Charge Density Wave Order and Fluctuations above T_{CDW} and below Superconducting T_{c} in the Kagome Metal CsV_{3}Sb_{5}.

The phase diagram of the kagome metal family AV_{3}Sb_{5} (A=K, Rb, Cs) features both superconductivity and charge density wave (CDW) instabilities, which have generated tremendous recent attention. Nonetheless, significant questions remain. In particular, the temperature evolution and demise of the CDW state has not been extensively studied, and little is known about the coexistence of the CDW with superconductivity at low temperatures. We report an x-ray scattering study of CsV_{3}Sb_{5} over a broad range of temperatures from 300 to ∼2 K, below the onset of its superconductivity at T_{c}∼2.9 K. Order parameter measurements of the 2×2×2 CDW structure show an unusual and extended linear temperature dependence onsetting at T^{*}∼160 K, much higher than the susceptibility anomaly associated with CDW order at T_{CDW}=94 K. This implies strong CDW fluctuations exist to ∼1.7×T_{CDW}. The CDW order parameter is observed to be constant from T=16 to 2 K, implying that the CDW and superconducting order coexist below T_{c}, and, at ambient pressure, any possible competition between the two order parameters is manifested at temperatures well below T_{c}, if at all. Anomalies in the temperature dependence in the lattice parameters coincide with T_{CDW} for c(T) and with T^{*} for a(T).

Improving thermoelectric performance of TiNiSn by mixing MnNiSb in the half-Heusler structure.

The thermoelectric properties of the n-type semiconductor TiNiSn were optimized by partial substitution with metallic MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the Ti1-xMnxNiSn1-xSbx system. The alloys were prepared by arc-melting and annealed at temperatures obtained from differential thermal analysis and differential scanning calorimetry results. The phases were characterized using powder X-ray diffraction patterns, energy-dispersive X-ray spectroscopy, and differential scanning calorimetry. After annealing, the majority phase was TiNiSn with some Ni-rich sites, and the minority phases were primarily Ti6Sn5, Sn and MnSn2. The Ni-rich sites were caused by Frenkel defects; this led to metal-like behavior in the semiconductor specimens at low temperature. For x ≤ 0.05 the samples showed an activated conduction, whereas for x > 0.05 they showed metallic character. The figure of merit for x = 0.05 was increased by 61% (zT = 0.45) in comparison with the pure TiNiSn.

Large noncollinearity and spin reorientation in the novel Mn2RhSn Heusler magnet.

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn(2)RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn(2)RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

Synthesis and properties of type-I clathrate phases Rb(8-x-t)K(x□t)Au(y)Ge(46-y).

Type-I clathrates Rb(8-x-t)K(x□t)Au(y)Ge(46-y) are synthesized from Rb4Ge9, K4Ge9, Au, and Ge. Crystal structures and compositions are determined by single-crystal and powder X-ray diffraction methods. The lattice parameters are 10.8103(2), 10.7956(2), 10.7850(2), and 10.7723(2) Šin space group Pm3n for Rb7.88(2)Au2.47(2)Ge43.53(2), Rb3.69(4)K4.31(4)Au2.17(2)Ge43.83(2), Rb1.66(5)K6.34(5)Au2.17(1)Ge43.83(1), and K6.71(4)Au2.28(2)Ge43.72(2), respectively. Bonding analysis for Rb8Au6Ge40 suggests ionic interaction of Rb with the framework besides covalent interactions between Ge and Au/Ge. Rb7.88Au2.47Ge43.53 and K6.71(4)Au2.28Ge43.72 are both diamagnetic. The heat capacity of K6.71Au2.28Ge43.72 is analyzed. Transport properties of Rb7.88Au2.47Ge43.53 reveal n-type conducting, and low thermal conductivity.

Charge-doping-driven evolution of magnetism and non-Fermi-liquid behavior in the filled skutterudite CePt4Ge(12-x)Sb(x).

