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Single crystal $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{V}}_{2}{\mathrm{PO}}_{5}$ was synthesized by chemical vapor transport method and characterized by transport, thermodynamic, neutron diffraction, nuclear magnetic resonance measurements and first-principles calculation. It was shown to be a semiconductor with a band gap of 0.48 eV, undergoing a charge ordering (unusual ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$) phase transition accompanied by a tetragonal to monoclinic structural distortion at 610 K and a paramagnetic to ferrimagnetic phase transition at 128 K with a propagation vector of $\mathbf{k}=0$. The easy axis is in the monoclinic $ac$ plane pointing 47(9)${}^{\ensuremath{\circ}}$ away from the monoclinic $a$ axis. This collinear ferrimagnetic structure and anisotropic isothermal magnetization measurements suggest weak magnetic anisotropy in this compound. The first-principles calculations indicate that the intrachain interactions in the face-sharing ${\mathrm{VO}}_{6}$ chains dominate the magnetic hamiltonian and identify the ${\mathrm{\ensuremath{\Gamma}}}_{5}^{+}$ normal mode of the lattice vibration to be responsible for the charge ordering and thus the structural phase transition.

We present the significant spin-driven dielectric anomaly ($\ensuremath{\sim}40%$ drop) and magnetoelectric coupling near the magnetic ordering temperature in single crystal ${\mathrm{Fe}}_{4}{\mathrm{Nb}}_{2}{\mathrm{O}}_{9}$. By combining neutron and x-ray single crystal diffraction techniques, we unambiguously determined its magnetic symmetry and studied the structural phase transition at ${T}_{S}$ = 70 K. The temperature-dependent static dielectric constant is strongly anisotropic, rendering two dielectric anomalies along the $a$ axis in the hexagonal lattice with the first one coupled to the magnetic ordering around ${T}_{N}$ = 97 K and the second one accompanying with a first-order structural transition around ${T}_{S}$ = 70 K. Below ${T}_{N}$, we found that the anomalous dielectric constant is practically proportional to the square of the magnetic moment from neutron diffraction data, indicating that the exchange striction is likely responsible for the strong spin-lattice coupling. Magnetic-field-induced magnetoelectric coupling was observed and is compatible with the determined magnetic structure that is characteristic of antiferromagnetically arranged ferromagnetic chains in the honeycomb plane. We propose that such magnetic symmetry should be immune to external magnetic fields to some extent favored by the freedom of rotation of moments in the honeycomb plane, laying out a promising system to control the magnetoelectric properties by magnetic fields.

The Dzyaloshinskii-Moriya (DM) interaction is present in the transition metal dichalcogenides (TMDC) magnets of form ${M}_{1/3}T{\mathrm{S}}_{2}$ ($M=3d$ transition metal, $T\ensuremath{\in}{\mathrm{Nb},\mathrm{Ta}}$), given that the intercalants $M$ form $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ superlattices within the structure of the parent materials $T{\mathrm{S}}_{2}$ and break the centrosymmetry. Competition between DM and ferromagnetic exchange interactions has been shown to stabilize a topological defect known as a chiral soliton in select intercalated TMDCs, initiating interest both in terms of fundamental physics and the potential for technological applications. In the current article we report on our study of the materials ${\mathrm{V}}_{1/3}{\mathrm{TaS}}_{2}$ and ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_{2}$, using a combination of x-ray powder diffraction, magnetization, and single crystal neutron diffraction. Historically identified as ferromagnets, our diffraction results instead reveal that vanadium spins in these compounds are arranged into an A-type antiferromagnetic configuration at low temperatures. Refined moments are 1.37(6) and 1.50(9) ${\ensuremath{\mu}}_{B}$ for ${\mathrm{V}}_{1/3}{\mathrm{TaS}}_{2}$ and ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_{2}$, respectively. Transition temperatures ${T}_{c}=32\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for ${\mathrm{V}}_{1/3}{\mathrm{TaS}}_{2}$ and 50 K for ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_{2}$ are obtained from the magnetization and neutron diffraction results. We attribute the small net magnetization observed in the low-temperature phases to a subtle ($\ensuremath{\sim}{2}^{\ensuremath{\circ}}$) canting of XY spins in the out-of-plane direction. These new results are indicative of dominant antiferromagnetic exchange interactions between the vanadium moments in adjacent $ab$ planes, likely eliminating the possibility of identifying stable chiral solitons in the current materials.

We report the single-crystal growth and characterization of a hexagonal phase ${\mathrm{Cu}}_{0.82}{\mathrm{Mn}}_{1.18}\mathrm{As}$, in the Cu-Mn-As system. This compound contains the same square-pyramidal ${\mathrm{MnAs}}_{5}$ units as the tetragonal and orthorhombic polymorphs of CuMnAs. Calorimetry, magnetometry, and neutron diffraction measurements reveal antiferromagnetic ordering at 270 K. The magnetic structure consists of a triangular arrangement of spins in the $ab$ plane. Hexagonal ${\mathrm{Cu}}_{0.82}{\mathrm{Mn}}_{1.18}\mathrm{As}$ shows resistivity that varies only weakly from 5 to 300 K, and is many times higher than tetragonal CuMnAs, indicative of a strongly scattering metal. First-principles calculations confirm the metallic band structure with a small density of states at the Fermi energy. The neutron-refined magnetic ground state is close to the computationally determined minimum energy configuration. This compound should serve as a clear control when disentangling the effects of current-driven N\'eel switching of metallic antiferromagnets since it exhibits in-plane spins but the magnetic ordering does not break degeneracy along the $a$ and $b$ directions, unlike tetragonal CuMnAs.

