YTTRIUMNITRIDOSILICATE - APATITE Y5(SiO4)3N - AN AUXILLIARY PHASE IN Si3N4 CERAMICS
W. W. Schmahl1, M. Györfi1, K.G. Nickel1, A. Hewat2
1Institut f. Mineralogie der
Universität Tübingen, Wilhelmstraße 56, D-72074 Tübingen
2A. Hewat, Institut Laue-Langevin,
Avenue des Martyrs, BP 156, F-38042 Grenoble
e-mail: wwschmahl@uni-tuebingen.de
Keywords: Yttriumnitridosilicate, Apatite, Silicon-Nitride Ceramics, Neutron Diffraction
Although Y5(SiO4)3N occurs as an auxilliary phase in Si3N4 ceramics as a host for sinter additives such as Y2O3, it's structure had so far not been examined in detail.
We heated a pelletized powder mixture of Si3N4, SiO2, and Y2O3 with nominal composition Y5(SiO4)3N in flowing nitrogen in a graphite furnace to 1600°C. The pellet was reground with mortar and pestle and reheated three times. The resulting product was examined with neutron powder diffraction on D1A at ILL.
We did not obtain a phase-pure product but a mixture of three phases:
1. "Apatite-type" Y5(SiO4)3N (P63/m, a
= 9.3644(6) A, c = 6.7699(2) A)
2. Cuspidine-type Y4Si2O7N2 (P21/c, a = 7.5598(5) A, b =
10.4525(7) A, c = 10.7519(8) A, b = 69.931(5)°),
3. Yttriumorthosilicate Y2SiO5 (I2/a, a = 10.425(1) A, b = 6.730(7) A, c = 12.495(1) A,
b =
102.766(9)°).
The presence of the two impurities with large monoclinic unit cells complicated the analysis considerably, but we were able to refine the structure of Y5(SiO4)3N and that of Y4Si2O7N2. The crystal structure data of Y2SiO5 were taken from [1] and were kept fixed.
In the apatite-related structure of Y5(SiO4)3N the nitride ion occupies a different void than the OH- or F- ions in phosphate apatites, i.e. it is located near (0,0, 0.25) rather than (0, 0, 0), a displacement of 1.5 A. Further, the nitride is positionally disordered (split), occupying z = 0.22 and z = 0.28.
The literature crystal data for cuspidine-type Y4Si2O7N2 [2] were quite wrong and based on wrong indexing of the powder pattern. We managed to index the Y4Si2O7N2 peaks correctly in the neutron powder pattern, having subtracted the Y5(SiO4)3N pattern by Rietveld refinement and found the correct lattice parameters. No structure data were previously available for this compound. We refined the structure with the parameters of cuspidine, Ca4Si2O7F2, as starting parameters [3].