Electron paramagnetic resonance of a copper doped [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite framework.

We report a continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) as well as pulse electron nuclear double resonance (ENDOR) study of Cu2+ doped [(CH3)2NH2][Zn(HCOO)3] hybrid perovskite which exhibits a structural phase transition. The multifrequency (X, Q and W-band) CW EPR measurements allow the temperature evolution of the Cu2+ ion local environment to be studied. The spectrum of the ordered (low-temperature) phase reveals an axially distorted octahedral Cu2+ site confirming the successful replacement of the Zn2+ ions and formation of the CuO6 octahedra. The CW EPR spectrum of the disordered (high-temperature) phase shows an additional broad line which gradually diminishes on cooling. The EPR linewidth of the axially symmetric Cu2+ ion site exhibits an anomaly at the phase transition point and Arrhenius-type behavior in the disordered phase. The temperature dependent Cu2+ spin Hamiltonian parameters change abruptly at the phase transition point indicating a strong first-order character of the transition. The X-band pulse ENDOR spectrum of the ordered phase reveals several protons in the vicinity of the Cu2+ center.

The effect of K+ cations on the phase transitions, and structural, dielectric and luminescence properties of [cat][K0.5Cr0.5(HCOO)3], where cat is protonated dimethylamine or ethylamine.

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of two novel heterometallic perovskite-type metal-organic frameworks (MOFs) of the following formula: [(CH3)2NH2][K0.5Cr0.5(HCOO)3] (DMAKCr) and [C2H5NH3][K0.5Cr0.5(HCOO)3] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R3[combining macron] space group) and undergoes an order-disorder phase transition to the monoclinic system (P1[combining macron] space group) at about 190 K. The dielectric studies confirm the presence of first-order relaxor-like structural transformation. In the high-temperature phase, the dimethylammonium cations are dynamically disordered over three equal positions and upon cooling the dynamical disorder evolves into a two-fold one. This partial ordering is accompanied by a small distortion of the metal-formate framework. EtAKCr crystallizes in a monoclinic structure (P21/n space group) with ordered EtA+ cations and does not experience any phase transition. The differences in the thermal behavior caused by the substitution of Na+ ions by larger K+ ions in the [cat]MIMIII (cat = DMA+, EtA+, MI = Na+, K+ and MIII = Cr3+ and Fe3+) heterometallic MOF family are discussed taking into account the impact of the hydrogen bond (HB) pattern and other factors affecting the stability of metal-formate frameworks. The optical studies show that DMANaCr and EtAKCr exhibit Cr3+-based emission characteristics for intermediate ligand field strength.

Perovskite metal formate framework of [NH2-CH(+)-NH2]Mn(HCOO)3]: phase transition, magnetic, dielectric, and phonon properties.

We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH2-CH(+)-NH2][Mn(HCOO)3] (FMDMn). The anionic framework of [(Mn(HCOO)3(-)] is counterbalanced by formamidinium (FMD(+)) cations located in the cavities of the framework. These cations form extensive N-H···O hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R3̅c symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD(+) cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH2 groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD(+) cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD(+) cations is associated with significant distortion of the anionic framework. On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature Tc = 8.0 K.

Novel bimetallic MOF phosphors with an imidazolium cation: structure, phonons, high- pressure phase transitions and optical response.

