Structures of ordered tungsten- or molybdenum-containing quaternary perovskite oxides

Study of mechanical, optical, and thermoelectric characteristics of Ba2 XMoO6 (X = Zn, Cd) double perovskite for energy harvesting

The double perovskites are become the emerging aspirant to fulfill the demand of energy. Therefore, the optoelectronic, elastic and transport characteristics of Ba2 XMoO6 (X = Zn, Cd) are addressed systemically. The elastic constants show the mechanical stability. The nature of Ba2 ZnMoO6 is brittle and Ba2 CdMoO6 is ductile with large values of Debye temperature covalent bonding. The electronic band structures exhibit band gaps of 2.81 and 2.98 eV, which increase their importance for optoelectronic applications. The absorption of light energy, optical loss, refractive index, polarization of light energy are addressed in the energy range zero to 14 eV. Furthermore, thermoelectric characteristics are computed against chemical potentials at 300, 600, and 900 K. The chemical potential decides the p-type nature, with holes as majority carriers. The increasing temperature increases the power factor and figure of merit. Therefore, the optoelectronic and thermoelectric characteristics reveals the importance of studied DPs for energy applications.

Characterization of the charge transfer luminescence of the [WO6]6- octahedron in Ca3WO6 and the [WO5]4- square pyramid in Ca3WO5Cl2

Charge transfer (CT) luminescence of different types of polyhedra, [WO₅]^4- in Ca₃WO₅Cl₂ and [WO₆]^6- in Ca₃WO₆, is characterized by spectroscopic experiments and ab initio calculations. According to the geometry optimization, W⁶⁺ ions form five-fold [WO₅]^4- square pyramids in Ca₃WO₅Cl₂ because of a large interatomic distance between W⁶⁺ and Cl^- of 3.266 Å. The analysis of the density of electronic states reveals the ionic character of Cl^- ions to the W⁶⁺ ions in the Ca₃WO₅Cl₂ lattice, resulting in the observed broad luminescence band peak at 488 nm of the single-crystal Ca₃WO₅Cl₂ sample being assigned to the CT transition in the [WO₅]^4- square pyramid. Compared with the [WO₆]^6- octahedron in Ca₃WO₆, the [WO₅]^4- square pyramid shows an inconsistent CT energy shift: higher CT absorption and lower luminescence energies. The larger bandgap brings about higher absorption energy due to the structural and compositional features of the orthorhombic Ca₃WO₅Cl₂. The redshifted CT luminescence band and small activation energy for the thermal quenching of the Ca₃WO₅Cl₂ sample are explained, assuming that the CT states of the anisotropic [WO₅]^4- square pyramid take a larger offset in the configurational coordinate diagram than the [WO₆]^6- octahedron.

Novel inorganic crystal structures predicted using autonomous simulation agents

We report a dataset of 96640 crystal structures discovered and computed using our previously published autonomous, density functional theory (DFT) based, active-learning workflow named CAMD (Computational Autonomy for Materials Discovery). Of these, 894 are within 1 meV/atom of the convex hull and 26826 are within 200 meV/atom of the convex hull. The dataset contains DFT-optimized pymatgen crystal structure objects, DFT-computed formation energies and phase stability calculations from the convex hull. It contains a variety of spacegroups and symmetries derived from crystal prototypes derived from known experimental compounds, and was generated from active learning campaigns of various chemical systems. This dataset can be used to benchmark future active-learning or generative efforts for structure prediction, to seed new efforts of experimental crystal structure discovery, or to construct new models of structure-property relationships.

Site-Selective d10/d0 Substitution in an S = 1/2 Spin Ladder Ba2CuTe1-xWxO6 (0 ≤ x ≤ 0.3)

Isovalent nonmagnetic d10 and d0 B″ cations have proven to be a powerful tool for tuning the magnetic interactions between magnetic B' cations in A2B'B″O6 double perovskites. Tuning is facilitated by the changes in orbital hybridization that favor different superexchange pathways. This can produce alternative magnetic structures when B″ is d10 or d0. Furthermore, the competition generated by introducing mixtures of d10 and d0 cations can drive the material into the realms of exotic quantum magnetism. Here, Te6+ d10 was substituted by W6+ d0 in the hexagonal perovskite Ba2CuTeO6, which possesses a spin ladder geometry of Cu2+ cations, creating a Ba2CuTe1-xWxO6 solid solution (x = 0-0.3). We find W6+ is almost exclusively substituted for Te6+ on the corner-sharing site within the spin ladder, in preference to the face-sharing site between ladders. The site-selective doping directly tunes the intraladder, Jrung and Jleg, interactions. Modeling the magnetic susceptibility data shows the d0 orbitals modify the relative intraladder interaction strength (Jrung/Jleg) so the system changes from a spin ladder to isolated spin chains as W6+ increases. This further demonstrates the utility of d10 and d0 dopants as a tool for tuning magnetic interactions in a wide range of perovskites and perovskite-derived structures.

Raman studies of A2MWO6 tungstate double perovskites

The Raman spectra of seven A(2)MWO(6) tungstate double perovskites are analysed. Ba(2)MgWO(6) is a cubic double perovskite with Fm3[combining macron]m symmetry and its Raman spectrum contain three modes that can be assigned in a straightforward manner. A fourth mode, the asymmetric stretch of the [WO(6)](6-) octahedron, is too weak to be observed. The symmetry of Ba(2)CaWO(6) is lowered to tetragonal I4/m due to octahedral tilting, but the distortion is sufficiently subtle that the extra bands predicted to appear in the Raman spectrum are not observed. The remaining five compounds have additional octahedral tilts that lower the symmetry to monoclinic P2(1)/n. The further reduction of symmetry leads to the appearance of additional lattice modes involving translations of the A-site cations and librations of the octahedra. Comparing the Raman spectra of fourteen different A(2)MWO(6) tungstate double perovskites shows that the frequency of the symmetric stretch (ν(1)) of the [WO(6)](6-) octahedron is relatively low for cubic perovskites with tolerance factors greater than one due to underbonding of the tungsten and/or M cation. The frequency of this mode increases rapidly as the tolerance factor drops below one, before decreasing gradually as the octahedral tilting gets larger. The frequency of the oxygen bending mode (ν(5)) is shown to be dependent on the mass of the A-site cation due to coupling of the internal bending mode with external A-site cation translation modes.