Holger Kohlmann

Hydrides of Alkaline Earth–Tetrel (AeTt) Zintl Phases: Covalent Tt–H Bonds from Silicon to Tin

Zintl phases form hydrides either by incorporating hydride anions (interstitial hydrides) or by covalent bonding of H to the polyanion (polyanionic hydrides), which yields a variety of different compositions and bonding situations. Hydrides (deuterides) of SrGe, BaSi, and BaSn were prepared by hydrogenation (deuteration) of the CrB-type Zintl phases AeTt and characterized by laboratory X-ray, synchrotron, and neutron diffraction, NMR spectroscopy, and quantum-chemical calculations. SrGeD4/3-x and BaSnD4/3-x show condensed boatlike six-membered rings of Tt atoms, formed by joining three of the zigzag chains contained in the Zintl phase. These new polyanionic motifs are terminated by covalently bound H atoms with d(Ge-D) = 1.521(9) Å and d(Sn-D) = 1.858(8) Å. Additional hydride anions are located in Ae4 tetrahedra; thus, the features of both interstitial hydrides and polyanionic hydrides are represented. BaSiD2-x retains the zigzag Si chain as in the parent Zintl phase, but in the hydride (deuteride), it is terminated by H (D) atoms, thus forming a linear (SiD) chain with d(Si-D) = 1.641(5) Å.

Recovery rate and homogeneity of doping europium into luminescent metal hydrides by chemical analysis

During the investigation of concentration dependent properties of phosphors, such as emission intensities, the knowledge of the activator ion concentration is of great importance. Herein we present a study on recovery rate and homogeneity of the activator ion concentration by chemical analysis in luminescent europium doped metal hydrides. The analysis method was established on the model system EuxSr1−xH2 and applied to brightly emitting hydridic perovskites LiMH3:Eu2+ and LiMD3:Eu2+ (M = Sr, Ba). The nominal activator ion concentrations calculated from initial weights are in good agreement with those determined via ICP-MS for 10−3% ≤ x both for the hydrides (deuterides) and the parent alloys. The synthesis protocol thus allows a reliable method for reproducibly doping europium into metal hydrides. Luminescence spectra of LiEuxSr1−xH3 with x = 0.0037% show a broad band emission at room temperature typical for Eu2+ 4f65d1–4f7 transitions. Below 80 K, a vibronic fine structure is observed with vibrational coupling frequencies of approximately 100, 370 and 970 cm−1.

Crystal structures and hydrogenation properties of palladium-rich compounds with elements from groups 12–16

Abstract We report on crystal structure data and hydrogenation properties of 24 palladium-rich intermetallic compounds with elements from groups 12–16 of the Periodic Table. Refined crystal structures based on X-ray powder diffraction data are presented for Pd3As (Fe3P type structure) and several members of the Pd5TlAs type structure family. Hydrogenation was studied in situ by differential scanning calorimetry (DSC) under 5.0 MPa hydrogen pressure up to 430 °C. Pd0.75Zn0.25, PdCd, PdHg, Pd2Sn, Pd5Pb3, Pd13Pb9, Pd3As, Pd20Sb7, Pd8Sb3, Pd5Sb2, PdSb, Pd5Bi2, Pd17Se15, Pd4Se, Pd5TlAs, Pd5CdSe, Pd5CdAs, Pd5HgSe, Pd5InAs, Pd8In2Se and Pd3Bi2Se2 do not show any sign of hydrogen uptake according to DSC and X-ray diffraction. For Pd3Sn and Pd3Pb a significant hydrogen uptake with unit cell volume increases of 0.4 and 0.6 %, respectively, with a retained structure type of the parent intermetallic was observed. Hydrogenation of Pd5InSe yields Pd3InH≈0.9 and a mixture of palladium selenides. Thermal analysis experiments in helium and in hydrogen atmosphere show that this is a multistep reaction with a decomposition of Pd5InSe to Pd3In and a liquid phase and subsequent hydrogenation of Pd3In.

Structural and Electronic Flexibility in Hydrides of Zintl Phases with Tetrel-Hydrogen and Tetrel-Tetrel Bonds

The hydrogenation of Zintl phases enables the formation of new structural entities with main-group-element-hydrogen bonds in the solid state. The hydrogenation of SrSi, BaSi, and BaGe yields the hydrides SrSiH5/3-x, BaSiH5/3-x and BaGeH5/3-x . The crystal structures show a sixfold superstructure compared to the parent Zintl phase and were solved by a combination of X-ray, neutron, and electron diffraction and the aid of DFT calculations. Layers of connected HSr4 (HBa4 ) tetrahedra containing hydride ions alternate with layers of infinite single- and double-chain polyanions, in which hydrogen atoms are covalently bound to silicon and germanium. The idealized formulae AeTtH5/3 (Ae=alkaline earth, Tt=tetrel) can be rationalized with the Zintl-Klemm concept according to (Ae2+ )3 (TtH- )(Tt2 H2- )(H- )3 , where all Tt atoms are three-binding. The non-stoichiometry (SrSiH5/3-x , x=0.17(2); BaGeH5/3-x , x=0.10(3)) can be explained by additional π-bonding of the Tt chains.

Eu(II) luminescence in the perovskite host lattices KMgH3, NaMgH3 and mixed crystals LiBaxSr1-xH3

Bright luminescence of Eu(II) doped into the cubic perovskites KMgH3 and mixed crystal compounds LiBaxSr(1-x)H(3) was observed and assigned to the 4f(6)5d-4f(7) emission of Eu2+. KMgH3: Eu2+ shows an extremely bright yellow emission, whereas the wavelength of the emission maximum in LiBaxSr(1-x)H(3): Eu2+ depends on the value of x and ranges from yellow to green. Furthermore, an extremely wide red shift in the emission energy is observed for the orthorhombically distorted perovskite NaMgH3: Eu2+. Additionally, we review the crystal structure of KMgH3 using density functional calculations.

