Paramagnetic defects induced by mechanical stress in calcium sulfide phosphor

Abstract

The effect of uniaxial pressing and grinding on pure and europium-doped CaS powders prepared by the alkaline polysulfide flux method is studied by electron spin resonance (ESR). F+ centers are generated in the bulk of CaS particles and their ESR spectra can be easily separated from those of other paramagnetic species by using a 90" out-of-phase detection. This is possible because of the very long transverse relaxation time Ts of F+ centers. It is thought that the primary defects induced by mechanical stress are bulk F centers, which are next partially converted into Ff centers by reduction of metal ion impurities (Eu3' ,Crs',Cu2') and native hole centers. Heat treatments in air of mechanically stressed powders show that F+ centers firstly diffuse to the particle surface forming low-symmetry F,f centers for temperatures ranging from 300 to 700 K. Atmospheric oxygen then transforms F: centers into ESR silent O-ions except if they are stabilized by Naf impurities originating from the flux, For temperatures higher than 700 K, these centers are then converted into paramagnetic SO, centers. The reason for the strong decrease of Eu2+ emission intensity (X=645 nm) following mechanical stress is discussed. The study of the fine structure of Mn2' natural impurity shows that local order is strongly disturbed in CaS particles by mechanical stress. 0 1995 American Institute of Physics.

Synthesis and Characterization of High-Efficiency Nanocrystal Up-Conversion Phosphors: Ytterbium and Erbium Codoped Lanthanum Molybdate

“…This was because grinding strongly influences the intrinsic point defect concentration, which leads to a decrease of the emission intensity. 26 This phenomenon has also been observed for europium-doped CaS 26 and rare-earth-metal oxysulfides. 27…”

CW EPR Investigation of Red‐Emitting CaS:Eu Phosphors: Rationalization of Local Electronic Structure

“…There is no clear evidence of any Mn 2+ signal initially present in the native sample, in contrast to previous reports. [12,15] A magnified view of the low field |m I | = 5/2 signal is provided in Figure S3 (Supporting Information) for reference.…”

“…Clearly, due to the influence of particle size, morphology, and levels of defects or impurities on the photoluminescent properties of the material, the synthesis method and any subsequent treatments require careful consideration, since subtle changes in the experimental conditions can have a substantial effect on the final functional properties of the material. A number of studies have reported the presence of various intrinsic point defects in these alkaline earth sulfides, [12,[15][16][17][18] such as S 2− vacancies, which can act as shallow electron traps (≈0.26 eV) below the conduction band of CaS. [16] Other S 2− vacancies, stabilized by Ca 2+ interstitial centers, were also evidenced by EPR and thermoluminescent measurements.…”

F?c Centres in Plastically Deformed CaS Polycrystals

“…The idea about the aggregates of cation± anion vacancies was only mentioned but not proved, as data on cation vacancies were absent at that time, and the centres observed had not been investigated thoroughly. Similar EPR signals were detected later [6] in plastically deformed CaS. The EPR lines of F À c centres were not resolved, and some other line of a defect with a different thermal stability was included into the spectrum.…”