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ZnAl2−xCrxO4 (x = 0 and 0.05) samples were synthesized via a high-temperature solid-state reaction method. The structure, photoluminescence properties, EPR measurements, thermal stability, and chromaticity diagram of the far-red phosphor ZnAl1.95Cr0.05O4 were investigated. These measurements have enabled us to study the Cr3+ transitions and the site symmetry of Cr3+ in the ZnAl2O4 host lattice and examine the suitability of ZnAl1.95Cr0.05O4 for plant growth application. According to optical and EPR measurements, Cr3+ ions substitute Al3+ ions with D3d symmetry in the ZnAl2O4 host. PLE measurement indicates that upon excitation at 390 nm and 530 nm, the far-red phosphor ZnAl1.95Cr0.05O4 exhibited bright far-red emission around 687 nm. Photoluminescence phenomena show a sharp R line origin from the sublevels of the 2Eg(2G) → 4A2(4F) transition in Cr3+ ions. The 2Eg level was split into 2Eg (g) and 2Eg (2Āg) levels in the distorted crystal field environment, and the sharp R line in the ZnAl2O4 matrix was split into R1 and R2 lines. In this paper, the temperature-dependent luminescence characteristics of ZnAl1.95Cr0.05O4 have been investigated. As the temperature increased from 300 K to 440 K, a slight decrease in the intensity of the R1 and R2 lines was observed under excitation at 390 nm. The experimental results show that the ZnAl1.95Cr0.05O4 phosphors exhibit a nearly zero-thermal-quenching behavior. The CIE chromaticity coordinates of the ZnAl1.95Cr0.05O4 phosphor were located at the boundary of the chromaticity diagram, signifying that the phosphors possessed high color purity. The emissions of the ZnAl1.95Cr0.05O4 phosphor match well with the PFR absorption of phytochromes in plants. The investigation indicates that ZnAl1.95Cr0.05O4 is a potential far-red phosphor matching ultraviolet (UV) LED chips for plant growth applications. [ABSTRACT FROM AUTHOR]

ZnAl 2- x Cr x O 4 ( x = 0 and 0.05) samples were synthesized via a high-temperature solid-state reaction method. The structure, photoluminescence properties, EPR measurements, thermal stability, and chromaticity diagram of the far-red phosphor ZnAl 1.95 Cr 0.05 O 4 were investigated. These measurements have enabled us to study the Cr 3+ transitions and the site symmetry of Cr 3+ in the ZnAl 2 O 4 host lattice and examine the suitability of ZnAl 1.95 Cr 0.05 O 4 for plant growth application. According to optical and EPR measurements, Cr 3+ ions substitute Al 3+ ions with D 3d symmetry in the ZnAl 2 O 4 host. PLE measurement indicates that upon excitation at 390 nm and 530 nm, the far-red phosphor ZnAl 1.95 Cr 0.05 O 4 exhibited bright far-red emission around 687 nm. Photoluminescence phenomena show a sharp R line origin from the sublevels of the 2 E g ( 2 G) → 4 A 2 F) transition in Cr 4 F) transition in Cr 3+ ions. The 2 E g level was split into 2 E g (Ē g ) and 2 E g (2Ā g ) levels in the distorted crystal field environment, and the sharp R line in the ZnAl 2 O 4 matrix was split into R 1 and R 2 lines. In this paper, the temperature-dependent luminescence characteristics of ZnAl 1.95 Cr 0.05 O 4 have been investigated. As the temperature increased from 300 K to 440 K, a slight decrease in the intensity of the R1 and R2 lines was observed under excitation at 390 nm. The experimental results show that the ZnAl 1.95 Cr 0.05 O 4 phosphors exhibit a nearly zero-thermal-quenching behavior. The CIE chromaticity coordinates of the ZnAl 1.95 Cr 0.05 O 4 phosphor were located at the boundary of the chromaticity diagram, signifying that the phosphors possessed high color purity. The emissions of the ZnAl 1.95 Cr 0.05 O 4 phosphor match well with the PFR absorption of phytochromes in plants. The investigation indicates that ZnAl 1.95 Cr 0.05 O 4 is a potential far-red phosphor matching ultraviolet (UV) LED chips for plant growth applications.

Solid-state and sol-gel procedures were used to prepare ZnAl 1.95 Cr 0.05 O 4 nanocrystal spinels. From the results obtained by X-ray diffraction (XRD) and transmission electron microscopy (TEM), it can be concluded that the samples prepared by sol-gel synthesis are better crystallized than the ones resulting from the solid-state method. Studies by spectroscopy of impedance were done in function of frequency (40-10 7 Hz) and temperature (540-680 K) in the sample prepared by sol-gel synthesis. The electrical conductivity spectra obey Jonscher's law and two models were observed studying the variation of the exponent 's' as a function of temperature, Correlated Barrier Hopping (CBH) and Non-overlapping Small Polaron Tunnelling (NSPT). The predominant conduction mechanism is bipolaron hopping. The scaling behavior of conductivity spectra was checked by Summerfield scaling laws. The time-temperature superposition principle (TTSP) points to a common transport mechanism working for the low and middle frequency ranges. The scaling mechanism fails in the high-frequency ranges suggesting that conduction dynamics, and the usual hopping distance of mobile species, have changed. The values obtained for the activation energy from the hopping frequency, conductivity σ dc , bulk resistance R gb , and relaxation ( f max ), in the temperature range of 540-680 K, are very close. A higher and negative temperature coefficient of resistivity (TCR coefficient) equal to -2.7% K -1 is found at 560 K. This result shows that our compound is suitable for uncooled infrared bolometric applications and infrared detectors., Competing Interests: There is no conflict of interest., (This journal is © The Royal Society of Chemistry.)