Synchrotron X-ray, Photoluminescence, and Quantum Chemistry Studies of Bismuth-Embedded Dehydrated Zeolite Y

For the first time, direct experimental evidence of the formation of monovalent Bi (i.e., Bi⁺) in zeolite Y is provided based on the analysis of high-resolution synchrotron powder X-ray diffraction data. Photoluminescence results as well as quantum chemistry calculations suggest that the substructures of Bi⁺ in the sodalite cages contribute to the ultrabroad near-infrared emission. These results not only enrich the well-established spectrum of optically active zeolites and deepen the understanding of bismuth related photophysical behaviors, but also may raise new possibilities for the design and synthesis of novel hybrid nanoporous photonic materials activated by other heavier p-block elements

Synchrotron X-ray, Photoluminescence, and Quantum Chemistry Studies of Bismuth-Embedded Dehydrated Zeolite Y

For the first time, direct experimental evidence of the formation of monovalent Bi (i.e., Bi⁺) in zeolite Y is provided based on the analysis of high-resolution synchrotron powder X-ray diffraction data. Photoluminescence results as well as quantum chemistry calculations suggest that the substructures of Bi⁺ in the sodalite cages contribute to the ultrabroad near-infrared emission. These results not only enrich the well-established spectrum of optically active zeolites and deepen the understanding of bismuth related photophysical behaviors, but also may raise new possibilities for the design and synthesis of novel hybrid nanoporous photonic materials activated by other heavier p-block elements

Low-Temperature Structural Modulations in CdMn₇O₁₂, CaMn₇O₁₂, SrMn₇O₁₂, and PbMn₇O₁₂ Perovskites Studied by Synchrotron X‑ray Powder Diffraction and Mössbauer Spectroscopy

Structural phase transitions in CdMn₇O₁₂, CaMn₇O₁₂, SrMn₇O₁₂, and PbMn₇O₁₂ perovskites are investigated by synchrotron X-ray powder diffraction between 113 and 583 K, differential scanning calorimetry (DSC), and Mössbauer spectroscopy on ⁵⁷Fe-doped samples. DSC is used to determine phase transition temperatures (<i>T</i>_OO). All the compounds crystallize in space group <i>R</i>-3 at room temperature. An incommensurate structural modulation is observed below <i>T</i>_OO = 265 K in SrMn₇O₁₂ with a propagation vector <b>q</b> = (0, 0, 0.9235) at 113 K, similar to the already reported case of CaMn₇O₁₂ (with <i>T</i>_OO = 258 K and <b>q</b> = (0, 0, 0.9215) at 113 K). However, superstructure reflections of SrMn₇O₁₂ are significantly weaker than those of CaMn₇O₁₂. On the other hand, a commensurate structural transition is found in CdMn₇O₁₂ below <i>T</i>_OO = 254 K and in PbMn₇O₁₂ below <i>T</i>_OO = 294 K; the transition can be described as from space group <i>R</i>-3 to space group <i>P</i>-3 with the same unit cell dimensions (<i>a</i> = 10.43306(2) Å and <i>c</i> = 6.33939(1) Å in CdMn₇O₁₂ at 113 K). CdMn₇O₁₂ shows quite strong superstructure reflections at 113 K, while PbMn₇O₁₂ has extremely small superstructure reflections. Below <i>T</i>_OO, the <i>c</i> lattice parameter of all the compounds increases with decreasing temperature (down to 113 K). Mössbauer spectroscopy shows that quadrupole splitting noticeably increases below <i>T</i>_OO, and we quantitatively explain this increase in CaMn₇O₁₂ by structural modulations. Structure parameters of the <i>R</i>-3 modification and high-temperature <i>Im</i>3̅ modification of CaMn₇O₁₂ and PbMn₇O₁₂ are also reported

Structural and Thermal Properties of Ternary Narrow-Gap Oxide Semiconductor; Wurtzite-Derived β‑CuGaO₂

