the high-temperature application for lead-free perovskite materials from the replacement of lead in MAPbCl3 (MA = methylammonium) with alkaline-earth cations. The alkaline-earth cations are of a stable positive divalent sort, which is the same as Pb, and most of them are abundant in nature. Therefore, by improving the dynamical stability, the Mg2+, Ca2+, and Sr2+ cations replacement of lead ions could stabilize the perovskite structure by decreasing the imaginary part of phonon density of states. Finally, the density functional theory results show that the MACaCl3 could be a dynamic stable lead-free methylammonium perovskite material with an ultrawide band gap (5.96 eV).

Electronic Structure and High-Temperature Thermochemistry of Oxygen-Deficient BaMO3 (M = Ti – Cu) Perovskites

Alkaline-earth perovskites are of particular interest for high-temperature energy conversion processes because of their mixed-valence charge states, which significantly affect their chemical and th...

Crystal chemistry, X-ray diffraction reference patterns, and bandgap studies for (BaxSr1–x)2CoWO6 (x = 0.1, 0.2, 0.3, 0.5, 0.7, and 0.9)

X-ray reference powder patterns and structures have been determined for a series of cobalt- and tungsten-containing cubic alkaline-earth perovskites, (BaxSr1–x)2CoWO6 (x = 0.1, 0.2, 0.3, 0.5, 0.7, and 0.9). The structure of the end members of the series, Sr2CoWO6 and Ba2CoWO6, were tetragonal and cubic, respectively, agreeing with the literature data. From Rietveld refinements, it was found that when x = 0.1 and 0.2, the structure was tetragonal I4/m (a = 5.60481(6) and 5.62305(11) Å and c = 7.97989(12) and 7.9847(2) Å, respectively; Z = 2). When x > 0.2, the structure was cubic (Fm$\bar{3}$m, No. 225; Z = 4) (from x = 0.3 to 0.9, a increases from 7.98399(13) to 8.08871(10) Å). This tetragonal series of compounds exhibit the characteristics of a distorted double-perovskite structure. The bond valence sum values for the alkaline-earth (Ba, Sr) sites in all (BaxSr1−x)2CoWO6 members are greater than the ideal value of 2.0, indicating over-bonding situation, whereas for the W sites, as x increases, a change from under-bonding to slightly over-bonding situation was observed. Density functional theory calculations revealed that while Sr2CoWO6 is a semiconductor, Ba2CoWO6 and SrBaCoWO6 are half-metals. Powder X-ray diffraction patterns of this series of compounds (BaxSr1−x)2CoWO6, with x = 0.1, 0.2, 0.3, 0.5, 0.7, and 0.9, have been submitted to be included in the Powder Diffraction File.

Microscopic Sources of Solid-State NMR Shielding in Titanate of Alkaline Earth Perovskite Metals

Nuclear magnetic resonance (NMR) parameters are calculated and analyzed in a series of titanate of alkaline earth perovskites to explore microscopic sources of their magnetic shieldings using a full-potential-based NMR scheme. In this method, there is no approximation to calculate the induced current density. The slope of the correlation between various approaches and available experimental data is successfully reproduced very close to the required ideal value (−1). Our NMR results are consistent with the experimental data and the available theoretical results calculated by the gauge-including projector augmented-wave (GIPAW) method. Moreover, we have predicted the chemical shifts of the compounds in which their experimental values have not been measured yet. Isotropic and anisotropic chemical shift parameters as well as associated asymmetries are analyzed. The analysis explores the relation between atomic and orbital characters of the valence and conduction bands wave functions as well as the 17O NMR shi...

Effect of metal cation replacement on the electronic structure of metalorganic halide perovskites: Replacement of lead with alkaline-earth metals

