Hydrothermal Synthesis of Pyrochlore-Type Pentavalent Bismuthates Ca2Bi2O7 and Sr2Bi2O7

Exploring the influence of pressure-induced semiconductor-to-metal transition on the physical properties of cubic perovskites FrXCl3 (X = Ge and Sn)

Even though lead halide perovskites have outstanding physiochemical properties and improved power conversion efficiency, most of these compounds threaten their future commercialization because of their instability and highly toxic nature. Thus, it is preferable to use stable alternative elements rather than lead to make environmentally friendly perovskite material that will have comparable optical and electronic properties to those constructed from Pb-based perovskites. However, devices constructed from lead-free perovskites typically display a lower power conversion efficiency. Applying hydrostatic pressure could be deemed an effective method to alter the physical properties of these compounds. This not only improves their performance in application but also reveals significant correlations between structure and properties. This work uses DFT to investigate the structural, electronic, optical, and elastic properties of non-toxic, francium-based halide perovskites FrXCl3 (X = Ge, Sn) at different levels of hydrostatic pressures that vary from 0 to 10 GPa. The estimated structural parameter's strong correlation with the data from earlier studies ensures the accuracy of the current findings. Pressure causes the Fr-Cl and Ge (Sn)-Cl bonds to shorten and become stronger. The electronic property calculations demonstrated that both compounds are direct band-gap semiconductors. The application of pressure leads to a linear reduction in the band gap (semiconducting to metallic state) and raises the electronic density of states around the Fermi level by forcing the valence band electrons upward, indicating that the optoelectronic device's performance can be tuned and improved. The values of the dielectric constant, absorptivity and reflectivity showed an increasing tendency with pressure. As the pressure applied to the compounds increases, the absorption spectra show a redshift. These findings suggested that the FrXCl3 (X = Ge and Sn) compound becomes more appropriate for usage in optoelectronic applications under pressure. Furthermore, our examination of the mechanical properties indicates that both FrGeCl3 and FrSnCl3 exhibit mechanically stability, and ductility. Interestingly, we observe an increase in ductility as pressure levels rise.

Pressure-driven modification of optoelectronic features of ACaCl3 (A = Cs, Tl) for device applications

Intending to advance the use of halide-perovskites in technological applications, in this research, we investigate the structural, electronic, optical, and mechanical behavior of metal-halide perovskites ACaCl3 (A = Cs, Tl) through first-principle analysis and assess their potential applications. Due to the applied hydrostatic pressure, the interaction between constituent atoms increases, thereby causing the lattice parameter to decrease. The band structure reveals that band gap nature transits from indirect to direct at elevated pressure. Moreover, at high pressure, the electronic band structure shows a notable band gap contraction from the insulator (>5.0 eV) to the semiconductor region, which makes them promising for electronic applications. The charge density map explores the ionic and covalent characteristics of Cs/Tl-Cl and Ca-Cl under pressured and unpressurized environments. Induced pressure enhances the optical conductivity as well as the optical absorption that moves toward the low-energy region (red shift), making ACaCl3 (A = Cs, Tl) advantageous for optoelectronic applications. Additionally, this study reveals that the mechanical properties of ductility and anisotropy were found to be improved at higher pressures than in ambient conditions. Overall, this study will shed light on the technological applications of lead-free halide perovskites in extreme pressure conditions.

Hydrothermal synthesis and crystal structure of a novel double-perovskite-type bismuth oxide with 3:1 ordering at the B-site

A low-temperature hydrothermal method was successfully used to synthesize a novel bismuth oxide Ba4Bi3NaO12. Here, NaBiO3·nH2O was used as one of the starting materials. Single-crystal X-ray diffraction revealed the triclinic...

Hydrothermal Synthesis and Crystal Structure of a Novel Bismuth Oxide: (K0.2Sr0.8)(Na0.01Ca0.25Bi0.74)O3

