Hydrogen order in hydrides of Laves phases

2020 ◽  
Vol 235 (8-9) ◽  
pp. 319-332 ◽  
Author(s):  
Holger Kohlmann
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Crystal Structures ◽  
Laves Phase ◽  
Crystallographic Group ◽  
Crystal Chemical ◽  
Hydrogen Atoms ◽  
Laves Phases ◽  
Structural Relationships ◽  
Chemical Description

AbstractMany Laves phases AM2 takes up hydrogen to form interstitial hydrides in which hydrogen atoms partially occupy A2M2, AM3, and/or M4 tetrahedral interstices. They often exhibit temperature-driven order-disorder phase transitions, which are triggered by repulsion of hydrogen atoms occupying neighboring tetrahedral interstices. Because of the phase widths with respect to hydrogen a complete ordering, i.e., full occupation of all hydrogen positions is usually not achieved. Order-disorder transitions in Laves phase hydrides are thus phase transitions between crystal structures with different degrees of hydrogen order. Comparing the crystal structures of ordered and disordered phases reveals close symmetry relationships in all known cases. This allows new insights into the crystal chemical description of such phases and into the nature of the phase transitions. Structural relationships for over 40 hydrides of cubic and hexagonal Laves phases ZrV2, HfV2, ZrCr2, ZrCo2, LaMg2, CeMg2, PrMg2, NdMg2, SmMg2, YMn2, ErMn2, TmMn2, LuMn2, Lu0.4Y0.6Mn2 YFe2, and ErFe2 are concisely described in terms of crystallographic group-subgroup schemes (Bärnighausen trees) covering 32 different crystal structure types, 26 of which represent hydrogen-ordered crystal structures.

The crystal structure of ZrCr2D≈4 at 50 K ≤ T ≤ 200 K

2020 ◽  
Vol 75 (11) ◽  
pp. 969-973
Author(s):  
Holger Kohlmann
Keyword(s):  
Crystal Structure ◽  
Phase Transition ◽  
Neutron Powder Diffraction ◽  
Laves Phase ◽  
Formula Unit ◽  
Powder Diffraction Data ◽  
Hydrogen Atoms ◽  
Monoclinic Modification ◽  
Laves Phases ◽  
Neutron Powder Diffraction Data

AbstractMany Laves phases take up considerable amounts of hydrogen to form metallic Laves phase hydrides. They frequently undergo phase transitions driven by ordering phenomena for the hydrogen atom distribution. The cubic Laves phase ZrCr2 takes up hydrogen to form a hydride with almost four hydrogen atoms per formula unit, which undergoes a phase transition to a monoclinic modification at a critical temperature Tc = 250.2 K. Its crystal structure was refined based on neutron powder diffraction data on the deuteride (ZrCr2D3.8 type [T = 1.6 K, C2/c]) at four temperatures in the range 50 K ≤ T ≤ 200 K. The monoclinic low-temperature modification features a strongly distorted square anti-prism ZrD8 and three CrD4 polyhedra with almost fully occupied deuterium sites in saddle-like, distorted tetrahedral and planar configurations. Zr–D distances are in the range 201.4(7) pm ≤ d(Zr–D) ≤ 208.5(8) pm and Cr–D distances in the range 172.9(7) pm ≤ d(Cr–D) ≤ 182.4(8) pm.

scholarly journals Crystal Chemical Relations in the Shchurovskyite Family: Synthesis and Crystal Structures of K2Cu[Cu3O]2(PO4)4 and K2.35Cu0.825[Cu3O]2(PO4)4

Crystals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 807
Author(s):  
Ilya V. Kornyakov ◽  
Sergey V. Krivovichev
Keyword(s):  
Crystal Structure ◽  
Crystal Structures ◽  
Structural Complexity ◽  
Crystal Chemical ◽  
X Ray Diffraction ◽  
Complexity Measures ◽  
X Ray ◽  
The Family ◽  
Similarities And Differences ◽  
Related Compounds

