Self-propagating metathesis routes to metastable group 4 phosphides

Group 4 phosphides, which are typically prepared at high temperatures (>800 degrees C) over several days, are synthesized in self-propagating metathes is (exchange) reactions in seconds. These reactions produce cubic forms of zirconium phosphide (ZrP) and hafnium phosphide (HfP) which are normally ma de at temperatures greater than 1425 degrees C and 1600 degrees C, respecti vely. To test whether the high temperatures reached in the metathesis react ions are responsible for the formation of the cubic phases, inert salts are added to lower the maximum reaction temperatures. The lower temperature re actions still result in cubic phosphides, although smaller crystallites for m. Further experiments with phosphorus addition indicate that the phosphoru s content is not responsible for cubic phase formation. Templating is ruled out using lattice mismatched KCl and hexagonal ZnS as additives. Therefore , the direct synthesis of the high-temperature cubic phase in metathesis re actions appears to be caused by nucleation of the metastable cubic form tha t is then trapped by rapid cooling. Heating the cubic phase of either ZrP o r HfP to 1000 degrees C for 18 h, or carrying out metathesis reactions in s ealed ampules at 1000 degrees C, results only in the hexagonal phase.