Structural, Spectroscopic, Electric and Magnetic Properties of New Trigonal K 5 FeHf(MoO 4 ) 6 Orthomolybdate.

A new multicationic structurally disordered K 5 FeHf(MoO 4 ) 6 crystal belonging to the molybdate family is synthesized by the two-stage solid state reaction method. The characterization of the electronic and vibrational properties of the K 5 FeHf(MoO 4 ) 6 was performed using density functional theory calculations, group theory, Raman and infrared spectroscopy. The vibrational spectra are dominated by vibrations of the MoO 4 tetrahedra, while the lattice modes are observed in a low-wavenumber part of the spectra. The experimental gap in the phonon spectra between 450 and 700 cm -1 is in a good agreement with the simulated phonon density of the states. K 5 FeHf(MoO 4 ) 6 is a paramagnetic down to 4.2 K. The negative Curie-Weiss temperature of -6.7 K indicates dominant antiferromagnetic interactions in the compound. The direct and indirect optical bandgaps of K 5 FeHf(MoO 4 ) 6 are 2.97 and 3.21 eV, respectively. The K 5 FeHf(MoO 4 ) 6 bandgap narrowing, with respect to the variety of known molybdates and the ab initio calculations, is explained by the presence of Mott-Hubbard optical excitation in the system of Fe 3+ ions.