Mössbauer Effect and Raman Studies of the Structural Phase Transitions in K₂[SnCl₆]

The 110 Sn Mössbauer spectrum and the Raman phonons in polycrystalline samples of K 2 [SnCl 5 ] have been investigated in the temperature range from 4 and 150 K, respectively, to 300 K. With decreasing temperature a discontinuous increase of the halfwidth of the Sn‐Mössbauer line and a concommitant change in the isomeric shift are observed. The former is attributed to the appearance of a quadrupole splitting at the first order crystallographic transition at T 1 = 262 K. The Raman spectrum of the low‐symmetry phase exhibits an external lattice mode located at 33 cm −1 at 150 K. It shows a pronounced softening following the relation ω( T ) ∼ (T 0 − T ) 1/2 where T 0 is close to T 1 . Two models are discussed in which alternatively a zone centre and a zone boundary rotary mode is considered to drive the crystallographic transition.