Arsenides continue to captivate the fields of inorganic chemistry and condensed matter physics. In this work we present the discovery of two new members of this family─U8Co42As25 and UCo3As2─which were synthesized in single crystalline form. U8Co42As25 is the first representative of the Y8Co41As25structure type (space group P63/m, Pearson symbol hP75, a = 17.7460(5) Å, c = 3.8120(1) Å), while UCo3As2 adopts HoCo3P2 type of structure (space group Pmmn, Pearson symbol oP36, a = 3.8611(2) Å, b = 10.8414(6) Å, c = 12.7297(6) Å). Both materials order antiferromagnetically─U8Co42As25 at TN = 26 K and UCo3As2 at TN = 67 K.

Noncentrosymmetric crystal structures are of significant interest due to the peculiar physical phenomena they frequently promote. Among mercury-based compounds, only a few noncentrosymmetric materials are known, with the vast majority remaining unexplored. In this work, we examine the family of noncentrosymmetric mercurides Na11Hg52 and A11-xHg54+x (A = Ca or Sr). All three materials are identified as new superconductors. The values of the critical temperatures are fairly modest, ranging from 1.7 K (Ca11-xHg54+x) to 3.1 K (Na11Hg52). We postulate that the superconductivity of noncentrosymmetric Na11Hg52 and A11-xHg54+x (A = Ca or Sr) compounds is likely conventional.

Cu12-xNixSb4S13 (x = 0, 0.05, 0.2, 0.5) tetrahedrites were synthesized by the polyol method. Measurements of magnetic susceptibility, specific heat capacity as well as electrical and thermal transport properties indicated a structural phase transition (SPT) of 1st order at Tk = 75-85 K, which is accompanied by strong changes in electronic density of states (EDOS) and entropy for x = 0, 0.05, and 0.2, whereas only weak changes were observed at x = 0.5. However, further temperature-dependent synchrotron high-resolution powder X-ray diffraction (HR PXRD) studies revealed compounds with Ni-content x = 0 and 0.5 to remain body-centered cubic [space group (SG) I43m, a approximate to 10.4(1) angstrom] down to 10 K, whereas crystal structures of x = 0.05 and 0.2 became tetragonal (SG P4c2, , ctetr approximate to acub) below Tk. Comparing changes of EDOS at the Fermi level [Delta N(EF)] deduced from magnetic and thermodynamic data with those calculated from density functional theory (DFT) in a rigid-band approximation, we found that decreases with Ni content and vanishes for x = 0.5, thus explaining the absence of the tetragonal instability at higher Ni dopings. This study shows that the SPT in tetrahedrites is facilitated by the presence of a minor amount of dopant.

In solid-state compounds, the valence of europium can sometimes be mixed, which is especially favored in structures with several positions for the europium atoms. In this work, we study the Eu-based intermetallic noncentrosymmetric system Eu11-x Hg54+x , which has 65 atoms per unit cell and 4 distinct crystallographic positions for europium and 14 positions for mercury. Our detailed analysis of the magnetism of large single crystals suggests that europium in Eu11-x Hg54+x might be present in two valence states, resulting in a fragile magnetic ground state. Due to the cage-like structure with a large distance between the Eu atoms, those atoms are weakly ferromagnetically coupled and Eu11-x Hg54+x orders at low temperatures, below T 1 = 5.5 K, with a subsequent spin reorientation at T 2 = 4.3 K. There is no sign of magnetic frustration. Interestingly, the magnetic ordering of the europium substructure results in a magnetization pole reversal with a delicate ferrimagnetic ground state. Additional magnetic phases can be induced by the application of a modest external magnetic field.

Understanding superconductivity requires a deep comprehension of the chemical structure. The discovery of a new unconventional superconductor UTe2 a few years ago prompted many detailed investigations of its physical properties. Despite its unconventional ground state being rather well-studied, a strong sample-to-sample variation of superconducting behavior as a result of different preparation conditions has remained largely unexplained until now. In this work, an in-depth analysis of the UTe2 crystal structure and resultant physical properties by implementing several types of synthetic routes was carried out. The difference between superconducting and non-superconducting UTe2 lies in the presence of uranium vacancies, on the order of 4%. As a result, the b and c lattice parameters vary, yielding a volume difference of about 0.51%. A subtler difference between samples exhibiting one and two superconducting transitions is driven by local deviations from the translational symmetry in the main atomic arrangement. Several well-pronounced maxima have been observed in the difference density map, predominantly located in the bc plane due to a local appearance of the similar atomic arrangements in different orientations. The extreme sensitivity of UTe2 to such a small number of defects re-emphasizes the unconventional nature of superconductivity in this compound. Furthermore, this work underscores the importance of a thorough, combined chemical and physical analysis of intriguing strongly correlated materials─in particular for compounds that are known to exhibit nontrivial ground states and exotic accompanying phenomena.