The filled skutterudite compound CePt(4)Ge(12) is situated close to the border between the intermediate valence of Ce and heavy-fermion behavior. Substitution of Ge by Sb drives the system into a strongly correlated and, ultimately, upon further increasing the Sb concentration, an antiferromagnetically ordered state. Our experiments evidence a delicate interplay of emerging Kondo physics and the formation of a local 4f moment. An extended non-Fermi-liquid region, which can be understood in the framework of a Kondo-disorder model, is observed. Band-structure calculations support the conclusion that the physical properties are governed by the interplay of electron supply via Sb substitution and the concomitant volume effects.

Structural transformation with "negative volume expansion": chemical bonding and physical behavior of TiGePt.

The synthesis and a joint experimental and theoretical study of the crystal structure and physical properties of the new ternary intermetallic compound TiGePt are presented. Upon heating, TiGePt exhibits an unusual structural phase transition with a huge volume contraction of about 10 %. The transformation is characterized by a strong change in the physical properties, in particular, by an insulator-metal transition. At temperatures below 885 °C TiGePt crystallizes in the cubic MgAgAs (half-Heusler) type (LT phase, space group F43m, a = 5.9349(2) Å). At elevated temperatures, the crystal structure of TiGePt transforms into the TiNiSi structure type (HT phase, space group Pnma, a = 6.38134(9) Å, b = 3.89081(5) Å, c = 7.5034(1) Å). The reversible, temperature-dependent structural transition was investigated by in-situ neutron powder diffraction and dilatometry measurements. The insulator-metal transition, indicated by resistivity measurements, is in accord with band structure calculations yielding a gap of about 0.9 eV for the LT phase and a metallic HT phase. Detailed analysis of the chemical bonding in both modifications revealed an essential change of the Ti-Pt and Ti-Ge interactions as the origin of the dramatic changes in the physical properties.

Room-temperature synthesis, hydrothermal recrystallization, and properties of metastable stoichiometric FeSe.

Room-temperature precipitation from aqueous solutions yields the hitherto unknown metastable stoichiometric iron selenide (ms-FeSe) with tetragonal anti-PbO type structure. Samples with improved crystallinity are obtained by diffusion-controlled precipitation or hydrothermal recrystallization. The relations of ms-FeSe to superconducting ß-FeSe(1-x) and other neighbor phases of the iron-selenium system are established by high-temperature X-ray diffraction, DSC/TG/MS (differential scanning calorimetry/thermogravimetry/mass spectroscopy), (57)Fe Mössbauer spectroscopy, magnetization measurements, and transmission electron microscopy. Above 300 °C, ms-FeSe decomposes irreversibly to ß-FeSe(1-x) and Fe(7)Se(8). The structural parameters of ms-FeSe (P4/nmm, a = 377.90(1) pm, c = 551.11(3) pm, Z = 2), obtained by Rietveld refinement, differ significantly from literature data for ß-FeSe(1-x). The Mössbauer spectrum rules out interstitial iron atoms or additional phases. Magnetization data suggest canted antiferromagnetism below T(N) = 50 K. Stoichiometric non-superconducting ms-FeSe can be regarded as the true "parent" compound for the "11" iron-chalcogenide superconductors and may serve as starting point for new chemical modifications.

Intrinsic electrical and thermal properties from single crystals of Na24Si136.

The observation of intrinsic structural, electrical, and thermal properties from measurements on single-crystal specimens of clathrate-II Na24Si136 is reported, revealing metallic conduction in agreement with electronic structure calculations. Low-temperature heat capacity measurements corroborate a substantial electronic density of states at the Fermi level, and reveal an Einstein-like mode that can be attributed to Na guest "rattling". The large thermal conductivity of Na24Si136, compared to literature data for other intermetallic clathrates, can be understood in terms of the predominant electronic contribution for the fully filled Na24Si136 composition.

Superfluid density and energy gap function of superconducting PrPt4Ge12.

