Large geometric polarization and magnetic behavior in the multiferroic quasi-2D SrNiF$_4$ fluoride

In the last decades, multifunctional single-crystals that show, for example, multiferroic and magnetoelectric responses have attracted considerable attention due to the potential applications and physical phenomena involved in the entanglement of the ferroic orders. This paper investigates the structural, ferroelectric, and magnetic properties of the unexplored layered SrNiF$_4$ fluoride compound. We show that, in terms of the vibrational phonon modes, this fluoride compound shows a tangible ferroelectric behavior with a spontaneous polarization as large as $P_s$ = 14.8 $\mu$C$\cdot$cm$^{-2}$ standing as one of the largest among its family. Besides, based on our findings such ferroelectric polarization presents a geometric origin that is strongly entangled with the octahedral rotations within the $\Gamma_2^-$ phonon mode and enhanced by the structure's $\Gamma_1^+$ mode. We also observed that the spontaneous polarization is coupled to the noncollinear $G$-type antiferromagnetic ordering in the structure allowing a switching of the weak antiferromagnetic component when the polarization is reversed.