Organic and inorganic lead halogen perovskites, and in particular, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, have during the last years emerged as a class of highly efficient solar cell materials. Herein we introduce metalorganic halogen perovskite materials for energy-relevant applications based on alkaline-earth metals. Based on the classical notion of Goldschmidt's rules and quantum mechanical considerations, the three alkaline-earth metals, Ca, Sr, and Ba, are shown to be able to exchange lead in the perovskite structure. The three alkaline-earth perovskites, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Ca}{\mathrm{I}}_{3},\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Sr}{\mathrm{I}}_{3}$, and $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Ba}{\mathrm{I}}_{3}$, as well as the reference compound, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, are in this paper investigated with density functional theory (DFT) calculations, which predict these compounds to exist as stable perovskite materials, and their electronic properties are explored. A detailed analysis of the projected molecular orbital density of states and electronic band structure from DFT calculations were used for interpretation of the band-gap variations in these materials and for estimation of the effective masses of the electrons and holes. Neglecting spin-orbit effects, the band gap of $\mathrm{MACa}{\mathrm{I}}_{3},\mathrm{MASr}{\mathrm{I}}_{3}$, and $\mathrm{MABa}{\mathrm{I}}_{3}$ were estimated to be 2.95, 3.6, and 3.3 eV, respectively, showing the relative change expected for metal cation exchange. The shifts in the conduction band (CB) edges for the alkaline-earth perovskites were quantified using scalar relativistic DFT calculations and tight-binding analysis, and were compared to the situation in the more extensively studied lead halide perovskite, $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$, where the change in the work function of the metal is the single most important factor in tuning the CB edge and band gap. The results show that alkaline-earth-based organometallic perovskites will not work as an efficient light absorber in photovoltaic applications but instead could be applicable as charge-selective contact materials. The rather high CB edge and the wide band gap together with the large difference of the electron and hole effective masses make them good candidates for $n$-type selective layers in hot carrier injection solar cell devices together with some light absorber candidates. The fact that they have similar lattice constants as the lead perovskite and suitable positions of the valence band edges open up the possibility to use them also as thin epitaxial $p$-type hole selective contacts in combination with the lead halogen perovskite materials. This can lead to both charge selectivity as well as to superior crystal growth of lead perovskite with less contact stress, which is interesting for further investigations.

Water pressure enhanced sintering of alkaline-earth perovskite ceramics

The alkaline earth-based perovskite ceramics are potential electrolytes for components of electrolysers, fuel cells and CO2/syngas converters. Their high refractariness is an advantage from the chemical stability point of view but involves high sintering temperature (>1450°C), detrimental to the co-sintering procedure required to assemble device components. Since the presence of protonic species is not intrinsic to the perovskite structure, the Ln-modified perovskites have to be successfully protonated by exposition of dense (95–99% theoretical density) ceramic pieces to (pressurized) water. We demonstrate that densification of a AZr1−xLnxO3−δ ceramic can be enhanced by prior exposition of 1200 to 1300°C pre-fired (porous) ceramics to high water pressure using an autoclave. Autoclave temperature and treatment duration are considered. Thermal expansion measurements reveal that the protonation (200–300°C under 15–80bar of water vapour) of previously fired (70–80% of the theoretical density) porous perovskite body (SrZr1−xYbxO3, x=0.1) enhances the sintering and allows densification at lower temperature. The gain in temperature reaches 200–300°C. IR and Raman studies show that the film formed on surface consists of (hydrated) alkali-earth oxyhydroxides. Sintering enhancement is assigned to the modification of the material structure close to the grain surface/boundary and to the enhancement of ions mobility at the grain surface/interface.

Proton conductors based on alkaline-earth substituted La28−xW4+xO54+3x/2

Lanthanum tungstates, "La6WO12", are mixed ion proton-electronic conductors with very interesting properties for technological applications and better phase stability compared to alkaline earth perovskites. A new series of compounds La(27.04-x)M(x)W(4.96)O(55.44-x/2□8.56+x/2) (M = Ca(2+), Sr(2+) and Ba(2+)) are investigated with the aim of increasing the concentration of oxygen vacancies and studying their effects on the structure and transport properties. The materials have been studied by high-resolution laboratory X-ray powder diffraction and scanning electron microscopy combined with energy dispersive spectroscopy (EDS). High temperature X-ray powder diffraction and thermal analysis in wet and dry N2 gas did not show any evidence of phase transition up to 800 °C. The total conductivity was studied by impedance spectroscopy under dry and wet atmospheres and as a function of the oxygen partial pressure. The electronic contribution to the conductivity was determined by the Hebb-Wagner polarization method. The generation of extrinsic vacancies in the lattice with alkaline earth doping leads to a decrease of the ionic conductivity for high doping level, suggesting a proton trapping mechanism.

Dielectric Properties of Thin Hf- and Zr-based Alkaline Earth Perovskite Layers

Thin layers of high dielectric constant materials are of special interest for memory, logic, and passive microelectronic applications. Memory storage capacitors based on these materials should exhibit high capacitance densities and low leakage currents. Here, we investigate a group of Hf- and Zr-based alkaline earth perovskites for this application. It is found that thin layers of polycrystalline BaHfO3, SrHfO3, and BaZrO3 can provide capacitance densities about 2 times higher than that of amorphous HfO2. Post deposition annealing above ~ 700{degree sign}C is a crucial processing step required for obtaining the cubic perovskite phase with high dielectric constant.

