Electron Paramagnetic Resonance spectroscopy (EPR) is a technique that is related to Nuclear Magnetic Resonance (NMR) in the sense that it involves particles with a magnetic moment (spin) that can absorb electromagnetic radiation. In NMR these particles are nuclei with a net magnetic moment (spin quantum number I = 1
2 n+
, with n = integer), which are separated in a spin up and spin down energy level by applying a strong magnetic field (Zeeman effect). As the upper energy level is slightly less occupied than the lower energy level, the lower energy state nuclei can absorb electromagnetic radiation to flip spin and occupy the high energy level. The frequency of the electromagnetic radiation in NMR is typically in the order of MHz (radio waves). In EPR it is unpaired electrons that are studied. Unpaired electrons have a magnetic moment and spin quantum number ½ and can therefore be split into two energy levels in a similar way. The energy difference between the two levels when applying an external magnetic field, is given by ∆ = ⋅ =E h ν ge⋅µB⋅B0, where ge the electron’s g-factor, µB the Bohr magneton and B0 the
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magnetic field. It can be seen that the energy required for changing spin state is directly proportional to the magnetic field. For EPR the required magnetic fields are much smaller than for NMR though and the electromagnetic radiation that will typically be absorbed is of higher energy, i.e. ν ~ 10 GHz frequency.
Figure 4-11: Representation of electron spin energy splitting when subjected to a magnetic field
As unpaired electrons have a chemical or electronic surrounding in materials, they experience some magnetic shielding and therefore a different magnetic field than the externally applied field. This can be expressed by formula (4.1). The energy required for changing spin state therefore changes slightly as well. This is finally represented by the overall g-factor, which accounts for the unpaired electron’s electronic environment.
(
)
0 1 eff B =B ⋅ −σ (4.1)(
)
0 1 0 e B eff e B B hν =g µ B =g µ B ⋅ −σ =gµ B (4.2)Because EPR is sensitive to unpaired electrons it is the ideal technique to study electronic defects in ceramic materials. The loss of hydrogen/deuterium that was observed in both neutron diffraction and thermogravimetric analysis as well as the colouration after reaction with NaH(D) suggests we might have incorporated electronic defects, namely F-centres. F-centres are unpaired electrons occupying anion sites. This would explain the remaining loss of hydrogen/deuterium apart from the creation of a small amount of hydride ion vacancies. In this case sodium will have acted as a hydrogen getter and the following defect reaction (equation (4.3)) would take place.
B0≠ 0 B0 = 0 Magnetic field Energy ms = + ½ ms = - ½ ∆E = hν = E+½ – E -½ ν ~ 10 GHz
80 1 2 2 ( ) x Na x H H H ⎯⎯→ H g +e (4.3)
EPR was performed on a sample of NaD doped SrD2 and as shown in Figure 4-12 it does show a typical F-centre response at g = 3416 – 3418 Gauss. There is actually a small shoulder on the peak and so it seems there are two unresolved peaks that resemble the two hydride ion sites, occupied by unpaired electrons. It must be noted that the response was very weak and the signal had to be boosted. A detailed analysis of the electronic defect structure was beyond the scope of this thesis and so no further studies have been performed. The result however does show F-centre formation and this seems a satisfactory explanation for the large deuterium/hydrogen losses observed after reactions with sodium.
3000 3200 3400 3600 3800 -6.0x105 -4.0x105 -2.0x105 0.0 2.0x105 4.0x105 6.0x105 G
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5 Electrical properties of doped CaH
2and SrH
2Whereas there are only few papers reporting structural properties of alkaline earth metal hydrides, the electrical properties of these hydrides have been even less studied. CaH2 has been reported to be an ionic conductor of hydride ions. Gorelov and Pal’guev [31] studied the conductivity of CaH2 doped with LiH and CaF2 using DC techniques. They claimed hydride ion conductivities up to 10 S/cm above 780°C at which temperature CaH2 supposedly adopts the fluorite structure. Further they report that they were able to dope CaH2 with up to 31 mol% LiH, which causes the phase transition to shift to lower temperatures. Some ambiguous publications by Zhu [99-101] further suggest good hydride ion conductivity in CaH2, but this was showed only by direct fuel cell testing, rather than conductivity tests on single phase materials. Strontium and barium hydride have been studied for their electrical resistances under high pressure by Wakamori et al [102], but to the author’s knowledge not under ambient pressures. None of the papers thoroughly study the conduction mechanism of those materials and prove pure hydride ion conductivity.
In this chapter it is tried to get a better understanding about the electrical properties of CaH2 and SrH2 in general and about the hydride ion conduction mechanism specifically, by creating extrinsic defects. To achieve this, both calcium and strontium hydride have been doped with sodium hydride to create hydride ion vacancies. The electrical properties of both undoped and sodium doped calcium/strontium hydride were tested to see the effect of the presence of hydride ion vacancies.