The filled skutterudite superconductor PrPt4Ge12 was studied in muon-spin rotation (muSR), specific heat, and electrical resistivity experiments. The continuous increase of the superfluid density with decreasing temperature and the dependence of the magnetic penetration depth lambda on the magnetic field obtained by means of muSR, as well as the observation of a T3 dependence of the electronic specific heat indicate the presence of pointlike nodes in the superconducting energy gap. The gap and the specific heat are found to be well described by two models with point nodes, similar to results obtained for the unconventional heavy fermion skutterudite superconductor PrOs4Sb12.

CeAsSe-synthesis, crystal structure, and physical properties.

Single crystals of CeAsSe were synthesized by a reaction of the elements using iodine as a mineralization agent at 900 degrees C. The crystal structure was established from single crystal X-ray diffraction data and obtained from a pseudomerohedrically twinned specimen (space group Pnma, a = 5.7969(1) A, b = 5.7664(1) A, and c = 17.8196(6) A; Z = 8). CeAsSe crystallizes in the GdPS type of structure and contains infinite cis-trans chains of arsenic atoms, packed between two-dimensional slabs of alternating Ce and Se atoms. The chemical composition of the investigated crystals was determined to be CeAs(1.01(1))Se(0.99(3)). High-resolution diffraction experiments with synchrotron radiation clearly evidence the orthorhombic metric. However, variations in composition or temperature profile in the synthesis procedure lead to vanishing distortion, that is, disappearance of reflection splitting and superstructure reflections, and thus to a tetragonal metric within the resolution of the synchrotron-based diffraction experiments. CeAsSe can be expressed as consisting of Ce(3+), As(1-), and Se(2-) as an electronic precise Zintl-type compound. This interpretation is consistent with the results of X-ray absorption spectroscopy at the Ce-L(III) edge and magnetic susceptibility data. The temperature dependence of a semiconductor was observed in electrical resistivity measurements.

EuTM(2)Ga(8) (TM = Co, Rh, Ir) - a contribution to the chemistry of the CeFe(2)Al(8)-type compounds.

The isostructural compounds EuTM(2)Ga(8) (TM = Co, Rh, Ir) were prepared by direct reaction of the elements by high-frequency thermal treatment. All three phases are isotypic with CeFe(2)Al(8) (space group Pbam, Pearson symbol oP44, Z = 4). The crystal structure was established from single-crystal X-ray diffraction data: a = 12.4322(7) A, b = 14.3814(9) A, and c = 4.0378(2) A for EuCo(2)Ga(8); a = 12.6001(6) A, b = 14.6757(7) A, and c = 4.1172(2) A for EuRh(2)Ga(8); and a = 12.6237(7) A, b = 14.6978(8) A, and c = 4.1486(2) A for EuIr(2)Ga(8), respectively. Analysis of the chemical bonding in EuRh(2)Ga(8) with the electron localizability tools reveals formation of the 3D [Rh(2)Ga(8)] polyanion build by polar covalent bonds. Europium interacts in two ways with the polyanion: mainly as a cation by charge transfer and additionally covalently by means of the electrons of the inner shells. Magnetic susceptibility measurements show Curie-Weiss paramagnetic behavior above 40 K with effective magnetic moments of 7.81, 8.05, and 8.27 micro(B)/f.u. for EuTM(2)Ga(8) (TM = Co, Rh, Ir). Antiferromagnetic ordering of Eu moments is observed in all three compounds below 20 K. Independently on the chemical composition of the coordination sphere, magnetic behavior and, especially, X-ray absorption spectra indicate predominantly the 4f(7) electronic configuration of europium with small admixture of the 4f(6) state.

Cationic clathrate I Si(46-x)P(x)Te(y) (6.6(1) < or = y < or = 7.5(1), x < or = 2y): crystal structure, homogeneity range, and physical properties.

