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The temperature evolution of Cu 2+ ion environment in the solid solutions of ((CH 3 ) 2 NH 2 ) 5 Cd 2 CuCl 11 is studied on the basis of absorption spectroscopy data. For the detailed analysis of experimental data the special program package Crys Tool 1.0 based on quantum-mechanical models, first of all on the model of normalized spherical harmonics (NSH), has been employed. It has been found that similarly to the crystal of ((CH 3 ) 2 NH 2 ) 5 Cd 3 Cl 11 (DMACC) the investigated solid solution contains tetragonally distorted octahedral metal-halogen complexes of two types and the degree of their distortion is changed considerably at the temperatures of phase transitions (PTs). The parameters of crystal field, angular overlap model, as well as the copper-chlorine distances, show continuous changes at T 1 = 176 K that should be related to the second-order transition, whereas the jump-like anomalies of the spectral parameters at T 2 = 115 K (on cooling) are characteristic of the first-order PTs. Introduction of the copper ions into the structure of the host DMACC crystal induces the shifts of these PTs toward low temperatures by 3.5 and 5 K, respectively. The observed structural changes around T 0 = 313 K are connected with a complex co-operative effect involving weakening of the hydrogen bonds and modification of the Jahn-Teller distortion with temperature.

The crystal structure of the 4-aminopyridinium perchlorate (4-apyH)ClO 4 has been determined at 100 K by means of x-ray diffraction as monoclinic, with space group P2 1 , with Z = 8. The crystal undergoes two structural phase transitions: one of first-order type, reversible, at 241/243 K (on cooling/heating respectively) and one of weakly first-order type, irreversible, at 277 K (on heating). The crystal dynamics is discussed on the basis of the temperature dependence of the 1 H nuclear magnetic resonance second moment (M 2 ) and spin-lattice relaxation time T 1 . Both phase transitions are interpreted in terms of the changes in the motional state of (4-apyH) + cations and ClO − 4 anions. The dielectric dispersion studies disclose a relaxation process over the high-temperature phase (above 241 K) in the audio-frequency region. The dielectric results are described by a Cole-Cole equation. The title crystal reveals pyroelectric properties below 241 K. The ferroelastic domain structure of (4-apyH)ClO 4 is observed over the whole temperature range studied.

The crystal structure of 2-cyanopyridinium perchlorate, [2-CNPyH][ClO4], has been determined at 100 (phase II) and 293 K (phase I). It is monoclinic P 21 at 100 K and orthorhombic P 212121 at 293 K. The dynamic properties of the crystal were studied by differential scanning calorimetry, dilatometry, pyroelectric, dielectric, proton (1H NMR), chlorine (35Cl NMR) magnetic resonance spectroscopies and the infrared method. The crystal undergoes a structural phase transition () at 170 K characterized by a complex mechanism involving both ‘order–disorder’ and ‘displacive’ contributions. It reveals pyroelectric properties below 170 K. The dielectric relaxation existing over phase I is due to the motion of the cyano group, whereas the dynamics of the [ClO4]− anions is reflected in the significant dielectric increment around the phase transition.