A temperature threshold for gas-bubble superlattice formation in molybdenum

He irradiation of metals at temperatures around 0.2 Tm (Tm is the melting point of the metal) commonly results in the formation of small (∼2 nm) bubbles in high concentration (∼1025 m-3) ordered on a superlattice. Previous work has established that there is an upper temperature limit for bubble ordering of ∼0.35 Tm. Here we report the results of the first comprehensive investigation of the possible existence of a lower temperature limit. Mo is implanted with He at temperatures in the range 0.07 Tm to 0.20 Tm. For the implantations above 0.16 Tm, TEM studies show that the bubbles are ordered on a superlattice. In contrast at lower implantation temperatures only random bubble arrays are found. It is demonstrated that there is a lower temperature threshold for bubble ordering in Mo in the temperature range 0.14 Tm to 0.16 Tm. This range correlates well with the 'onset' temperature for thermally stimulated vacancy motion in Mo. (orig.)