Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Phosphors and Fluorescent Powders

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_141



Phosphor (fluorescent powder) is one kind of inorganic materials which absorbs the energy from electrons, photons, or even other forms of microscopic particles and converts it into visible light. Phosphor (fluorescent powder) has a history of about 100 years and today plays one of the key roles in lighting, display, and imaging. So far there are varieties of phosphors finding their applications in cathode-ray tubes (CRTs), fluorescent lamps, x-ray films, plasma display panels (PDPs), and recently developed white light-emitting diodes (w-LEDs).

The Mechanism of Luminescence

Although phosphors are available in diversity, the mechanisms of luminescence in them are similar, including the following primary items:
  • Transitions of rare-earth ions and transition metal ions

  • Crystal field splitting and nephelauxetic effect

  • Energy transfer

Transitions of Rare-Earth Ions and Transition Metal Ions

Essentially, luminescence of phosphors is a process of electron transitions, which occurs in luminescence center ions (active centers). But not all of the ions can be considered to act as optically active centers, which are usually based on rare-earth ions or transition metal ions. In this section, we will concentrate on these luminescent centers and discuss their energy levels.

Electronic Configuration, Energy Levels, and Transitions of Rare-Earth Ions

The rare-earth ions usually refer to lanthanide ions, Sc3+ and Y3+. The electronic configurations of both trivalent and divalent rare-earth ions in the ground states are shown in Table 1. It can be seen that the 4f orbit is empty for Sc3+, Y3+, and La3+ and fully filled for Lu3+, leading to no energy levels required for luminescence, while the lanthanide ions from Ce3+ to Yb3+ have partially filled 4f orbitals, so that they exhibit their characteristic energy levels, which can induce luminescence processes. Additionally, there are three divalent lanthanide ions that are stable, Sm2+, Eu2+, and Yb2+, and their electronic configurations are the same as those of Eu3+, Gd3+, and Lu3+, respectively. The electronic state is denoted as 2S+1LJ, where S, L, and J are the spin angular momentum, orbital angular momentum, and total angular momentum, respectively.
Phosphors and Fluorescent Powders, Table 1

Electronic configurations and ground state of rare-earth ions


4f electrons

Ground state


4f electrons

Ground state















2 F5/2



















7 F0



7 F0










7 F6




















2 F7/2







The experimentally measured energy levels of trivalent lanthanide ions in lanthanum fluoride (LaF3) are illustrated in Dieke diagram (as shown in Fig. 1). It presents the energy of electronic states 2S+1LJ for trivalent ions in LaF3 and almost any other crystals. The number of levels is determined by the symmetry of the crystal field surrounding the lanthanide ions, and the width of each level is a measure of the crystal field splitting. By taking advantage of the Dieke diagram, one can explain the absorption and emission spectra and assign each spectral peak of rare-earth ions in luminescent materials. That is to say, the transitions of trivalent rare-earth ions can be pointed out according to Dieke diagram, especially the luminescence of rare-earth ions with f-f transitions. But for the rare-earth ions, such as Ce3+ and Eu2+ with f-d transitions, it should be deserved much more attentions for their higher luminous efficiency, which will be discussed as following.
Phosphors and Fluorescent Powders, Fig. 1

Characteristic energy levels of trivalent lanthanide ions, proposed by Dieke and coworkers (Reproduced with permission from Carnall et al. [3]. Copyright 1989, AIP Publishing LLC)

Ce 3+ : The Ce3+ ion has the simplest electron configuration among the rare-earth ions. The 4f1 ground-state configuration is divided into two sublevels, 2F5/2 and 2F7/2, and these two sublevels are separated by about 2,000 cm−1 as a result of spin–orbit coupling. This is the reason for the double structure usually observed in the Ce3+ emission band. The 5d1 excited state configuration is split into two to five components by the crystal field, with the splitting number depending on the crystal field symmetry. The Ce3+ emission is strongly affected by the host lattice through the crystal field splitting of the 5d orbital and the nephelauxetic effect and usually varies from the blue to the red spectral region.

