Reaction synthesis and characterisation of lanthanum silicon nitride

The synthesis of lanthanum silicon nitride (LaSi3N5) by direct nitridation of LaSi/Si/Si3N4 powder mixture at 1400 °C is described. Stepwise heat treatment schedule was adopted for the synthesis of LaSi3N5 powder, on the basis of TG–DTA measurements. Except temperature and nitrogen pressure also the ratio of constituents LaSi/Si/Si3N4 has an influence on the exothermicity of the reaction and consequently on the final phase composition of the product. Silicon particle size d ≤ 1 μm is preferable to decrease the time of nitridation. On the other hand, the reaction system must be diluted with Si3N4 to avoid the overheating of powder mix and melting of silicon. When starting from binary LaSi/Si3N4 powder mixture, additional nitridation at higher temperature (1550 °C) was necessary to obtain single phase LaSi3N5 powder. The oxygen content of as-synthesized powders was in the range 1.0–1.2 wt%. ttice parameters and diffraction pattern of LaSi3N5 have been calculated and compared with experimental data. The infrared spectra of LaSi3N5 have been measured and partly described.