7789-78-8Relevant academic research and scientific papers
Structural Distortion in Perovskite Type KCaH3–xFx (0.54 ≤ x ≤ 3)
Kohlmann, Holger,Pflug, Christian
, p. 175 - 179 (2020)
Representatives of the solid solution series KCaH3–xFx were synthesized by solid state reactions from binary metal hydrides and fluorides. Crystal structures were analyzed by Rietveld refinement based on X-ray powder diffraction. The degree of substitution was determined by refinement of site occupancy factors as well as elemental analysis for hydrogen. Three sections of x in KCaH3–xFx can be distinguished. For x 3 and the solid solution starts only at x = 0.54. The tetragonal SrTiO3 type structure with partial ordering of hydrogen and fluorine atoms is found for 0.54 ≤ x ≤ 1.7. Both anion positions show mixed occupation with some preference of hydrogen atoms for 8h and fluorine atoms for 4a sites (I4/mcm, SrTiO3 type). For fluorine-rich compounds a solid solution with orthorhombic GdFeO3 type structure (Pnma) and a perfectly statistical distribution of hydrogen and fluorine atoms is found (1.8 ≤ x ≤ 3). Interatomic distances resulting from the structure refinements are in the range of typical K–H, K–F, Ca–H, and Ca–F distances for mainly ionic compounds.
CsCaF3 - //xH//xSOLID SOLUTION (0 less than equivalent to x less than equivalent to 1. 70): STRUCTURAL CHARACTERISTICS AND HYDROGEN DIFFUSION INVESTIGATION.
Park,Senegas,Reau,Pezat,Darriet,Hagenmuller
, p. 1127 - 1138 (1988)
A study of the substitution of hydrogen for fluorine in CsCaF//3 shows the existence of a solid solution CsCaF//3// minus //x//H//x (0 less than equivalent to x less than equivalent to 1. 70) with cubic perovskite type structure. Both X-ray powder diffraction and wideline NMR show a statistical distribution of hydrogen and fluorine anions. The thermal evolution of the line width in the NMR spectra for **1H and **1**9F has been studied. The hydride ion is the only mobile one. Ionic-conductivity measurements seem to confirm a diffusion mechanism based on anionic hydrogen vacancies.
Synthesis of high-purity calcium hydride
Bulanov,Troshin,Balabanov
, p. 875 - 877 (2004)
A procedure for synthesizing high-purity calcium hydride in high yield was suggested. The admixture composition of the resulting CaH2 was determined by laser mass spectrometry.
Structural and vibrational properties of Ca2FeH6 and Sr2RuH6
Hagemann,Danna,Lawson Daku,Gomes,Renaudin,Yvon
, p. 286 - 289 (2011)
The structural and vibrational properties of the isostructural compounds Ca2FeH6 and Sr2RuH6 are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A2TH6 (A=Mg, Ca, Sr; T=Fe, Ru) in order to identify correlations between selected frequencies and the TH bond length. The bulk moduli of Ca2FeH6 and Sr2RuH6 have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg2FeH6 and several other compounds of this structure.
X-ray and neutron powder diffraction study of the order-disorder transition in Eu2IrH5 and the mixed crystal compounds Eu2-xAxIrH5 (A = Ca, Sr; x = 1.0, 1.5)
Kohlmann,Moyer Jr.,Hansen,Yvon
, p. 35 - 43 (2003)
The title compounds and their deuterides have been prepared by solid-state and solid-gas reactions from the elements and investigated by X-ray and neutron powder diffraction as a function of temperature. At room temperature they crystallize with an anion-deficient cubic K2PtCl6-type structure (space group Fm3m) in which five hydrogen (deuterium) atoms surround iridium randomly on six octahedral sites with average bond distances of Ir-D=169-171pm. At low temperature they undergo a tetragonal deformation (space group I4/mmm) to the partially ordered Sr2IrD5 (T=4.2K)-type structure in which four hydrogen (deuterium) atoms occupy planar sites with full occupancy (Ir-D=166-170pm) and two hydrogen (deuterium) atoms axial sites (Ir-D=174-181pm) with ～50% occupancy, i.e., the data are consistent with a mixture of square-pyramidal [IrD5]4- complexes pointing in two opposite directions. The transitions occur at ～240K (Eu0.5Ca1.5IrD5, Eu0.5Sr1.5IrD5), ～210K (EuSrIrD5), ～200K (EuCaIrD5, Eu2IrD5), and are presumably of first order.
