Structures of α‑K₃MoO₃F₃ and α‑Rb₃MoO₃F₃: Ferroelectricity from Anion Ordering and Noncooperative Octahedral Tilting

The room temperature crystal structures of α-K₃MoO₃F₃ and α-Rb₃MoO₃F₃ have been solved via combined Rietveld refinements of synchrotron and neutron powder diffraction data. These two compounds are part of a broader family of <i>A</i>₂<i>BM</i>O₃F₃ compounds that have been studied for their dielectric properties, but until now the complex crystal structures of the ferroelectric phases of these compounds were not known. At room temperature and below, these two isostructural compounds are tetragonal with <i>I</i>4₁ space group symmetry and unit cell parameters of <i>a</i> = 19.38613(3) Å, <i>c</i> = 34.86739(8) Å for α-K₃MoO₃F₃ and <i>a</i> = 20.0748(4) Å, <i>c</i> = 36.1694(1) Å for α-Rb₃MoO₃F₃. Their structures are related to the cubic double perovskite structure but are considerably more complicated due to noncooperative octahedral tilting and long-range orientational ordering of the polar MoO₃F₃^3– units. The pattern of octahedral tilting is equivalent to that seen in the α-K₃AlF₆ structure, which has <i>I</i>4₁/<i>a</i> symmetry, but orientational ordering of MoO₃F₃^3– units lowers the symmetry to <i>I</i>4₁. The polar space group symmetry is consistent with earlier reports of ferroelectricity in these compounds. Hence orientational ordering of the MoO₃F₃^3– units is directly responsible for the ferroelectric behavior

Study of Anion Order/Disorder in RTaN₂O (R = La, Ce, Pr) Perovskite Nitride Oxides

Symmetries and model structures are given for ABN₂O (and ABNO₂) perovskites that possess long-range ordering of anions in combination with <i>a</i>⁰<i>a</i>⁰<i>c</i>^–, <i>a</i>^–<i>b</i>⁰<i>a</i>^–, and <i>a</i>^–<i>b</i>⁺<i>a</i>^– octahedral tilting. The stabilities of competing structures have been evaluated using density functional theory (DFT) calculations, which show that <i>cis</i>-ordered models are more stable than competing <i>trans</i>-ordered polymorphs. To test the validity of these predictions, the perovskite nitride oxides LaTaN₂O, CeTaN₂O, and PrTaN₂O have been synthesized and characterized using neutron powder diffraction. CeTaN₂O and PrTaN₂O crystallize with orthorhombic <i>Pnma</i> symmetry (Ce: <i>a</i> = 5.69666(8), <i>b</i> = 8.03272(9), and <i>c</i> = 5.70893(7) Å; Pr: <i>a</i> = 5.6868(1), <i>b</i> = 8.0153(1), and <i>c</i> = 5.68057(8) Å) as a result of <i>a</i>^–<i>b</i>⁺<i>a</i>^– tilting of the octahedra. The structure of LaTaN₂O is re-examined and found to possess orthorhombic <i>Imma</i> symmetry (<i>a</i> = 5.7093(1), <i>b</i> = 8.0591(2), and <i>c</i> = 5.7386(2) Å) as a result of <i>a</i>^–<i>b</i>⁰<i>a</i>^– tilting. No evidence for long-range anion order is found in any of the three compounds. Optical band gaps for these compounds are measured to be 2.0 eV (LaTaN₂O), 1.9 eV (CeTaN₂O), and 2.0 eV (PrTaN₂O). These values are 0.6–0.7 eV smaller than CaTaNO₂ where the Ta-centered octahedra tilt by a similar amount. As the nitrogen content increases, there is an increase in the overlap of the anion 2p orbitals, which increases the energy of the valence band maximum and narrows the band gap

Evaluating NaREMgWO₆ (RE = La, Gd, Y) Doubly Ordered Double Perovskites as Eu³⁺ Phosphor Hosts

Three doubly ordered double perovskites NaREMgWO₆ (RE = La, Gd, Y) have been synthesized via traditional solid-state methods, doped with Eu³⁺, and characterized to evaluate their promise as Eu³⁺ phosphor hosts. NaYMgWO₆, a new member of the family, was found to crystallize in the <i>P</i>2₁ space group and is isostructural with NaGdMgWO₆. Emissions characteristic of Eu³⁺ ions (⁵D₀ → ⁷F_4,3,2,1,0) were observed, with the most intense transition being the ⁵D₀ → ⁷F₂ transition near 615 nm. Substitution of Eu³⁺ onto a more compressed RE site in the NaY_1–<i>x</i>Eu_<i>x</i>MgWO₆ and NaGd_1–<i>x</i>Eu_<i>x</i>MgWO₆ hosts results in a blue shift of the charge-transfer excitation band and an increase in the intensity of the ⁵D₀ → ⁷F₂ transition compared to NaLa_1–<i>x</i>Eu_<i>x</i>MgWO₆. All of the hosts can incorporate high concentrations of Eu³⁺ before concentration quenching is observed. When the rare-earth ion is either Gd³⁺ or Y³⁺, good energetic overlap between the Eu³⁺ charge-transfer band and the absorption of the host lattice results in sensitization and energy transfer from the perovskite host lattice to the Eu³⁺ activator sites. These hosts display comparable if not better luminescence than Y₂O₃:Eu³⁺, a commonly used commercial standard, demonstrating their promise as red phosphors