Eu 2+ : The Eu2+ ion has the ground state of 4f7 (8S7/2) and the excited state of 4f65d1. The luminescence is strongly dependent on the host lattice, with the emission color varying in a very broad range, from ultraviolet to red. The abundant emission colors are due to the changes in covalency (nephelauxetic effect) and crystal field splitting from one host to another. With increasing the covalency or crystal field splitting, the 5d energy level of Eu2+ is lowered greatly, resulting in the red shift of the absorption and emission bands.

Transitions of Transition Metal Ions

Transition metal ions are an important class of dopants in luminescent materials and solid-state laser materials, and their emission colors are very abundant. Among the transition metal ions, Mn2+ and Mn4+ are two of the most important and useful luminescent centers in phosphors, which will be discussed below.

Mn 2+ : The emission color of Mn2+ varies from green to orange red, depending on the crystal field strength of the host crystal. The emission spectrum of Mn2+ displays a broad band, and the emission corresponds to the 4T1 (4G) → 6A1 (6S) transition. In general, the emission color is green when Mn2+ is tetrahedrally coordinated, and it is orange red when Mn2+ is octahedrally coordinated. The absorption of Mn2+ is usually low due to the spin-forbidden transition. To improve the absorption of Mn2+, energy transfer mechanisms are employed to sensitize Mn2+. Commonly used sensitizing ions are Sb3+, Pb2+, Sn2+, Ce3+, and Eu2+, which absorb the ultraviolet (UV) light efficiently and then transfer the energy to Mn2+.

Mn 4+ : The emission of Mn4+ is usually in the red spectral region, due to the 2E → 4A2 transition, while the excitation spectral shows broad bands. In CaAl12O19, the emission peak is located at ~656 nm, and it is positioned at ~715 nm in YAlO3. Mn4+-doped K2TiF6 can absorb blue or violet light efficiently and emits at ~632 nm, enabling it to be used in high-color-rendering white LEDs.

Crystal Field Splitting and Nephelauxetic Effect

As it is known, when aforementioned luminescence ions are incorporated in a crystal, the surrounding anions will affect them, especially the luminescence ions with d-electron orbits and transitions. Certainly, it affects the luminescence ions with f-f transitions as well by getting rid of forbidden transitions, which would result in luminescence and even high luminescence efficiency. This effect is called crystal field effect, which shown great influence on luminescence ions. Crystal field splitting is, in fact, the energy level splitting of luminescence ions by the influence of crystal field. Generally, the crystal field splitting is affected by symmetry of coordination anions and the distances between luminescence ions and coordination anions. And crystal field splitting is usually described by the point charge electrostatic model (PCEM), which can be usefully applied especially in the case of coordination in the form of an octahedron, cube, tricapped trigonal prism, and cuboctahedron.

Some electrons of the ligands move into the orbitals of the central ion and reduce the cationic valency. Due to this reduction, the d-electron wave functions expand toward the ligands to increase the distances between electrons, reducing the interaction between them. This effect is called the nephelauxetic effect. This effect may be considered to increase with ligands anion in the order: F < O2− < N3− < Cl < Br < I < S2−.

Energy Transfer

Energy transfer means energy migrating from a higher energy ion to another lower energy ion or even from a host crystal to activators. Thus, three kinds of energy transfer should be included as following:
  • The energy transfer between the same ion species. When the ionic separation becomes smaller, the energy transfer probabilities increases by means of resonant, exchange interaction, and multipolar interaction. As results, the optical excitation is trapped at defects or impurity sites, enhancing non-radiative relaxation, which is the reason for concentration quenching.

  • The energy transfer between different ion species can take place when they have closely matched energy levels. The energy transfer result is that the luminescence of energy-accepted activators enhanced and the luminescence of energy-donated activators weaken. Typically, Ce3+ ions transfer energy to Tb3+ ions in the phosphors CeMgAl11O19:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; etc.

  • Theenergy transfer from a host crystal to activators mainly refers to energy transfer from an excited oxy-anion complex to lanthanide ions. This kind of energy transfer is commonly seen in vanadate, molybdate, tungstate, etc., such as YVO4:Eu3+, CaMoO4: Eu3+, and Y2WO6:Eu3+.


