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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.)