Low-energy structures of zinc borohydride Zn(BH${}_4$)${}_2$

We present a systematic study of the low-energy structures of zinc borohydride, a crystalline material proposed for the hydrogen storage purpose. In addition to the previously proposed structures, many new low-energy structures of zinc borohydride are found by utilizing the minima-hopping method. We identify a new dynamically stable structure which belongs to the $I{4}_{1}22$ space group as the most stable phase of zinc borohydride at low temperatures. A low transition barrier between $I{4}_{1}22$ and $P1$, the two lowest-lying phases of zinc borohydride is predicted, implying that a coexistence of low-lying phases of zinc borohydride is possible at ambient conditions. An analysis based on the simulated X-ray diffraction pattern reveals that the $I{4}_{1}22$ structure exhibits the same major features as the experimentally synthesized zinc borohydride samples.