Low-temperature incommensurate-commensurate phase sequence of (${\mathrm{C}}_3$${\mathrm{H}}_7$${\mathrm{ND}}_3$${)}_2$${\mathrm{MnCl}}_4$

The perovskite-type layer-structure compound (${\mathrm{C}}_3$ ${\mathrm{H}}_7$ ${\mathrm{ND}}_3$ ${)}_2$ ${\mathrm{MnCl}}_4$ [bis-(propylammon ium)manganese tetrachloride (PAMC)] exhibits an extraordinary rich structural phase sequence with six different phases (α,β,γ,δ,ε,ζ), two of which (γ and ε) are incommensurately modulated. In this contribution we focus on the δ-ε-ζ sequence which shows a peculiar switching of the modulation wave vector from ((1/3+δ${)\mathrm{a}}^{\mathrm{*}}$ to (${\mathrm{a}}^{\mathrm{*}}$±${\mathrm{b}}^{\mathrm{*}}$)/3. The study was performed by means of ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ nuclear quadrupole resonance (NQR) and deuteron and proton NMR, as well as elastic neutron scattering. It was possible to determine the rotational components of the polarization vector for the N-C bond. A microscopic model is proposed which takes couplings between lattice waves and fluctuations of the rotational disorder of the propyl-ammonium (PA) chains into account. The origin of the incommensurate modulation of the ε phase is the competition between the ${\mathrm{ND}}_3$ ordering in the Cl-octahedra cavities and the PA-chain packing due to van der Waals interactions. Because of intralayer short-range interactions via the hydrogen bonds the ${\mathrm{ND}}_3$ ordering is incompatible with a continuous incommensurate modulation. This leads to an additional symmetry breaking of the translational symmetry, which was observed in the ${}_{}{}^{35}{}_{}{}^{}\mathrm{NQR}$ spectra. The complete ordering of the ${\mathrm{ND}}_3$ groups reduces the PA dynamics in such a way that the interlayer interaction through the PA chains changes sign, which leads to the switching of the wave vector.