On the possibility of metal hydride formation -: Part I.: The synthesis of MgNi3B2 by mechanical milling and sintering

In this first part of an ongoing investigation, we present results on the crystal structure determination of MgNi3B2. We also describe the novel procedure that was used to synthesize this compound. Using powder X-ray diffraction, MgNi3B2 was indexed in the space group P6(2)22. A Rietveld refinement of the X-ray profile determined the structure to be of the CeCo3B2 type. This is a variation of the CaCu5 (P6/mmm) structure of the classic hydride forming family of LaNi5 compounds. MgNi3B2 also has an elemental composition which is characteristic of hydride-forming intermetallic compounds. That is, the combination of a rare or alkaline earth metal which has a high affinity for hydrogen (Mg), with a transition metal (Ni). However, gaseous hydrogen and electrochemical treatments of MgNi3B2 did not produce a hydride. This study led us to develop a very useful procedure for synthesizing compounds which are difficult to produce using more standard techniques. Because of the high melting point of B (2030 degrees C) and the low boiling point of Mg (1107 degrees C at 1 atm) it was not possible to prepare MgNi3B2 by melting. We were able to successfully produce this alloy by ball milling for 30 min and sintering for 10 min at 800 degrees C. The method is fast and simple, and should prove very useful in the synthesis of other similar intermetallic compounds. (C) 1998 Elsevier Science S.A.