A novel distorted perovskite-type (K_0.2Sr_0.8)(Na_0.01Ca_0.25Bi_0.74)O₃ was prepared by a hydrothermal method using the starting materials NaBiO₃·nH₂O, Sr(OH)₂·8H₂O, Ca(OH)₂, and KOH. Single-crystal X-ray diffraction of the novel compound revealed a GdFeO₃-related structure belonging to the monoclinic system of the space group Cc with the following cell parameters: a = 11.8927 (17) Å, b = 11.8962 (15) Å, c = 8.4002 (10) Å, and β = 90.116 (9)°. The final R-factors were obtained as R ₁ = 0.0354 and wR ₂ = 0.0880 (using all the data). K⁺ and Sr²⁺ ions were distributed at four types of A-sites. On the other hand, four Bi⁵⁺-sites (Bi1, Bi2, Bi3, and Bi4) were occupied by four Ca²⁺ ions (Ca1, Ca2, Ca3, and Ca4), and the first three B-sites were occupied predominantly by Bi⁵⁺ with Na⁺ ions. The forth B-site was occupied predominantly by the Ca²⁺ ion with Bi⁵⁺ ions. Two types of B-sites, thus forming tilted distorted (Na/Ca/Bi)O₆ and (Bi/Ca)O₆ octahedra, have an ordering of 3:1 represented as (K/Sr)₄(Na/Ca/Bi)₃(Bi/Ca)O₁₂. The distorted (Na/Ca/Bi)O₆ and (Ca/Bi)O₆ octahedra formed a perovskite-type network by corner sharing with features closely matching those of a GdFeO₃-type structure. The novel compound is the first example of a perovskite-type bismuth oxide containing only Bi⁵⁺ in a system without a Ba atom and has a unique ordering (3:1) of the B site. The compound showed photocatalytic activity for phenol degradation under visible light irradiation.

Hydrothermal magic for the synthesis of new bismuth oxides

A variety of bismuth oxides have been synthesized by hydrothermal reactions using NaBiO₃·nH₂O as a starting material.

Hydrothermal Synthesis and Crystal Structure of a Mixed-Valence Bismuthate, Na3Bi3O8

Four types of bismuth oxides, Na₃Bi₃O₈, NaBiO₃, α-Bi₂O₃, and ε-Bi₂O₃, were obtained by hydrothermal reactions using NaBiO₃·nH₂O in NaOH solution. The crystal structure of a new phase (Na₃ Bi³⁺)Bi₂⁵⁺O₈ ((Na_0.75Bi_0.25)₂BiO₄) was determined by using single crystal X-ray diffraction data, and this compound was found to show a Na₂MnCl₄-related structure with a monoclinic system (space group, Pm) with the following lattice parameters: a = 5.990 (2) Å, b = 3.335 (2) Å, c = 10.108 (2) Å, and β = 91.08 (3)°. The final R-factors R₁ and wR₂ were 0.041 and 0.090 (all data), respectively. The new phase was composed of mixed valence states of Bi (Bi³⁺ and Bi⁵⁺, with a mean Bi valence of 4.30) with five distinct Bi sites, where two Bi⁵⁺ (Bi1 and Bi2) fully occupied the distorted octahedral sites and three Bi³⁺ (Bi3, Bi4, and Bi5) were statistically distributed at the split sites with Na⁺ (Na3, Na4, and Na5). The Na6 site is fully occupied. The distorted Bi⁵⁺O₆ octahedra formed one-dimensional chains via edge-sharing along the b-axis, with the chains held by Bi³⁺/Na⁺ split sites. The structural feature except for the split distribution of Bi³⁺/Na⁺ was classified as a Na₂MnCl₄-type structure. DFT calculations based on a model discounting the split distribution of Bi³⁺/Na⁺ indicated that Bi 6s and O 2p orbitals form sp hybridization at the conduction band. This new mixed valence bismuth oxide exhibited photocatalytic activity for phenol degradation under visible light irradiation. In addition to Na₃Bi₃O₈, the hydrothermal reaction using NaBiO₃·nH₂O in NaOH solution yielded micrometer-sized single crystals of an ilmenite-type NaBiO₃ and two polymorphs of bismuth oxides with monoclinic (α-Bi₂O₃) and orthorhombic (ε-Bi₂O₃) structures, depending on the reaction temperature and NaOH concentration.

Tolerance factor, phase stability and order–disorder of the pyrochlore structure

The tolerance factor is a structural indicator used to predict the phase stability of pyrochlore structures from their chemical compositions.

Hydrothermal Synthesis and Crystal Structure of a (Ba0.54K0.46)4Bi4O12 Double-Perovskite Superconductor with Onset of the Transition Tc ∼ 30 K

A new superconducting double perovskite was successfully synthesized by a low-temperature hydrothermal reaction at 240 °C. The crystal structure refinement of this double perovskite was done by single-crystal X-ray diffraction, and it had a cubic unit cell of a = 8.5207(2) Å with space group Im3̅m (No. 229). This superconducting double-perovskite chemical composition was estimated by electron probe microanalysis and was similar to the refined data. The superconducting transition temperature of the double perovskite was ∼30 K; the electrical resistivity began to fall at ∼25 K, and zero resistivity occurred below 7 K. Moreover, temperature-dependent resistivity under various magnetic fields and isothermal magnetization measurements ensured the nature of a type II superconductor for the sample. Finally, the metallic nature of the material was investigated by a first-principles study.