The many phases of CaC2
Konar, Sumit,Nylén, Johanna,Svensson, Gunnar,Bernin, Diana,Edén, Mattias,Ruschewitz, Uwe,H?ussermann, Ulrich
, p. 204 - 213 (2016)
Polymorphic CaC2 was prepared by reacting mixtures of CaH2 and graphite with molar ratios between 1:1.8 and 1:2.2 at temperatures between 700 and 1400 °C under dynamic vacuum. These conditions provided a well controlled, homogeneous, chemical environment and afforded products with high purity. The products, which were characterized by powder X-ray diffraction, solid state NMR and Raman spectroscopy, represented mixtures of the three known polymorphs, tetragonal CaC2-I and monoclinic CaC2-II and -III. Their proportion is dependent on the nominal C/CaH2 ratio of the reaction mixture and temperature. Reactions with excess carbon produced a mixture virtually free from CaC2-I, whereas high temperatures (above 1100 °C) and C-deficiency favored the formation of CaC2-I. From first principles calculations it is shown that CaC2-I is dynamically unstable within the harmonic approximation. This indicates that existing CaC2-I is structurally/dynamically disordered and may possibly even occur as slightly carbon-deficient phase CaC2-δ. It is proposed that monoclinic II is the ground state of CaC2 and polymorph III is stable at temperatures above 200 °C. Tetragonal I represents a metastable, heterogeneous, phase of CaC2. It is argued that a complete understanding of the occurrence of three room temperature modifications of CaC2 will require a detailed characterization of compositional and structural heterogeneities within the high temperature form CaC2-IV, which is stable above 450 °C. The effect of high pressure on the stability of the monoclinic forms of CaC2 was studied in a diamond anvil cell using Raman spectroscopy. CaC2-II and -III transform into tetragonal CaC2-I at about 4 and 1GPa, respectively.
Hydrogen impurity effects. A5Tt3 intermetallic compounds between A = Ca, Sr, Ba, Eu and Tt = Si, Ge, Sn with Cr5B3-like structures that are stable both as binary and as ternary hydride and fluoride phases
Leon-Escamilla,Corbett, John D.
, p. 149 - 162 (2001)
All of the binary systems Ca, Sr, Ba, or Eu (A) with Tt (tetrel) = Si or Ge as well as Sr-Sn form both binary Cr5B3-type A5Tt3 phases and the corresponding ternary hydrides with stuffed Cr5B3- (Ca5Sn3F-) type structures. All of those tested, Ca-Si, Ba-Si, Ca-Ge, also yield the isotypic A5Tt3Fx phases. The tetragonal structures of Ca5Si3, Ca5Si3F0.42, Sr5Si3, Eu5Si3Hx, Ca5Ge3, Ca5Ge3Hx, Ca5Ge3F0.66(I4/mcm, No. 140) and of Ba5Si3F0.16 (P4/ncc, Ba5Si3-type) were refined from single-crystal X-ray diffraction data. The interstitial H, F atoms are bound in a constricted tetrahedral (A2+)4 cavity in the Cr5B3-type heavy atom structure, which can be described ideally as (A2+)5(Tt2)6-(Tt)4-. Many of 14 previous reports of the phases reported here were apparently hydrides according to lattice constant differences or, for Sr5Si3, the fractional coordinates of Sr2 about the tetrahedral site. An articulated model is developed that allows description of the relationship between the dimensions of the tetrahedral interstitial site and the cation cavity about Tt2 and for some matrix effects in this structure type. The model suggests limitations on the stability of these binary A5Tt3 compounds for the heavier tetrels, as observed. The resistivities of Ca5Ge3 and Ca5Ge3Hx are both characteristic of poor metals, and Pauli-like magnetic susceptibilities are exhibited by Ca5Ge3, Ca5Ge3Hx, Ca5Ge3F0.66, Sr5Ge3, and Sr5Sn3. The characteristic ideal Tt26- dimers are evidently not realistic descriptions for these phases; rather at least some of the η*4 electrons in the dimers are delocalized in a conduction band. This effect appears to be greater in two europium salts. Bond lengths of dimers in the Ca-Si and Ca-Ge families appear to shorten slightly in three instances of their oxidation to form the hydride or the fluoride, as might be expected.