A new cationic clathrate I Si(46-x)P(x)Te(y) (6.6(1) < or = y < or = 7.5(1), x < or = 2y at 1375 K) was synthesized from the elements and characterized by X-ray powder diffraction, thermal analysis, scanning electron microscopy, wavelength dispersive X-ray spectroscopy (WDXS), neutron powder diffraction, and (31)P NMR spectroscopy. The thermal behaviors of the magnetic susceptibility and resistivity were investigated as well. Si(46-x)P(x)Te(y) reveals a wide homogeneity range due to the presence of vacancies in the tellurium guest positions inside the smaller cage of the clathrate I structure. The vacancy ordering in the structure of Si(46-x)P(x)Te(y) causes the change of space group from Pm3n (ideal clathrate I) to Pm3 accompanied by the redistribution of P and Si atoms over different framework positions. Neutron powder diffraction confirmed that P atoms preferably form a cage around the vacancy-containing tellurium guest position. Additionally, (31)P NMR spin-spin relaxation experiments revealed the presence of sites with different coordination of phosphorus atoms. Precise determination of the composition of Si(46-x)P(x)Te(y) by WDXS showed slight but noticeable deviation (x < or = 2y) of phosphorus content from the Zintl counting scheme (x = 2y). The compound is diamagnetic while resistivity measurements show activated behavior or that of heavily doped semiconductors. Thermal analysis revealed high stability of the investigated clathrate: Si(46-x)P(x)Te(y) melts incongruently at approximately 1460 K in vacuum and is stable in air against oxidation up to 1295 K.

Sn(x)Pt4Sn(y)Sb(12-y): a skutterudite with covalently bonded filler.

A new phase, Sn(x)Pt4Sb(12-y)Sn(y), has been prepared from the elements. It exhibits a wide range of homogeneity with 0.3(2) < or = x < or = 1.0(2) and 4.2(2) < or = y < or = 7.0(2). The crystal structure and the composition were established by single-crystal and powder X-ray diffraction as well as wavelength-dispersive X-ray analysis measurements and were supported by nuclear magnetic resonance experiments. The compound is the first representative of the filled-skutterudite family with the filler atoms not located at the center of the cavity but covalently bonded to the cavity's wall, as confirmed by the analysis of chemical bonding with the electron localizability indicator. The Sn and Sb atoms share the framework site with different coordinate parameters caused by the difference in atomic size; additional tin atoms are located in the cavities of the framework. The material is a diamagnet in the whole composition range. In agreement with the calculated electronic density of states, the material reveals a metallic behavior in electronic transport. The absolute values of electrical resistivity vary with the tin-to-antimony ratio.

Elastic and anelastic relaxation behaviour of perovskite multiferroics II: PbZr0.53Ti0.47O3 (PZT)-PbFe0.5Ta0.5O3 (PFT).

Elastic and anelastic properties of ceramic samples of multiferroic perovskites with nominal compositions across the binary join PbZr0.53Ti0.47O3-PbFe0.5Ta0.5O3 (PZT-PFT) have been assembled to create a binary phase diagram and to address the role of strain relaxation associated with their phase transitions. Structural relationships are similar to those observed previously for PbZr0.53Ti0.47O3-PbFe0.5Nb0.5O3 (PZT-PFN), but the magnitude of the tetragonal shear strain associated with the ferroelectric order parameter appears to be much smaller. This leads to relaxor character for the development of ferroelectric properties in the end member PbFe0.5Ta0.5O3. As for PZT-PFN, there appear to be two discrete instabilities rather than simply a reorientation of the electric dipole in the transition sequence cubic-tetragonal-monoclinic, and the second transition has characteristics typical of an improper ferroelastic. At intermediate compositions, the ferroelastic microstructure has strain heterogeneities on a mesoscopic length scale and, probably, also on a microscopic scale. This results in a wide anelastic freezing interval for strain-related defects rather than the freezing of discrete twin walls that would occur in a conventional ferroelastic material. In PFT, however, the acoustic loss behaviour more nearly resembles that due to freezing of conventional ferroelastic twin walls. Precursor softening of the shear modulus in both PFT and PFN does not fit with a Vogel-Fulcher description, but in PFT there is a temperature interval where the softening conforms to a power law suggestive of the role of fluctuations of the order parameter with dispersion along one branch of the Brillouin zone. Magnetic ordering appears to be coupled only weakly with a volume strain and not with shear strain but, as with multiferroic PZT-PFN perovskites, takes place within crystals which have significant strain heterogeneities on different length scales.

