Some Orthophosphate Phosphors
The orthophosphate group, is the most common phosphorus oxoanion also known as phosphoric acid. All four oxygen atoms, are usually coordinated to cations in solid orthophosphates leading to strongly bonded, extended structures. The acid orthophosphate ani
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Some Orthophosphate Phosphors
5.1 Introduction The orthophosphate group, PO4 3− , is the most common phosphorus oxoanion also known as phosphoric acid. All four oxygen atoms, are usually coordinated to cations in solid orthophosphates leading to strongly bonded, extended structures. The acid orthophosphate anions, hydrogen phosphate, HPO4 2− , dihydrogen phosphate, and H2 PO4 − are also found in many materials. Almost every metallic element forms an orthophosphate, and a range of oxidation states is stabilized for transition elements. The orthophosphates were the last family of phosphors to be introduced before the present material yttrium vanadate came into service. They are all activated with tin, which must be in the divalent state to give luminescence. All tin-activated phosphors are characterized by their very broad spectral energy emission. Typically, they cannot achieve such a deep red emission as it is possible from previously mentioned materials, but their quantum efficiency is remarkably high, as much as 87 % for magnesium orthophosphate. They are colorless so the light transmission is high, and because the emission is at wavelengths to which the human eye is more sensitive, lamps with remarkably high luminous efficacy can be fabricated. They do, however, leave a lot to be desired in terms of their color rendering properties, but from the early 1960s until the late 1980s many manufacturers offered two different kinds of mercury lamp—types with good color rendering, and types with high efficacy at the expense of color properties. Strontium orthophosphate, (Sr,Mg)3 (PO4 )2 :Sn2+ modified with Mg to adjust the crystal lattice structure was the most commonly employed material in high-efficacy lamps. Its emission peak lies at 630 nm at room temperature, decreasing to shorter wavelengths quite dramatically at lamp operating temperatures but the light emission remains high. By 330 ◦ C the red emission peak has shifted to 570 nm and thus it is clearly evident that color rendering of these lamps suffers. While a magnesium germanate 400 W lamp gives a red ratio of about 7.5 %, the strontium orthophosphate lamps only manage about 5 %. Similar performance was attained from calcium orthophosphate, modified with Mg and this material found preference with some of the Eastern European manufacturers. It has slightly higher K. N. Shinde et al., Phosphate Phosphors for Solid-State Lighting, Springer Series in Materials Science 174, DOI: 10.1007/978-3-642-34312-4_5, © Springer-Verlag Berlin Heidelberg 2013
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5 Some Orthophosphate Phosphors
efficiency again, but red ratio falls further to 4.4 %, really not better than the first MAF type lamps. Meanwhile, Sylvania in the USA developed materials based on calcium-zinc orthophosphates, and a particular 400 W lamp attained an efficacy of 59.5l m/W. This was excellent by comparison with the 49l m/W attainable from redemitting phosphors at the time, and is still impressive compared with the 55l m/W of present-day lamps with vanadate-based coatings. When vanadate coatings
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