Cl35NQR study of the structural order-disorder transition in(CH3NH3)2MnCl4

The temperature dependence of the ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ nuclear quadrupole resonance (NQR) for the two chemically inequivalent chlorine sites in the perovskitic layer-structure compound ${\left(\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\right)}_2$Mn${\mathrm{Cl}}_4$ has been measured around the second-order phase transition at 393.7 K. A value of the critical exponent $\beta$ of the order parameter was determined to be $\beta =0.250\mathrm{}\pm 0.005$, which is intermediate between the corresponding values for the three- and the two-dimensional Ising model. A microscopic model of the phase transition is proposed, involving an order-disorder transition of the C${\mathrm{H}}_3$N${\mathrm{H}}_3$ groups. Above $T_c$ these groups move on a conical surface between four equivalent potential wells which become inequivalent below $T_c$. The mechanism of this phase transition has been computer simulated in a point-charge model from which a quadratic dependence of the ${}_{}{}^{35}{}_{}{}^{}\mathrm{Cl}$ NQR frequencies on the order parameter is derived. Quantitative agreement between theory and experimental data is obtained by choosing appropriate values for the lattice parameters and by defining how the probability amplitudes for the four potential wells vary with the order parameter.