Single crystals of two novel shchurovskyite-related compounds, K2Cu[Cu3O]2(PO4)4 (1) and K2.35Cu0.825[Cu3O]2(PO4)4 (2), were synthesized by crystallization from gaseous phase and structurally characterized using single-crystal X-ray diffraction analysis. The crystal structures of both compounds are based upon similar Cu-based layers, formed by rods of the [O2Cu6] dimers of oxocentered (OCu4) tetrahedra. The topologies of the layers show both similarities and differences from the shchurovskyite-type layers. The layers are connected in different fashions via additional Cu atoms located in the interlayer, in contrast to shchurovskyite, where the layers are linked by Ca2+ cations. The structures of the shchurovskyite family are characterized using information-based structural complexity measures, which demonstrate that the crystal structure of 1 is the simplest one, whereas that of 2 is the most complex in the family.

scholarly journals Effect of Nd, Pr substitutional atoms on the magnetic and magnetostrictive properties in (Tb-Dy)(Fe-Co)2 Laves phases

2021 ◽  
Vol 2103 (1) ◽  
pp. 012196
Author(s):  
G A Politova ◽  
M A Ganin ◽  
A B Mikhailova ◽  
D A Morozov ◽  
K E Pankov ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Magnetic Properties ◽  
Laves Phase ◽  
Type Structure ◽  
Partial Substitution ◽  
Laves Phases ◽  
Temperature Range ◽  
Homogenizing Annealing ◽  
Arc Melting ◽  
Magnetostrictive Materials

Abstract Polycrystalline TbxDy1-xR0.1Fe2-zCoz (R = Nd, Pr, x = 0.2, 0.3; z = 0, 1.3) cubic Laves phase alloys with MgCu2-type structure were prepared by arc melting followed by homogenizing annealing. The crystal structure, magnetic properties, and magnetostriction have been investigated. Compounds with high values of magnetostrictive susceptibility were found in the temperature range 150-300 K. Compounds with partial substitution of cobalt for iron demonstrate a change in the sign of anisotropic magnetostriction. This work continues the search for magnetostrictive materials with inexpensive neodymium and praseodymium.

79Br, 127I NQR of Diammoniumalkyl Halides and Piperazinium Halides. Crystal Structure of Piperazinium Dibromide Monohydrate and Piperazinium Monoiodide

1989 ◽  
Vol 44 (1) ◽  
pp. 41-55 ◽  
Author(s):  
Jutta Hartmann ◽  
Shi-Qi Dou ◽  
Alarich Weiss
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Hydrogen Bonds ◽  
Crystal Structures ◽  
Monoclinic Space Group ◽  
Orthorhombic Space Group ◽  
Space Group ◽  
Temperature Range

Abstract The 79Br and 127I NQR spectra were investigated for 1,2-diammoniumethane dibromide, -diiodide, 1,3-diammoniumpropane dibromide, -diiodide, piperazinium dibromide monohydrate, and piperazinium monoiodide in the temperature range 77 ≦ T/K ≦ 420. Phase transitions could be observed for the three iodides. The temperatures for the phase transitions are: 400 K and 404 K for 1,2-diammoniumethane diiodide, 366 K for 1,3-diammoniumpropane diiodide, and 196 K for piperazinium monoiodide.The crystal structures were determined for the piperazinium compounds. Piperazinium dibromide monohydrate crystallizes monoclinic, space group C2/c, with a= 1148.7 pm, 0 = 590.5 pm, c= 1501.6pm, β = 118.18°, and Z = 4. For piperazinium monoiodide the orthorhombic space group Pmn 21 was found with a = 958.1 pm, b = 776.9 pm, c = 989.3 pm, Z = 4. Hydrogen bonds N - H ... X with X = Br, I were compared with literature data.

scholarly journals Assessment of the Compositional Influences on the Toughness of TiCr2-Base Laves Phase Alloys

1996 ◽  
Vol 460 ◽  
Author(s):  
Katherine C. Chen ◽  
Samuel M. Allen ◽  
James D. Livingston
Keyword(s):  
Crystal Structure ◽  
Single Phase ◽  
Laves Phase ◽  
Second Phase ◽  
Vickers Indentation ◽  
Toughening Mechanisms ◽  
Two Phase ◽  
Laves Phases ◽  
Effective Factors ◽  
Alloying Effects

ABSTRACTSystematic studies of alloys based on TiCr2 have been performed in order to improve the toughness of Laves phase intermetallics. The extent to which alloy compositions and annealing treatments influence the toughness was quantified by Vickers indentation. The single-phase Laves behavior was first established by studying stoichiometric and nonstoichiometric TiCr2. Next, alloying effects were investigated with ternary Laves phases based on TiCr2. Different microstructures of two-phase alloys consisting of (Ti,Cr)-bcc+TiCr2 were also examined. Various toughening theories based on vacancies, site-substitutions, crystal structure (C14, C36, or C15) stabilization, and the presence of a second phase were evaluated. The most effective factors improving the toughness of TiCr2 were determined, and toughening mechanisms are suggested.

