Nuclear magnetic resonance evidence for the occurrence of a low-temperature pseudo-cubic phase in the Nb--H system near NbH/sub 0/. /sub 75/

Journal Article · Thu Jun 01 04:00:00 EDT 1978 · Scr. Metall.; (United States)

DOI:https://doi.org/10.1016/0036-9748(78)90197-7· OSTI ID:6376783

Hwang, Y S; Torgeson, D R; Barnes, R G

The phase diagram of the niobium--hydrogen system continues to be actively investigated. At low temperatures in particular, considerable uncertainty exists as to the locations of phase boundaries and the structures of the different phases. In the range of high hydrogen concentration, i.e., approximately 0.7 less than or equal to H/Nb less than or equal to 1.0, three low-temperature phases exist. These are the orthorhombic epsilon phase with ideal stoichiometry Nb/sub 4/H/sub 3/, the lambda phase with an as yet unknown structure and ideal stoichiometry Nb/sub 5/H/sub 4/, and the pseudo-cubic ..gamma.. phase with ideal stoichiometry NbH. The first measurements of the nuclear magnetic resonance (NMR) of (93)Nb in niobium hydrides at low temperature (4.2K and 77K) are reported. These measurements reveal clearly the occurrence of a pseudo-cubic phase, i.e., one in which the Nb sites possess axial point symmetry. Since (93)Nb possesses a substantial electric quadrupole moment, its magnetic resonance spectrum is particularly sensitive to the symmetry and magnitude of the crystalline electric field (CEF) at the Nb site. At low temperatures, the random inhomogeneity of the CEF generated by the diffusive motion of the hydrogen is effectively ''frozen out,'' and well-resolved spectra are obtained as is also the case at high temperature (T approx. 370K) in the orthorhombic ..beta.. phase and cubic ..cap alpha..' phase, where the hydrogen motion is sufficiently fast to average the inhomogeneity to a negligible value (2,3). At intermediate temperatures, 100K less than or equal to T less than or equal to 330K, the large inhomogeneity of the CEF causes the (93)Nb spectra to be very difficult to interpret.

• Research Organization: Ames Lab., IA
• OSTI ID: 6376783
• Journal Information: Scr. Metall.; (United States), Journal Name: Scr. Metall.; (United States) Vol. 12:6; ISSN SCRMB
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360202* -- Ceramics
Cermets
& Refractories-- Structure & Phase Studies
CRYSTAL STRUCTURE
CRYSTAL-PHASE TRANSFORMATIONS
HYDRIDES
HYDROGEN COMPOUNDS
MAGNETIC RESONANCE
NIOBIUM COMPOUNDS
NIOBIUM HYDRIDES
NUCLEAR MAGNETIC RESONANCE
PHASE TRANSFORMATIONS
POWDERS
RESONANCE
TRANSITION ELEMENT COMPOUNDS