Hf- and Zr-based alkaline earth perovskite dielectrics for memory applications

We investigate a group of Hf- and Zr-based dielectrics crystallizing in the cubic perovskite structure for memory applications. The dielectrics are deposited in the form of thin layers (<30 nm) onto TiN substrates using molecular beam deposition. Prepared layers are characterized with respect to their dielectric constant, crystallization temperature as well as some aspects of leakage current flowing across group-II hafnates and zirconates. We show that dielectric constants of about 40 can be achieved for BaHfO 3, BaZrO 3, and SrHfO 3 on TiN substrates. On an example of BaHf 1− x Ti x O 3, we demonstrate that a significant gain in the dielectric constant and capacitance density can be achieved by partial substitution of Hf atoms by Ti in BaHfO 3. This process leads also to a reduction of the crystallization temperature of the layers.

Crystal and defect chemistry influences on band gap trends in alkaline earth perovskites

A number of perovskites with A-site alkaline earth chemistries being Ca, Sr, and Ba, and tetravalent cations including Ce, Zr, and Ti are measured for optical band gap and found to vary systematically with tolerance factor and lattice volume within limits defined by the chemistry of the octahedral site. This paper also focuses on the BaTiO3 system, considering equilibrated nonstoichiometries, and determines the changes in band gap with respect to Ba∕Ti ratios. It was found that the optical band gap changes in the solid solution regime and is invariant in the second phase regions, as would be expected. In the cases of Ba∕Ti&amp;lt;1.0, the variation in band gap scales with lattice volume, but in the Ba∕Ti&amp;gt;1.0 stoichiometries, there is a distinct Urbach tail and the trend with lattice volume no longer holds. It is inferred that the VTi⁗−2VO•• partial Schottky complex controls the band gap trend with Ba-rich nonstoichiometries.

Thermochemical and thermophysical properties of alkaline-earth perovskites

In order to contribute to safety evaluation of high burnup oxide fuels, we studied the thermochemical and thermophysical properties of alkaline-earth perovskites known as oxide inclusions. Polycrystalline samples of alkaline-earth perovskites, BaUO 3, BaZrO 3, BaCeO 3, BaMoO 3, SrTiO 3, SrZrO 3, SrCeO 3, SrMoO 3, SrHfO 3 and SrRuO 3, were prepared and the thermal expansion coefficient, melting temperature, elastic moduli, Debye temperature, microhardness, heat capacity, and thermal conductivity were measured. The relationship between some physical properties was studied.

Preparation of PbZrO3/ASO4 Composites (A: Ca, Sr, Ba) and PbZrO3 by Metathetic Reactions in the Solid State: Metathetic Exchange of Divalent Species.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation of PbZrO3/ASO4 Composites (A = Ca, Sr, Ba) and PbZrO3 by Metathetic Reactions in the Solid State: Metathetic Exchange of Divalent Species

Metathetic solid-state reactions of alkaline earth perovskite zirconates (AZrO3, A = Ca, Sr, Ba) and lead sulfate (PbSO4) result in the relatively quick formation of perovskite PbZrO3 and the corresponding alkaline earth sulfate ASO4 at temperatures as low as 1023 K. The product can be described as a composite dispersion of small (<300 nm) particles of PbZrO3 on larger crystals of the alkaline earth sulfates. The solid-state reaction at 973 K between ZrO2, PbSO4, and K2CO3 followed by washing in water to remove K2SO4 provides a quick route to submicron (<1 μm) PbZrO3 powders starting from inexpensive precursors. In all the reactions, we believe the driving force to be the high enthalpy of formation of ASO4 or K2SO4. The reactions presented here provide proof of concept for the preparation of a number of important polar lead-based perovskite phases under relatively mild conditions: (i) as composites, (ii) as small particles, and (iii) possibly as porous monoliths.

Microwave Dielectric Properties of Certain Simple Alkaline Earth Perovskite Compounds as a Function of Tolerance Factor

Microwave dielectric properties of some simple alkaline earth perovskite compounds were studied with respect to the variation in the tolerance factor. It was found in the present study that the variation in dielectric constant (ε), Q·f o product and the temperature coefficient of polarizability with tolerance factor is quite opposite to what is observed in some complex perovskite materials. It was also observed that the temperature coefficient of dielectric constant follows the same trend as in some complex perovskite materials with respect to the tolerance factor.

Crystal stability limits at positive and negative pressures, and crystal-to-glass transitions.

The direct crystal-to-glass transformation, i.e., spontaneous amorphization, which was first observed by thermal annealing of stishovite ${\mathrm{SiO}}_{2}$ at ambient pressure, has now been observed as an isothermal phenomenon during both compression and decompression of initially stable crystals. While counterintuitive, and dependent on kinetically controlled metastable events, the phenomenon is of broad interest and potential importance in materials science and geophysics. In this paper we use a combination of molecular dynamics simulations and analyses of laboratory data to explore the metastable crystal ranges, including the negative pressure range, for key compounds such as the ices, silicas, and alkaline earth perovskites. Our focus is on the establishment of phenomenological patterns rather than on specific metastability-terminating mechanicsms. We find that a simple quadratic law, P-${\mathit{P}}_{\mathit{s}}$\ensuremath{\sim}(V-${\mathit{V}}_{\mathit{s}}$${)}^{2}$ (where ${\mathit{P}}_{\mathit{s}}$ and ${\mathit{V}}_{\mathit{s}}$ are the values of the pressure P and volume V on the spinodal), well approximates the equations of state over much of the metastable and even the stable range---and implies the existence of an isochoric boundary line for stability to isotropic density fluctuations. We delineate the conditions under which amorphization occurs, usually substantially before the stability limit is reached. (c) 1995 The American Physical Society

