The crystal structure of $\mathrm{B}{\mathrm{a}}_4\mathrm{NbR}{\mathrm{u}}_3{\mathrm{O}}_{12}$ is based on triangular planes of elongated $\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{12}$ trimers oriented perpendicular to the plane. We report that it is semiconducting, that its Weiss temperature and effective magnetic moment are −155 K and $2.59{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, respectively, and that the magnetic susceptibility and specific-heat data indicate that it exhibits magnetic ordering near 4 K. The presence of a high density of low energy states is evidenced by a substantial Sommerfeld-like T-linear term $\mathrm{[}\gamma =31\left(2\right)\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}]$ in the specific heat. Electronic-structure calculations reveal that the electronic states at the Fermi energy reside on the $\mathrm{R}{\mathrm{u}}_3{\mathrm{O}}_{12}$ trimers and that the calculated density of electronic states is high and continuous around the Fermi energy—in other words density functional theory calculates the material to be a metal. The results imply that $\mathrm{B}{\mathrm{a}}_4\mathrm{NbR}{\mathrm{u}}_3{\mathrm{O}}_{12}$ is a geometrically frustrated trimer-based Mott insulator.

Two-dimensional van der Waals magnetic semiconductors display emergent chemical and physical properties and hold promise for novel optical, electronic and magnetic "few-layers" functionalities. Transition-metal iodides such as CrI₃ and VI₃ are relevant for future electronic and spintronic applications; however, detailed experimental information on their ground state electronic properties is lacking often due to their challenging chemical environment. By combining X-ray electron spectroscopies and first-principles calculations, we report a complete determination of CrI₃ and VI₃ electronic ground states. We show that the transition metal-induced orbital filling drives the stabilization of distinct electronic phases: a wide bandgap in CrI₃ and a Mott insulating state in VI₃. Comparison of surface-sensitive (angular-resolved photoemission spectroscopy) and bulk-sensitive (X-ray absorption spectroscopy) measurements in VI₃ reveals a surface-only V²⁺ oxidation state, suggesting that ground state electronic properties are strongly influenced by dimensionality effects. Our results have direct implications in band engineering and layer-dependent properties of two-dimensional systems.

$\mathrm{B}{\mathrm{a}}_4\mathrm{NbI}{\mathrm{r}}_3{\mathrm{O}}_{12}$, a material with a triangular planar geometry of $\mathrm{I}{\mathrm{r}}_3{\mathrm{O}}_{12}$ trimers, is described. Magnetic susceptibility measurements show it to be paramagnetic with no magnetic ordering down to 1.8 K despite the Curie-Weiss temperature $\left({\theta }_{\mathrm{CW}}\right)$ of −13 K. The material has a very low effective magnetic moment $\left({\mu }_{\mathrm{eff}}\right)$ of 0.80 ${\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ To look at the lower-temperature behavior, the specific heat $\left(C_p\right)$ was measured down to 0.35 K; it shows no indication of magnetic ordering and fitting a power law to $C_p$ vs $T$ below 2 K yields the power $\alpha =3/4$. Comparison to the trimer compound made with the $4d$ element Rh in place of the $5d$ element Ir, $\mathrm{B}{\mathrm{a}}_4\mathrm{NbR}{\mathrm{h}}_3{\mathrm{O}}_{12}$, which is also paramagnetic (${\mu }_{\mathrm{eff}}=1.48{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}., {\theta }_{\mathrm{CW}}=-23\mathrm{K}$), is presented. The analysis suggests that $\mathrm{B}{\mathrm{a}}_4\mathrm{NbI}{\mathrm{r}}_3{\mathrm{O}}_{12}$ is a candidate spin liquid material.

Magnetic susceptibility, magnetization, and specific-heat measurements under both in-plane and out-of-plane field on ${\mathrm{K}}_2{\mathrm{Co}}_2{\left(\mathrm{Se}{\mathrm{O}}_3\right)}_3$ single crystals are reported. The Co dimers in this compound, which are made from face-sharing effective spin-1/2 $\mathrm{Co}{\mathrm{O}}_6$ octahedra, sit on the corners of a geometrically frustrating triangular planar lattice. No long-range magnetic ordering is found down to 0.35 K, which, together with the large negative Curie-Weiss temperature obtained from fitting the temperature-dependent magnetic susceptibility, indicates that the system is magnetically frustrated. Analysis of the field and orientation dependence of the magnetic and thermodynamic properties suggests the presence of an easy-axis anisotropy. The magnetic fluctuations present due to geometric frustration are more resistant to an in-plane field than a perpendicular-to-plane field.