Structural phase transitions in the perovskite-type layer compounds NH3(CH2)3NH3CdCl4, NH3(CH2)4NH3MnCl4, and NH3(CH2)5NH3CdCl4

Abstract

The structural phase transitions in the perovskite-type layer-structure compounds [N${\mathrm{H}}_{3}$${(\mathrm{C}{\mathrm{H}}_{2})}_{3}$N${\mathrm{H}}_{3}$]Cd${\mathrm{Cl}}_{4}$ (${T}_{c}=375 \mathrm{K}$), [N${\mathrm{H}}_{3}$${(\mathrm{C}{\mathrm{H}}_{2})}_{4}$N${\mathrm{H}}_{3}$]Mn${\mathrm{Cl}}_{4}$ (${T}_{c}=382 \mathrm{K}$), and [N${\mathrm{H}}_{3}$${(\mathrm{C}{\mathrm{H}}_{2})}_{5}$N${\mathrm{H}}_{3}$]Cd${\mathrm{Cl}}_{4}$ (${T}_{c}=338 \mathrm{K}$) have been studied by $^{35}\mathrm{Cl}$ and deuteron quadrupole resonance spectroscopy, birefringence, and dilatation measurements, optical-domain investigations and group-theoretical considerations. The results show that this transition, which is for all three compounds of second order, is basically a dynamic order-disorder transition of the alkylenediammonium chains, each of which can take on four different states (two all-trans states and two twisted states). The high-temperature phase is characterized by a dynamical disorder of the chains between the four possible states, the twisted states being less populated because of their higher potential energy. In the low-temperature phase, well below the phase transition, the chains are completely ordered in one of the two all-trans states. A strong even-odd effect, concerning the number of carbon atoms in the alkylene chains, has been observed in the temperature dependence of the static dielectric constant and in the critical exponent $\ensuremath{\beta}$ of the order parameter. This is connected with the symmetry of the chains in the all-trans state which is $\mathrm{mm}2$ for odd chains and $\frac{2}{m}$ for even chains. In contrast to the even chains the odd chains can thus carry a permanent electric dipole moment. As a consequence the phase transition in the two Cd compounds is of antiferrodistortive nature and leads from a paraelectric phase to an antiferroelectric phase, whereas the Mn compound undergoes a proper ferroelastic transition, although the microscopic mechanism is the same for both types. It is shown that the order-disorder transition in these compounds can be described adequately with a microscopic rigid-lattice model in the mean-field approximation.