Crystal structure and dielectric properties of the [(CH3)(2)NH2](3)Sb2(1-x)Bi2xCl9 (DMACAB) mixed crystals

(P)hase transitions in [(CH3)(2)NH2](3)Sb-2((1-x))Bi2xCl9 (DMACAB) n-dxed s alts in the composition range 0 less than or equal to x less than or equal to 0.41 have been investigated by the pyroelectric method and dielectric me asurements over the frequency range from 75 kHz to 900 MHz. The phase situa tion is additionally confirmed by the differential scanning calorimetry (DS C) and dilatometric techniques. A transition from the paraelectric (PE) to the ferroelectric (FE) phase is observed for crystals with 0 less than or e qual to x less than or equal to 0.14. Pyroelectric measurements support the presence of polar phases. The dynamic dielectric behaviour of ferroelectri c systems is found to be determined by the existence of two independent rel axators. The low-frequency relaxator reveals a critical slowing down of its dynamics in the close vicinity of the PE --> FE transition. For compositio ns in the range 0.17 < x < 0.41 a new sequence of phase transitions appears without any polar phases being present. The disappearance of ferroelectric properties is related to the fundamental changes in the crystal structure of the mixed compounds. The characteristic feature of the ferroelectric mix ed crystals (0.0 less than or equal to x less than or equal to 0.14) is the presence of polyanionic (M2Cl93-)(n) (M = Sb, Bi) layers, whereas non9 fer roelectric ones are characterized by a different anionic sublattice compose d of polyanionic one-dimensional double chains.