Structural and Spectral Studies of 2-Pyridineformamide Thiosemicarbazone and its Complexes Prepared with Zinc Halides

2000 ◽  
Vol 55 (6) ◽  
pp. 511-518 ◽  
Author(s):  
Alfonso Castiñeiras ◽  
Isabel Garcia ◽  
Elena Bermejo ◽  
Douglas X. West
Keyword(s):  
Crystal Structure ◽  
Crystal Structures ◽  
Intermolecular Interactions ◽  
Trigonal Bipyramid ◽  
Spectral Studies ◽  
Hydrogen Atoms ◽  
Pyridyl Nitrogen ◽  
Imine Nitrogen

Reduction of 2-cyanopyridine by sodium in dry methanol in presence of thiosemicarbazide produces 2-pyridineformamide thiosemicarbazone, HAm4DH. The crystal structure of HAm4DH, which has a number of intermolecular interactions involving its five NH hydrogen atoms, has been solved. Crystal structures of the complexes prepared by reaction of HAm4DH with zinc(II) chloride, bromide and iodide have also been obtained. Neutral HAm4DH is coordinated via the pyridyl nitrogen, imine nitrogen and thione sulfur atoms, and each complex is five-coordinate with two halogen ligands. The structures of the three complexes are best described as square pyramidal with [Zn(HAm4DH)l2] having the largest distortion toward a trigonal bipyramid.

scholarly journals Structural Study of Ferroelectric Phase in Acid–Base Supramolecule

2014 ◽  
Vol 70 (a1) ◽  
pp. C553-C553
Author(s):  
Akiko Nakao ◽  
Reiji Kumai ◽  
Sachio Horiuchi ◽  
Yoshinori Tokura ◽  
Takashi Ohhara ◽  
...  
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Structure Analysis ◽  
Ferroelectric Phase ◽  
Structural Parameters ◽  
Imaging Plate ◽  
Hydrogen Atoms ◽  
Acid Base ◽  
Space Group ◽  
Successive Phase

Supramolecular ferroelectric cocrystal of phenazine (Phz) with chloranilic acid (H2ca), which exhibits three successive phase transitions, have been characterized by the interplay between their structural transformations and solid-state acid–base (proton transfer) reactions (Figure) [1]. This material undergoes a ferroelectric phase (FE-I phase) transition of displacive-type at 253 K followed by successive phase transitions to the lattice modulated phases with incommensurate periodicities and with commensurate 2-fold periodicity (FE-II phase) at lower temperature [2]. To elucidate the origin of the ferroelectricity in the FE-I phase, it is crucial to study the crystal structure using single crystals. The synchrotron x-ray diffraction experiment was carried out on the imaging-plate diffractometer at BL-8A of Photon Factory in KEK. Superstructure reflections with the modulation wave vector q=(1/2 1/2 1/2) were clearly observed below 103 K. Considering the preserved 2/m Laue symmetry, the lattice can be transformed to a C-centered monoclinic lattice, which is related by (-2a, -2b, a + c) or (2a, -2b, -a - c) with the FE-I structure. Although the lattice distortion and the intensities of the superlattice reflections are consistent with the 2/m Laue symmetry, the space group C1 is deduced from the polar nature and a subgroup symmetry of the FE-I structure. Moreover, we performed single-crystal neutron diffraction experiments at SENJU of MLF/J-PARC in order to determine the displacement of the hydrogen atom. The crystal structure analysis at 10 K was carried out using the reflections measured in a half-sphere of reciprocal space at d > 0.4. The structure analysis was performed on the basis of the space group C1, where four Phz and four H2ca become crystallographically inequivalent. Finally, all the structural parameters including all hydrogen atoms were successfully refined. In the FE-II phase, the neutral and ionic molecules alternately align along the π-molecular stack.