In solid-state compounds, the valence of europium can sometimes be mixed – which is especially favored in structures with several positions for the europium atoms. In this work, we study the Eu-based intermetallic noncentrosymmetric system Eu_10Hg_55 which has 65 atoms per unit cell and 4 distinct crystallographic positions for europium and 17 positions for mercury. Our detailed analysis of magnetism of large single crystals suggests that europium in Eu_10Hg_55 might be present in two valence states, resulting in a fragile magnetic ground state. Due to the cage-like structure with a large distance between the Eu atoms, those atoms are weakly ferromagnetically coupled and Eu_10Hg_55 orders at low temperatures, below T_1 = 5.5 K, with a subsequent spin re-orientation at T_2 = 4.3 K. There is no sign of magnetic frustration. Interestingly, the magnetic ordering of europium sub-lattices results in a magnetization pole reversal with a weak ferrimagnetic ground state. Additional magnetic phases can be induced by application of a modest external magnetic field.

The composition of a natural single crystalline specimen from the province of Potosí in Bolivia is found to be Fe0.8Mn0.2WO4. It crystallizes with the primitive monoclinic NiWO4 structure type [space group P2/c, a = 4.74751(6) Å, b = 5.71335(7) Å, c = 4.96847(5) Å, β = 90.15(1)°]. Magnetic susceptibility and specific heat capacity measurements indicated that the mineral undergoes multiple magnetic transitions: TN1 ≈ T\,cpN1 = 67(1) K, TN2 = 28(3) K, and T\,cpN2 = 8(1) K. The reduced magnetic entropy of ≈R ln 3 upon the high-temperature antiferromagnetic ordering suggests the failure of the simplified LS-coupling scheme in the description of the magnetism. Fe0.8Mn0.2WO4 is characterized by enlarged electrical resistivity showing an exponential decrease with temperature for T > 300 K, from which an energy gap of 310 meV is deduced. The well-pronounced maximum occurring in the phononic thermal conductivity just below the TN1 is described by the Debye-Callaway model, indicating the dominance of phonon scattering on defects as well as umklapp processes.

This work presents a study on a new thorium iron arsenide, Th2Fe12As7, a second discovered compound in the ternary system. It crystallizes in the noncentrosymmetric hexagonal Zr2Fe12P7 structure type (space group P6, a = 9.5506(4) Å and c = 3.8645(2) Å). The bonding analysis of Th2Fe12As7 reveals that the structure can be represented by a trigonal prismatic environment of the As atoms, which are the geometric locations of anionic components of the structure. The cationic components of the structure are represented by the Th species. Furthermore, the bonding analysis revealed a new feature─a three-dimensional framework of two-atomic bonds between the Fe atoms, which─in the sense of effective charges─is essentially neutral. It plays the role of a bonding mediator within the [Fe-As] framework. Th2Fe12As7 is a metallic paramagnet down to the lowest measured temperature, 0.4 K.

Yb5Rh6Sn18 crystallizes with a unique structural arrangement [space group P42/nmc, a = 9.6997(4) & Aring;, c = 13.7710(7) & Aring;], which is related with primitive cubic Yb3Rh4Sn13 and body-centered tetragonal (Sn1-xTbx)Tb4Rh6Sn18 types. X-ray absorption spectroscopy showed that Yb atoms exhibit temperature-dependent valence fluctuations (VF) (i.e., intermediate valence state). Its complex mechanism is corroborated by the fact that the well-pronounced maximum in magnetic susceptibility can only be fairly described by the Bickers-Cox-Wilkins model developed for a J = 3/2 multiplet, atypical for Yb ions. Both Hall and Seebeck coefficients revealed a switch of the sign, indicating the change of charge carrier type from electrons to holes between 120 and 220 K. Both these effects together with electrical resistivity and theoretical DFT calculations confirm Yb5Rh6Sn18 to be a metal, which disobeys the free electron gas theory. 'Rattling' motion of Sn1 atoms within the enlarged 16-vertices distorted Frank-Kasper polyhedra, concluded from the specific heat measurements, is argued to be the main reason for the appearance of a phonon resonance behavior, resulting in an ultra-low thermal conductivity in the studied stannide.

In the Yb-Co-Sn system a Remeika phase Yb3+xCo4Sn13-x is found with solid solution extending within 0 <= x 0.5. All these compounds crystallize with primitive cubic (space group Pm3n, a approximate to 9.54 angstrom) strongly disordered Sc3Ir4Si13+x type. Stoichiometric Yb3Co4Sn13 is a superconductor with critical temperature Tc = 3.1(2) K, lower- [Bc1 = 1.90(2) mT] and upper [Bc2 = 2.16(5) T] critical fields. The specific-heat jump Delta cp/gamma Tc = 1.72(9) together with the fact that Bc2(Tc) dependence is described by the Werthamer-Helfand-Hohenberg (WHH) model hint toward a conventional mechanism with a weak electron-phonon coupling. Although a more precise analysis is hampered by the presence of multiple superconducting transitions observed in the specific heat of this Remeika phase, the nonstoichiometric Yb3.2Co4Sn12.8 reveals Tc = 2.4(2) K and Bc2 = 4.79(9) T exceeding the classical Pauli limit. Strong electron-phonon coupling in this superconductor is confirmed by the high value of the lambda AD = 0.92(2) parameter determined from the Allen-Dynes formula. However, the electronic specific heat of Yb3.2Co4Sn12.8 follows the exponential law [cel(T ) proportional to e-Delta(0)/kBT ] and can be described by the alpha-model [alpha equivalent to Delta(0)/kBTc = 2] indicating this stannide to be a conventional BCS-superconductor. Both Yb3+xCo4Sn13-x (x = 0, 0.2) compounds reveal complex phonon spectra with possible "rattling" behavior. They are also found to be metallic systems, some aspects of which can be described by a free-electron-gas model.

A procedure for quantifying the U 5 f electronic covalency and degree of localization in U intermetallic compounds is presented. To this end, bulk sensitive hard and soft x-ray photoelectron spectroscopy were utilized in combination with density-functional theory (DFT) plus dynamical mean-field theory (DMFT) calculations. The energy dependence of the photoionization cross sections allows the disentanglement of the U 5 f contribution to the valence band from the various other atomic subshells so the computational parameters in the DFT + DMFT can be reliably determined. Applying this method to UGa2 and UB2 as model compounds from opposite ends of the (de)localization range, we have achieved excellent simulations of the valence band and core-level spectra. The width in the distribution of atomic U 5 f configurations contributing to the ground state, as obtained from the calculations, quantifies the correlated nature and degree of localization of the U 5 f. The findings permit answering the longstanding question why different spectroscopic techniques give seemingly different numbers for the U 5 f valence in intermetallic U compounds.

Amalgams have played an important role in fundamental and applied solid-state chemistry and physics because of the diversity of crystallographic features and properties that they have to offer. Moreover, their peculiar chemical properties can sometimes give rise to unconventional superconducting or magnetic ground states. In the current work, we present an in-depth analysis of single crystals of YHg3 and LuHg3 (Mg3Cd structure type, space group P63/mmc). Both compounds show superconductivity below Tc = 1 ± 0.1 K (YHg3) and Tc = 1.2 ± 0.1 K (LuHg3). Given the high air-sensitivity and toxicity of these compounds, this study was only possible using a number of dedicated experimental techniques.

During the investigation of the binary system Be-Ru two new phases - Be7 Ru4 and Be12 Ru7 - with similar compositions (63.6 at. % Be and 63.2 at. % Be, respectively), are discovered. They both represent new structural prototypes. The phases are located between Be2 Ru (Fe2 P-type structure) and Be3 Ru2 (U3 Si2 -type structure) in the phase diagram. This explains why their crystal structures, solved and refined from single crystal X-ray diffraction data, are described as 2D intergrowth of Fe2 P and U3 Si2 motives. The calculated electronic density of stats (DOS) reveals pronounced minima in the vicinity of the Fermi level for both compounds. Position-space analysis of chemical bonding exhibits the formation of three- and four-atomic polar bonds, involving both, Ru and Be, atoms, and a strong charge transfer from Be to the more electronegative Ru.

