I came across a paper which talks about $Cs_2SnI_6$ the material is well researched and there are dozens of experimental papers which say that the material exists in Fm3m space group(FCC), the particular paper I am talking about talks about the phase transition from cubic to monocilinic as the tempreature decreases although this theoretical calculation does not really go with experimental evidence. My point is that the they show the phonon band structure as
where the red lines show unstable modes. So is this calculation wrong(as I have heard this would be a dynamically unstable mode, but the material does exist) or can materials with negative phonon modes also exist?
P.S there are studies which talk about the anharmonic lattice dynamics of Cs2SnI6 saying that the material shows quite a lot of anharmonicity.