Kristallographische Konsequenzen von Pseudosymmetrie in Kristallstrukturen

2000 ◽  
Vol 215 (3) ◽  
Author(s):  
M. Ruck
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Crystal Structures ◽  
Structure Determination ◽  
Stacking Faults ◽  
Spatial Arrangement ◽  
Crystal Structure Determination ◽  
Small Deviations ◽  
New Class

The term pseudo-symmetry means a spatial arrangement that feigns a symmetry without fulfilling it. In crystal structures pseudo-symmetry is a more common feature than often recognized. In case of small deviations a variety of phenomena results: polytypism and stacking faults, disorder and twinning, commensurate and incommensurate super-structures, phase transitions and unusual reflection conditions. Most of these effects complicate crystal structure determination considerably. A series of examples with focus on the new class of ternary bismuth subhalides illustrates the different crystallographic consequences of pseudo-symmetry in crystal structures.

Crystal chemistry of orthosilicates and their analogs: the classification by topological types of suprapolyhedral structural units

2002 ◽  
Vol 58 (6) ◽  
pp. 948-964 ◽  
Author(s):  
G. D. Ilyushin ◽  
V. A. Blatov ◽  
Yu. A. Zakutkin
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Crystal Structures ◽  
Crystal Chemistry ◽  
Topological Classification ◽  
Structural Units ◽  
Coordination Sequence ◽  
Coordination Spheres

A method is developed for the analysis and classification of orthosilicates and their analogs Mx (TO4) y containing M cations and tetrahedral TO4 anions. The method uses the concepts of coordination sequence and crystal structure `reduced' graphs and is optimized for orthostructures of any complexity. First, the suprapolyhedral level of crystal structure organization was studied, where T tetrahedra were considered as templates for condensing M polyhedra, constructing as a result T polyhedral microensembles. Using this methodology, the crystal structures of 54 orthosilicates and orthogermanates were analyzed within the first 12 coordination spheres of T nodes and were arranged into 21 topological types. The topological types were expanded with the analogs found within the orthostructures of phosphates, sulfates etc. T polyhedral microensembles were used for the topological classification of reconstruction mechanisms of thermal and baric phase transitions of orthosilicates.

scholarly journals Hydration and Dehydration Transformation Mechanism of Cefaclor Pseudopolymorphs

2014 ◽  
Vol 70 (a1) ◽  
pp. C1574-C1574
Author(s):  
Ryosuke Toyoshima ◽  
Akiko Sekine ◽  
Hidehiro Uekusa
Keyword(s):  
Crystal Structure ◽  
Phase Transitions ◽  
Crystal Structures ◽  
Pharmaceutical Research ◽  
Three Dimensional ◽  
Crystal Form ◽  
Transformation Mechanism ◽  
Polymorphic Transitions ◽  
Block Pattern ◽  
Hydration And Dehydration

Hydration/dehydration phase transitions of active pharmaceutical ingredients (API) are often accompanied with changes of physicochemical properties, such as solubility, stability, and bioavailability. Therefore, three dimensional structural investigation of the hydration / dehydration mechanism of API is important for pharmaceutical research and development. By relative humidity control, Cefaclor hydrate crystal dehydrates non-stoichiometrically from dihydrate to anhydrous form A. Unexpectedly, its monohydrate form transformed into new 1.9 hydrate by slurry treatment (methanol / water) which dehydrated into another anhydrous form B through hemihydrate by heating. In this study, these hydration and dehydration presudo-polymorphic transitions of Cefaclor are investigated by the crystal structure analyses. Crystal structures of anhydrous and partially dehydrated forms were determined by structure determination from powder diffraction data technique because such dehydration phase transitions were resulted in a disintegration of single crystal form. In the first dehydration route, hydrates and the anhydrous form A have similar crystal structure, which is referred as `isomorphic desolvation'. Interestingly, the anhydrous form A has void spaces which corresponds to the water molecule position in the hydrate form. Thus, in hydration / dehydration phase transitions, water molecules move in and out of the void without changing the crystal structures, and the anhydrous form A can hydrate even in low R.H. condition. In the second route, the 1.9 hydrate, hemihydrate and the anhydrate form B have three crystallographically independent molecules forming similar T-shape building block pattern. There are tunnel spaces along b axis between the blocks. In the hydration / dehydration process, the blocks slide each other to open and close the channel. This mechanism explains another non-stoichiometric dehydration in this route.

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