Structural, optical and phonon properties of formate-based MOF phosphors with ethylammonium cations.

We report the synthesis, crystal structure, IR, Raman and luminescence studies of four metal-organic frameworks of the following formulas: [CH3CH2NH3]Y1-x-yYbyErx(HCOO)4 (x = 1, y = 0; x = 0.2, y = 0.8; x = 0.02, y = 0.07) and [CH3CH2NH3]Y0.92Eu0.08(HCOO)4. All the compounds are isostructural and crystallize in a polar and non-centrosymmetric monoclinic system (P21 space group). They have been characterized by single crystal and powder X-ray diffraction methods as well as by vibrational spectroscopy (IR and Raman). The assignment of the external and internal modes has been discussed and presented. Furthermore, the optical properties of the Er3+ and Eu3+ ions have been assessed using diffuse absorption, excitation and emission spectra. The site symmetry of the Eu3+ ions has been analyzed using the emission spectrum and the luminescence decay of the red emission.

Dielectric relaxation behavior in antiferroelectric metal organic framework [(CH3)2NH2][Fe(III)Fe(II)(HCOO)6] single crystals.

The fundamental aspects of the relaxation dynamics in niccolite-type, mixed valence metal-organic framework, multiferroic [(CH3)2NH2][Fe(3+)Fe(2+)(HCOO)6] single crystals have been reported using dielectric relaxation spectroscopy covering eight decades in frequency (10(-2) ≤ f ≤ 10(6)) in the temperature range 120 K ≤ T ≤ 250 K. The compound shows antiferroelectric to paraelectric phase transition near T = 154 K with the relaxor nature of electric ordering. The temperature dependent dielectric response in modulus representation indicates three relaxation processes within the experimental window. The variable range hopping model of small polarons explains the bulk non-Debye type conductivity relaxation. The fastest relaxation with activation energy Ea = 0.17 eV is related to progressive freezing of the reorientation motions of DMA(+) cations. X-ray diffraction data revealed that complete freezing of orientational and translational motions of DMA(+) cations occurs well below phase transition temperature. These experimental observations are fundamentally important for the theoretical explanation of relaxation dynamics in niccolite-type metal-organic frameworks.

A new approach in the synthesis of La(1-x)Gd(x)FeO3 perovskite nanoparticles--structural and magnetic characterization.

A series of highly crystalline orthoferrite nanoparticles (type La(1-x)Gd(x)FeO3, where x = 0 to 1) were prepared using the self-combustion method. Extensive studies including X-ray diffraction, Rietveld refinement and Fourier transform infrared spectroscopy as well as Raman spectroscopy confirmed the orthorhombic space group Pnma of the obtained materials. The calculated average grain size for powders is in the range of 30 to 80 nm. Magnetic characterization of the La(1-x)Gd(x)FeO3 series, performed at 1.72 K, indicated an antiferromagnetic state characterized by some canting of iron magnetic moments, in good agreement with the data reported for similar fine-particle systems.