Three-dimensional
methylhydrazinium (CH3NH2NH2
+, MHy+) lead halides, related
to the famous methylammonium (CH3NH3
+, MA+) and formamidinium (CH(NH2)2
+, FA) perovskites, are attractive optoelectronic materials
crystallizing in polar structures. In this work, temperature-dependent 1H and 207Pb magic-angle spinning (MAS) NMR, Raman
as well as high-pressure Raman studies of CH3NH2NH2PbCl3 (MHyPbCl3) are reported.
Raman spectroscopy reveals many similarities between phonon properties
of MHy lead halides and the MA and FA analogues. In particular, these
families of compounds show an increase in wavenumber of cage modes
when large I– ions are replaced by smaller Br– and then Cl– ones. They also show
strong sensitivity of the CH3 torsional mode on size of
the cavity occupied by MHy+ cation that decreases with
decreasing size of the halide anion. The cage modes of MHyPbCl3 are, however, observed at significantly lower wavenumbers
than similar modes of MAPbCl3 and FAPbCl3, indicating
higher softness of MHyPbCl3. Temperature-dependent Raman
and NMR studies demonstrate that the MHy+ cations in MHyPbCl3 are significantly less affected by the temperature-induced
phase transition than the Pb–Cl framework. This suggests a
displacive type of the phase transition dominated by tilting and deformation
of the PbCl6 octahedra. Analysis of the 207Pb
MAS NMR spectra reveals the presence of two differently distorted
PbCl6 octahedra and diminishing (increasing) distortion
of the less (more) distorted octahedra in the high-temperature phase.
Pressure-dependent Raman studies reveal the presence of a single first-order
pressure-induced phase transition between 0.72 and 1.27 GPa. Analysis
of the spectra indicates that the driving forces for the pressure-induced
phase transition in MHyPbCl3 are tilting and distortion
of the PbCl6 octahedra accompanied by reorientation of
MHy+ cations. Raman spectra do not show evidence of any
additional phase transition or amorphization up to 6.95 GPa.
Abstract:In this work, a set of nanoparticles of Nb 2 O 5 nanoparticles were grown by both the Pechini and the sol-gel methods. The amorphous materials were calcined at 650 • C or at 750 • C. X-ray diffraction, scanning electron microscopy, luminescence and Raman spectroscopy were used in order to characterize the materials. From the study, it is possible to state that the method of production of nanoparticles, beyond the temperature of synthesis, has a great influence on whether the phase produced is hexagonal or orthorhombic. Additionally, compared to de Sol-gel method, the Pechini method produced samples with smaller particle sizes. The photoluminescence spectra of niobium pentoxide nanostructure materials show that the emission peaks are positioned between 334 to 809 nm and there is a change of intensity which varies depending on the synthesis route used. High pressure Raman spectra at room temperature were obtained from two samples grown by the sol-gel method. Up to 6 GPa, where it is possible to observe the Raman bands, no modification other than the increase of disorder was observed, and this can be associated with a change of phase.
By combining two methods, Raman spectroscopy and high-pressure single-crystal X-ray diffraction, we demonstrate that the disordered, ambient pressure phase α of [NH 2 CHNH 2 ]Mn(H 2 POO) 3 is less compressible than the methylhydrazinium analog due to the formation of strong hydrogen bonds between the formamidinium cations and the framework. Above 4.0 GPa, the ordering of formamidinium cations and the collapse of perovskite cages lead to triclinic phase γ of space group P1̅ . Phase γ exhibits a rare and important mechanical property, i.e., a negative linear compressibility of −7.82 ± 0.6 TPa −1 along the c axis. Raman spectroscopy indicates the presence of other phase transitions to phases δ and ε above 4.7 and 6.0 GPa, respectively. These transformations occur due to the cumulative distortions of the chemical bonds and structural components of the manganese-hypophosphite framework.
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