The crystal structure of the wurtzite-derived β-CuGaO₂ was refined by Rietveld analysis of high-resolution powder diffraction data obtained from synchrotron X-ray radiation. Its structural characteristics are discussed in comparison with the other I–III–VI₂ and II–VI oxide semiconductors. The cation and oxygen tetrahedral distortions of the β-CuGaO₂ from an ideal wurtzite structure are small. The direct band-gap nature of the β-CuGaO₂, unlike β-Ag­(Ga,Al)­O₂, was explained by small cation and oxygen tetrahedral distortions. In terms of the thermal stability, the β-CuGaO₂ irreversibly transforms into delafossite α-CuGaO₂ at >460 °C in an Ar atmosphere. The transformation enthalpy was approximately −32 kJ mol^–1, from differential scanning calorimetry. This value is close to the transformation enthalpy of CoO from the metastable zincblende form to the stable rock-salt form. The monovalent copper in β-CuGaO₂ was oxidized to divalent copper in an oxygen atmosphere and transformed into a mixture of CuGa₂O₄ spinel and CuO at temperatures >350 °C. These thermal properties indicate that β-CuGaO₂ is stable at ≤300 °C in both reducing and oxidizing atmospheres while in its metastable form. Consequently, this material could be of use in optoelectronic devices that do not exceed 300 °C

High-Pressure Synthesis, Crystal Structures, and Properties of CdMn₇O₁₂ and SrMn₇O₁₂ Perovskites

We synthesize CdMn₇O₁₂ and SrMn_7–<i>x</i>Fe_<i>x</i>O₁₂ (<i>x</i> = 0, 0.08, and 0.5) perovskites under high pressure (6 GPa) and high temperature (1373–1573 K) conditions and investigate their structural, magnetic, dielectric, and ferroelectric properties. CdMn₇O₁₂ and SrMn₇O₁₂ are isostructural with CaMn₇O₁₂: space group <i>R</i>3̅ (No. 148), <i>Z</i> = 3, and lattice parameters <i>a</i> = 10.45508(2) Å and <i>c</i> = 6.33131(1) Å for CdMn₇O₁₂ and <i>a</i> = 10.49807(1) Å and <i>c</i> = 6.37985(1) Å for SrMn₇O₁₂ at 295 K. There is a structural phase transition at 493 K in CdMn₇O₁₂ and at 404 K in SrMn₇O₁₂ to a cubic structure (space group <i>Im</i>3̅), associated with charge ordering as found by the structural analysis and Mössbauer spectroscopy. SrMn_6.5Fe_0.5O₁₂ crystallizes in space group <i>Im</i>3̅ at 295 K with <i>a</i> = 7.40766(2) Å and exhibits spin-glass magnetic properties below 34 K. There are two magnetic transitions in CdMn₇O₁₂ with the Néel temperatures <i>T</i>_N2 = 33 K and <i>T</i>_N1 = 88 K, and in SrMn₇O₁₂ with <i>T</i>_N2 = 63 K and <i>T</i>_N1 = 87 K. A field-induced transition is found in CdMn₇O₁₂ from about 65 kOe, and <i>T</i>_N2 = 58 K at 90 kOe. No dielectric anomalies are found at <i>T</i>_N1 and <i>T</i>_N2 at 0 Oe in both compound, but CdMn₇O₁₂ exhibits small anomalies at <i>T</i>_N1 and <i>T</i>_N2 at 90 kOe. In pyroelectric current measurements, we observe large and broad peaks around magnetic phase transition temperatures in CdMn₇O₁₂, SrMn₇O₁₂, and SrMn_6.5Fe_0.5O₁₂; we assign those peaks to extrinsic effects and compare our results with previously reported results on CaMn₇O₁₂. We also discuss general tendencies of the AMn₇O₁₂ perovskite family (A = Cd, Ca, Sr, and Pb)

Mn Self-Doping of Orthorhombic RMnO₃ Perovskites: (R_0.667Mn_0.333)MnO₃ with R = Er–Lu

