Crystal Structure of a Lightweight Borohydride from Submicrometer Crystallites by Precession Electron Diffraction

We demonstrate that precession electron diffraction at low-dose conditions can be successfully applied for structure analysis of extremely electron-beam-sensitive materials. Using LiBH₄ as a test material, complete structural information, including the location of the H atoms, was obtained from submicrometer-sized crystallites. This demonstrates for the first time that, where conventional transmission electron microscopy techniques fail, quantitative precession electron diffraction can provide structural information from submicrometer particles of such extremely electron-beam-sensitive materials as complex lightweight hydrides. We expect the precession electron diffraction technique to be a useful tool for nanoscale investigations of thermally unstable lightweight hydrogen-storage materials

Copper(II)-Binding Ability of Stereoisomeric <i>cis-</i> and <i>trans</i>-2-Aminocyclohexanecarboxylic Acid–l-Phenylalanine Dipeptides. A Combined CW/Pulsed EPR and DFT Study

With the aim of an improved understanding of the metal-complexation properties of alicyclic β-amino acid stereoisomers, and their peptides, the complex equilibria and modes of coordination with copper­(II) of l-phenylalanine (F) derivatives of <i>cis</i>/<i>trans</i>-2-aminocyclohexanecarboxylic acid (<i>c</i>/<i>t</i>ACHC), <i>i.e</i>. the dipeptides F-<i>c</i>/<i>t</i>ACHC and <i>c/t</i>ACHC-F, were investigated by a combination of CW and pulsed EPR methods. For the interpretation of the experimental data, DFT quantum-chemical calculations were carried out. Simulation of a pH-dependent series of room-temperature CW-EPR spectra revealed the presence of EPR-active complexes ([Cu­(aqua)]²⁺, [CuL]⁺, [CuLH_–1], [CuLH_–2]⁻, and [CuL₂H_–1]⁻), and an EPR-inactive species ([Cu₂L₂H_–3]⁻) in aqueous solutions for all studied cases. [CuLH]²⁺ was included in the equilibrium model for the <i>c</i>/<i>t</i>ACHC-F–copper­(II) systems, and [CuL₂], together with two coordination isomers of [CuL₂H_–1]⁻, were also identified in the F-<i>t</i>ACHC–copper­(II) system. Comparison of the complexation properties of the diastereomeric ligand pair F-(1<i>S</i>,2<i>R</i>)-ACHC and F-(1<i>R</i>,2<i>S</i>)-ACHC did not reveal significant differences. Considerably lower formation constants were obtained for the <i>trans</i> than for the <i>cis</i> isomers for both the F-<i>c</i>/<i>t</i>ACHC and the <i>c</i>/<i>t</i>ACHC-F pairs in the case of [CuLH_–1] involving tridentate coordination by the amino, the deprotonated peptide, and the carboxylate groups. A detailed structural analysis by pulsed EPR methods and DFT calculations indicated that there was no significant destabilization for the complexes of the <i>trans</i> isomers. The lower stability of their complexes was explained by the limitation that only the conformer with donor groups in equatorial–equatorial ring positions can bind to copper­(II), whereas both equatorial-axial conformers of the <i>cis</i> isomers are capable of binding. From a consideration of the proton couplings obtained with X-band ¹H HYSCORE, ²H exchange experiments, and DFT, the thermodynamically most stable cyclohexane ring conformer was assigned for all four [CuLH_–1] complexes. For the F-<i>cAC</i>HC case, the conformer did not match the most stable conformer of the free ligand

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1

Incommensurate Modulation and Luminescence in the CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) Red Phosphors

Scheelite related compounds (<i>A</i>′,<i>A</i>″)_<i>n</i>[(<i>B</i>′,<i>B</i>″)­O₄]_<i>m</i> with <i>B</i>′, <i>B</i>″ = W and/or Mo are promising new light-emitting materials for photonic applications, including phosphor converted LEDs (light-emitting diodes). In this paper, the creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescent properties. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> (0 ≤ <i>x ≤</i> 1, 0 ≤ <i>y ≤</i> 1) solid solutions with scheelite-type structure were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder X-ray diffraction. Within this series all complex molybdenum oxides have (3 + 2)­D incommensurately modulated structures with superspace group <i>I</i>4₁/<i>a</i>(α,β,0)­00­(−β,α,0)­00, while the structures of all tungstates are (3 + 1)­D incommensurately modulated with superspace group <i>I</i>2/<i>b</i>(<i>αβ</i>0)­00. In both cases the modulation arises because of cation-vacancy ordering at the <i>A</i> site. The prominent structural motif is formed by columns of <i>A</i>-site vacancies running along the <i>c</i>-axis. These vacant columns occur in rows of two or three aligned along the [1̅10] direction of the scheelite subcell. The replacement of the smaller Gd³⁺ by the larger Eu³⁺ at the <i>A</i>-sublattice does not affect the nature of the incommensurate modulation, but an increasing replacement of Mo⁶⁺ by W⁶⁺ switches the modulation from (3 + 2)­D to (3 + 1)­D regime. Thus, these solid solutions can be considered as a model system where the incommensurate modulation can be monitored as a function of cation nature while the number of cation vacancies at the <i>A</i> sites remain constant upon the isovalent cation replacement. All compounds’ luminescent properties were measured, and the optical properties were related to the structural properties of the materials. CaGd_{2(1–<i>x</i>)}Eu_2<i>x</i>(MoO₄)_{4(1–<i>y</i>)}(WO₄)_4<i>y</i> phosphors emit intense red light dominated by the ⁵D₀–⁷F₂ transition at 612 nm, along with other transitions from the ⁵D₁ and ⁵D₀ excited states. The intensity of the ⁵D₀–⁷F₂ transition reaches a maximum at <i>x</i> = 0.5 for <i>y</i> = 0 and 1