The oxonitridosilicates, also called sions (Si + O + N) or silicon-oxynitrides are inorganic ceramic compounds in which oxygen and nitrogen atoms are bound to a silicon atom. A common variant also has aluminium replacing some silicon. They can be considered as silicates in which nitrogen partially replaces oxygen, or as nitridosilicates with oxygen partly replacing nitrogen.
Formation A possible way to make these compounds is to heat metal oxides or carbonates with silicon nitride and silica in a nitrogen atmosphere.
Properties Oxonitridosilicates have diverse structures. At the small scale silicon forms tetrahedral shapes surrounded by four oxygen or nitrogen atoms. With oxygen, one point of a tetrahedron can be bridged to another denoted O, or the oxygen can be a terminal atom denoted O. With nitrogen, it can be shared in more ways: terminal N, bridged between two tetrahedra N, or even three or four: N, N. With nitrogen, tetrahedra can be fused on an edge, or a point, whereas oxygen only connects tetrahedra at a vertex.
The condensation ratio, denoted by K indicates how condensed the tetrahedra are. K is the number of silicon atoms divided by the sum of oxygen and nitrogen. For SiN48− or SIO24+ K is 1/4 and it increases to 1/2 for SiO2.
Use Oxonitridosilicates doped with europium have been used as phosphors in white LEDs, as they convert some blue light to other colours that can mix to white.
Listing nameformulacrystalsystemspacegroupunit cellvolumephosphor colour Eu2+commentsrefLiSiNOLi3SiNO2Li6SiN2O2 Li7SiN3OFm3ma=4.6994Li5SiNO3Na3SiNO2CaSi2O2N2Ca2.89Si2N1.76O4.24 Ca3Si2O4N2cubicPaa = 15.0626 Å V = 3417.45 Å3 Z=24greencolourlessSrSi2O2N2greenLi4Sr4[Si4O4N6]OtetragonalP42/nmca=7.4833, c = 9.8365 V=552.81 Z = 2orangeband gap 3.6 eVSr3Si13Al3O2N21 orthorhombica = 9.037, b = 14.734, c = 7.464 greenY4SiO7N2BaSi2O2N2bluish greenBa3Si6O9N4bluish greenBa3Si6O12N2trigonalP3a = 7.5046 c = 6.4703 greenLa5Si3O12NhexagonalP63/ma=9.7051 c=7.2546 Z=1591.76white UV>261La4SiO7N2monoclinicP121/C1a=8.036 b=10.992 c=11.109 β=110.92 Z=4916.75white UV>283LaSiO2NhexagonalP6c2a=7.310 c=9.550 V=441.95 Z=6white UV>263La3Si8O4N11orthorhombicC2/ca=15.850 b=4.9029 c=18.039 Z=41271.05white UV>243La3[SiON3]OtetragonalI4/mcma = 6.822, c = 11.074 Å, Z = 4La16[Si8N22][SiON3]2triclinicPa = 5.718, b = 11.391, c = 13.435, α = 112.02, β = 90.19, γ = 90.58 Z=2811.1Ce4[Si4O4N6]Ogreen-yellowCe3[SiON3]OtetragonalI4/mcma = 6.723, c = 11.069 Å, Z = 4dark redSr3Ce10Si18Al12O18N36Pr3[SiON3]OtetragonalI4/mcma = 6.6979, c = 11.005 Å, Z = 4Pr4Si2O7N2Gd3[SiON3]OtetragonalI4/mcmSr3Pr10Si18Al12O18N36Sr3Nd10Si18Al12O18N36Sr10Sm6Si30Al6O7N54EuSi2O2N2triclinicP1a=7.095 b=7.246 c=7.256 α=88.69 β=84.77 γ=75.84 Z=4ytterbium silicon oxynitrideYb4Si2O7N2
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