We have synthesized single crystals of ${\mathrm{Na}}_{3}{\mathrm{Co}}_{2}{\mathrm{SbO}}_{6}$ and characterized the structure and magnetic order by measuring anisotropic magnetic properties, heat capacity, and x-ray and neutron single crystal diffraction. Magnetic properties and specific heat of polycrystalline ${\mathrm{Na}}_{3}{\mathrm{Co}}_{2}{\mathrm{SbO}}_{6}$ were also measured for comparison. ${\mathrm{Na}}_{3}{\mathrm{Co}}_{2}{\mathrm{SbO}}_{6}$ crystallizes in a monoclinic structure (space group $C2/m)$ with ${[{\mathrm{Co}}_{2}{\mathrm{SbO}}_{6}]}^{3\ensuremath{-}}$ layers separated by ${\mathrm{Na}}^{+}$ ions. The temperature dependence of magnetic susceptibility shows significant anisotropic behavior in the whole temperature range 2--350 K investigated in this work. An effective moment of about $5.5{\ensuremath{\mu}}_{B}/{\mathrm{Co}}^{2+}$ from a Curie-Weiss fitting of the magnetic susceptibility is larger than the spin only value and signals significant orbital contribution. ${\mathrm{Na}}_{3}{\mathrm{Co}}_{2}{\mathrm{SbO}}_{6}$ single crystal undergoes a transition into a long-range antiferromagnetically ordered state below ${T}_{N}=5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Neutron single crystal diffraction confirmed the zigzag magnetic structure with a propagation vector $k=(0.5,0.5,0)$. The ordered moment is found to be $0.9{\ensuremath{\mu}}_{B}$ at 4 K and align along the crystallographic $b$ axis. Density functional theory calculations suggest that the experimentally observed zigzag order is energetically competing with the N\'eel order. It is also found that the covalency between Co $d$ and O $p$ is quite strong and competes with the local spin-orbit coupling, suggesting a ${J}_{\mathrm{eff}}=1/2$ ground state may not be realized in this compound.

We report a detailed investigation of ${\mathrm{K}}_{2}{\mathrm{MnS}}_{2}$ and ${\mathrm{K}}_{2}{\mathrm{MnSe}}_{2}$ from the ${\mathrm{K}}_{2}{\mathrm{MnS}}_{2}$ structure type and their magnetic solid solution ${\mathrm{K}}_{2}{\mathrm{MnS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ and find that compounds of this structure type consist of strongly coupled pseudo-one-dimensional antiferromagnetic chains that collectively represent a frustrated two-dimensional triangular antiferromagnet. Bulk samples of ${\mathrm{K}}_{2}{\mathrm{MnS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ with $0\ensuremath{\le}x\ensuremath{\le}2$ are characterized using x-ray diffraction, neutron diffraction, magnetization, and heat-capacity measurements. An incommensurate cycloid magnetic structure with a magnetic propagation vector $k=[0.58\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}1]$ is observed for all samples in ${\mathrm{K}}_{2}{\mathrm{MnS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$, and the ordering is robust despite a 12% increase in cell volume. Geometric frustration of chains results in incommensurability along $a$ and a two-step magnetic transition. The varying geometries accessible in compounds of this structure type are presented as promising avenues to tune frustration.

The magnetic behavior of Sn-substituted ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ has been examined through magnetization measurements and single-crystal neutron diffraction on ${\mathrm{Sr}}_{2}{\mathrm{Ir}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}{\mathrm{O}}_{4}$ for $0.05\ensuremath{\lesssim}x\ensuremath{\lesssim}0.35$. For all $x$ examined, the magnetic ground state is similar to that induced in ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$ by application of a magnetic field. This magnetic state is characterized by a small net moment due to the alignment of canted moments within an otherwise antiferromagnetic structure. The weak ferromagnetic behavior is observed in the magnetization data for both unique crystallographic orientations. Diffraction data reveal an anisotropic response in the lattice parameters, with an overall lattice expansion, as well as a small reduction in the rotation of the ${\mathrm{IrO}}_{6}$ octahedra. The magnetic ordering temperature is continually suppressed with increasing $x$, and long-range magnetic order persists to the highest Sn concentrations synthesized.

The non-centrosymmetric Weyl semimetal candidate, MoTe$_2$ was investigated through neutron diffraction and transport measurements at pressures up to 1.5 GPa and at temperatures down to 40 mK. Centrosymmetric and non-centrosymmetric structural phases were found to coexist in the superconducting state. Density Functional Theory (DFT) calculations reveal that the strength of the electron-phonon coupling is similar for both crystal structures. Furthermore, it was found that by controlling non-hydrostatic components of stress, it is possible to mechanically control the ground state crystal structure. This allows for the tuning of crystal symmetry in the superconducting phase from centrosymmetric to non-centrosymmetric. DFT calculations support this strain control of crystal structure. This mechanical control of crystal symmetry gives a route to tuning the band topology of MoTe$_2$ and possibly the topology of the superconducting state., 16 pages, 4 figures, supplement included