Structural, Spectroscopic, Electric and Magnetic Properties of New Trigonal K5FeHf(MoO4)6 Orthomolybdate

 https://doi.org/10.3390/molecules28041629

A new multicationic structurally disordered K₅FeHf(MoO₄)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 K₅FeHf(MoO₄)₆ was performed using density functional theory calculations, group theory, Raman and infrared spectroscopy. The vibrational spectra are dominated by vibrations of the MoO₄ 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. K₅FeHf(MoO₄)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 K₅FeHf(MoO₄)₆ are 2.97 and 3.21 eV, respectively. The K₅FeHf(MoO₄)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 Fe³⁺ ions.