The crystal growth and properties of novel magnetic double molybdate RbFe5(MoO4)7 with mixed Fe3+/Fe2+ states and 1D negative thermal expansion

Abstract

Single crystals of new compound RbFe₅(MoO₄)₇ were successfully grown by the flux method, and their crystal structure was determined using the X-ray single-crystal diffraction technique. The XRD analysis showed that the compound crystallizes in the monoclinic space group P2₁/m, with unit cell parameters a = 6.8987(4), b = 21.2912(12) and c = 8.6833(5) Å, β = 102.1896(18)°, V = 1246.66(12) ų, Z (molecule number in the unit cell) = 2, R-factor (reliability factor) = 0.0166, and T = 293(2) K. Raman spectra were collected on the single crystal to show the local symmetry of MoO₄ tetrahedra, after the confirmation of crystal composition using energy dispersive X-ray spectroscopy (EDS). The polycrystalline samples were synthesized by a solid-state reaction in the Ar atmosphere; the particle size and thermal stability were investigated by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses. The compound decomposes above 1073 K in an Ar atmosphere with the formation of Fe(III) molybdate. The thermal expansion coefficient along the c direction has the value α = −1.3 ppm K⁻¹ over the temperature range of 298–473 K. Magnetic measurements revealed two maxima in the magnetization below 20 K, and paramagnetic behavior above 50 K with the calculated paramagnetic moment of 12.7 μB per formula unit is in good agreement with the presence of 3Fe³⁺ and 2Fe²⁺ in the high-spin (HS) state. The electronic structure of RbFe₅(MoO₄)₇ is comparatively evaluated using X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations.