  Be-B/B-Ta based compounds are very attractive to researchers because of their
high density and ultra-hardness. But ternary Be-B-Ta compounds are neither
synthesized nor predicted. In this paper, variable composition evolutionary
crystal structure prediction calculations based on first-principles method have
been performed to find the stable crystal structure containing Be-B-Ta at
ambient condition. The predicted five compounds BeB$_2$Ta, BeB$_3$Ta$_2$
(high-pressure phase), BeBTa, BeBTa$_2$, and Be$_2$B$_2$Ta have been found to
be highly dense and very hard materials. All these compounds are metallic and
spin-orbit coupling (SOC) effect is significant in them. Only two of them
(BeB$_2$Ta and Be$_2$B$_2$Ta) have been found to be superconductors within
Migdal-Eliashberg theory. The calculated critical temperature including SOC
effect is 8 and 9 K for BeB$_2$Ta and Be$_2$B$_2$Ta, respectively. Because of
their energetic and dynamic stability, these compounds might be favorable to
synthesize in the laboratory.
