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Measuring Motional Dynamics of (CH$_3$)$_2$ NH$_2^+$ in the Perovskite-Like Metal--Organic Framework [(CH$_3$)$_2$ NH$_2$][Zn(HCOO)$_3$]: The Value of Low-Frequency Electron Paramagnetic Resonance

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 Added by Sylvain Bertaina
 Publication date 2018
  fields Physics
and research's language is English




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Dimethylammonium zinc formate (DMAZnF) is the precursor for a large family of multiferroics, materials which display co-existing magnetic and dielectric ordering. However, the mechanism underlying these orderings remains unclear. While it is generally believed that the dielectric transition is related to the freezing of the order-disorder dynamics of the dimethylammonium (DMA+) cation, no quantitative data on this motion are available. We surmise that this is due to the fact that the timescale of this cationic motion is on the borderline of the timescales of experimental techniques used in earlier reports. Using multifrequency EPR, we find that the timescale of this motion is ~ 5 x 10 -9 s. Thus, S-band (4 GHz) EPR spectroscopy is presented as the technique of choice for studying these motional dynamics. This work highlights the value of the lower-frequency end of EPR spectroscopy. The data are interpreted using DFT calculations and provide direct evidence for the motional freezing model of the ferroelectric transition in these metal-organic frameworks with the ABX3 perovskite-like architecture.



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The bulk single crystals of of low-dimensional magnet (CH$_3$)$_2$NH$_2$CuCl$_3$ (DMACuCl$_3$ or MCCL) are grown by a slow evaporation method with different kinds of solvents, different degrees of super-saturation of solution and different temperatures of solution, respectively. Among three kinds of solvent, methanol, alcohol and water, alcohol is found to be the best one for growing MCCL crystals because of its structural similarity to the raw materials and suitable evaporation rate. The best growth temperature is in the vicinity of 35 $^{circ}$C. The problem of the crystals deliquescing in air has been solved through recrystallization process. The crystals are characterized by means of x-ray diffraction, specific heat and magnetic susceptibility.
We present the observational result of a glycine precursor, methylamine (CH$_3$NH$_2$), together with methanol (CH$_3$OH) and methanimine (CH$_2$NH) towards high-mass star-forming regions, NGC6334I, G10.47+0.03, G31.41+0.3, and W51~e1/e2 using ALMA. The molecular abundances toward these sources were derived using the rotational diagram method and compared with our state-of-the-art chemical model. We found that the observed ratio of CH$_3$NH$_2$/CH$_3$OH is in between 0.11 and 2.2. We also found that the observed CH$_3$NH$_2$/CH$_3$OH ratio agrees well with our chemical model by considering the formation of CH$_3$NH$_2$ on the grain surface via hydrogenation process to HCN. This result clearly shows the importance of hydrogenation processes to form CH$_3$NH$_2$. NGC63343I MM3, where CH$_3$NH$_2$ was not detected in this study and showed CH$_3$NH$_2$/CH$_3$OH ratio of less than 0.02, is clearly distinguished from the other cores.
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The organic-inorganic lead halide perovskites are composed of organic molecules imbedded in an inorganic framework. The compounds with general formula CH$_{3}$NH$_{3}$PbX$_{3}$ (MAPbX$_{2}$) display large photovoltaic efficiencies for halogens $X$=Cl, Br, and I in a wide variety of sample geometries and preparation methods. The organic cation and inorganic framework are bound by hydrogen bonds that tether the molecules to the halide anions, and this has been suggested to be important to the optoelectronic properties. We have studied the effects of this bonding using time-of-flight neutron spectroscopy to measure the molecular dynamics in CH$_3$NH$_3$PbCl$_3$ (MAPbCl$_3$). Low-energy/high-resolution neutron backscattering reveals thermally-activated molecular dynamics with a characteristic temperature of $sim$ 95,K. At this same temperature, higher-energy neutron spectroscopy indicates the presence of an anomalous broadening in energy (reduced lifetime) associated with the molecular vibrations. By contrast, neutron powder diffraction shows that a spatially long-range structural phase transitions occurs at 178,K (cubic $rightarrow$ tetragonal) and 173,K (tetragonal $rightarrow$ orthorhombic). The large difference between these two temperature scales suggests that the molecular and inorganic lattice dynamics in MAPbCl$_3$ are actually decoupled. With the assumption that underlying physical mechanisms do not change with differing halogens in the organic-inorganic perovskites, we speculate that the energy scale most relevant to the photovoltaic properties of the lead-halogen perovskites is set by the lead-halide bond, not by the hydrogen bond.
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