Magnetism and electrical and thermal transport in the natural Fe1−xMnxWO4 (x = 0.2) mineral from Potosí, Bolivia

Abstract

The composition of a natural single crystalline specimen from the province of Potosí in Bolivia is found to be Fe0.8Mn0.2WO4. It crystallizes with the primitive monoclinic NiWO4 structure type [space group P2/c, a = 4.74751(6) Å, b = 5.71335(7) Å, c = 4.96847(5) Å, β = 90.15(1)°]. Magnetic susceptibility and specific heat capacity measurements indicated that the mineral undergoes multiple magnetic transitions: TN1 ≈ T\,cpN1 = 67(1) K, TN2 = 28(3) K, and T\,cpN2 = 8(1) K. The reduced magnetic entropy of ≈R ln 3 upon the high-temperature antiferromagnetic ordering suggests the failure of the simplified LS-coupling scheme in the description of the magnetism. Fe0.8Mn0.2WO4 is characterized by enlarged electrical resistivity showing an exponential decrease with temperature for T > 300 K, from which an energy gap of 310 meV is deduced. The well-pronounced maximum occurring in the phononic thermal conductivity just below the TN1 is described by the Debye–Callaway model, indicating the dominance of phonon scattering on defects as well as umklapp processes.