EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

EPR and photoluminescence properties of green light emitting LaAl11O18:Mn phosphors

The LaAl₁₁O₁₈:Mn²⁺ powder phosphor has been prepared using a self-propagating synthesis. Formation and homogeneity of the LaAl₁₁O₁₈:Mn²⁺ phosphor has been verified by X-ray diffraction and energy dispersive X-ray analysis respectively. The EPR spectra of Mn²⁺ ions exhibit resonance signals with effective g values at g≈4.8 and g≈1.978. The signal at g≈1.978 exhibits six-line hyperfine structure and is due to Mn²⁺ ions in an environment close to tetrahedral symmetry, whereas the resonance at g≈4.8 is attributed to the rhombic surroundings of the Mn²⁺ ions. It is observed that the number of spins participating in resonance for g≈1.978 increases with decreasing temperature obeying the Boltzmann law. Upon 451 nm excitation, the photoluminescence spectrum exhibits a green emission peak at 514 nm due to ⁴T₁ (G)→⁶A₁ (S) transition of Mn²⁺ ions. The crystal field parameter Dq and Racah inter-electronic repulsion parameters B and C have been evaluated from the excitation spectrum.     

Enhanced blue emission and EPR study of LaMgAl11O19:Eu phosphors

Europium doped LaMgAl₁₁O₁₉ phosphor was prepared by the combustion method. The as-prepared and post-treated (1350 °C 10 h 5% H₂+95% N₂) phosphors were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), photoluminescence (PL) and electron paramagnetic resonance (EPR) techniques. XRD patterns show that LaMgAl₁₁O₁₉:Eu phosphors have hexagonal structure. FT-IR spectrum exhibits absorption bands corresponding to the stretching vibration of AlO₄ and AlO₆. Morphological studies reveal that this phosphor has faceted plates of varying sizes and shapes. The as-prepared LaMgAl₁₁O₁₉:Eu phosphor consists of both Eu³⁺ and Eu²⁺ ions. The phosphor exhibits a bright blue emission at 450 nm (4f⁶5d→4f⁷ transition of Eu²⁺). On post-treating the phosphor we are able to enhance the blue emission efficiency by 330%. The process was detected from the evolution of excitation, emission and EPR spectra and the results are discussed.     

Green luminescence and EPR studies on Mn-activated yttrium aluminum garnet phosphor

Yttrium aluminum garnet (Y₃Al₅O₁₂) and Mn activated Y₃Al₅O₁₂ phosphors have been prepared by urea combustion route in less than 5 min. The phosphors are well characterized by powder X-ray diffraction, Scanning electron microscopy and Fourier-transform infrared spectroscopic techniques. Photoluminescence tests on the pure Y₃Al₅O₁₂ showed a strong green emission at 525 nm (2.36 eV) attributed to the strongly allowed transition of F⁺ center whereas in Mn²⁺ activated YAG the green emission at 519 nm is due to the ⁴T₁ (G)→⁶A₁ (S) transition of Mn²⁺ ions. EPR studies have been carried out on Mn²⁺ activated Y₃Al₅O₁₂ phosphor at 300 and 110 K. From EPR spectra the spin-Hamiltonian parameters have been evaluated. The magnitude of the hyperfine splitting (A) indicates that the Mn²⁺ ions are in a moderately ionic environment. The spin concentration (N) and paramagnetic susceptibility (χ) have been evaluated and discussed.     

A Low Temperature (10 K) High-Frequency (208 GHz) EPR Study of the Non-Kramers Ion Mn3+ in a MnMo6Se8 Single Crystal

A high-frequency (208 GHz) electron paramagnetic resonance (EPR) study on Mn³⁺ (3d⁴, S = 2) ions embedded in a MnMo₆Se₈ single crystal has been performed at 10 K. The experimental spectra reveal the presence of only one set of EPR lines from Mn³⁺ ions, whose magnetic axes are oriented along the crystal axes. The spin-Hamiltonian parameters are evaluated by the method of least-squares, fitting all the observed line positions simultaneously, for the three orthogonal orientations of the external magnetic field. The symmetry of the spin Hamiltonian at the site of the Mn³⁺ ions has been deduced from the EPR spectra.     

Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

The red emitting Cr³⁺ activated α-Al₂O₃ powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al₂O₃ phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to ²E _g →⁴A_2g transition of Cr³⁺ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr³⁺ ions are occupying in Al³⁺ sites in octahedral coordination.     

EPR study of Fe3+ and Mn2+ in CeO2 and ThO2

Attempts were made to grow CeO₂ and ThO₂ single crystals doped with transition metal ions. Only Fe³⁺ and Mn²⁺ could be detected by the EPR technique. The EPR spectrum of Fe³⁺ in CeO₂ exhibits the well-known fine structure in cubic fields. The parameters areg=2.0044(1) anda=15.6(1)·10^?4 cm^?1. The hyperfine constantA for⁵⁷Fe in hexahedral coordination was found to be 8.9(1)·10^?4 cm^?1. The EPR spectrum of Mn²⁺ in CeO₂ reveals two cubic Mn²⁺ centers. The parameters for center 1 areg=1.9999(1) andA=86.9(1)·10^?4 cm^?1 and for center 2g=1.9984(1) andA=87.0(1)·10^?4 cm^?1. Heating the Mn doped CeO₂ samples in hydrogen, the Mn²⁺ centers transform from cubic into trigonal centers with approximate values ofg=1.9988(2),A=84.5(6)·10^?4 cm^?1 andD=203(1)·10^?4 cm^?1. The two observed Mn²⁺ centers in ThO₂ exhibita priori axial symmetry with approximate values ofg=2.0006(2),A=88.9(4)·10^?4 cm^?1 andD=33(3)·10^?4 cm^?1.     

Characterization, EPR and luminescence studies of ZnAl2O4:Mn phosphors

ZnAl₂O₄:Mn green light emitting powder phosphors have been prepared by urea combustion technique involving furnace temperatures about 500 °C in a short time (<5 min). The prepared powders were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectrometry and the surface area measurements by a Brunauer-Emmet-Teller (BET) adsorption isotherms. The EPR spectrum exhibits a resonance signal at g≈2.0, which shows a six-line hyperfine structure (hfs). From the EPR spectra the spin-Hamiltonian parameters have been evaluated at room temperature as well as at 110 K. EPR and photoluminescence (PL) studies revealed that manganese ions were present in divalent state and the site symmetry around Mn²⁺ ions is distorted tetrahedral. The spin concentration (N), the paramagnetic susceptibility (χ) and the zero-field splitting parameter (D) have been evaluated and discussed. The green emission at 511 nm in ZnAl₂O₄:Mn phosphor is assigned to a transition from the upper ⁴T₁→⁶A₁ ground state of Mn²⁺ ions.     

EPR and luminescence properties of LiGa5O8:Mn green emitting phosphor

Green emitting LiGa₅O₈:Mn powder phosphor has been prepared in a short time by solution combustion method. Powder X-ray diffraction pattern indicated a dominant phase of LiGa₅O₈ with another secondary LiGaO₂ phase. Morphology aspects were studied by using field emission scanning electron microscopy. Upon UV light excitation (296 nm), the phosphor exhibits a strong green luminescence (510 nm), which corresponds to the ⁴T₁→⁶A₁ transition of Mn²⁺ ions in an environment close to tetrahedral symmetry. EPR spectrum exhibits resonance signals characteristic of Mn²⁺ ions. It is observed that the spin-Hamiltonian parameters g and A do not vary with temperature. The magnitude of the hyperfine splitting constant (A) in the present study indicates that there exists a moderately covalent bonding between Mn²⁺ ions and the surrounding ligands. The zero-field splitting parameter (D), spin concentration (N) and paramagnetic susceptibility (χ) have also been evaluated.     

