Formation and Structural Properties

The niobium-hydrogen system has been investigated extensively and its structure and thermodynamic properties are well recorded. A comprehensive phase diagram of the system exists, as shown in figure 4.1 after Kobler and Welter [4.1] who have looked specifically at the low temperature region using magnetic susceptibility studies to differentiate the phase regions of the peritectoid cascade. Reviews of the entire system with greater detail of the structure of each phase, which were not determined by the susceptibility studies of Kfibler and Welter, are given in numerous papers [4.2 to 4.6]. The hydrogens move around on interstitial sites behaving like a gas in the a-phase and like a liquid in the a ’-phase. The ¡3-phase results from ordering of the hydrogens due to H-H interactions which also produces a distortion of the niobium lattice. The ¡3- phase is an ordered solid solution of hydrogen, where the niobium atoms form a face- centred orthorhombic structure. This was determined using x-ray diffraction which also shows that the fco lattice parameters, a. b and c, vary linearly with hydrogen concentration [4.3 and references therein]. The slight distortion of the bcc niobium lattice on hydriding to this phase is shown in figure 4.2 where the fco cell parameters are a- 4.82 - 4.84A. b - 4.86 - 4.92A and c- 3.42 - 3.47A with ij- 0.4 - 0.9® for x between 0.7 and 1.0 at room temperature. It is partly the large concentration of the |3-phase with only a slight associated lattice distortion and the availability of relatively large niobium crystals that make it a suitable system for this nmr study.

Hydrogen has been shown to occupy an ordered subset of T-sites within the niobium lattice. This was initially observed using neutron diffraction [4.7] and later confirmed by electron microscopy [4.8] where it was shown that the hydrogens form linear chains along the <110> directions on [ll2 ] planes of the original bcc lattice, see figure 4.3. Within a single crystal o f niobium, six possible hydrogen chain directions exist due to the inequivalence of T-sites within a unit cell. There are three pairs o f T-sites, each pair having a principal symmetiy axis parallel with either the [100], [010] or [001] original bcc niobium axes. Note that this axis system is referred to throughout, see figure 4.4. The separation of nearest T sites along one chain is a/V2 (ie along the [110] direction), and between adjacent chains it is V3a/2 and a along [111] and [001]

directions respectively. Identical chains tend to group forming domains which vary in size depending on the formation process. The growth of the domains tends to be on {100} planes which minimises the enthalpy of formation of the ordered phase since boundaries are shared with inequivalent T-site chains which do. however, share the same principal symmetry axis. These pairs are shown in figure 4.4 as: sites 1 and 2. sites 3 and 4, and sites 5 and 6. This formation of coherent domain boundaries gives rise to twinning throughout the lattice of say 1-2-1-2-1 domain sequences as observed by Schober and Linke [4.9]. However, a single crystal of niobium once hydrided will consist of an array of all six domains unless the sample is prepared in a manner which gives an increased population of a specific domain. This causes a problem in the analysis of nmr results and is discussed later, in particular in section 4.1.6.

Figure 4.5 shows the hydrogen solubility' curves and in conjunction with the phase diagram we can see that in forming a single crystal a minimum hydriding temperature of 450K must be observed in order to avoid the ot + o i region. In any event it is found that the most effective hydriding temperature is around 700-800K. The (3- phase is formed on decreasing the temperature from this range which leads to decomposition of the supersaturated solution a'-phase into the ordered hydride. On exposure to the atmosphere a thin oxide layer seals the niobium surface effectively trapping the hydrogen up to temperatures of the order of 500K.

Formation of the (3-phase can be achieved by gaseous doping since niobium reacts exothermically with hydrogen. It has intermediate stability in relation to other metal hydrogen systems due to a relatively low dissociation pressure at room temperature, however, it is found that a surface oxide layer readily forms effectively increasing the surface barrier potential inhibiting either absorption or desorption of hydrogen [4.3]. To remove this layer prior to hydriding a suitable etchant was found to be a mixture of HFiHNQyfyO in the ratio of about 1:1:6.

Theoretical Knight Shift Behaviour

From this known structure the angular dependence of the hydrogen Knight shift can be calculated. Equation 2.10 gives the general expression for the anisotropic Knight shift. Initially ignoring the lattice distortion and assuming each T-site has axial symmetry then

since the bcc T-site has 42m symmetiy and therefore K* = Ky = -1/2 K*. Here 0 defines the angle of the T-site symmetry axis with respect to B0. Having ignored the lattice distortion there are only three inequivalent domains whose symmetry axes are orthogonal. Rotation of a single crystal, which comprises these domains, about a [110] type direction, that is with a (110) plane in the plane of B0, enables the orientation of [100], [111] and [110] type directions along B0. For one of the domains this rotation will change the T-site symmetiy axis from parallel to perpendicular to B0. Thus 0j = 0* for <100>//Bo and 0j - 90* for <110> parallel to B„. For the other two domains this is not the case and it can be shown that

Allowing for the lattice distortion means that the T-sites no longer have axiai symmetry, ie Kx#Ky, and therefore