Synthesis and characterization of a new ternary imide-Li 2Ca(NH)2
Wu, Guotao,Xiong, Zhitao,Liu, Tao,Liu, Yongfeng,Hu, Jianjiang,Chen, Ping,Feng, Yuanping,Wee, Andrew T. S.
, p. 517 - 521 (2007)
The ternary imide Li2Ca(NH)2 was successfully synthesized by dehydrogenating a mixture of LiNH2 and CaH2 at a molar ratio of 2:1 in a stream of purified argon at 300°C. A powder X-ray diffraction measurement revealed that Li2Ca(NH)2 was of the trigonal anti-La2O3 structure (space group P3m1) with lattice constants of a = 3.5664(3)A and c = 5.9540(8) A. Ca occupied the 1b site (0, 0, 1/2), Li occupied the 2d site (1/3, 2/3, 0.8841(22)), and N occupied the 2d site (1/3, 2/3, 0.2565(15)). Nuclear magnetic resonance and X-ray absorption fine structure analyses demonstrated that each Li ion was coordinated with four imide ions and each Ca ion was coordinated with six imide ions.
Hydrogen storage of metal nitride by a mechanochemical reaction
Kojima, Yoshitsugu,Kawai, Yasuaki
, p. 2210 - 2211 (2004)
Metal imides (Li2NH, CaNH), a metal amide (LiNH2) and metal hydrides (LiH, CaH2) were synthesized by ball milling of their respective metal nitrides (Li3N, Ca3N2) in a H2 atmosphere at 1 MPa and at room temperature.
Synthesis and Photoluminescence Properties of Rare-Earth-Activated Sr3- xAxAlO4H (A = Ca, Ba; x = 0, 1): New Members of Aluminate Oxyhydrides
Fujii, Kotaro,Matsuishi, Satoru,Murakami, Taito,Wu, Tong,Yashima, Masatomo
, p. 15384 - 15393 (2020)
A series of aluminate-based oxyhydrides, Sr3-xAxAlO4H (A = Ca, Ba; x = 0, 1), has been synthesized by high-temperature reaction of oxide and hydride precursors under a H2 atmosphere. Their crystal structures determined via X-ray and neutron powder diffraction are isostructural with tetragonal Sr3AlO4F (space group I4/mcm), consisting of (Sr1-x/3Ax/3)2H layers and isolated AlO4 tetrahedra. Rietveld refinement based on the diffraction patterns and bond-valence-sum analysis show that Ba preferentially occupies the 10-coordinated Sr1 sites, while Ca strongly prefers to occupy the 8-coordinated Sr2 sites. Luminescence owing to the 4f-5d transition of Eu2+ or Ce3+ was observed from Eu- and Ce-doped samples, Sr3-x-yAxByAlO4H (A = Ca, Ba; B = Eu, Ce; x = 0, 1, y = 0.02), under excitation of near-ultraviolet light. Compared with its fluoride analogue, Sr3AlO4H:Ce3+ shows red shifts of both the excitation and emission bands, which is consistent with the reported hydride-based phosphors and can be explained by the covalency of the hydride ligands. The observed luminescence spectra can be decomposed into two sets of sub-bands corresponding to Ce3+ centers occupying Sr1 and Sr2 sites with distinctly different Stokes shifts (1.27 and 0.54 eV, respectively), as suggested by the results of constrained density functional theory (cDFT). The cDFT results also suggest that the large shift for Ce3+ at Sr1 is induced by large distortion of the coordinated structure with shortening of the H-Ce bond in the excited state. The current findings expand the class of oxyhydride materials and show the potential of hydride-based phosphors for optical applications.
