  Multiferroics with coupling between ferroelectricity and magnetism have been
pursued for decades. However, their magnetoelectric performances remain limited
due to the common trade-off between ferroelectricity and magnetism. Here, a
family of nitride perovskites is proposed as multiferroics with prominent
physical properties and nontrivial mechanisms. Taking GdWN$_3$ as a prototype,
our first-principles calculations found that its perovskite phases own large
polarizations (e.g. $111.3$ $\mu$C/cm$^2$ for the $R3c$ phase) and a magnetic
moment $7$ $\mu_{\rm B}$/Gd$^{3+}$. More interestingly, its ferroelectric
origin is multiple, with significant contributions from both Gd$^{3+}$ and
W$^{6+}$ ions, different from its sister member LaWN$_3$ in which the
ferroelectricity almost arises from W$^{6+}$ ions only. With decreasing size of
rare earth ions, the A site ions would contribute more and more to the
ferroelectric instability. Considering that small rare earth ions can be
primary origins of both proper ferroelectricity and magnetism in nitride
perovskites, our work provides a route to pursuit more multiferroics with
unconventional mechanisms and optimal performances.
