  We searched for stable crystal structures of YFe$_{12}$ using a crystal
structure prediction technique based on a genetic algorithm and
first-principles calculations. We obtained two monoclinic $C2/m$ structures as
metastable phases that are different from the well-known ThMn$_{12}$ structure.
These two phases have advantages in their magnetism over the ThMn$_{12}$
structure: The total magnetization $M$ is increased from 25.6
$\mu_{\text{B}}$/f.u. up to 26.8 $\mu_{\text{B}}$/f.u. by the transformations.
We also calculated Curie temperature $T_{\text{C}}$ for these structures within
the mean-field approximation and predicted the increase of $T_{\text{C}}$ from
792 K up to 940 K, which is mainly caused by the increase of intersite magnetic
couplings within the distance of 2.3--3.1\r{A}. The similar enhancements of $M$
and $T_{\text{C}}$ are also obtained in the pseudo-binary system
Y(Fe$_{1-x}$Co$_{x}$)$_{12}$ with $x$ of 0--0.7.
