Phonon properties and mechanism of order-disorder phase transition in formamidinium manganese hypophosphite single crystal
Introduction
In the last few years many coordination polymers were studied due to their interesting physical properties including ferroelectricity [[1], [2], [3], [4], [5], [6]], multiferroicity [[7], [8], [9], [10], [11]], unique magnetic [7,12], dielectric [2,13], and non-linear optical properties [14]. They offer a multifunctionality due to their structural features allowing the occurrence of order-disorder phase transitions induced by temperature or pressure. Initially, the interests were focused on perovskite-like formates with the general formula [A]MII(HCOO)3, where A denotes the protonated amine accommodated in the voids of the metal-formate 3D framework and balancing its negative charge. The confined ammonium cations are stabilized mainly by hydrogen bonds (HBs), therefore in specific physical conditions they can exhibit many types of disorder. The scientists realized quickly that small formate ion as a linker can be replaced by other small anions (azide, dicyanamide, cyanide, etc.) [[15], [16], [17]] offering different flexibility of the framework, sizes of voids, patterns of created HBs and ability of metal coordination. Thus, the properties of the coordination polymers can be tuned either by substitution of linkers or by accommodated molecules. One of the proposed linkers is hypophosphite ion, H2POO−, which has very similar size to the formate ion [4,[18], [19], [20]]. Therefore, a new group of perovskite-type hypophosphite polymers with analogical general formula to formates, [A]MII(H2POO)3, has been studied. It has been found that despite of some chemical and geometrical similarities between these two linkers, the structures and properties of final coordination polymers are significantly different [4,[18], [19], [20]].
The [DMA]Mn(HCOO)3 (DMA+ denotes dimethylammonium cation) perovskite adopts at room temperature (RT) the trigonal symmetry with disordered DMA+ cations, which transforms below 181 K to the low-temperature (LT) Cc monoclinic symmetry with ferroelectric order of DMA+ cations [8,21,22]. On the other hand, [DMA]Mn(H2POO)3 perovskite crystallizes in the P21/c monoclinic symmetry with the DMA+ cation shifted from the center of the voids [20]. [GUA]Mn(HCOO)3 (GUA+ denotes guanidinium cation) adopts the orthorhombic Pnna structure [23,24] while its hypophosphite analogue forms at RT two stable polymorphs with ordered organic cations: triclinic and monoclinic I2/m [[18], [19], [20]]. A very recent study of mixed guanidinium formate-hypophosphite perovskites showed other monoclinic symmetry (C2/c) for nearly equimolar ratio of two linkers in [GUA]Mn(HCOO)1.47(H2POO)1.53 [19]. The HT phase of [FA]Mn(HCOO)3 (FA+ denotes formamidinium cation) is also trigonal but it transforms below 335 K to the monoclinic C2/c phase [12]. The hypophosphite analogue, [FA]Mn(H2POO)3, also crystallizes in the C2/c symmetry but it undergoes at about 175 K the structural phase transition to the LT phase described by other monoclinic P21/c symmetry [18,20]. The mechanism of this transition remains unsolved.
It is widely known that IR and Raman spectroscopy, along with x-ray diffraction studies, are very powerful tools in studying disordered coordination polymers. However, these spectroscopic method have not been yet employed in studies of any known organic-inorganic hypophosphite frameworks. We have decided, therefore, to analyze phonon properties [FA]Mn(H2POO)3. Based on the literature data and previous Density Functional Calculations (DFT) for the simple metal hypophosphites [25,26], we propose assignment of the observed IR and Raman bands. We have also employed the factor group analysis and temperature-dependent vibrational studies to obtain deep insight into phase transition mechanism in [FA]Mn(H2POO)3.
Section snippets
Synthesis of single crystals
In order to grow single crystals of [FA]Mn(H2POO)3, 5 mmol of MnCO3 (Sigma-Aldrich) was dissolved in excess 5.2 mL of aqueous solution of hypophosphorous acid (50%, Sigma-Aldrich) (equivalent of 48 mmol of H3PO2) using bath sonification followed by stirring at 50 °C for 1 h. In the next step, 10 mmol of formamidinium acetate (Sigma-Aldrich) was added and stirred until complete dissolution for 2 h. Then heating was switched off and the solution was left for crystallization. Colorless single
Materials and methods
Polycrystalline IR spectra in the mid-IR (500–4000 cm−1) and far-IR ranges (100–600 cm−1) were measured using a Nicolet iS50 FT-IR with resolution of 2 cm−1 in KBr disc and nujol mull in a polyethylene plate, respectively. The temperature-dependent IR spectra in the 550–4000 cm−1 spectral range were collected using a Nicolet iN10 FTIR microscope and Linkam THMS600 cryostat cell equipped with ZnSe windows. Polycrystalline Raman spectrum in the range of 50–4000 cm−1 was measured using a Bruker
Crystal structure
To understand the spectroscopic properties of the studied perovskite, its crystal structure needs to be briefly described. In the HT monoclinic phase the FA+ cations occupy the perovskite-like pseudo-cubic cages formed by the MnO6 octahedra connected by hypophosphite linkers. The negative charge of the manganese-hypophosphite framework is balanced by the FA+ cations stabilized by medium HBs. In the HT phase these cations exhibit positional disorder, however only one terminal NH2 groups is
Conclusions
We have studied phonon properties of synthesized manganese-hypophosphite framework templated by formamidinium cations using the temperature-dependent IR and Raman spectroscopies. We have presented selection rules and correlation diagram for the LT (P21/n) and HT monoclinic (C2/c) phases. For the first time, we have proposed assignment of the observed IR and Raman bands to the respective internal and external (lattice) vibrations. The temperature-dependent Raman and IR studies allowed us to
Declarations of interest
None.
Acknowledgment
This research was supported by the National Science Center (Narodowe Centrum Nauki) in Poland under project No. 2018/31/B/ST5/00455.
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