Orthorhombic rare-earth trivalent manganites RMnO₃ (R = Er–Lu) were self-doped with Mn to form (R_0.667Mn_0.333)­MnO₃ compositions, which were synthesized by a high-pressure, high-temperature method at 6 GPa and about 1670 K from R₂O₃ and Mn₂O₃. The average oxidation state of Mn is 3+ in (R_0.667Mn_0.333)­MnO₃. However, Mn enters the A site in the oxidation state of 2+, creating the average oxidation state of 3.333+ at the B site. The presence of Mn²⁺ was confirmed by hard X-ray photoelectron spectroscopy measurements. Crystal structures were studied by synchrotron powder X-ray diffraction. (R_0.667Mn_0.333)­MnO₃ crystallizes in space group <i>Pnma</i> with <i>a</i> = 5.50348(2) Å, <i>b</i> = 7.37564(1) Å, and <i>c</i> = 5.18686(1) Å for (Lu_0.667Mn_0.333)­MnO₃ at 293 K, and they are isostructural with the parent RMnO₃ manganites. Compared with RMnO₃, (R_0.667Mn_0.333)­MnO₃ exhibits enhanced Néel temperatures of about <i>T</i>_N1 = 106–110 K and ferrimagnetic or canted antiferromagnetic properties. Compounds with R = Er and Tm show additional magnetic transitions at about <i>T</i>_N2 = 9–16 K. (Tm_0.667Mn_0.333)­MnO₃ exhibits a magnetization reversal or negative magnetization effect with a compensation temperature of about 16 K

Complex Structural Behavior of BiMn₇O₁₂ Quadruple Perovskite

Structural properties of a quadruple perovskite BiMn₇O₁₂ were investigated by laboratory and synchrotron X-ray powder diffraction between 10 and 650 K, single-crystal X-ray diffraction at room temperature, differential scanning calorimetry (DSC), second-harmonic generation, and first-principles calculations. Three structural transitions were found. Above <i>T</i>₁ = 608 K, BiMn₇O₁₂ crystallizes in a parent cubic structure with space group <i>Im</i>3̅. Between 460 and 608 K, BiMn₇O₁₂ adopts a monoclinic symmetry with pseudo-orthorhombic metrics (denoted as <i>I</i>2/<i>m</i>(o)), and orbital order appears below <i>T</i>₁. Below <i>T</i>₂ = 460 K, BiMn₇O₁₂ is likely to exhibit a transition to space group <i>Im</i>. Finally, below about <i>T</i>₃ = 290 K, a triclinic distortion takes place to space group <i>P</i>1. Structural analyses of BiMn₇O₁₂ are very challenging because of severe twinning in single crystals and anisotropic broadening and diffuse scattering in powder. First-principles calculations confirm that noncentrosymmetric structures are more stable than centrosymmetric ones. The energy difference between the <i>Im</i> and <i>P</i>1 models is very small, and this fact can explain why the <i>Im</i> to <i>P</i>1 transition is very gradual, and there are no DSC anomalies associated with this transition. The structural behavior of BiMn₇O₁₂ is in striking contrast with that of LaMn₇O₁₂ and could be caused by effects of the Bi³⁺ lone electron pair

High-Pressure Synthesis, Crystal Structures, and Magnetic Properties of 5d Double-Perovskite Oxides Ca₂MgOsO₆ and Sr₂MgOsO₆

