High-pressure phase research on Nb₂O₅

Abstract

Experimental results indicate that the direct transitions from the H form to the T form and from the H form to the B form of Nb₂O₅ are possible under high pressures. Both highly pure and less pure reagents were used. The distinct TT form was obtained only when less pure reagents were used so that the TT form seems to be stabilised by means of trace impurities. Previously the TT form was indexed as having a pseudohexagonal unit cell. However, the present distinct TT form has a monoclinic unit cell,a=7.23₈Å,b=15.7₉Å,c=7.18₈Å,β=119°5₉′,V=711_.7(Å)³. Eight molecules are contained per cell. Probably only B, T and H have stable regions; the other polymorphs of Nb₂O₅ may be metastable forms or forms stabilised by impurities.