Intermetallic germanides with non-centrosymmetric structures derived from the Yb3Rh4Sn13 type.

New germanides with composition RE3Pt4Ge13 (RE = Y, Pr, Sm, Gd, Tb, Tm) have been prepared by high-pressure, high-temperature synthesis. Their crystal structures have been refined, and the relationship of this new rhombohedral and monoclinic structure types with the primitive cubic Yb3Rh4Sn13 prototype is discussed. Band structure calculations within density functional theory confirm the distorted rhombohedral and monoclinic structural arrangements to be energetically more favorable than the simple cubic one. X-ray absorption spectroscopy and magnetic susceptibility measurements indicate that the RE-atoms are in the +3 oxidation state in all studied compounds.

Elastic and magnetoelastic relaxation behaviour of multiferroic (ferromagnetic + ferroelectric + ferroelastic) Pb(Fe0.5Nb0.5)O3 perovskite.

Resonant Ultrasound Spectroscopy has been used to characterize elastic and anelastic anomalies in a polycrystalline sample of multiferroic Pb(Fe(0.5)Nb(0.5))O(3) (PFN). Elastic softening begins at ~550 K, which is close to the Burns temperature marking the development of dynamical polar nanoregions. A small increase in acoustic loss at ~425 K coincides with the value of T(*) reported for polar nanoregions starting to acquire a static or quasi-static component. Softening of the shear modulus by ~30-35% through ~395-320 K, together with a peak in acoustic loss, is due to classical strain/order parameter coupling through the cubic → tetragonal → monoclinic transition sequence of ferroelectric/ferroelastic transitions. A plateau of high acoustic loss below ~320 K is due to the mobility under stress of a ferroelastic microstructure but, instead of the typical effects of freezing of twin wall motion at some low temperature, there is a steady decrease in loss and increase in elastic stiffness below ~85 K. This is attributed to freezing of a succession of strain-coupled defects with a range of relaxation times and is consistent with a report in the literature that PFN develops a tweed microstructure over a wide temperature interval. No overt anomaly was observed near the expected Néel point, ~145 K, consistent with weak/absent spin/lattice coupling but heat capacity measurements showed that the antiferromagnetic transition is actually smeared out or suppressed. Instead, the sample is weakly ferromagnetic up to ~560 K, though it has not been possible to exclude definitively the possibility that this could be due to some magnetic impurity. Overall, evidence from the RUS data is of a permeating influence of static and dynamic strain relaxation effects which are attributed to local strain heterogeneity on a mesoscopic length scale. These, in turn, must have a role in determining the magnetic properties and multiferroic character of PFN.

Physical properties and valence state of cerium in the filled skutterudite CePt4Ge12.

Electronic, magnetic, and transport properties of the filled platinum-germanium skutterudite CePt4Ge12 are investigated. High resolution x-ray absorption spectroscopy measurements at the cerium L(III) edge demonstrate that CePt4Ge12 in this compound has a temperature-independent valence close to three. However, magnetic susceptibility, thermopower, Hall effect, and electronic specific heat reveal a broad maximum at Tmax D 65-80 K, suggesting the presence of valence fluctuations. The Sommerfeld coefficient γ = 105 mJ mol⁻¹ K⁻², deduced from specific heat, indicates moderately enhanced band masses for CePt4Ge12. We discuss these findings and conclude that CePt4Ge12 represents a system at the border between intermediate valence (IV) and Kondo lattice behavior. In addition, the lattice specific heat and the thermal conductivity are discussed with respect to the vibrational dynamics of Ce in the [Pt4Ge12] framework.