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Magnetic Order in YbMn2Si2 - Neutron Scattering Investigation
S. J. Campbell,M. Hofmann,R. A. Mole,K. Prokes,D. Wallacher,J. L. Wang 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.63 No.3
Several mechanisms have been proposed to account for the structural and magnetic behaviour ofthe rare earth intermetallic compound YbMn2Si2 below the transition that occurs around 30 K. We have carried out detailed neutron diffraction measurements over the temperature range 0.3 -52 K and confirm both an antiferromagnetic structure with ferromagnetic Mn (001) planes coupledantiferromagnetically along the c-axis above 30 K and the absence of ordering of the Yb sublatticedown to 0.3 K. The decrease in intensity of the (111) reflection around 30 K together with theappearance of satellite reflections of propagation vector k = 00 12 confirm that the magnetic unitcell doubles along the c axis below TN2. The resultant molecular field acting on half of the ionsbelow 30 K removes the degeneracy of the Kramers doublets of the Yb3+ ions and accounts forthe additional excitations observed below 30 K compared with the inelastic neutron scatteringabove 30 K.
The effect of Al on the hydrogen sorption mechanism of LiBH<sub>4</sub>
Friedrichs, O.,Kim, J. W.,Remhof, A.,Buchter, F.,Borgschulte, A.,Wallacher, D.,Cho, Y. W.,Fichtner, M.,Oh, K. H.,Zü,ttel, A. Royal Society of Chemistry 2009 Physical chemistry chemical physics Vol.11 No.10
<P>We demonstrate the synthesis of LiBH<SUB>4</SUB> from LiH and AlB<SUB>2</SUB> without the use of additional additives or catalysts at 450 °C under hydrogen pressure of 13 bar to the following equation: 2LiH + AlB<SUB>2</SUB> + 3H<SUB>2</SUB>↔ 2LiBH<SUB>4</SUB> + Al. By applying AlB<SUB>2</SUB> the kinetics of the formation of LiBH<SUB>4</SUB> is strongly enhanced compared to the formation from elemental boron. The formation of LiBH<SUB>4</SUB> during absorption requires the dissociation of AlB<SUB>2</SUB>, <I>i.e.</I> a coupled reaction. The observed low absorption-pressure of 13 bar, measured during hydrogen cycling, is explained by a low stability of AlB<SUB>2</SUB>, in good agreement with theoretical values. Thus starting from AlB<SUB>2</SUB> instead of B has a rather low impact on the thermodynamics, and the effect of AlB<SUB>2</SUB> on the formation of LiBH<SUB>4</SUB> is of kinetic nature facilitating the absorption by overcoming the chemical inertness of B. For desorption, the decomposition of LiBH<SUB>4</SUB> is not indispensably coupled to the immediate formation of AlB<SUB>2</SUB>. LiBH<SUB>4</SUB> may decompose first into LiH and elemental B and during a slower second step AlB<SUB>2</SUB> is formed. In this case, no destabilization will be observed for desorption. However, due to similar stabilities of LiBH<SUB>4</SUB> and LiBH<SUB>4</SUB>/Al a definite answer on the desorption mechanism cannot be given and neither a coupled nor decoupled desorption can be excluded. At low hydrogen pressures the reaction of LiH and Al gives LiAl under release of hydrogen. The formation of LiAl increases the total hydrogen storage capacity, since it also contributes to the LiBH<SUB>4</SUB> formation in the absorption process.</P> <P> </P> <P>Graphic Abstract</P><P>LiBH<SUB>4</SUB> (LiBD<SUB>4</SUB>) is synthesized by hydrogenation of LiH (LiD) and AlB<SUB>2</SUB> without use of additional additives or catalysts. The reversible reaction mechanism is investigated. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=b814282c'> </P>