Effect of H on the crystalline and magnetic structures of theYCo3−H(D)system. II.

Abstract

The crystalline and magnetic structures of the YCo 3 -H͑D͒ system have been investigated by means of x-ray and neutron diffraction with the objective of understanding the complex magnetic changes that are observed in this system as hydrogen is added. Synchrotron x-ray diffraction ͑XRD͒ patterns were first refined to yield the lattice parameters and coordination of Y and Co atoms in the metal and two ␤-hydride phases while XRD was used for the ␥ phase. In situ neutron powder diffraction measurements of YCo 3 D x were then made in all four phases to determine the deuterium site occupancies and magnetic structures. The site occupancies were also rationalized using the Westlake geometric model. The highest hydrogen concentration measured was YCo 3 H 4.6 . Using the Westlake model, we conclude that the saturated hydrogen content would be YCo 3 H 5 . Our results reported here and in Part I ͓Phys. Rev. B 76, 184443 ͑2007͔͒ have enabled us to rationalize the changes in the magnetic structures in terms of changes in the cobalt-cobalt distance caused by the addition of hydrogen. In particular, in the antiferromagnetic ␥ phase, we observe Co atomic displacements that enable the structure to adopt a particular antiferromagnetic structure in a manner that is reminiscent of a Peierls distortion as observed in transitions from the conducting to nonconducting hydrides on addition of hydrogen in YH 3 .

Directional metal–hydrogen bonding in interstitial hydrides

“…Deuterium contents were estimated from X-ray measurements of rhombohedral cell volumes that varied in the ranges V = 554-559Å 3 and 574-577Å 3 for the ␤ 1 and ␤ 2 -phases, and V = 623-635Å 3 for the ␥-phase. The coexistence of ␤ 1 and ␤ 2 phases in some samples confirmed that these were separate phases, such as those reported in the YCo 3 -D system [2].…”

“…Only few hydrides have been structurally characterized. The most complete study is that on the cobalt-based YCo 3 -D system, which displays three rhombohedral hydride phases (␤ 1 , ␤ 2 and ␥ [2]). All have centrosymmetric structures (R3m).…”

Quintessence and effective RN de Sitter brane geometries

“…Since closed strings are tangential to a D p -brane, Einstein a e-mail: kppandey@physics.du.ac.in b e-mail: abhishek151983@gmail.com c e-mail: sunita@physics.du.ac.in d e-mail: richa.phy@du.ac.in e e-mail: skkar@physics.du.ac.in gravity is believed to decouple from a D p -brane world volume [2]. Nevertheless a non-linear U (1) gauge field is well known to describe an effective open string metric on a D pbrane and leads to a number of near horizon geometries [3][4][5][6][7][8][9][10][11][12][13][14][15]. In fact a constant Neveu-Schwarz (NS) two form background in an open string theory, in combination with an electromagnetic field, forms a gauge invariant field strength which governs the non-linear U (1) charge on a D p -brane.…”

“…The background metric for a D 4 -brane gauge dynamics presumably describes a hiddenD 4 -brane in the formalism. A priori, the D 4 -brane vacuum may be seen to be governed by an effective black hole essentially sourced by the global modes of a NS two form [3][4][5][6][7][8][9][10][11][12][13][14][15]. Then the two form in the gauge theory may be seen to vacuum create a pair of (DD) 3 -branes at the past horizon of a background pure de Sitter.…”

Effect of H on the crystalline and magnetic structures of theYCo3−H(D)system. I.YCo3Cui1, Liu2, Georgiev3 et al. 2007Phys. Rev. BSelf Cite13010View full textAdd to dashboardCite

“…As one can see below, even for the parent crystal YCo 3 , there are conflicting views as to whether the magnetic ground state is ferromagnetic or ferrimagnetic. In Part II, 6 we report on investigations into the crystal and magnetic structures of this system as a function of increasing hydrogen content by means of synchrotron x-ray and neutron diffraction distinguishing between the so-called ␤ 1 and ␤ 2 phases with the objective of understanding what drives the dramatic changes in magnetization that are observed.…”