Gemei Cai

A peculiar layered 12-fold cationic coordination compound LiInTi2O6: phase relations, crystal structure and color-tunable photoluminescence

Developing new hosts is one of the significant and intriguing aspects in the field of luminescence materials. Herein, we report a novel LiInTi2O6 host for phosphors in solid state lighting. The sub-solidus phase relationships, thermal stability, crystal structure, as well as composition- and temperature-dependent luminescence were investigated and discussed by means of various analytical techniques, including powder X-ray diffraction (XRD), differential scanning calorimetry (DSC), structure solution, photoluminescence excitation (PLE) and emission (PL) spectra, decay lifetime, high-temperature luminescence and chromaticity coordinates. LiInTi2O6 crystallizes in a trigonal unit cell with lattice parameters of a = b = 5.1050(1) A, c = 28.5622(4) A, and Z = 6 in space group Rm (No. 166), consisting of an unusual 12-fold coordination hexagonal prism structural framework. Taking LiInTi2O6 as the host, a series of Dy3+/Tm3+ singly-doped and co-doped phosphors was successfully synthesized. With increasing Dy3+ concentration, the emission colors of LiInTi2O6:Tm3+,Dy3+ phosphors can be appropriately tuned from blue to yellow, going through the white region, based on the principle of energy transfer. Energy transfer efficiencies were calculated and the mechanism was confirmed to follow a resonant-type electric dipole–dipole interaction. A configuration coordinate diagram was employed to explain the thermal quenching behaviour of the white lighting phosphor. Structural characteristics and evolution trends of representative titanates were summarized, providing available information for photo/electrical materials designing.

Isothermal section of Al–Ti–Zr ternary system at 1073 K

Through alloy sampling combined with diffusion triple technique, phase equilibria in Al–Ti–Zr ternary system at 1073 K were experimentally determined with electron probe microanalysis (EPMA). Experimental results show that there is a solid solution β(Ti,Zr) which dissolves Al up to 16.3% (mole fraction). Ti and Zr can substitute each other in most Ti–Al and Al–Zr binary intermediate phases to a certain degree while the maximum solubility of Zr in Ti3Al and TiAl reaches up to 17.9% and 4.0% (mole fraction), respectively. The isothermal section consists of 16 single-phased regions, 27 two-phased regions and 14 three-phased regions. No ternary phase was detected.

Improvement of thermoelectric performance of copper-deficient compounds Cu2.5+δIn4.5Te8 (δ = 0–0.15) due to a degenerate impurity band and ultralow lattice thermal conductivity

Cu–In–Te ternary chalcogenides have unique crystal and band structures; hence they have received much attention in thermoelectrics. In this work we have observed an enhancement in Hall carrier concentration (nH) and ultralow lattice thermal conductivity (κL) when Cu was added to ternary Cu2.5+δIn4.5Te8 (δ = 0–0.15) compounds. The enhancement in nH is attributed to a degenerate impurity band at the G point in the valence band maximum (VBM), while the extremely low κL results from the increased lattice disorder. We thus obtained the minimum κL value of only 0.23 W K−1 m−1 in the sample at δ = 0.1 and 820 K, which is in good agreement with the calculation using the Callaway model. The highest thermoelectric figure of merit ZT is 0.84 for the material at δ = 0.1, which is about 0.38 higher than that of the pristine Cu2.5In4.5Te8.

Increased effective mass and carrier concentration responsible for the improved thermoelectric performance of the nominal compound Cu2Ga4Te7 with Sb substitution for Cu

Although the ternary chalcopyrite compound Cu2Ga4Te7 has relatively high thermal conductivity and electrical resistivity, it has a high carrier concentration, thus making it a good thermoelectric candidate. In this work we substitute Sb for Cu in this compound, aiming at engineering both the electrical and thermal properties. Rietveld refinement revealed that the nominal compounds Cu2−xSbxGa4Te7 (x = 0–0.1) crystallize with the crystal structure of CuGaTe2 with the real compositions deviating from those of their nominal ones. Besides, Sb resides in Cu sites, which increases both the effective mass and the Hall carrier concentration. Therefore, the Seebeck coefficient increases at high temperatures, and the lattice thermal conductivity reduces due to increased phonon scattering from point defects and electron–phonon interactions. As a consequence, the thermoelectric (TE) performance improves with the highest TE figure of merit (ZT) of 0.58 at 803 K. This value is about 0.21 higher than that of the pristine Cu2Ga4Te7.