Hierarchical C@MoS ₂ @C hollow spheres with active MoS ₂ nanosheets being sandwiched by carbon layers have been produced using a modified template method.

Single crystalline ammonium trivanadate NH₄V₃O₈ with variable morphologies, including shuttles, flowers, belts, and plates, was synthesized by the hydrothermal treatment of NH₄VO₃ with acetic acid. The crystals optimally grow under gentle conditions of 140 °C for 48 h. The resulting NH₄V₃O₈ microcrystals were characterized by means of X-ray diffraction, scanning electron microscopy, infrared and Raman spectroscopy, static magnetization studies, and thermal analysis. The key factors to control the size and morphology of the crystals are the pH value and the vanadium concentration. A tentative microscopic growth mechanism is proposed and it is demonstrated how shape and morphology of the resulting microcrystalline material can be tuned by appropriate synthesis parameters.

Thermodynamic properties and low-energy magnon excitations of $S=1$ honeycomb-layered ${\mathrm{Na}}_3{\mathrm{Ni}}_2{\mathrm{SbO}}_6$ have been investigated by high-resolution dilatometry, static magnetization, and high-frequency electron spin resonance studies in magnetic fields up to 16 T. At $T_{\mathrm{N}}$ = 16.5 K, there is a tricritical point separating two distinct antiferromagnetic phases, AF1 and AF2, from the paramagnetic regime. In addition, our data imply short-range antiferromagnetic correlations at least up to $\sim 5T_{\mathrm{N}}$. Well below $T_{\mathrm{N}}$, the magnetic field $B_{\text{C}1}\approx 9.5$ T is needed to stabilize AF2 against AF1. The thermal expansion and magnetostriction anomalies at $T_{\mathrm{N}}$ and $B_{\mathrm{C}1}$ imply significant magnetoelastic coupling, both of which are associated with a sign change of $\partial L/\partial B$. The transition at $B_{\mathrm{C}1}$ is associated with softening of the antiferromagnetic resonance modes observed in the electron-spin-resonance spectra. The anisotropy gap $\mathrm{\Delta }=360$ GHz implies considerable uniaxial anisotropy. We deduce the crucial role of axial anisotropy favoring the AF1 spin structure over the AF2 one. While the magnetostriction data disprove a simple spin-flop scenario at $B_{\mathrm{C}1}$, the nature of a second transition at $B_{\mathrm{C}2}$ $\approx$ 13 T remains unclear. Both the sign of the magnetostriction and Grüneisen analysis suggest that the short-range correlations at high temperatures are of AF2 type.

The magnetic structure of $\mathrm{L}{\mathrm{i}}_3\mathrm{N}{\mathrm{i}}_2\mathrm{Sb}{\mathrm{O}}_6$ has been determined by low-temperature neutron diffraction, and the crystal structure has been refined by a combination of synchrotron and neutron powder diffraction. The monoclinic ($C2/m$) symmetry, assigned previously to this pseudohexagonal layered structure, has been unambiguously proven by peak splitting in the synchrotron diffraction pattern. The structure is based on essentially hexagonal honeycomb-ordered $\mathrm{N}{\mathrm{i}}_2\mathrm{Sb}{\mathrm{O}}_6$ layers alternating with $\mathrm{L}{\mathrm{i}}_3$ layers, all cations and anions being in an octahedral environment. The compound orders antiferromagnetically below $T_{\mathrm{N}}=15\mathrm{K}$, with the magnetic supercell being a $2a\times 2b$ multiple of the crystal cell. The magnetic structure within the honeycomb layer consists of zigzag ferromagnetic spin chains coupled antiferromagnetically. The ordered magnetic moment amounts to $1.62\left(2\right){\mu }_{\mathrm{B}}/\mathrm{Ni}$, which is slightly lower than the full theoretical value. Upon cooling below $T_{\mathrm{N}}$, the spins tilt from the $c$ axis, with a maximum tilting angle of $15.6^{\circ }$ at $T=1.5\mathrm{K}$. Our data imply non-negligible ferromagnetic interactions between the honeycomb layers. The observed antiferromagnetic resonance modes are in agreement with the two-sublattice model derived from the neutron data. Orthorhombic anisotropy shows up in zero-field splitting of $\mathrm{\Delta }=198\pm 4$ and $218\pm 4\mathrm{GHz}$. Above $T_{\mathrm{N}}$, the electron spin resonance data imply short-range antiferromagnetic order up to about 80 K.

We investigate in detail the low-temperature structural and physical properties of the trilayer nickelates $R_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($R=\mathrm{La}$, Pr, and Nd), which crystallize with a monoclinic symmetry (space group $P{2}_{1}a,Z=4$) and undergo a metal-to-metal transition (MMT) near $T_{\mathrm{MMT}}$= $135\mathrm{K}\left(\mathrm{La}\right), 156\mathrm{K}$ (Pr), and $160\mathrm{K}$ (Nd). Using a high-resolution synchrotron powder x-ray diffraction technique, we show that the lattice parameters in all cases exhibit an anomalous behavior at $T_{\mathrm{MMT}}$, however, without any sign of change in the lattice symmetry. Unambiguous signature of MMT is also observed in the magnetic and transport data, suggesting a strong coupling between the electronic, magnetic, and structural degrees of freedom. Analysis of thermal expansion yields hydrostatic pressure dependence of MMT in close agreement with previous high-pressure experiments. In ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, the ${\mathrm{Pr}}^{3+}$ ions located in the rocksalt (RS) layers order magnetically near $5\mathrm{K}$, which is significantly suppressed compared to ${\theta }_p\sim -36\mathrm{K}$. In contrast, ${\mathrm{Pr}}^{3+}$ ions in the perovskite-block (PB) layers exhibit a crystal field (CF) induced nonmagnetic singlet ground state. In ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$, on the other hand, the CF ground state of ${\mathrm{Nd}}^{3+}$ ions in both RS and PB layers is a Kramers doublet. The heat capacity of ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ shows a pronounced Schottky-like anomaly near $40\mathrm{K}$, and a sharp upturn indicating short-range correlations between the Nd-moments below $10\mathrm{K}$. However, no signs of long-range ordering of Nd-moments could be found down to $2\mathrm{K}$ despite a sizable value of ${\theta }_p\sim -40\mathrm{K}$. The strongly suppressed magnetic long-range ordering in both $R=\mathrm{Pr}$ and Nd suggests the presence of strong magnetic frustration in these compounds. In the presence of an overwhelming Schottky contribution, the electronic term in the specific heat of ${\mathrm{Pr}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ and ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ appears highly inflated, which can be falsely interpreted as a sign of heavy fermion behavior as is done in a recent study on ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$. Accordingly, the low-temperature resistivity of these compounds is found to follow a $-T^{0.5}$ rather than a $-\ln T$ dependence.