The Jahn-Teller distortion and the resulting staggered orbital-ordering phenomena typically lead to ferromagnetism in perovskite-structured compounds. Herein, we demonstrate the role of strong Jahn-Teller distortion in explaining the ground-state structure and quasi-two-dimensional behavior of the fluoride perovskite ${\mathrm{CsMnF}}_4$. Despite its theoretical robustness, comprehensive experimental verification has remained incomplete in these fluoride perovskites. Hence, a comprehensive investigation combining both experimental and theoretical approaches was conducted on ${\mathrm{CsMnF}}_4$, which was synthesized $via$ a coprecipitation method in hydrofluoric acid. Structural characterization using variable-temperature synchrotron x-ray diffraction (SXRD) and neutron powder diffraction (NPD) confirmed a layered tetragonal framework. Thermal analysis revealed its phase stability and decomposition behavior. Magnetometry and temperature-dependent NPD measurements indicate long-range magnetic ordering below 20 K, with a ferromagnetic ground state confirmed through Curie-Weiss analysis and hysteresis studies. Complementary theoretical investigations provided insights into the magnetic structure and electronic properties, aligning well with experimental observations. Dynamical analysis of the undistorted prototype phase (i.e., $P4/mmm$), elucidated the underlying mechanism for the structural phase transitions in ${\mathrm{CsMnF}}_4$. We confirmed the experimentally observed distorted (i.e., $P4/n$) phase to be the stable phase, which is achieved by condensing a robust Jahn-Teller distortion mode coupled to two in-phase octahedral rotations along the $x$ and $y$ axes $\left(a^{+}{a}^{+}{c}^0\right)$. Our analysis reveals quasi-two-dimensional ferromagnetism with an estimated $T_c$ of 22 K and the spin-phonon coupling parameter $\lambda =2.25\mathrm{c}{\mathrm{m}}^{-1}$ associated with an $A_g$ phonon mode at $526.07\mathrm{c}{\mathrm{m}}^{-1}$. This work provides a complete experimental and theoretical insight into the structure-magnetism properties of ${\mathrm{CsMnF}}_4$, with theoretical insights shedding light on the structural phase transitions and quasi-two-dimensional ferromagnetism based on Jahn-Teller distortion.

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• Electronic structure
• Magnetic order
• Magnetism
• Ferroelectrics
• Ferromagnets
• Layered crystals
• Magnetic systems
• Metal halides
• Crystal structures
• First-principles calculations
• Magnetization measurements
• Neutron diffraction
• X-ray diffraction