Besides the mechanism of luminescence, characterization of the prepared phosphors is vital to evaluate the practical application. The principal characterization of phosphors can be listed as following:
  • Spectrum

  • Quantum efficiency

  • Thermal quenching

  • Fluorescent lifetime


When light interacts with a phosphor, refraction, absorption, scattering, and luminescence will occur. The luminescence process – excitation and emission – will be discussed and characterized by excitation and emission spectra.

Emission Spectrum

During the luminescence process, the distribution of emission photon number varies with emission wavelength or frequency, that is, the so-called emission spectrum, which is important to judge what color of light emitted by the phosphor and to calculate the color coordinates. Generally, two kinds of emission spectrum are presented: line spectrum for activators with f-f transitions and band spectrum for activators with d-electron transitions. No matter what kinds of emission spectrum, it shows difference with activator species and compositions of host.

Excitation Spectrum

The distribution of emission photon number of the selected wavelength varies with excitation wavelength or frequency. Thus, it, in fact, is a specific emission spectrum, which is used to characterize the effectiveness of excitation. From this spectrum, it is convenient to judge the effective excitation wavelength for phosphors, which is important for users.

Quantum Efficiency

The emission and excitation spectra let people know what light the phosphor emits and the effectiveness of different excitation wavelength, while it still does not know how efficient the energy conversion is. Thus, the quantum efficiency is introduced. The quantum efficiency of a phosphor is how much light (photons) absorbed by the phosphor is converted into luminescence, which is in the range of 0 ~ 1. This is also called internal quantum efficiency. The external quantum efficiency denotes the ratio of the number of photons of the emitted light to that of the incident light on a phosphor. The internal (ηi) and external (η0) quantum efficiency can be computed by the following equations:
$$ {\eta}_{\mathrm{i}}=\frac{{\displaystyle \int \lambda \bullet P\left(\lambda \right)d\lambda }}{{\displaystyle \int \lambda \left\{E\left(\lambda \right)-R\left(\lambda \right)\right.\left.\right\}d\lambda }} $$
$$ {\eta}_0=\frac{{\displaystyle \int \lambda \bullet P\left(\lambda \right)d\lambda }}{{\displaystyle \int \lambda \bullet E\left(\lambda \right)d\lambda }} $$
where E(λ)/, R(λ)/, and P(λ)/ are the number of photons in the spectrum of excitation, reflectance, and emission of the phosphor, respectively.

Thermal Quenching

Thermal quenching of luminescence is the phenomenon in which the luminescence efficiency decreases as the temperature increases due to the increasing phonon–electron interaction. This phenomenon can be explained by configuration coordinate (as shown in Fig. 2). The excited electron can go across the crossing point of ground state and excited state and relax to the ground state non-radiatively, the probabilities of which will increase with the increasing of temperature. The energy difference between the lowest vibrational level of excited state and the crossing point is called activation energy for thermal quenching, which is used to judge the thermal quenching properties of phosphors. The larger value of activation energy means higher energy barrier for thermal quenching. The activation energy can be calculated by the Arrhenius equation:
Phosphors and Fluorescent Powders, Fig. 2

Configurational coordinate diagram for thermal quenching

$$ {I}_T=\frac{I_0}{1+c\ \exp \left(\frac{-\Delta E}{kT}\right)} $$
Here I 0 is the initial emission intensity, I T is the intensity at different temperatures, ΔE is the activation energy of thermal quenching, c is a constant for a certain host, and k is the Boltzmann constant.

Fluorescent Lifetime

Fluorescent lifetime is another important factor, which is not used to judge the phosphors good or bad but limits the applications of phosphors. For display and imaging applications, short fluorescent lifetime is needed as well as high luminescence efficiency and narrow full width at half maximum (FWHM) of emission spectrum. The decay process of the luminescence intensity I(t) after the termination of excitation at t = 0 is generally represented by an exponential function of the elapsed time after the excitation:
$$ I(t)={I}_0 \exp \left(-t/\alpha \right) $$
where α is the decay time constant of the emission and τ = −1/α is named fluorescent lifetime. For convenience, fluorescent lifetime is defined as the duration of I(t) decreasing from I 0 to 1/e I 0 . Generally, light emission due to a forbidden transition has a long fluorescence lifetime, while that due to an allowed transition usually shows a short fluorescence lifetime, but that is not absolute since the electron may be captured by a trap or lattice defect, leading to long afterglow luminescence. Fluorescent lifetime is usually determined by measuring the time-resolved emission spectroscopy, which provides information on crystal structure and the kinetic behavior of luminescent excited states and intermediates as well.