Spectral studies on Mn ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption, and luminescence spectral studies of Mn²⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xMnO₂ (x=1.0, 2.0, 3.0, 4.0, and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g value at g_eff≈2.02 with six line hyperfine structure. A weak resonance signal with effective g value at g_eff≈4.3 is also observed for higher concentrations of Mn²⁺ ions. The EPR spectra of x =3.0 mol% of Mn²⁺ in sodium-lead borophosphate glass sample have been studied at various temperatures. It is observed that the resonance signal intensity decreases with increase in temperature. The optical absorption spectrum exhibits bands characteristic of Mn²⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibits single broad band in the green region.     

Luminescent properties of La2LiTaO6:Mn4+ and its application as red emission LEDs phosphor

A novel phosphor, Mn⁴⁺ doped La₂LiTaO₆, was developed by solid-state reaction method. The luminescent spectra and emission efficiencies of La₂LiTaO₆:Mn _x ⁴⁺ (x = 0.001, 0.003, 0.005 and 0.01) were discussed. The effects of co-doped charge compensation ions, M = Mg²⁺, Ca²⁺, Na⁺, were investigated, respectively. The excitation spectra indicated that La₂LiTaO₆:Mn⁴⁺ could be effectively excited by both NUV light and blue light. The emission spectra of the phosphor exhibit a broadband ranging from 670 to 720 nm with the maximum at about 709 nm in deep red region. The co-doping of Mg²⁺ could significantly improve the luminescent properties of La₂LiTaO₆:Mn⁴⁺. Thus, phosphor La₂LiTaO₆:Mn⁴⁺, Mg²⁺ can serve as a key component to improve color rendering of blue-chip white-LEDs.     

EPR and photoluminescence properties of Mn-activated zinc gallate phosphor prepared by urea combustion route and post heat treatment

Mn²⁺ activated ZnGa₂O₄ powder phosphors have been prepared by urea combustion route. Powder X-ray diffraction and scanning electron microscopy (SEM) techniques have been used to characterize the as-prepared and post-treated (900 °C, 3 h) phosphors. The morphology shows small particles, voids and pores, with non-uniform shapes and sizes. The EPR spectrum exhibits an intense resonance signal at g≈1.985 with a sextet hyperfine structure (hfs) besides a weak broad signal at g≈4.05 and a hump near g≈2.27. The g≈1.985 resonance is due to Mn²⁺ ions in an environment close to tetrahedral symmetry. The resonances at g≈4.05 and 2.27 are attributed to the rhombic surroundings of the Mn²⁺ ions. The spin concentration (N) and the paramagnetic susceptibility (χ) are evaluated and discussed. It is observed that the intensity of the resonance signal at g≈1.985 increases with decrease in temperature obeying the Boltzmann law. Upon post treatment the intensity of the green emission (λ_em=528 nm,⁴T₁→⁶A₁ transition of Mn²⁺ ions) has been increased to 3.35 times and a red shift has been observed.     

Mn activated MgSrAl10O17 green-emitting phosphor—A luminescence and EPR study

This paper reports on the luminescence and electron paramagnetic resonance (EPR) investigations on MgSrAl₁₀O₁₇:Mn²⁺ green-emitting phosphor. Single-phase MgSrAl₁₀O₁₇ was successfully synthesized by the one-step solution combustion route without the need for post-annealing at a higher temperature. Crystallization of the powder was confirmed by X-ray diffraction. The luminescence of Mn²⁺- activated MgSrAl₁₀O₁₇ shows a strong green-emission peak around 515 nm due to the ⁴T₁→⁶A₁ transition of Mn²⁺ ions under the excitation (453 nm). The EPR spectra of Mn²⁺ ions exhibit a sextet hyperfine structure centered at g ≈1.995. The Mn²⁺ ion occupies Mg sites which are in tetrahedral symmetry. The magnitude of the hyperfine splitting (A) indicates that Mn²⁺ is in a moderately ionic environment. The number of spins participating in resonance (N), the paramagnetic susceptibility (χ) and the zero-field splitting parameter (D) have been evaluated and discussed.     