Alkoxide‐Hydroxide Route to Syntheltize BaTiO3‐Based Powders

When preparing homogeneous, fine barium titanate powders, the major difficulty is to avoid the spontaneous self‐condensation between the Ti‐OH groups. In the usual way of preparing fine barium titanate powders, chelating agents (citrate, oxalate) or simply unidentate ligands (alkoxy or carboxyl groups) are used to complex titanium atoms. Another way is to mix barium and titanium precursors in a strongly basic medium. The condensation between the Ti(OH)2‐6Ba2+ species directly gives the perovskite compound. Using an alkoxide‐hydroxide route, a homogeneous Ba‐Ti solution was prepared that completely advanced by condensation between the Ti(OH)2‐6Ba2+ species and led to a controlled‐stoichiometry powder. Concerning pure barium titanate, dried powders exhibited the cubic perovskite structure, and a direct sintering at 1150°C, without calcination, led to highly dense BaTiO3 bodies with fine‐grained uniform microstructure (1 μm) that exhibited a high permittivity value at room temperature (K= 5400). The alkoxide‐hydroxide method was also used to prepare dense alkaline‐earth perovskite ceramics with complex compositions.

Chemical processing of (Ba, Sr)(Ti, Zr)O3 perovskite ceramics

Stoichiometric high-purity submicrometre-size perovskite powders have long been of interest for the preparation of dense ferroelectric bodies, thin film electronic components piezoelectrics and electro-optic materials. In recent years, numerous methods of preparing fine perovskite powders have been developed, such as the thermal decomposition of oxalates, citrates and carbonates [1-3], the organic sol-gel technique from metal alkoxides [4-6] and more recently the metallo-organic decomposition [7, 8]. The organic techniques which lead to homogeneity, stoichiometry and high purity in fine particles, offer a great flexibility because of the large variety of organic precursors that are available. However, concerning the very interesting barium or strontium perovskites, the use of the expensive alkaline earth alkoxides is generally very inconvenient because of their high reactivity to moisture and of the high molecular complexity which decreases their solubility in alcohols. We have studied the synthesis of alkaline earth perovskite ceramics from a sol-gel route, previously proposed by Kiss et al. [9] for barium titanate, in which titanium and zirconium alkoxides are hydrolysed by an aqueous solution of barium and/or strontium hydroxides. The method first makes possible the synthesis of a cubic perovskite phase from 100 ° C [10, 11], second, a low sintering temperature (1150°C in the barium titanate) of the powder compact to near theoretical density, and finally the preparation of uniform grain-size bodies. Fig. 1 shows the diagram of preparation of perovskites by the alkoxide-hydroxide method. The organic mixture containing titanium and/or zirconium normal butoxides and 2-butanol in the equimolar proportions is hydrolysed, at about 80 ° C, in a carbon dioxide-free atmosphere, by an aqueous solution containing barium and/or strontium hydroxides with a concentration of about 2.5moll-L For all the compositions, doping elements (Nb, Sb, La, etc.) can be easily introduced by mixing alkoxides in inorganic salts with the organic solution of butoxides. Reactions which take place during this method have been previously described by Flashen for the barium titanate [10]. When a pH of 11 to 14 is maintained, in the presence of hydroxyl ions, four molecules of water and two hydroxyl groups attach through their oxygen atoms to the titanium of Ti(OR)4 in a nucleophilic process and, following an electronic rearrangement, four molecules of alcohol are expelled (Fig. 2). The result of the initial step is an anion of the formula Ti(OH)~ or Zr(OH)~ which is neutralized by the Ba 2+ or Sr 2+ ions in the solution following Equation 1.

Resputtering effects in Ba(Pb, Bi)O3 perovskites

A resputtering effect in Ba(Pb, Bi)O3 has been found to vary from complete film and partial substrate erosion to slightly altered film surface microstructure and bulk composition. It is strongly dependent on the sputtering conditions; only the films prepared at the higher gas pressures (≳200 mTorr) show no resputtering, apparently due to gas phase interactions. The most striking result, other than the obvious surface erosion, is the Ba deficiency, ranging from 10 to 70 at. %, that is directly related to the degree of resputtering. Similar results have been seen for other alkaline earth perovskites.