Evolution of the spin-flop antiferromagnetic transition and the emergence of frustration in saw-tooth lattice Mn₂SiS_4-xSe_x (x = 0 - 4) chalcogenides

The olivine A₂BX₄ structure takes Pnma symmetry where the A site forms the triangle-based one dimensional saw-tooth chain structure and are known for optoelectronics, thermoelectric and magentic applications. We studied the olivines, Mn₂SiS_4-xSe_x (0 < x < 4), using magnetization, specific heat, thermal conductivity and ab initio density functional theory (DFT) calculations and tracked the evolution of spin-flop transition and frustration. It is seen that the antiferromagnetic transition temperature T_N shows a linear decrease from 86 K for Mn₂SiS₄ towards 66 K for Mn₂SiSe₄ as x varied; whereas the critical field for spin-flop varied non-linearly. The magnetic phase transitions characterized using specific heat reveal very low magnetic entropy evolved at the T_N, suggesting the underlying frustrated nature of the Mn²⁺ ions in the saw-tooth like triangular arrangement. A semiconducting-like thermal conductivity is revealed by the olivines where the DFT calculations show a small energy gap (~ 0.5 eV) between the valence and conduction bands. Our results are explained based on the emergence of frustration in the triangular units of the saw-tooth Mn lattice.