Electronic structure of ternary rhodium hydrides with lithium and magnesium.

Chemical bonding in and electronic structure of lithium and magnesium rhodium hydrides are studied theoretically using DFT methods. For Li3RhH4 with planar complex RhH4 structural units, Crystal Orbital Hamilton Populations reveal significant Rh−Rh interactions within infinite one-dimensional ∞ 1 [RhH4] stacks in addition to strong rhodium−hydrogen bonding. These metal−metal interactions are considerably weaker in the hypothetical, heavier homologue Na3RhH4. Both compounds are small-band gap semiconductors. The electronic structures of Li3RhH6 and Na3RhH6 with rhodium surrounded octahedrally by hydrogen, on the other hand, are compatible with a classical complex hydride model according to the limiting ionic formula (M+)3[RhH6]3− without any metal−metal interaction between the 18-electron hydridorhodate complexes. In MgRhH, building blocks of the composition (RhH2)4 are formed with strong rhodium−hydrogen and significant rhodium−rhodium bonding (bond lengths of 298 pm within Rh4 squares). These units are linked together to infinite two-dimensional layers ∞ 2 [(RhH2/2)4] via common hydrogen atoms. Li3RhH4 and MgRhH are accordingly examples for border cases of chemical bonding where the classical picture of hydridometalate complexes in complex hydrides is not sufficient to properly describe the chemical bonding situation.

Magnetic Structure of SmCo5 from 5 K to the Curie Temperature

The crystal and magnetic structure of SmCo5 is determined by neutron powder diffraction between 5 K and the Curie temperature. In order to overcome the enormous neutron absorption of samarium, a 154Sm isotopically enriched sample was used. The ordered magnetic moments of both crystallographically distinct cobalt atoms are not significantly different over the whole temperature range. They decrease from 2.2 μB at 5 K to about 0.6 μB at 1029 K. Samariums ordered magnetic moment decreases from 1.0 μB at 5 K, runs through a minimum of 0.2 μB around 650 K, and becomes larger than cobalts ordered magnetic moment above 950 K. No sign or orientation change of the samarium and cobalt ordered magnetic moments is found between the Curie temperature and 5 K. SmCo5 is thus a ferromagnet and does not switch to a ferrimagnetic state as discussed in the literature.

Synthesis and Crystal Structure of Pd5InSe

Pd5InSe has been prepared from the elements. The use of iodine as a mineralizing agent enables the synthesis of single-phase powder samples as well as of single crystals. Pd5InSe is stable in cold air, but reacts to give Pd, In2O3 and Pd4Se at 400 °C. The crystal structure of Pd5InSe was determined from single-crystal X-ray diffraction data (space group P4=mmm, a = 4.0255(7), c = 6.972(1) Å, z(Pd2) = 0.28111(8)) and belongs to the Pd5TlAs-type structure with full occupation of all atomic sites. EDX analysis on the single crystal (Pd5.0(1)In0.99(3)Se1.0(1)) confirms the stoichiometric composition. The relationship to the cubic close packing (Cu-type structure), which may be visualized by the crystal chemical formula Pd4PdTlAs⃞, is proven by a Bärnighausen symmetry tree. Graphical Abstract Synthesis and Crystal Structure of Pd5InSe

The lanthanide hydride oxides SmHO and HoHO

Abstract Metal hydride oxides are an interesting class of compounds with potential for hydride ion conduction and as host materials for luminescence. SmHO and HoHO were prepared from mixtures of the sesquioxides Ln2O3 and the hydrides LnH2+x at 1173 K as gray powders (Ln=Sm, Ho). They crystallize in a fluorite type crystal structure with disordered anion distribution (Fm3̅m; SmHO: a=5.46953(6) Å, V=163.625(5) Å3; HoHO: a=5.27782(3) Å, V=147.016(2) Å3, based on powder X-ray diffraction) and show stability towards air. Lanthanide-oxygen and -hydrogen distances are 2.36838(3) Å in SmHO and 2.28536(1) Å in HoHO and comparable to those in binary lanthanide oxides and hydrides. Elemental analyses confirm the composition LnHO. Quantum-mechanical calculations show a negative enthalpy for the reaction RE2O3+REH3→3 REHO for all lanthanides and Y, with increasing values for decreasing ionic radii.

LiSr2SiO4H, an Air-Stable Hydride as Host for Eu(II) Luminescence

LiSr2SiO4H is synthesized by solid-state reaction of LiH and α-Sr2SiO4. It crystallizes in space group P21/ m ( a = 658.63(4) pm, b = 542.36(3) pm, c = 695.01(4) pm, β = 112.5637(9)°) as proven by X-ray and neutron diffraction, is isotypic to LiSr2SiO4F, and exhibits isolated SiO4 tetrahedra. Hydride anions are located in Li2Sr4 octahedra, which share faces to form columns, with H-H distances of 271.18(2) pm. NMR, IR, and Raman spectroscopy, density measurements, elemental analysis, and theoretical calculations confirm these results. Despite its hydridic nature, it is stable in air up to 550 K. When doped with europium, it emits bright yellow-green light with an intensity maximum at 560 nm for LiSr1.98Eu0.02SiO4H. Even after treatment in water for several hours, the solid shows luminescence. The broad emission peak is attributed to the allowed 4f65d → 4f7 transition of divalent europium. LiSr2SiO4H is the first silicate hydride, a class of compounds that might have potential as host for luminescent materials.