Cs₂AgBiX₆ (X = Br, Cl): New Visible Light Absorbing, Lead-Free Halide Perovskite Semiconductors

The double perovskites Cs₂AgBiBr₆ and Cs₂AgBiCl₆ have been synthesized from both solid state and solution routes. X-ray diffraction measurements show that both compounds adopt the cubic double perovskite structure, space group <i>Fm</i>3̅<i>m</i>, with lattice parameters of 11.2711(1) Å (X = Br) and 10.7774(2) Å (X = Cl). Diffuse reflectance measurements reveal band gaps of 2.19 eV (X = Br) and 2.77 eV (X = Cl) that are slightly smaller than the band gaps of the analogous lead halide perovskites, 2.26 eV for CH₃NH₃PbBr₃ and 3.00 eV for CH₃NH₃PbCl₃. Band structure calculations indicate that the interaction between the Ag 4d-orbitals and the 3p/4p-orbitals of the halide ion modifies the valence band leading to an indirect band gap. Both compounds are stable when exposed to air, but Cs₂AgBiBr₆ degrades over a period of weeks when exposed to both ambient air and light. These results show that halide double perovskite semiconductors are potentially an environmentally friendly alternative to the lead halide perovskite semiconductors

Metal-to-Metal Charge Transfer in AWO₄ (A = Mg, Mn, Co, Ni, Cu, or Zn) Compounds with the Wolframite Structure

Using a combination of UV–visible spectroscopy and electronic structure calculations, we have characterized the electronic structures and optical properties of AWO₄ (A = Mn, Co, Ni, Cu, Zn, or Mg) tungstates with the wolframite structure. In MgWO₄ and ZnWO₄, the lowest energy optical excitation is a ligand to metal charge transfer (LMCT) excitation from oxygen 2p nonbonding orbitals to antibonding W 5d orbitals. The energy of the LMCT transition in these two compounds is 3.95 eV for ZnWO₄ and 4.06 eV for MgWO₄. The charge transfer energies observed for the other compounds are significantly smaller, falling in the visible region of the spectrum and ranging from 2.3 to 3.0 eV. In these compounds, the partially occupied 3d orbitals of the A²⁺ ion act as the HOMO, rather than the O 2p orbitals. The lowest energy charge transfer excitation now becomes a metal-to-metal charge transfer (MMCT) excitation, where an electron is transferred from the occupied 3d orbitals of the A²⁺ ion to unoccupied antibonding W 5d states. The MMCT value for CuWO₄ of 2.31 eV is the lowest in this series due to distortions of the crystal structure driven by the d⁹ configuration of the Cu²⁺ ion that lower the crystal symmetry to triclinic. The results of this study have important implications for the application of these and related materials as photocatalysts, photoanodes, pigments, and phosphors

Na_1.5Ag_1.5MO₃F₃ (M = Mo, W): An Ordered Oxyfluoride Derivative of the LiNbO₃ Structure

Na_1.5Ag_1.5MoO₃F₃ and Na_1.5Ag_1.5WO₃F₃ have been synthesized by solid state reactions and structurally characterized using synchrotron X-ray and neutron powder diffraction. Unlike the vast majority of salts containing [MO₃F₃]^3– anions (M = Mo, W) the oxyfluoride groups in Na_1.5Ag_1.5MoO₃F₃ and Na_1.5Ag_1.5WO₃F₃ are orientationally ordered, so that the Na⁺ ions are coordinated by fluorine and the Ag⁺ ions by oxygen. The resulting structure type, which has not previously been reported, is related to the LiNbO₃ structure, but the combination of Na/Ag ordering and orientational ordering of the [MO₃F₃]^3– anions produces a supercell that doubles the <i>c</i>-axis and changes the space group symmetry from <i>R</i>3 to <i>R</i>3̅. The use of hard (Na⁺) and soft (Ag⁺) cations to direct the orientational ordering of polar oxyfluoride building units provides a new approach to the design of polar materials

Na_1.5Ag_1.5MO₃F₃ (M = Mo, W): An Ordered Oxyfluoride Derivative of the LiNbO₃ Structure

