The three-dimensional charge density wave (CDW) compound Lu 2 Ir 3 Si 5 undergoes a first-order CDW phase transition at around 200 K. An atypical CDW state is found, that is characterized by an incommensurate CDW with q = [0.2499(3), 0.4843(4), 0.2386(2)] at 60 K, and a large orthorhombic-to-triclinic lattice distortion with β = 91.945(2) • . We present the modulated crystal structure of the incommensurate CDW state. Structural analysis shows that the CDW resides on the zigzag chains of iridium atoms along c. The structural distortions are completely similar between nonmagnetic Lu 2 Ir 3 Si 5 and previously studied isostructural magnetic Er 2 Ir 3 Si 5 with the small differences explained by the different values of the atomic radii of Lu and Er. Such a similarity is unique to R 2 Ir 3 Si 5 (R = rare earth). It differs from, for example, the rare-earth CDW compounds R 5 Ir 4 Si 10 for which Lu 5 Ir 4 Si 10 and Er 5 Ir 4 Si 10 possess entirely different CDW states. We argue that the mechanism of CDW formation, thus, is different for R 2 Ir 3 Si 5 and R 5 Ir 4 Si 10 .

EuAl4 possesses the BaAl4 crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge density wave (CDW) transition at T CDW = 145 K and features four consecutive antiferromagnetic phase transitions below 16 K. Here we use single-crystal X-ray diffraction to determine the incommensurately modulated crystal structure of EuAl4 in its CDW state. The CDW is shown to be incommensurate with modulation wave vector q = (0,0,0.1781 (3)) at 70 K. The symmetry of the incommensurately modulated crystal structure is orthorhombic with superspace group Fmmm(00σ)s00, where Fmmm is a subgroup of I4/mmm of index 2. Both the lattice and the atomic coordinates of the basic structure remain tetragonal. Symmetry breaking is entirely due to the modulation wave, where atoms Eu and Al1 have displacements exclusively along a, while the fourfold rotation would require equal displacement amplitudes along a and b. The calculated band structure of the basic structure and interatomic distances in the modulated crystal structure both indicate the Al atoms as the location of the CDW. The temperature dependence of the specific heat reveals an anomaly at T CDW = 145 K of a magnitude similar to canonical CDW systems. The present discovery of orthorhombic symmetry for the CDW state of EuAl4 leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

Ho 2 Ir 3 Si 5 belongs to the family of three-dimensional (3D) R 2 Ir 3 Si 5 (R = Lu, Er, Ho) compounds that exhibit a first-order, charge-density-wave (CDW) phase transition, where there is a strong orthorhombic-to-triclinic distortion of the lattice accompanied by superlattice reflections. The analysis by single-crystal X-ray diffraction (SXRD) has revealed that the Ir−Ir zigzag chains along c are responsible for the CDW in all three compounds. The replacement of the rare earth element from nonmagnetic Lu to magnetic Er or Ho lowers T CDW , where T CDWLu = 200 K, T CDWEr = 150 K, and T CDWHo = 90 K. Out of the three compounds, Ho 2 Ir 3 Si 5 is the only system where second-order superlattice reflections could be observed, indicative of an anharmonic shape of the modulation wave. The CDW transition is observed as anomalies in the temperature dependencies of the specific heat, electrical conductivity, and magnetic susceptibility, which includes a large hysteresis of 90 to 130 K for all measured properties, thus corroborating the SXRD measurements. Similar to previously reported Er 2 Ir 3 Si 5 , there appears to be a coupling between CDW and magnetism such that the Ho 3+ magnetic moments are influenced by the CDW transition, even in the paramagnetic state. Moreover, earlier investigations on polycrystalline material revealed antiferromagnetic (AFM) ordering at T N = 5.1 K, whereas AFM order is suppressed and only the CDW is present down to at least 0.1 K in our highly ordered single crystal. First-principles calculations predict Ho 2 Ir 3 Si 5 to be a metal with coexisting electron and hole pockets at the Fermi level. The Ho and Ir atoms have spherically symmetric metallic-type charge density distributions that are prone to CDW distortion. Phonon calculations affirm that the Ir atoms are primarily responsible for the CDW distortion, which is in agreement with the experiment.

Intercalation and stacking‐order modulation are two active ways in manipulating the interlayer interaction of transition metal dichalcogenides (TMDCs), which lead to a variety of emergent phases and allow for engineering material properties. Herein, the growth of Pb‐intercalated TMDCs–Pb(Ta1+xSe2)2, the first 124‐phase, is reported. Pb(Ta1+xSe2)2 exhibits a unique two‐step first‐order structural phase transition at around 230 K. The transitions are solely associated with the stacking degree of freedom, evolving from a high‐temperature (high‐T) phase with ABC stacking and R3m symmetry to an intermediate phase with AB stacking and P3m1, and finally to a low‐temperature (low‐T) phase again with R3msymmetry, but with ACB stacking. Each step involves a rigid slide of building blocks by a vector [1/3, 2/3, 0]. Intriguingly, gigantic lattice contractions occur at the transitions on warming. At low‐T, bulk superconductivity with Tc ≈ 1.8 K is observed. The underlying physics of the structural phase transitions are discussed from first‐principle calculations. The symmetry analysis reveals topological nodal lines in the band structure. The results demonstrate the possibility of realizing higher‐order metal‐intercalated phases of TMDCs and advance the knowledge of polymorphic transitions, and may inspire stacking‐order engineering in TMDCs and beyond.

This work reports reversible, single-crystal-to-single-crystal phase transitions of commensurately modulated sodium saccharinate 1.875-hydrate [Na(sac)(15/8)H2O]. The phases were studied in the temperature range 298 to 20 K. They exhibit complex disordered states. An unusual reentrant disorder has been discovered upon cooling through a phase transition at 120 K. The disordered region involves three sodium cations, four water molecules and one saccharinate anion. At room temperature, the structure is an eightfold superstructure that can be described by the superspace group C2/c(0σ20)s0 with q = (0, 3/4, 0). It demonstrates maximum disorder with the disordered chemical entities having slightly different but close to 0.50:0.50 disorder component ratios. Upon cooling, the crystal tends to an ordered state, smoothly reaching a unified disorder component ratio of around 0.90:0.10 for each of the entities. Between 130 and 120 K a phase transition occurs involving a sudden increase of the disorder towards the disorder component ratio 0.65:0.35. Meanwhile, the space group and general organization of the structure are retained. Between 60 and 40 K there is another phase transition leading to a twinned triclinic phase. After heating the crystal back to room temperature its structure is the same as before cooling, indicating a complete reversibility of the phase transitions.