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Spinel-type CoV2O4 compound shows cubic-to-tetragonal phase transition with a very small distortion (1 – c/a

Green-emitting Tb3+-doped pollucite (Cs1-3xTbx)AlSi2O6 (x = 0.01–0.13) phosphors were synthesized by the high-temperature solid-state reaction method. Structural characterization of the prepared (Cs1-3xTbx)AlSi2O6 phosphors was performed using powder X-ray diffraction and X-ray photoelectron spectroscopy. The excitation and emission spectra showed green emission at ~543 nm because of the 5D4→7F5 transition under excitation at 238 nm. The optimum doping concentration of the Tb3+ ions was x = 0.07, and the quantum efficiency was 83.6%. The temperature dependence of emission spectra for x = 0.07 revealed that 92.7% and 78.4% of the initial value of the emission intensity at 300 K are retained at 400 K and 500 K, respectively. These results show that the green-emitting phosphors synthesized herein have good performance and potential applications in ultraviolet-C excited white light-emitting diodes.

The high-pressure (HP) crystal polymorph of a fluorinated ionic liquid (fIL) was determined using X-ray diffraction and Raman spectroscopy. The irreversible crystal polymorph of 1-butyl-3-methylimidazolium perfluorobutanesulfonate ([C4mim][PFBS]) was observed upon compression and decompression using X-ray diffraction. Conformational variance/invariance of [C4mim]+ and [PFBS]− in the Raman spectra corresponded to the HP crystal polymorph. By the degrees of freedom of the conformations and their conformational flexibilities, the conformational variance was switched at the specific pressure for a volume contraction under HP, at which point the crystal system changed. The HP crystal polymorph was entirely different from the low-temperature crystal polymorph.

A series of Eu2+ substituted pollucite [(Cs1-xEu1-2x)AlSi2O6 (CASO:xEu) (x = 0.001–0.08)] phosphors were synthesized by a high-temperature solid-state reaction method in a reducing atmosphere and characterized by X-ray diffraction, photoluminescence (PL), and PL excitation (PLE) measurements. Under excitation at 275 nm, CASO:xEu phosphors exhibit an emission band that shifts from the bright blue region with color coordinates of (0.159, 0.247) to the bright blue-green region with color coordinates of (0.225, 0.421) with increasing x. Such tunable luminescence color was attributed to the crystal field splitting effect of Eu2+ ions because the substitution of Eu2+ created vacancies at Cs sites, and fluctuations occurred at the positions occupied by the Eu2+ ions. Studies on the temperature dependence of luminescence for x = 0.01, 0.03 0.05, and 0.07 in CASO:xEu reveal that 92.5%, 85.3%, 74.9%, and 79.8% of the initial value of the luminescence intensity at 300 K are retained at 400 K, respectively. Tuning the emission spectra was possible by varying the value of x while maintaining a high thermal stability.

Hidden crystal phases of fluorinated ionic liquids (fILs) at low temperature were clarified by simultaneous measurements using X-ray diffraction and differential scanning calorimetry. The fILs studied are 1-alkyl-3-methylimidazolium perfluorobutanesulfonate, [Cnmim][PFBS] (n = 4, 6, and 8). The crystal polymorphs of all [Cnmim][PFBS] appeared both upon cooling and heating. The phase varieties of fILs increased with an increase in the alkyl chain length, n. Density functional theory calculations provided the anisotropic electrostatic potential surface of the [PFBS]− anion. The complicated phase behaviors of fILs, including metastable phases, were derived from two conformations and the anisotropic electrostatic potential surface of the [PFBS]− anion.

We report reversible structural transformation that occurs in two ladder compounds: Cu2CO3(ClO4)2(NH3)6 (1) and Cu2CO3(ClO4)2(NH3)5(H2O) (2), when they are exposed to gaseous vapors. The ladder structures of both 1 and 2 consist of two Cu2+ ions and one CO32&minus, ion. In 1, the Cu2+ ions are coordinated by three NH3 molecules on each side, while those in 2 are coordinated by three NH3 molecules on one side, and two NH3 molecules and one H2O molecule on the other side. We demonstrated reversible transformation of 1 and 2 via the exposure of 1 to H2O vapor and the exposure of 2 to NH3 vapor using a simple bench-scale method. The minor structural change observed led to a significant difference in physical properties, which we observed using several methods.

In frustrated spinel oxide $\mathrm{Co}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, the knowledge of orbital, lattice, and spin structures has been fragmentary thus far. To investigate the structural and magnetic phase transitions in $\mathrm{Co}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, we performed high-resolution neutron powder diffraction, single crystal synchrotron radiation diffraction, and magnetization and specific heat measurements for high-quality samples. Extremely small crystal distortion from cubic to tetragonal phase ($1\ensuremath{-}c/al0.06%$) was observed using the high-resolution neutron diffraction measurement below ${T}^{*}\ensuremath{\sim}95\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where a cusp was observed in the temperature dependence of magnetization. The single crystal diffraction experiment revealed that the structural phase transition accompanied by a change in the space group from $I{4}_{1}/amd$ to $I{4}_{1}/a$ occurred at ${T}_{2}=59\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where the phase transition was observed in the specific heat measurement. Crystal and magnetic structure analysis was carried out with the neutron data, and the results suggest that the magnetic phase transition from collinear to noncollinear magnetic ordering occurs at ${T}^{*}$ and the antiferro-orbital ordering occurs below ${T}_{2}$. We discuss the observed small crystal distortion and orbital characteristics in the light of the boundary between localization and delocalization.