The binary Be2Fe, which crystallizes in the C14-structure type hexagonal Laves phase, can dissolve a large amount of Os as a third element, without changing the crystal structure (about 3/4 of Fe can be replaced by Os). A study of the homogeneity range for the Be2Fe1-xOsx Laves phase, shows an increase of the lattice parameters with an increase of the Os content. When Ru substitutes Fe in Be2Fe, a temperature- and composition-dependent transformation from the C14 to the cubic C15 Laves phase is observed, with a narrow homogeneity range. Formation energy calculations revealed that the C14-structure type is energetically favored when Os substitutes Fe in Be2Fe, while a structural phase transition is observed between Be2Fe (C14) and Be2Fe0.5Ru0.5 (C15), all in good agreement with the experimental results. A strong charge transfer from Be to Os or Ru is observed from the analysis of electron density within the Quantum Theory of Atoms in Molecules (QTAIM) framework. A formation of multiatomic bonds is found from the electron localizability approach. Be2Fe1-xOsx undergoes a transition to a disordered ferromagnetic state at T-C = 350 K and behaves like a ferromagnet with a non-collinear magnetic structure (not all domains polarized along field). Be2Fe1-xRux orders ferromagnetically below T-C = 340 K and the very narrow hysteresis curves show that Be2Fe1-xRux is a soft ferromagnet.

Structural and physical properties of the Yb3Rh4Sn13 Remeika phase are investigated on large single crys-tals grown from Sn-flux. It crystallizes with disordered Y3Co4Ge13 structure type [space group Pm3 over bar n, a = 9.6709(2) angstrom], where the 24k crystallographic site occupied by Sn atoms is split. Yb3Rh4Sn13 is a superconductor (SC) with the critical temperature Tc = 7.63(5) K, lower [Bc1 = 14.5(5) mT] and upper [Bc2(0) = 2.89(5) T] critical fields, as well as a clear peak effect with B*(0) = 1.96(9) T observed in the M(H) loops. Bc2(Tc) can be described by the sum of two Werthamer-Helfand-Hohenberg equations. A gamma(B) proportional to B0.75 dependency is found. The electronic specific heat below Tc follows an exponential function including 87% of a strongly coupled [⠃1/kBTc = 3.52(1)] and 13% of a conventional s-wave-like [⠃2/kBTc = 1.32(1)] gap. The observations are in line with Yb3Rh4Sn13 being a two-gap SC. The Remeika phase reveals a complex electronic band structure studied by Hall coefficient (RH) measurements and calculations performed within the density functional theory. A "rattling" effect in Yb3Rh4Sn13 is discussed based on structural refinements and phononic contributions to its specific heat capacity.

The structural features of the hexagonal layered crystal structure of Be2 Ru (a=5.7508(3) Å, c=3.0044(2) Å, space group P 6 ‾ ${\bar{6}}$ 2m) were investigated by single crystal X-ray diffraction and transmission electron microscopy (TEM). The residual electron density and high-resolution TEM images show that the real structure can be described as an intergrowth of the main hexagonal matrix of the Fe2 P type with minor orthorhombic inclusions of its stacking variants. Such atomic arrangement is stabilized by the charge transfer from Be to Ru and by a system of polar three- and four-atomic bonds involving both components. The calculated electronic density of states (DOS) of Be2 Ru revealed, contrarily to typical intermetallic compounds, a pseudo gap (dip) in the vicinity of the Fermi level. The temperature dependence of the electrical resistivity of Be2 Ru shows metal behaviour in agreement with the non-zero DOS at the Fermi level.

The structural and physical properties of Y5Ir6Sn18 grown from Sn-flux as large single crystals are studied. Y5Ir6Sn18 crystallizes with a unique structure [space group Fm3̄m, a = 13.7706(1) Å], which is characterized by a strong disorder. A transmission electron microscopy (TEM) study indicated that the structural model of Y5Ir6Sn18 obtained from X-ray diffraction methods is an average description of a complex intergrowth of domains with different structural arrangements. The studied stannide is a type-II superconductor with a critical temperature Tc = 2.1 K, a rather weak electron-phonon coupling and conventional s-wave BCS-like mechanisms. Performed theoretical electronic band structure calculations indicated the inconsistency of an idealized structural model earlier reported for Y5Ir6Sn18.

We carried out electrical resistivity and X-ray diffraction (XRD) studies on the filled skutterudite superconductors LaPt4Ge12 and PrPt4Ge12 under hydrostatic pressure. The superconducting transition temperature Tc is linearly suppressed upon increasing pressure, though the effect of pressure on Tc is rather weak. From the analysis of the XRD data, we obtain bulk moduli of B=106 GPa and B=83 GPa for LaPt4Ge12 and PrPt4Ge12, respectively. The knowledge of the bulk modulus allows us to compare the dependence of Tc on the unit-cell volume from our pressure study directly with that found in the substitution series La1−xPrxPt4Ge12. We find that application of hydrostatic pressure can be characterized mainly as a volume effect in LaPt4Ge12 and PrPt4Ge12, while substitution of Pr for La in La1−xPrxPt4Ge12 yields features going beyond a simple picture.