Practical Phosphors

As mentioned above, phosphors are various and have shown varieties of applications in lighting, display, and imaging. However, the lighting industry constitutes the largest consumer of phosphors and produces the largest quantity of phosphor-related products. Thus, the widely used phosphors for fluorescent lamps and w-LEDs are chosen and introduced as following:
  • Phosphors for fluorescent lamps

  • Phosphors for w-LEDs

Phosphors for Fluorescent Lamps [1]

The invention of fluorescent lamp can be traced back to the nineteenth century while the calcium halophosphate phosphor was developed, and the age of fluorescent lamps was truly entered since 1950s. However, the problems of poor color-rendering and high color temperature of this lamp prompted the development of three-band fluorescent lamp, which shows higher color-rendering and even higher luminescence efficiency. The practical phosphors for fluorescent lamps will be introduced briefly as following:

Calcium Halophosphate Phosphors for Traditional Fluorescent Lamps

Crystal structure: These phosphors have an apatite-type structure belonging to the hexagonal system. The Ca2+ ions have two different sites: Ca(I) coordinated by six oxygen and Ca(II) coordinated by halogen ions. Sb3+ and Mn2+ ions are capable of replacing Ca2+ ions at both types of sites. In addition, the Ca2+ ions are possible to be partially replaced by Sr2+, Ba2+, and Cd2+ ions.

Luminescent properties: These series of phosphors are doubly activated by Sb3+ and Mn2+. The Sb3+ emission peak is located at ~480 nm and is not influenced by the kind of halogen in the host. The emission peak of Mn2+ is located at 575 ~ 585 nm, depending on the F:Cl ratio. Energy transfer from Sb3+ to Mn2+ can be observed and show great influence on emission colors by turning Sb3+:Mn2+ ratio. By adjusting appropriate compositions of these phosphors, the color temperatures of the white light can be obtained ranging from 3,000 to 6,500 K. In addition, these series phosphors have almost 90 % of the emission intensity retained, even at 200 °C, without a shift in the emission peak.

Phosphors for Three-Band Fluorescent Lamps

Phosphors utilized for three-band lamps can be grouped into three categories as following:
  1. 1.

    Red phosphor: Y2O3:Eu3+ is the only available red phosphor for three-band lamps mainly due to its nearly perfect luminescence properties, except for the high price.

    Crystal structure: The crystal structure of this phosphor is cubic and Eu3+ ions occupy the two Y3+ sites of C2 and S6 symmetry.

    Luminescent properties: The excitation spectrum has a broad band ranging from 200 to 300 nm, which matches perfectly with 254 nm light emitted by mercury vapor. The emission spectra contains a very narrow and intense emission band at 611 nm (originates from the 5D07F2 transition of Eu3+) with a full width at half maximum of ~7 nm. This phosphor exhibits high color purity, high quantum efficiency (nearly 100 %), excellent thermal quenching properties, and stable chemical properties.

  2. 2.

    Green phosphors: There are three kinds of practicable green phosphors being developed successively, which are CeMgAl11O19:Ce3+,Tb3+; LaPO4:Ce3+,Tb3+; and GdMgB5O10:Ce3+,Tb3+.

    Crystal structures: CeMgAl11O19:Ce3+,Tb3+ belongs to the hexagonal system and has the same magnetoplumbite structure as the PbFe12O19 crystal. LaPO4:Ce3+,Tb3+ belongs to the monazite crystal group. GdMgB5O10:Ce3+,Tb3+ has monoclinic symmetry and is isostructural with LaCoB5O10.