We report the synthesis, crystal structure, phonons and luminescence properties of three novel heterometallic metal organic frameworks (MOFs) with perovskite-like topology of the following formulas: [C3H5N2]Na0.5Cr0.5(HCOO)3 (ImNaCr), [C3H5N2]Na0.5Al0.5(HCOO)3 (ImNaAl) and [C3H5N2]Na0.5Al0.475Cr0.025(HCOO)3 (ImNaAlCr with 5 mol% of Cr3+). ImNaCr crystallizes in a monoclinic system (P2/n space group) with one imidazolium cation (Im+) in an asymmetric unit forming six N-HO and four C-HO hydrogen bonds. In contrast to other known heterometallic MOFs, the complete substitution of Cr3+ ions with smaller Al3+ ions leads to a change of the crystal symmetry. ImNaAl adopts a monoclinic P21/n space group with two distinct Im+ cations and different H-bonding patterns. The DSC measurements and XRD single-crystal studies show that the studied crystals do not undergo structural phase transitions in the 80-400 K range. The high-pressure Raman studies of ImNaCr reveal the presence of two reversible structural instabilities, first in the 0.4-1.1 GPa range and second near 4 GPa. The first pressure-induced phase transition involves weak distortion of the metal-formate framework, while the second one is associated with partial and reversible amorphization of the sample. We discuss the stability of heterometallic formate MOFs depending on their building blocks. The luminescence measurements show that both the fully concentrated crystal (ImNaCr) and the diluted one (ImNaAlCr) exhibit a Cr3+-based emission characteristic of intermediate ligand field strength. We also show that the spectroscopic properties of heterometallic MOFs depend strongly on the templated cation, i.e. the decreasing size of the organic cation leads to an increase in the crystal field.

Synthesis and characterization of novel niccolites [(CH3)2NH2][Fe(III)M(II)(HCOO)6] (M(II) = Zn, Ni, Cu).

We report the synthesis, X-ray diffraction, thermal, magnetic, Raman and IR studies of three heterometallic MOFs, [(CH3)2NH2][Fe(III)M(II)(HCOO)6] with M = Zn (DMFeZn), Ni (DMFeNi) and Cu (DMFeCu), crystallizing in the niccolite type structure. DMFeZn and DMFeNi crystallize in the trigonal structure (space group P3[combining macron]1c) while DMFeCu crystallizes in the monoclinic structure (space group C2/c). Magnetic investigation shows that DMFeZn remains paramagnetic down to the lowest temperature obtained in our experiment while DMFeNi and DMFeCu exhibit ferromagnetic order below 42 and 28.5 K, respectively. IR and Raman data confirm the structural model of the monoclinic DMFeCu and show evidence for stronger hydrogen bonds when compared to trigonal DMFeZn and DMFeNi. A different hydrogen bond network in the monoclinic DMFeCu when compared to trigonal DMFeZn and DMFeNi is responsible for the different behavior of these compounds upon cooling, that is, DMFeCu exhibits a sign of short range ordering of dimethylammonium cations at low temperatures while the trigonal analogues show evolution of dynamic disorder into static disorder.

Temperature-dependent studies of [(CH3)2NH2][Fe(III)M(II)(HCOO)6] frameworks (M(II) = Fe and Mg): structural, magnetic, dielectric and phonon properties.

Novel heterometallic formate [(CH3)2NH2][Fe(III)Mg(II)(HCOO)6] (DMFeMg) was prepared and characterized by single crystal X-ray diffraction, DSC, dielectric, magnetic susceptibility, Raman and IR methods. We also report thermal, Raman and IR studies of the known compound [(CH3)2NH2][Fe(III)Fe(II)(HCOO)6] (DMFeFe). DMFeMg crystallizes in the niccolite structure (P3[combining macron]1c space group). In contrast to the known DMFeFe, [(CH3)2NH2][Fe(III)Mn(II)(HCOO)6] (DMFeMn) and [(CH3)2NH2][Fe(III)Co(II)(HCOO)6] (DMFeCo) formates, the metal ions in DMFeMg are distributed statistically over the two available octahedral sites. Temperature-dependent studies show that whereas DMFeFe exhibits an order-disorder phase transition at 151.8 K upon cooling, freezing-in of re-orientational motions of DMA(+) cations does not lead to any structural phase transition in DMFeMg. We discuss the origin of this difference. The low-temperature studies also show that DMFeMg orders magnetically at TC = 13.5(5) K and the shape of M(T) measured in the field-cooling regime suggests ferromagnetic character of the ordering.