Van der Waals and repulsive interactions in the crystal structure of heavy alkali metals

1980 ◽  
Vol 58 (6) ◽  
pp. 905-911 ◽  
Author(s):  
J. C. Upadhyaya ◽  
S. Wang ◽  
R. A. Moore
Keyword(s):  
Crystal Structure ◽  
Binding Energy ◽  
Alkali Metal ◽  
Alkali Metals ◽  
High Pressures ◽  
Van Der Waals ◽  
Energy Difference ◽  
Repulsive Interactions

The present work deals with the calculation of a screened van der Waals contribution to the binding energy of heavy alkali metals, namely K, Rb, and Cs. Our calculations show that the van der Waals contribution is of the order of energy difference between different phases for a heavy alkali metal. Further we study the effect of the Born–Mayer type repulsive interactions on the crystal structure for heavy alkalis. It appears from this study that these repulsive interactions are important in the determination of the crystal structure, particularly for a heavy alkali metal under high pressures.

scholarly journals Spotlight on Alkali Metals: The Structural Chemistry of Alkali Metal Thallides

Crystals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1013
Author(s):  
Stefanie Gärtner
Keyword(s):  
Crystal Structure ◽  
Alkali Metal ◽  
Crystal Structures ◽  
Alkali Metals ◽  
Valence Electron ◽  
Oxidation States ◽  
Valence Electron Concentration ◽  
Base Metals ◽  
Charge Balancing

Alkali metal thallides go back to the investigative works of Eduard Zintl about base metals in negative oxidation states. In 1932, he described the crystal structure of NaTl as the first representative for this class of compounds. Since then, a bunch of versatile crystal structures has been reported for thallium as electronegative element in intermetallic solid state compounds. For combinations of thallium with alkali metals as electropositive counterparts, a broad range of different unique structure types has been observed. Interestingly, various thallium substructures at the same or very similar valence electron concentration (VEC) are obtained. This in return emphasizes that the role of the alkali metals on structure formation goes far beyond ancillary filling atoms, which are present only due to charge balancing reasons. In this review, the alkali metals are in focus and the local surroundings of the latter are discussed in terms of their crystallographic sites in the corresponding crystal structures.

Structural characterization of a new high-pressure phase of GaAsO4

2006 ◽  
Vol 62 (6) ◽  
pp. 1019-1024 ◽  
Author(s):  
David Santamaría-Pérez ◽  
Julien Haines ◽  
Ulises Amador ◽  
Emilio Morán ◽  
Angel Vegas
Keyword(s):  
Crystal Structure ◽  
High Pressure ◽  
High Pressures ◽  
Ambient Conditions ◽  
Hexagonal Cell ◽  
Octahedral Sites ◽  
New Phase ◽  
Pressure Phase

As in SiO2 which, at high pressures, undergoes the α-quartz → stishovite transition, GaAsO4 transforms into a dirutile structure at 9 GPa and 1173 K. In 2002, a new GaAsO4 polymorph was found by quenching the compound from 6 GPa and 1273 K to ambient conditions. The powder diagram was indexed on the basis of a hexagonal cell (a = 8.2033, c = 4.3941 Å, V = 256.08 Å3), but the structure did not correspond to any known structure of other AXO4 compounds. We report here the ab initio crystal structure determination of this hexagonal polymorph from powder data. The new phase is isostructural to β-MnSb2O6 and it can be described as a lacunary derivative of NiAs with half the octahedral sites being vacant, but it also contains fragments of the rutile-like structure.

An explanation of the crystal structure of the rare gas solids

1974 ◽  
Vol 336 (1606) ◽  
pp. 365-377 ◽  
Keyword(s):  
Crystal Structure ◽  
Band Structure ◽  
Excited States ◽  
Solid Helium ◽  
Van Der Waals ◽  
Energy Difference ◽  
Relative Energy ◽  
Rare Gas ◽  
Interatomic Potentials ◽  
Starting Point

A new explanation of why the crystal structure of the rare gas solids, Ne, Ar, Kr and Xe is f. c. c. rather than h. c. p. is offered. The magnitude of the relative energy difference, ∆ = ( E f. c. c. – E h. c. p. )/ E t. c. c. , is estimated and it is shown that the effect is numerically large enough in all these solids ( ∆ ≳ + 1 x 10 –3 ) to overcome the small preference of two-body interatomic potentials for the h. c. p. structure ( ∆ ≃ – 10 –4 ). The effect is much weaker in helium and so the h. c. p. structure of solid helium emerges naturally as a consequence of the two-body potential. The explanation depends on the modification of the (long-range) van der Waals energy by the (short-range) overlap of atomic excited states with the neighbouring atoms in the crystal. The resulting crystal field in the f. c. c. and h. c. p. structures splits excited d-states by different amounts. The f. c. c. structure is favoured because the energy split is wider in f. c. c. (which is centrosymmetric) than in h. c. p. (which does not have a centre of symmetry at the atomic sites); the resulting van der Waals attractive energy is thereby greater in f. c. c. An alternative approach is also developed, which uses the band states of the crystal as a starting-point, and yields a similar result. We expect that, if good enough band structure calculations of h. c. p. rare gas solids were available, the best way to estimate the value of ∆ would be to calculate the van der Waals energy in the solid in terms of band structure energies for the excited states and gas phase values for the dipole matrix elements. Preliminary estimates of the size of the effect, based on currently available band structure data, suggest that ∆ ranges from approximately 12 x 10 –4 for Ne to 27 x 10 –4 for Xe; these values are quite sufficient to explain the stability of the f. c. c. structure.

Hydrothermal Liquefaction of Enteromorpha prolifera for Bio-Crude Production: An Experimental Study on Aqueous Phase Recirculation

2019 ◽  
Vol 62 (5) ◽  
pp. 1113-1119 ◽  
Author(s):  
Jixiang Zhang ◽  
Cheng Qian ◽  
Kelei Yan ◽  
Jianfei Song ◽  
Baohui Jiang
Keyword(s):  
Experimental Study ◽  
Alkali Metal ◽  
Aqueous Phase ◽  
Alkali Metals ◽  
Hydrothermal Liquefaction ◽  
Enteromorpha Prolifera ◽  
Catalyst Recycling ◽  
Catalyst Dosage

Abstract. In this study, catalytic hydrothermal liquefaction (HTL) of with aqueous phase recirculation (APR) was tested in order to improve bio-crude yield and reduce catalyst dosage. Bio-crude yields of 17.0% and 18.5% were obtained at 300°C in 30 min of non-catalytic HTL and catalytic HTL with 5.0% NaOH, respectively. Determination of alkali metals showed that 76.7% of the Na was recovered in the aqueous phase, indicating that APR provided the possibility for catalyst recycling. Bio-crude yields were increased to 20.0% and 22.7% when performing APR for non-catalytic and catalytic HTL, respectively. To investigate the effects of recycled catalyst and volatile aqueous products (VAPs), a different APR experiment with VAPs removal for catalytic HTL was conducted, and a bio-crude yield of 17.8% was achieved. It was concluded that bio-crude yield increased with APR, mainly due to the effects of VAPs. Keywords: Alkali metal, Aqueous phase recirculation, Hydrothermal liquefaction.

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