High-field EPR Study of the Effect of Chloride on Mn2+ Ions in Frozen Aqueous Solutions

The effect of chloride concentration on Mn²⁺ (S = 5/2, I = 5/2) ions in frozen aqueous solutions is studied by high-field high-frequency electron paramagnetic resonance (HFEPR). The usually six sharp lines characteristic of Mn²⁺ ions, arising from the m _s  = ?1/2 → 1/2 transition, is modified by the addition of Cl^? anions and the six resonances become much broader and more complex. This new feature likely arises from the ligation of one Cl^? anion to a hydrated Mn²⁺ ion forming a [Mn(H₂O)₅Cl]^? complex. This complex increases linearly with Cl^? concentration with an association constant of K _{a, apparent} = 61 M^?1. The structure of the putative chloride complex was studied using density functional theory calculations and the expected zero-field interaction of such a manganese center was calculated using the superposition model. The predicted values were similar to those determined from the simulation of the spectrum of the m _s  = ?5/3 → ?3/2 transition of the chloride complex. This effect of Cl^? anions occurs at biologically relevant concentration and can be used to probe the Mn²⁺ ions in cellular and protein environments.     

EPR and Optical Absorption Study of Cr3+-Doped Dipotassium Tetrachloropalladate

X-band electron paramagnetic resonance (EPR) study of Cr³⁺-doped dipotassium tetrachloropalladate single crystal is done at liquid nitrogen temperature. EPR spectrum shows two sites. The spin-Hamiltonian parameters have been evaluated by employing hyperfine resonance lines observed in EPR spectra for different orientations of crystal in externally applied magnetic field. The values of spin-Hamiltonian and zero-field splitting (ZFS) parameters of Cr³⁺ ion-doped DTP for site I are: g _x  = 2.096 ± 0.002, g _y  = 2.167 ± 0.002, g _z  = 2.220 ± 0.002, D = (89 ± 2) × 10^?4 cm^?1, E = (16 ± 2) × 10^?4 cm^?1. EPR study indicates that Cr³⁺ ion enters the host lattice substitutionally replacing K⁺ ion and local site symmetry reduces to orthorhombic. Optical absorption spectra are recorded at room temperature. From the optical absorption study, the Racah parameters (B = 521 cm^?1, C = 2,861 cm^?1), cubic crystal field splitting parameter (Dq = 1,851 cm^?1) and nephelauxetic parameters (h = 2.06, k = 0.21) are determined. These parameters together with EPR data are used to discuss the nature of bonding in the crystal.     

EPR study of the impurity paramagnetic centres in (CaO−Ga2O3−GeO2) glasses

The X-band EPR spectra of Cr³⁺, Mn²⁺, and Fe³⁺ impurity ions in glasses of (CaO?Ga₂O₃?GeO₂) system are investigated in the 77÷300 K temperature range. The experimental data analysis yields the following results: (i) Impurity chromium ions are incorporated into the (CaO?Ga₂O₃?GeO₂) glasses network in Cr³⁺ (3d³,⁴F_3/2) paramagnetic valence state only and occupy the strong distorted oxygen coordinated octahedral sites. (ii) For all activated and non-activated (CaO?Ga₂O₃?GeO₂) glasses the iron impurity is present at concentration roughly 0.01 wt.%. Isotropic EPR signals atg _eff=4.29 andg _eff=2.00 are assigned to Fe³⁺ (3d⁵,⁶S_5/2) ions in the sites with strong rhombic distortion and in the sites with nearly cubic symmetry respectively. (iii) The manganese EPR spectrum in (CaO?Ga₂O₃?GeO₂) glasses is weakly dependent on temperature, doping procedure as well as manganese concentration. EPR spectra of impurity manganese ions in glasses with Ca₃Ga₂Ge₃O₁₂ and Ca₃Ga₂Ge₄O₁₄ compositions are virtually identical and belong to Mn²⁺ (3d⁵,⁶S_5/2) ions. Impurity manganese ions are incorporated into the (CaO?Ga₂O₃?GeO₂) glass network as isolated Mn²⁺ centres and clusters of Mn²⁺ ions.     