    Luminescent properties: The common characteristics of these phosphors are activated by Tb3+ and adopting Ce3+ as sensitizer, because the absorption lines of Tb3+ in the ultraviolet region does not match the light emitted by mercury vapor, but Ce3+ can absorb the light emitted by mercury vapor efficiently and transfer the energy to Tb3+ with a high efficiency by overlapping the emission spectra of Ce3+ and absorption lines of Tb3+. Thus, these phosphors exhibit the emission of Tb3+ with emission peak at ~543 nm (corresponding to the 5D47F5 transition) and a full width at half maximum of ~10 nm. But by comparing other properties, the CeMgAl11O19:Ce3+,Tb3+ presents much more excellent in quantum efficiency (as high as 97 %), thermal quenching properties, and chemical stability; therefore, the CeMgAl11O19:Ce3+,Tb3+ phosphor is still one of the most important and widely used green phosphors for three-band fluorescent lamps.

  3. 3.

    Blue phosphors: (Sr,Ca,Ba)5(PO4)3Cl:Eu2+ and BaMgAl10O17:Eu2+ are two main classes of blue-emitting phosphors.

    Crystal structures: (Sr,Ca,Ba)5(PO4)3Cl:Eu2+ has an apatite-type structure belonging to the hexagonal system. BaMgAl10O17:Eu2+ has a crystal structure similar to hexagonal β-alumina.

    Luminescent properties: (Sr,Ca,Ba)5(PO4)3Cl:Eu2+ gives a sharp emission spectrum peaking at 447 nm and a broad band excitation spectrum overlapping 200 ~ 400 nm region. Partial replacement of Sr2+ by Ca2+ or Ba2+ results in red shift or blue shift, mainly ascribed to the change of crystal field. Incorporation of a small amount of Ba2+ serves to improve the degradation characteristics; incorporation of a small amount of borate may increase the emission intensity. BaMgAl10O17:Eu2+ also has a broad band excitation spectrum in the ultraviolet region and an emission band with the peak located at ~450 nm. Generally, the luminescent properties can be adjusted by controlling the Al2O3 ratio, Eu2+ concentration, and Sr2+ replacing part of Ba2+. By comparison, BaMgAl10O17:Eu2+ shows higher quantum efficiency (appears to exceed 100 % measured by conventional methods). The relative poor thermal quenching properties are still challenges for both blue phosphors.


Phosphors for w-LEDs

W-LEDs are thought to have brought about a revolution in energy-efficient lighting due to the outstanding advantages such as long lifetime, high energy efficiency, and environmental friendliness. Currently, w-LEDs are commonly generated by the combination of the blue LED chips and yellow-emitting phosphors. But, the white light obtained by this approach would result in low color-rendering index and high correlated color temperature due to the red and green emission deficiency in the visible spectrum. Generally, red-emitting phosphors are added to make up for this deficiency. Thus, the widely used phosphors for w-LEDs can be divided in to two groups as following:

Oxide Yellow Phosphors

  1. 1.

    YAG:Ce3+ garnet phosphor

    Crystal structure: YAG is the abbreviation of Y3Al2Al3O12, which has the classical garnet structure represented by the general formula A3B2X3O12, where A, B, and X are eight, six, and four coordinated with the surrounding O, forming dodecahedron, octahedron, and tetrahedron, respectively. The octahedron and tetrahedron do not share any edge among themselves, but they share edges with at least one dodecahedron.

    Luminescent properties: The excitation spectrum of YAG:Ce3+ is mainly composed of two broad bands with excitation peaks located at 340 and 460 nm, respectively. The emission spectrum shows a very broad band with emission peak at ~535 nm and a full width at half maximum (FWHM) of ~100 nm. The high external quantum efficiency (more than 83 %), excellent thermal quenching properties (more than 87 % of the initial emission intensity remained at 150 °C), and the strong absorption of blue light emitted by blue LED chip made YAG:Ce3+ one of the most important and widely used phosphors for white LEDs. Actually, YAG:Ce3+ is a green-yellow-emitting phosphor, and the emission spectrum in the red region is very weak, which results in the produced white light having a high color temperature. Generally, La3+ and Gd3+ replacing part of Y3+ or increasing Ce3+ concentration may turn the emission spectrum shift toward longer wavelength, but the quantum efficiency decreases radically; meanwhile, the thermal quenching increases.

  2. 2.

    Sr2SiO4:Eu2+ silicate phosphor

    Orthosilicate phosphor has outstanding advantages such as high luminescence efficiency, low cost, and strong excitation in blue to UV region, which have attracted lots of attentions.

