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The perovskite‐type layer compound NH3(CH2)3NH3MnCl4 undergoes two discontinuous structural phase transitions at 336 and 305 K, respectively. Optical, dielectric, and 35Cl NQR measurements in all three phases as well as differential thermal analysis of the phase transitions are performed. The partially deuterated compound ND3(CH2)3ND3MnCl4 is investigated by deuteron NMR–NQR spectroscopy in order to analyze the dynamics of the propylenediammonium chains in each phase. It is shown that the phase transitions are governed by the rotational motion of the rigid chains around their long axis. Above 305 K a dynamic disorder of the chains between two orientations is observed, whereas below this temperature the chains are almost completely ordered. From this the following phase sequence is derived: Imma 336 K Fmmm 305 K Pnma. In the “family tree” of the hypothetical parent phase P4/mmm the intermediate phase Fmmm does not belong to the same branch as Imma and Pnma which explains the first order character of the two transitions.

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