Spinel phosphors

Ceramics in aluminate systems are usually formed from cubic crystal systems and this includes spinel and garnet. Rare earth aluminate garnets include the phase YAG (yttrium aluminium garnet), which is an important laser host when doped with Nd(III) and more recently Yb(III). Associated applications include applications as scintillators and phosphors. 

For reasons which will become clear later, we cannot use chromium-spinels or iron-spinels as a phosphor base. Thus, we are limited to aluminum, tin and the like. We will limit our discussion to tin and gallium-based compounds, i.e.- Mga SnO and Mg2Ga04. 

Both of these materials can be "activated" by Mn which also forms a spinel, i.e.- Mn2Sn04. The latter as a phosphor, i.e.- Mg2GKi04 Mn , is used as the source of green light in most copsdng machines today. It is interesting to note that the cubic spinel structure is formed by a majority of dibasic cations like Mg and that many trivalent or quadrivalent cations can form oxide-based anions like stannate. aluminate, titanates, vanadates and the like. 

What this means is that these UV-radiation wavelengths will excite the phosphor which then emits visible green light. When the phosphor is prefired in air, the resulting phosphor does not respond to UV excitation. However, cathode-ray excitation (an electron-beam like a television tube) produces the same green emission. Emission occurs from Mn centers in the spinel structure. These centers are not Intrinsic defects as such since the divalent manganese is able to substitute directly at the Mg sites in the spinel structure. The only difference is the radius of the two cations at the tetrahedral site. It is because of this difference that increasing the Mn concentration leads to less efficient, i.e.- "duller , phosphors. 

A number of other -alumina related phases have been prepared. In some of these the spinel blocks have an increased thickness, the so-called P, P" and P " phases, while in others, the Na or A1 components have been replaced with similar species. Related structures, such as BaMgAlnOiy doped with Eu +, are widely used as phosphors. Crystal-structure studies on such materials show that the defects present depend sensitively upon both temperature and the constituents of the phase. Large replacement ions, lanthanide or alkali metals, tend to occupy the interlayer regions as interstitial defects, but surprisingly, some also enter the spinel blocks as substitutional defects, in association with oxide ion vacancies. Smaller ions occupy the spinel blocks as substitutional point defects. The delicate balance between oxygen interlayer interstitials and spinel block cation vacancies varies with composition. These defect interactions can often be successfully explored by using simulation techniques. Ordering occurs at lower temperatures see Ionic Conductors). 

Lu C.H., Hong H.C., Jagannathan R. Sol-gel synthesis and photoluminescent properties of cerium-ion doped yttrium aluminium garnet powders. J. Mater. Chem. 2002 12 2525-2530 Marsh P.J., Silver J., Vecht A., Newport A. Cathodoluminescence studies ofyttrium silicate cerium phosphors synthesized by a sol-gel process. J. Lumin. 2002 97 229-236 Meyer F., Hempehnann R, Mathur S., Veith M. Microemulsion mediated sol-gel synthesis of nano-scaled MAI2O4 (M = Co, Ni, Cu) spinels from single-source heterobimetallic alkoxide precursors. J. Mater. Chem. 1999 9 1755-1763... 

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