EPR and Optical Absorption Study of Cu2+-Doped Diammonium Hexaaqua Magnesium Sulphate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺ ions doped in diammonium hexaaqua magnesium sulphate single crystal over the temperature range of 4.2–320 K is reported. Copper enters the lattice substitutionally and is trapped at two magnetically equivalent sites. The spin Hamiltonian parameters are evaluated at 320, 300, 77, and 4.2 K. The angular variations of the resonance lines in three mutually perpendicular planes ab, bc* and c*a are used to determine principal g and A values. The observed spectra are fitted to a spin Hamiltonian of rhombic symmetry with parameters of Cu²⁺ at 77 and 4.2 K: g _xx  = 2.089, g _yy  = 2.112, g _zz  = 2.437 (±0.002) and A _xx  = 38, A _yy  = 14, A _zz  = 110 (±2) × 10^?4 cm^?1. The ground state wave function of Cu²⁺ ion in this lattice is determined. The g-factor anisotropy is calculated and compared with the experimental value. The optical absorption spectra of the crystal are also recorded at room temperature. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. By correlating the optical and EPR data, the nature of bonding in the complex is discussed. The temperature dependence of the g values is explained to conclude the occurrence of both static and dynamic Jahn–Teller effects over the temperature range of investigation.     

Green and red photoluminescence from ZnAl2O4:Mn phosphors prepared by sol–gel method

Luminescence investigations of Mn-activated ZnAl₂O₄ phosphors prepared by using sol–gel method were described. The phosphor was characterized by X-ray diffraction (XRD) and electronic paramagnetic resonance (EPR). The EPR spectra of the samples suggested that Mn ions possessed homogeneous distribution in ZnAl₂O₄ phosphors. Photoluminescence studies of the prepared phosphors showed green and red emissions. The red emission became weaker, and vanished at last with sintering temperature increasing from 600 to 900 °C in reducing atmosphere, while the intensity of green emission peak increased. Furthermore, when the phosphor was sintered at 900 °C in air, the intensity of red and green emissions decreased, but the value of intensity ratio increased. It suggested that the green emission resulted from Mn²⁺ and the red emission resulted from Mn⁴⁺.     

The valence states of manganese ion in potassium-borate oxide glasses

EPR and magnetic susceptibility measurements have been performed on xMnO·(1-x) [2B₂O₃·K₂O] with 0?x?50 mol %. The X-ray diffraction analysis showed that in this glass system homogeneous glasses are formed up to x = 70 mol %.EPR and magnetic susceptibility data have shown that in the glasses with x ? 5 mol % only Mn²⁺ ions are present as magnetically isolated species. EPR spectra are modified with the increasing of manganese ions content. In the concentration range 0.5 ? x ? 5 mol %, the spectra are characterized by appearance of three resonance absorptions at g ? 4.3 and g ? 3.3 without hyperfine structure, and at g ? 2.0 with hyperfine structure. For the glasses with x >62; 5 mol %, the resonance spectra are characterized by the appearance of the broad line at g ? 2, characteristic for clustered ions. The magnetic susceptibility data suggest the appearance of superexchange interactions for x >62; 5 mol %. From Curie constant values and qualitative chemical analysis we have established that in the glasses with x ? 10 mol % both, Mn²⁺ and Mn³⁺ ions are present.     

Luminescence and EPR studies of Eu doped BaAl12O19 blue light emitting phosphors

Europium doped BaAl₁₂O₁₉ powder phosphors have been synthesized by combustion process within few minutes. The phosphors have been characterized by XRD, SEM, FT-IR, EPR and PL techniques. The EPR spectrum exhibits an intense resonance signal at g=1.96 characteristic of Eu²⁺ ions. In addition to this two weak resonance signals have been observed at g=2.28 and g=4.86. The population of the spin levels (N) for the resonance signal at g=1.96 is calculated as a function of temperature. By post-treating the phosphor at 1350 °C under a reducing atmosphere, it is observed that the population of spin levels has been increased five times. The excitation spectrum shows a peak at 326 nm with a shoulder at 290 nm. Upon excitation at 326 nm, the emission spectrum exhibits a well defined broad band with maximum at 444 nm emitting a blue light corresponding to 4f⁶5d→4f⁷ transition. The luminescence intensity also has been enhanced to 60% by post-treating the phosphor at 1350 °C under a reducing atmosphere.