DFT-Guided Crystal Structure Redetermination and Lattice Dynamics of the Intermetallic Actinoid Compound UIr.

UIr has been discussed as a rare example of a noncentrosymmetric, ferromagnetic superconductor crystallizing in the acentric PdBi structure type (P21, mP16). Here we present a new structure model for UIr. By means of single-crystal and powder X-ray diffraction we find UIr to crystallize in the centrosymmetric space group P21/c, in line with previous ab initio calculations. The discrepancy with the previous noncentrosymmetric model in space group P21 is explained by the occurrence of twinning. The observed twinning hints toward a high-temperature displacive phase transition of UIr to the CrB structure type (Cmcm, oS8): we discuss the lattice dynamics corresponding to this transition by crystallographic symmetry mode analysis and by density functional theory (DFT). We find that spin-orbit coupling is essential to understand this phase transition. We apply our theoretical considerations for a critical judgment of the structure models of UPt and NpIr that have been reported to crystallize isotypically with UIr. We confirm that UPt is isotypic to UIr (P21/c), whereas we predict NpIr to crystallize in the CrB structure type. Our report on the centrosymmetric crystal structure of UIr has an effect on all those theoretical models that investigated potentially novel superconducting coupling mechanisms of this compound on the basis of the noncentrosymmetric structure model.

Binary Lead Fluoride Pb3 F8.

The binary lead fluoride Pb3 F8 was synthesized by the reaction of anhydrous HF with Pb3 O4 or by the reaction of BrF3 with PbF2 . The compound was characterized by single-crystal and powder X-ray diffraction, IR, Raman, and solid-state MAS 19 F NMR spectroscopy, as well as thermogravimetric analysis, XP and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Solid-state quantum-chemical calculations are provided for the vibrational analyses and band assignments. The electronic band structure offers an inside view of the mixed valence compound.

A Revised Structure Model for the UCl6 Structure Type, Novel Modifications of UCl6 and UBr5 , and a Comment on the Modifications of Protactinium Pentabromides.

The redetermination of the crystal structure of trigonal UCl6 , which is the eponym for the UCl6 structure type, showed that certain atomic coordinates had been incorrectly reported. This led to noticeably different U-Cl distances within the octahedral UCl6 molecule (2.41 and 2.51 Å). Within the revised structure model presented here, which is based on single crystal data as well as quantum chemical calculations, all U-Cl distances are essentially equal within standard uncertainty (2.431(5), 2.437(5), and 2.439(6) Å). This room temperature modification, called rt-UCl6 , crystallizes in the trigonal space group P 3 ‾ m1, No. 164, hP21, with a=10.907(2), c=5.9883(12) Å, V=616.9(2) Å3 , Z=3 at T=253 K. A new low-temperature (lt) modification of UCl6 is also presented that was obtained by cooling a single crystal of rt-UCl6. The phase change occurs between 150 and 175 K. lt-UCl6 crystallizes isotypic to a low-temperature modification of SF6 in the monoclinic crystal system, space group C2/m, No. 12, mS42, with a=17.847(4), b=10.8347(18), c=6.2670(17) Å, ß=96.68(2)°, V=1203.6(5) Å3 , Z=6 at 100 K. The Cl anions form a close-packed structure corresponding to the α-Sm type with uranium atoms in the octahedral voids. During the synthesis of UBr5 a new modification was obtained that crystallizes in the triclinic crystal system, space group P 1 ‾ , No. 2, aP36, with a=10.4021(6), b=11.1620(6), c=12.2942(7) Å, α=68.3340(10)°, ß=69.6410(10)° and γ=89.5290(10)°, V=1231.84(12) Å3 , Z=3 at T=100 K. In this structure the UBr5 units are dimerized to U2 Br10 molecules. The Br anions also form a close-packed structure of the α-Sm type with adjacent uranium atoms in the octahedral voids. Comparisons of the crystal structures of the compounds MX5 (M=Pa, U; X=Cl, Br) show that the crystal structure of monoclinic α-PaBr5 is probably not correct.

UF4 and the High-Pressure Polymorph HP-UF4.