Double-perovskite oxides Ca₂MgOsO₆ and Sr₂MgOsO₆ have been synthesized under high-pressure and high-temperature conditions (6 GPa and 1500 °C). Their crystal structures and magnetic properties were studied by a synchrotron X-ray diffraction experiment and by magnetic susceptibility, specific heat, isothermal magnetization, and electrical resistivity measurements. Ca₂MgOsO₆ and Sr₂MgOsO₆ crystallized in monoclinic (<i>P</i>2₁/<i>n</i>) and tetragonal (<i>I</i>4/<i>m</i>) double-perovskite structures, respectively; the degree of order of the Os and Mg arrangement was 96% or higher. Although Ca₂MgOsO₆ and Sr₂MgOsO₆ are isoelectric, a magnetic-glass transition was observed for Ca₂MgOsO₆ at 19 K, while Sr₂MgOsO₆ showed an antiferromagnetic transition at 110 K. The antiferromagnetic-transition temperature is the highest in the family. A first-principles density functional approach revealed that Ca₂MgOsO₆ and Sr₂MgOsO₆ are likely to be antiferromagnetic Mott insulators in which the band gaps open, with Coulomb correlations of ∼1.8–3.0 eV. These compounds offer a better opportunity for the clarification of the basis of 5d magnetic sublattices, with regard to the possible use of perovskite-related oxides in multifunctional devices. The double-perovskite oxides Ca₂MgOsO₆ and Sr₂MgOsO₆ are likely to be Mott insulators with a magnetic-glass (MG) transition at ∼19 K and an antiferromagnetic (AFM) transition at ∼110 K, respectively. This AFM transition temperature is the highest among double-perovskite oxides containing single magnetic sublattices. Thus, these compounds offer valuable opportunities for studying the magnetic nature of 5d perovskite-related oxides, with regard to their possible use in multifunctional devices

High-Pressure Synthesis, Crystal Structure, and Semimetallic Properties of HgPbO₃

The crystal structure of HgPbO₃ was studied using single-crystal X-ray diffraction and powder synchrotron X-ray diffraction. The structure was well characterized as a centrosymmetric model with a space group of <i>R</i>-3<i>m</i> [hexagonal setting: <i>a</i> = 5.74413(6) Å and <i>c</i> = 7.25464(8) Å] rather than as a noncentrosymmetric model as was expected. It was found that Pb⁴⁺ is octahedrally coordinated by six oxygen atoms as usual, while Hg²⁺ is coordinated by three oxygen atoms in a planar manner, this being a very rare coordination of Hg in a solid-state material. The magnetic and electronic transport properties were investigated in terms of the magnetic susceptibility, magnetization, Hall coefficient, and specific heat capacity of polycrystalline HgPbO₃. Although HgPbO₃ has a carrier concentration (=7.3–8.5 × 10²⁰ cm^–3) that is equal to that of metallic oxides, the very weak temperature dependence of the electrical resistivity (residual-resistivity ratio ∼1.5), the significant diamagnetism (= –1.02 × 10^–4 emu mol^–1 at 300 K) that is in the same order of that of Bi powder and the remarkably small Sommerfeld coefficient [=1.6(1) × 10^–3 J mol^–1 K^–2] implied that it is semimetallic in nature. HgPbO₃ does not have a cage structure; nevertheless, at temperatures below approximately 50 K, it clearly exhibits phonon excitation of an anharmonic vibrational mode that is as significant as those of RbOs₂O₆. The mechanism of the anharmonic mode of the HgPbO₃ has yet to be identified, however

High-Pressure Synthesis, Structures, and Properties of Trivalent A‑Site-Ordered Quadruple Perovskites RMn₇O₁₂ (R = Sm, Eu, Gd, and Tb)

A-site-ordered quadruple perovskites RMn₇O₁₂ with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group <i>I</i>2/<i>m</i> at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic <i>Im</i>3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at <i>T</i>_N1 ≈ 82–87 K at zero magnetic field, but additional magnetic transitions below <i>T</i>_N2 ≈ 12 K were observed in SmMn₇O₁₂ and EuMn₇O₁₂ at high magnetic fields. Very weak kinklike dielectric anomalies were observed at <i>T</i>_N1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18–35 K)these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn₇O₁₂ shows signs of nonstoichiometry expressed as (Tb_1–<i>x</i>Mn_<i>x</i>)­Mn₇O₁₂, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn₇O₁₂ and EuMn₇O₁₂ were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn₇O₁₂ and (Tb_0.88Mn_0.12)­Mn₇O₁₂ were studied by synchrotron X-ray powder diffraction at 295 K