A 35 Cl NQR Study of [(CH 3 ) 2 NH 2 ] 2 SeCl 6 Crystals

The temperature dependence of the 35 Cl NQR frequencies in [(CH 3 ) 2 NH 2 ] 2 SeCl 6 is measured between 78 and 300 K. Two NQR lines with intensity ratio of 2:1 are observed at 196 K. The low‐frequency line fades out below about 80 K, while the other line is still clearly observed. This arises from a second‐order structural phase transition in which the librational modes of [SeCl 6 ] 2− anions become soft. Both lines fade out at higher temperature, however, the low‐frequency one at 26 K lower than the other one. This is explained by the C 4 reorientional motions of the octahedral anions having a relative low activation energy of 18 kJ mol −1 .