A laboratory-scale synthesis of UF4 is presented that utilizes the reduction of UF6 with sulfur in anhydrous hydrogen fluoride. An excess of sulfur can be removed by vacuum sublimation, yielding pure UF4 , as shown by powder X-ray diffraction, micro X-ray fluorescence analysis, infrared and Raman spectroscopy, as well as magnetic measurements. Furthermore, a single-crystalline, high-pressure modification of UF4 was obtained in a multi-anvil press at elevated temperatures. The high-pressure polymorph HP-UF4 was characterized by means of single-crystal and powder X-ray diffraction, as well as by magnetic measurements, and presents a novel crystal structure type. Quantum-chemical calculations show the HP-modification to be 10 kJ mol-1 per formula unit higher in energy compared to UF4 .

Half-metallicity in uranium intermetallics: crystal structure prediction of a high-pressure phase of UCo.

Evolutionary crystal structure prediction methods combined with density functional theory (DFT) calculations reveal a high-pressure (hp) phase of the intermetallic compound UCo crystalizing in the NaTl structure type ([Formula: see text]). We predict this compound to be formed at pressures below 9 GPa. Hp-UCo shows the same structural trends as the two experimentally known pseudo-binary compounds UCo0.2Rh0.8 and UNi0.8Pt0.2. We classify them as ordered solid solutions of a bcc lattice following Vegard's law. We predict hp-UCo and its adjacent phases UFe and UNi to be itinerant magnets. In the limit of vanishing spin-orbit interactions UFe and hp-UCo are half-metallic ferrimagnets. Spin-orbit coupling generally reduces the spin-polarization at the Fermi level. In case of hp-UCo the decrease depends on the applied DFT functional making the prediction of actual occurrence of half-metallicity problematic. In case of UFe the results are less dependent on the DFT functional. We demonstrate with these calulations that against 'common sense' also materials with heavy elements can be interesting candidates for half-metallicity. We highlight that the NaTl structure type should be an interesting candidate for further investigations of half-metallicity.

Forcing substitution of tantalum by copper in 1T-TaS2: synthesis, structure and electronic properties of 1T-Cu x Ta1-x S2.

We investigated the compound 1T-Cu x Ta1-x S2 with respect to its synthesis, homogeneity range, structure and electronic properties. The average structure of 1T-Cu x Ta1-x S2 resembles that of the high-temperature phase of the layered transition metal dichalcogenide 1T-TaS2 in which tantalum is partially substituted by copper. 1T-Cu x Ta1-x S2 readily decomposes at elevated temperatures and can only be prepared and stabilized by a sufficiently high amount of sulfur excess. XPS and NEXAFS measurements reveal that copper has the oxidation state +I in 1T-Cu x Ta1-x S2, which is supported by quantum chemical calculations. The disorder introduced by copper doping causes an Anderson-type localization of the conduction electrons as manifested by a strong increase of the electrical resistivity and a Curie-type paramagnetism at low temperatures as in other doped systems 1T-M x Ta1-x S2 with higher valent metals. Quantum chemical calculations support this interpretation.

LiNi(0.5)Mn(1.5)O4 high-voltage cathode coated with Li4Ti5O12: a hard X-ray photoelectron spectroscopy (HAXPES) study.

A Li4Ti5O12 (LTO) film was coated as buffer layer onto a LiNi0.5Mn1.5O4 (LNMO) high-voltage cathode, and after cycling of the cathode in a battery electrolyte, the LTO film was investigated by means of synchrotron radiation based hard X-ray photoelectron spectroscopy (HAXPES). By tuning the photon energy between 2 keV and 6 keV, we obtained non-destructive depth profiles of the coating material with probing depths ranging from 6 nm to 20 nm. The coating was found to be covered by a few nanometers thin surface layer resulting from electrolyte decomposition. This layer consisted predominantly of organic polymers as well as metal fluorides and fluorophosphates. A positive influence of the Li4Ti5O12 coating with regard to the size and stability of the surface layer was found. The coating itself consisted of a uniform mixture of Li(I), Ti(IV), Ni(II) and Mn(IV) oxides that most likely adopted a spinel structure by forming a solid solution of the two spinels LiNi0.5Mn1.5O4 and Li4Ti5O12 with Li, Mn, Ni and Ti cations mixing on the spinel octahedral sites. The diffusion of Ni and Mn ions into the Li4Ti5O12 lattice occurred during the heat treatment when preparing the cathode. The doping of Li4Ti5O12 with the open d-shell ions Ni(2+) (d(8)) and Mn(4+) (d(3)) should increase the electronic conductivity of the coating significantly, as was found in previous studies. The complex signal structure of the Ti 2p, Ni 2p and Mn 2p core levels provides insight into the chemical nature of the transition metal ions.