Polymorphism of the Tb2Ni7 binary compound solubility of lithium and magnesium in α-phase

Abstract

The solubility of lithium and magnesium in the binary intermetallic compound Tb 2 Ni 7 and its polymorphism were investigated by X-ray powder diffraction, X-ray fluorescent spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The alloys for investigation were synthesized by arc melting of pure components taken in stoichiometric composition with the exception of Li and Mg which were taken in excess of 5 wt. % under purified argon atmosphere and further annealing at 400 °C for two months in sealed evacuated silica tubes. Morphology of the alloys surfaces was studied using scanning electron microscope TESCAN Vega3 LMU. Quantitative compositions of the powders were established using Oxford Instruments energy-dispersive X-ray analyser (Aztec ONE system). X-ray fluorescent spectroscopy (spectrometer ElvaX Pro) was used for investigation of the integral composition of the alloys. X-ray phase analysis was carried out on powder data obtained on diffractometer DRON-2.0M (Fe K α -radiation). We confirmed that the Tb 2 Ni 7 binary compound exists in two polymorphic modifications: low-temperature α -Tb 2 Ni 7 (structure type Ce 2 Ni 7 , space group P 6 3 / mmc ) and high-temperature β -Tb 2 Ni 7 (structure type Gd 2 Co 7 , space group R -3 m ). The phase transition α -Tb 2 Ni 7 <=> β -Tb 2 Ni 7 according to the results of differential thermal analysis occurs at 931 °C. The solubility of lithium and magnesium is observed in both modifications however it is larger in α -phase. Substitution of Ni-atoms by the statistical mixture of Li and Mg in α -Tb 2 Ni 7 and formation of the solid solution α -Tb 2 Ni 7- 2 х Li х Mg x ( х = 0–0.5) causes an increase of the unit cell parameters. This solid solution is in equilibrium with the CaCu 5 -type (space group P 6/ mmm ) and PuNi 3 -type (space group R -3 m ) phases. Keywords: polymorphism, solid solution, X-ray diffraction, energy-dispersive X-ray spectroscopy.