Na_1.5Ag_1.5MoO₃F₃ and Na_1.5Ag_1.5WO₃F₃ have been synthesized by solid state reactions and structurally characterized using synchrotron X-ray and neutron powder diffraction. Unlike the vast majority of salts containing [MO₃F₃]^3– anions (M = Mo, W) the oxyfluoride groups in Na_1.5Ag_1.5MoO₃F₃ and Na_1.5Ag_1.5WO₃F₃ are orientationally ordered, so that the Na⁺ ions are coordinated by fluorine and the Ag⁺ ions by oxygen. The resulting structure type, which has not previously been reported, is related to the LiNbO₃ structure, but the combination of Na/Ag ordering and orientational ordering of the [MO₃F₃]^3– anions produces a supercell that doubles the <i>c</i>-axis and changes the space group symmetry from <i>R</i>3 to <i>R</i>3̅. The use of hard (Na⁺) and soft (Ag⁺) cations to direct the orientational ordering of polar oxyfluoride building units provides a new approach to the design of polar materials

Na_1.5Ag_1.5MO₃F₃ (M = Mo, W): An Ordered Oxyfluoride Derivative of the LiNbO₃ Structure

Na_1.5Ag_1.5MoO₃F₃ and Na_1.5Ag_1.5WO₃F₃ have been synthesized by solid state reactions and structurally characterized using synchrotron X-ray and neutron powder diffraction. Unlike the vast majority of salts containing [MO₃F₃]^3– anions (M = Mo, W) the oxyfluoride groups in Na_1.5Ag_1.5MoO₃F₃ and Na_1.5Ag_1.5WO₃F₃ are orientationally ordered, so that the Na⁺ ions are coordinated by fluorine and the Ag⁺ ions by oxygen. The resulting structure type, which has not previously been reported, is related to the LiNbO₃ structure, but the combination of Na/Ag ordering and orientational ordering of the [MO₃F₃]^3– anions produces a supercell that doubles the <i>c</i>-axis and changes the space group symmetry from <i>R</i>3 to <i>R</i>3̅. The use of hard (Na⁺) and soft (Ag⁺) cations to direct the orientational ordering of polar oxyfluoride building units provides a new approach to the design of polar materials

Cs_1–<i>x</i>Rb_<i>x</i>PbCl₃ and Cs_1–<i>x</i>Rb_<i>x</i>PbBr₃ Solid Solutions: Understanding Octahedral Tilting in Lead Halide Perovskites

The structures of the lead halide perovskites CsPbCl₃ and CsPbBr₃ have been determined from X-ray powder diffraction data to be orthorhombic with <i>Pnma</i> space group symmetry. Their structures are distorted from the cubic structure of their hybrid analogs, CH₃NH₃PbX₃ (X = Cl, Br), by tilts of the octahedra (Glazer tilt system <i>a</i>^–<i>b</i>⁺<i>a</i>^–). Substitution of the smaller Rb⁺ for Cs⁺ increases the octahedral tilting distortion and eventually destabilizes the perovskite structure altogether. To understand this behavior, bond valence parameters appropriate for use in chloride and bromide perovskites have been determined for Cs⁺, Rb⁺, and Pb²⁺. As the tolerance factor decreases, the band gap increases, by 0.15 eV in Cs_1–<i>x</i>Rb_<i>x</i>PbCl₃ and 0.20 eV in Cs_1–<i>x</i>Rb_<i>x</i>PbBr₃, upon going from <i>x</i> = 0 to <i>x</i> = 0.6. The band gap shows a linear dependence on tolerance factor, particularly for the Cs_1–<i>x</i>Rb_<i>x</i>PbBr₃ system. Comparison with the cubic perovskites CH₃NH₃PbCl₃ and CH₃NH₃PbBr₃ shows that the band gaps of the methylammonium perovskites are anomalously large for APbX₃ perovskites with a cubic structure. This comparison suggests that the local symmetry of CH₃NH₃PbCl₃ and CH₃NH₃PbBr₃ deviate significantly from the cubic symmetry of the average structure

The Incommensurately Modulated Structures of the Perovskites NaCeMnWO₆ and NaPrMnWO₆

The structures of the doubly ordered perovskites NaCeMnWO₆ and NaPrMnWO₆, with rock salt ordering of the Mn²⁺ and W⁶⁺ <i>B</i>-site cations and layered ordering of the Na⁺ and (Ce³⁺/Pr³⁺) <i>A</i>-site cations, have been studied by transmission electron microscopy, electron diffraction, neutron and synchrotron X-ray powder diffraction. Both compounds possess incommensurately modulated crystal structures. In NaCeMnWO₆ the modulation vector (with reference to the ideal <i>ABX</i>₃ perovskite subcell) is <i>q</i> ≈ 0.067<i>a</i>* (∼58.7 Å) and in NaPrMnWO₆ <i>q</i> ≈ 0.046<i>a</i>* (∼85.3 Å). In both compounds the superstructures are primarily the two-dimensional chessboard type, although some crystals of NaCeMnWO₆ were found with one-dimensional stripes. In some crystals of NaPrMnWO₆ there is a coexistence of chessboards and stripes. Modeling of neutron diffraction data shows that octahedral tilting plays an important role in the structural modulation