Averievite-type compounds with the general formula (MX)[Cu₅O₂(TO₄)], where M = alkali metal, X = halogen and T = P, V, have been synthesized by crystallization from gases and structurally characterized for six different compositions: 1 (M = Cs; X = Cl; T = P), 2 (M = Cs; X = Cl; T = V), 3 (M = Rb; X = Cl; T = P), 4 (M = K; X = Br; T = P), 5 (M = K; X = Cl; T = P) and 6 (M = Cu; X = Cl; T = V). The crystal structures of the compounds are based upon the same structural unit, the layer consisting of a kagome lattice of Cu²⁺ ions and are composed from corner-sharing (OCu₄) anion-centered tetrahedra. Each tetrahedron shares common corners with three neighboring tetrahedra, forming hexagonal rings, linked into the two-dimensional [O₂Cu₅]⁶⁺ sheets parallel to (001). The layers are interlinked by (T⁵⁺O₄) tetrahedra (T⁵⁺ = V, P) attached to the bases of the oxocentered tetrahedra in a “face-to-face” manner. The resulting electroneutral 3D framework {[O₂Cu₅](T⁵⁺O₄)₂}⁰ possesses channels occupied by monovalent metal cations M⁺ and halide ions X⁻. The halide ions are located at the centers of the hexagonal rings of the kagome nets, whereas the metal cations are in the interlayer space. There are at least four different structure types of the averievite-type compounds: the P-3m1 archetype, the 2 × 2 × 1 superstructure with the P-3 space group, the monoclinically distorted 1 × 1 × 2 superstructure with the C2/c symmetry and the low-temperature P2₁/c superstructure with a doubled unit cell relative to the high-temperature archetype. The formation of a particular structure type is controlled by the interplay of the chemical composition and temperature. Changing the chemical composition may lead to modification of the structure type, which opens up the possibility to tune the geometrical parameters of the kagome net of Cu²⁺ ions.

Pressure evolution of the crystal structure and magnetism of the honeycomb $\alpha \text{−}{\mathrm{RuBr}}_3$ is studied using high-pressure x-ray diffraction, magnetometry, and density-functional band-structure calculations. Hydrostatic compression transforms antiferromagnetic $\alpha \text{−}{\mathrm{RuBr}}_3$ $\left(R\overline{3}\right)$ into paramagnetic ${\alpha }^{\prime }\text{−}{\mathrm{RuBr}}_3$ $\left(P\overline{1}\right)$ where short Ru-Ru bonds cause magnetism collapse above 1.3 GPa at 0 K and 2.5 GPa at 295 K. Below this critical pressure, the Néel temperature of $\alpha \text{−}{\mathrm{RuBr}}_3$ increases with the slope of 1.8 K/GPa. Pressure tunes $\alpha \text{−}{\mathrm{RuBr}}_3$ away from the Kitaev limit, whereas increased third-neighbor in-plane coupling and interlayer coupling lead to a further stabilization of the collinear zigzag state. Both $\alpha$- and ${\alpha }^{\prime }\text{−}{\mathrm{RuBr}}_3$ are metastable at ambient pressure, but their transformation into the thermodynamically stable $\beta$ polymorph is kinetically hindered at room temperature.

Using infrared and Raman spectroscopies combined with high-resolution x-ray diffraction, we address several controversial aspects of altermagnetic α -MnTe. We show that mechanical stress applied to crystals of this material causes a drastic broadening of Bragg peaks that conceals signatures of additional phases present in the sample. Indeed, spatially resolved Raman spectroscopy reveals that the modes around 175 cm − 1 often reported in α -MnTe are not reproducible across different positions and samples and originate from the secondary phase of MnTe 2 . By combining spectroscopic probes with calculations, we establish the IR-active optical phonon of α -MnTe around 155 cm − 1 ( E 1 u ) and the Raman-active optical phonon around 100 cm − 1 ( E 2 g ) at room temperature. Two intense Raman modes around 120 and 140 cm − 1 are shown to be intrinsic, even though they can not be assigned to Γ -point optical phonons. These modes couple to magnetic order in α -MnTe and also to the transient reflectivity, resulting in coherent oscillations. Both sixfold rotation symmetry and inversion symmetry are preserved in bulk α -MnTe within our experimental resolution.