    Crystal structure: Sr2SiO4 has two types of crystal structures, which are orthorhombic corresponding to α′ phase and monoclinic corresponding to β phase. The α′ and β phases of Sr2SiO4 are isostructural with Ba2SiO4 and β-Ca2SiO4, respectively. Generally, in order to stable the α′-Sr2SiO4 phase, Ba2+ ions were introduced to replace part of Sr2+ ions. For α′-Sr2SiO4, Sr2+ ions have two kinds of sites, Sr(1) and Sr(2), coordinated by 10 and 9 oxygen atoms, respectively.

    Luminescent properties: The excitation spectrum of Sr2SiO4:Eu2+ shows a broad band covering the spectral region of 200–550 nm with excitation peak at about 387 nm. The emission spectrum shows two bands centered at about 490 and 560 nm, assigning to the luminescence centers occupying two types of Sr sites: Sr(1) and Sr(2), respectively. Due to the weak 490 nm emission peak, the emission spectra of Sr2SiO4:Eu2+ shows emission peak at about 560 nm, which can be shift to longer wavelength by increasing the SiO2 content. Thermal quenching of this kind of silicate phosphor is unsatisfactory for the emission intensity decreases by 62 % when the temperature rises to 150 °C compared to the initial intensity measured at room temperature. Though the luminescence efficiency of Sr2SiO4:Eu2+ is comparable with YAG:Ce3+, the thermal stability and structure stability are still limitations for its applications.


Nitride Red Phosphors [2]

  1. 1.

    M2Si5N8:Eu2+ (M = Ca, Sr, Ba) phosphors

    Crystal structure: Ca2Si5N8 has a monoclinic crystal system with the space group of Cc, whereas both Sr2Si5N8 and Ba2Si5N8 belong to orthorhombic crystal system with the space group of Pmn21. There are two different crystallographic sites for alkaline earth metals. Each Ca2+ ion in Ca2Si5N8 is coordinated by seven nitrogen ions, while Sr2+ and Ba2+ in Sr2Si5N8 and Ba2Si5N8 are coordinated by eight or nine nitrogen ions.

    Luminescent properties: The M2Si5N8:Eu2+ (M = Ca, Sr, Ba) phosphors show very intense red emission under blue light excitation. The emission color changes from orange to deep red depending on the alkaline earth metal ion ratio. The emission spectra all exhibit a broad and asymmetrical band extending from 550 to 850 nm. Upon the 450 nm excitation, Sr2Si5N8:Eu2+ shows an absorption and external quantum efficiency of 87 and 71 %, respectively.

  2. 2.

    CaAlSiN3:Eu2+ phosphor

    Crystal structure: CaAlSiN3 belongs to orthorhombic crystal system with Cmc21 space group. The Ca2+ ions have only one crystallographic site, which is coordinated by five nitrogen ions.

    Luminescent properties: The excitation spectrum covers a broad spectral range of 250–600 nm, showing an extremely broad band and strong absorption of visible light. The emission spectrum extends from 550 to 800 nm, showing a broad band centered at 655 nm with a FWHM value of 93 nm. When the phosphor is excited by the blue light (λ = 450 nm), the external quantum efficiency is up to 70 %. In addition, it has a small thermal quenching. The luminescence is quenched by 10 % of the initial intensity when the phosphor is heated to 150 °C and excited at 450 nm.



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    Yen, W.M., Hajime, Y. (eds.): Phosphor Handbook. CRC Press, Boca Raton (2012)Google Scholar
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    Xie, R.J., Li, Y.Q., Hirosaki, N., Yamamoto, H.: Nitride Phosphors and Solid-State Lighting. Taylor & Francis, Boca Raton (2011)Google Scholar
  3. 3.
    Carnall, W.T., Goodman, G.L., Rajnak, K., Rana, R.S.: A Systematic Analysis of the Spectra of the Lanthanides Doped into Single Crystal LaF3. J. Chem. Phys. 90, 3443 (1989)ADSCrossRefGoogle Scholar

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Authors and Affiliations

  1. 1.National Engineering Research Center for Rare Earth MaterialsGeneral Research Institute for Nonferrous MetalsBeijingChina