The detailed temperature-dependent IR and Raman spectra were used to study and understand the mechanism of structural phase transition occurring at 175 K in manganese hypophosphite templated with formamidinium (FA+) ions, [FA]Mn(H2POO)3, which adopts a perovskite-like architecture. The structural transformation between the C2/c and the P21/c monoclinic phases has a complicated nature and is mainly driven by re-orientational motions of the FA+ cations but it is also accompanied by a significant distortion of the MnO6 octahedral units as well as steric-forced changes of the PH2 groups determining the off-center shifts of FA+ cations in the cages. The re-orientational motions of formamidinium cations at 175 K are followed by slight changes of their geometry and re-arrangement of hydrogen bonds (HBs). The strong temperature-dependences of bands corresponding to vibrations involving hydrogen bonding reveal the highly-dynamic character of this phase transition and strong nature of created HBs. The most pronounced changes are observed for the modes corresponding to the formamidinium cation, proving that the phase transition has an order-disorder character.

中文翻译：

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甲ami次磷酸锰单晶的声子性质及有序无序相变机理。

详细的温度依赖性IR和拉曼光谱用于研究和了解在以甲K（FA +）离子[FA] Mn（H2POO）3为模板的次磷酸盐锰中175 K发生的结构相变机理，该结构采用钙钛矿-像建筑。C2 / c和P21 / c单斜晶相之间的结构转变具有复杂的性质，主要是由FA +阳离子的重新定向运动驱动的，但同时还会伴随MnO6八面体单元和空间的明显变形。 PH2基团的强力变化决定了笼中FA +阳离子的偏心移位。甲K阳离子在175 K的重新定向运动后，其几何形状和氢键（HBs）的重新排列发生了细微变化。与涉及氢键的振动相对应的能带具有很强的温度依赖性，这揭示了这种相变的高度动态特性以及所产生的HBs的强特性。对于与甲ami阳离子相对应的模式，观察到最明显的变化，证明相变具有有序-无序特性。