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Environmental-friendly lead-free double perovskite (DP) with superior photoluminescence (PL) properties are prospective candidates for advanced lighting and display devices. Herein, a series of Cs 2 NaLnCl 6 (Ln = Ho3+, Er3+, Tm3+) DP microcrystals (MCs) with pumping power-dependent up-conversion (UC) luminescence chroma are synthesized by simple hydrothermal method. XRD and EDS results demonstrate the formation of pure phase Cs 2 NaLnCl 6 (Ln = Ho3+, Er3+, Tm3+). The calculated bond lengths by Generalized-gradient approximation (GGA)-Perdew-Burke-Ernzerhof (PBE) method and XPS binding energy reveal a competition between [NaCl 6 ]3- and [LnCl 6 ]3- octahedra for Cl-. The projected density of state (PDOS) verifies that the electrons mainly concentrate at the [LnCl 6 ]3- octahedra, providing sufficient electrons for GSA process of Ln3+. Under 980 nm excitation, Cs 2 NaHoCl 6 exhibits pure red emission (Ho3+: 5F 5 → 5I 8) and the color vary from orange to red with increase of pump power. Cs 2 NaErCl 6 displays intense green (Er3+: 2H 11/2 /4S 3/2 → 4I 15/2) and red (Er3+: 4F 9/2 → 4I 15/2) emissions, the increase of pump power results in color variation from orange to green. Cs 2 NaTmCl 6 demonstrates blue (Tm3+: 1D 2 → 3F 4 and 1G 4 → 3H 6), green (Tm3+: 1D 2 → 3H 5) and red (Tm3+: 1G 4 → 3F 4 and 3F 2,3 → 3H 6) emissions, with a color regulation from blue to red by increase of pumping power. Variable UC emission chroma based on Cs 2 NaLnCl 6 (Ln = Ho3+, Er3+, Tm3+) DPs by excitation power regulation are proposed to validate their application in lighting and displaying. [Display omitted] • Lanthanide double perovskite Cs 2 NaLnCl 6 microcrystals are synthesized. • Up-conversion luminescence (UCL) under 980 nm excitation have been obtained. • Excitation power regulated emission chromaticity is achieved. • First principles calculations are used to elucidate the UCL mechanisms. [ABSTRACT FROM AUTHOR]

The potential thermoelectric Cu 2 GeSe 3 is limited by the unfavorable exorbitant electrical conductivity and the subsequent high electrical thermal conductivity. In this regard, we highlight a compensatory compositing strategy to synergistically optimize its thermoelectric properties by adding GeSe second-phase. Experimental results demonstrated that the GeSe additive compensated the carrier concentration and mobility to some extent, which suppressed the exorbitant electrical conductivity and led to much reduced electrical thermal conductivity and the final low thermal conductivity in (GeSe) x (Cu 2 GeSe 3) 1–x composites. Meanwhile, due to the decreased carrier mobility, the GeSe additive could also increase the effective mass of the Cu 2 GeSe 3 matrix, which significantly enhanced the Seebeck coefficient and power factor. Finally, with the synergistically optimized electrical and thermal transport properties of Cu 2 GeSe 3 via GeSe addition, the final figure-of-merit ZT values of composites were significantly improved. The peak ZT of the average of three times trials of 0.49 was achieved at 723 K for (GeSe) 0.02 (Cu 2 GeSe 3) 0.98 composite, which was 188% higher than its pristine counterpart. • Compensatory compositing strategy is used to lessen the inherent high κ e of Cu 2 GeSe 3. • The κ tot of composites and ratio of κ e /κ tot remarkably decreased by GeSe composite. • ZT max of ~0.49 at 723 K was achieved for (GeSe) 0.02 (Cu 2 GeSe 3) 0.98 composite. • ZT of the composite increased 1.88 times more than that of the pristine Cu 2 GeSe 3. [ABSTRACT FROM AUTHOR]