Magnetic ordering of the Mo$_3$O$_8$-type cluster Mott insulator Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$ with spin-1/2 triangular lattice prepared via optimal synthesis

We detail the optimized synthesis of the Mo$_3$O$_8$-type cluster Mott insulator (CMI) Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$, which has been considered a candidate for realizing the spin liquid ground state. The optimized Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$, characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, and magnetic and heat capacity measurements, exhibited an effective magnetic moment close to the ideal 1.73 $\mu_{\mathrm B}$ for $S = 1/2$ spin and magnetic ordering at $\sim$ 5 K. These observations categorize Na$_3$Sc$_2$(MoO$_4$)$_2$Mo$_3$O$_8$ as the second Mo$_3$O$_8$-type CMI to achieve a magnetic ground state, following Li$_2$InMo$_3$O$_8$. They highlight the stabilization of the magnetic ground state over the theoretically anticipated quantum spin liquid state through precise valence and chemical disorder tuning. Our findings challenge the existing theory that the magnetic ground state of Mo$_3$O$_8$-type CMIs is determined by the breathing parameter, instead showing that magnetic order is suppressed by spin defects. This study underscores the crucial role of chemical precision in investigating quantum magnetism. It suggests that precise tuning of valence states could induce magnetic ordering in previously nonmagnetic Mo$_3$O$_8$-type CMIs. Additionally, the negative findings regarding the existence of quantum spin liquids highlight the need for applied research and a reevaluation of our fundamental understanding of electronic states from both theoretical and experimental aspects.
Comment: 8 pages, 5 figures, accepted in Physical Review Materials