Phase transitions of Cs3Sb2I9, Cs3Bi2I9, and Cs3Bi2Br9 crystals

The phase transitions of Cs3Sb2I9 Cs3Bi2I9, and Cs3Bi2Br9 crystals were investigated by Cs-133 nuclear magnetic resonance. Cs3Bi2Br9 was also studied by Br-87 nuclear quadrupole resonance. The parameters of the Cs electric-field gradient tensors were determined. The phase transitions were shown to produce significant changes in the nearest neighbor environment of only one of two inequivalent Cs sites. In Cs3Sb2I9, the tensors remain axisymmetric below the transition at 85 K, in line with the assumption relying on powder x-ray diffraction data that, below the transition temperature, Cs3Sb2I9 has a trigonal structure. The observed temperature variation in the shape of the Cs-133 NMR and I-127 NQR signals suggests that Cs3Bi2I9 may undergo a transition to an incommensurate phase.