ANEXOS

From examining the results of single crystal growth, it is apparent that Na2BH4NH2 readily

precipitates out of the liquid phase of the sample during cooling. As the sample cools and crystallizes it forms single crystals of the cubic phase. At room temperature single crystals were identified using a polarising microscope quantifying candidates for diffraction according to their optical quality – in particular, optical isotropy as expected for a cubic crystal and lack of strain or cracking. Only crystals of the cubic phase were found, As mentioned above crystals with potential for x-ray diffraction were then quick frozen under 100K nitrogen this had the effect of freezing in the crystal in MC-phase seen in figure 3.11,

68 as the same powder sample was used to produce these single crystals. At low temperature as seen in the 0kl plane of reciprocal space with all the observable Bragg peaks lying on a cubic lattice, figure 3.14. The results from the refinement of this lattice are given in table 3.2. These results agree well with the results from the structural powder analysis presented by M. Somer et al. [2] eventhough this crystal is in a MC-phase and not the C-phase examined in [2].

69

Figure 3.15 The 0kl plane from single crystal diffraction of the O-phase of Na2BH4NH2 at 100K

Crystal System Cubic Orthorhombic Space Group Pm-3m Pbcm

Lattice parameter, a, (Å) 4.665(1) 6.475(1) Lattice parameter, b, (Å) -- 5.253(1) Lattice parameter, c, (Å) -- 10.741(2) Unit Cell Volume (Å3) 101.5(1) 365.4(1)

Table 3.2 The results of lattice fitting of single crystal diffraction

As all the crystals formed from cooling the melt are in the cubic phase, to form an orthorhombic crystal, a cubic crystal was first removed from the oil and stored under a protective atmosphere for several days after which time the crystal was examined using single crystal XRD to determine its crystal lattice. Figure 3.15 shows the reciprocal lattice plane 0kl from the O-phase. The results of the lattice refinement from both polymorphs are presented in Table 3.2. The results for the MC-phase crystal are not consistent with a contraction of the cubic, C-phase, lattice described by the HT-PXRD data in figure 3.10, the unit cell volume in table 3.2 does not follow the linear expansion trend seen in the figure it is considerably higher than would be expected if the graph was extrapolated down to the single crystal measurement temperature. This is further proof of this cubic phase being the metastable, MC-phase, and deviant from the general trends of the C-phase crystal.

70 Comparing the lattice parameters of this crystal with the MC-phase results given in figure 3.11 the results are consistent with the lattice expansion described there. The results for the O-phase crystal show that the lattice parameters are as expected given the direction of the thermal lattice expansion described above and LT-PXRD results. However, it is difficult to extrapolate a line of fit to the data in figure 3.7 and so the expected lattice parameters at this temperature are difficult to determine. There is a large discrepancy between the expected lattice volume of the O-phase based on the volume thermal expansion given in table 3.1 and the calculated unit cell volume given in table 3.2. This discrepancy could be a result of the unreliable determination of the trend in volume expansion given the low resolution results of the LT-PXRD experiment. Furthermore the relation between the unit cell volumes of the MC-phase and O-phase polymorphs at 100k seems to suggest that there are less than 4 formula units percent in the O-phase or that the MC-phase unit cell volume is higher than it should be- this is most likely the case as the MC-phase is not the native C- phase at these temperatures. Extrapolating the C-phase volume expansion given in figure 3.10 the cubic lattice volume should be much lower at this temperature. The structural refinement of this MC-phase has proved difficult, whilst being mindful of the total experiment length- to reduce the impact of any sample degradation due to exposure to oxygen and moisture, it has been possible to collect data to a significant resolution, down to 0.6Å in d-spacing, with a good level of redundancy over the entire Ewald sphere. However, the internal R-factor for this data has proved less than desirable with an average value of 0.21. This indicates that the quality of the crystal is questionable; in addition to this the internal R-value is greater at lower d-spacing this is due to the very low intensity of these reflections- as the material contains only light elements. Even using the structure of the C-phase as a starting point, it has been not been possible to get a fit with a fitting parameter (R-value) less than 0.25, which makes it impossible to confirm the structure of the single crystal. The most likely cause of this discrepancy is the inherent disorder in the crystal due to the only 2/3 filling of the octahedra; whilst the O-phase would indicate that the partially-filled octahedra are ordered. This is not seen in the single crystal structure most likely as the crystals were not derived from the O-phase and instead were grown out of the melt which will have resulted in a random distribution of filled octahedra sites within the crystal giving the averaged cubic structure seen in figure 3.1(a). The consistency of the results from the O-phase is much better with an average internal r-value of 0.055 and whilst the resolution of the experiment is similar to that of the MC-phase data the redundancy of this data is much lower which may mask the actual quality of the data.

71 Again, the internal R-values of this data increase at higher d-spacing which is to be expected with a weakly diffracting sample, such as this, over a relatively short experimental exposure time. Attempts to solve the crystal structure of this O-phase have proved as fruitless as the MC-phase. Whist the structure is not expected to be as disordered as the MC-phase; it has still proved difficult to solve the structure either from the expected structure or from scratch with the best R-values of 0.6 and 0.2, respectively.