Amorphous Al1−xTix, Al1−xVx, and Al1−xFex phases in the hydrogen cycled TiCl3, VCl3 and FeCl3 enhanced NaAlH4 systems

A structural review of nanoscopic Al1−xTMx phase formation in the TMCln enhanced NaAlH4 system

“…Such a large deviation is indicative of a change in the NaAlH 4 structure. Several factors must be considered before any deductions are made about what could change the NaAlH 4 unit cell dimensions so dramatically: (i) the diffracted intensities of the 'compressed' NaAlH 4 phase are not able to be analysed for compositional changes at proportions in the order of 2 mol% of the sample, (ii) all NaCl expected by mechanochemical reduction is accounted for, and we do not expect Cl − anions have substituted into the NaAlH 4 unit cell (the same can be said for every TM and RE species in this study), (iii) the 'compressed' NaAlH 4 lineshape is similar to the 'normal NaAlH 4 lineshape, indicating that microstructure plays no role [15], and (iv) the c-Al 87.6 Sc 12.4 solid solution accounts for 79.16% of the originally added Sc atoms by itself, and is highly likely embedded on the powder grain surface [15], similar to the case for Ti [11,20] and V [20]. We expect that Al 3 Sc will always co-exist with the c-Al 87.6 Sc 12.4 solid solution.…”

“…By itself, this phase accounts for 79.16% of the originally added Sc atoms. At high mol% TiCl 3 content, we have observed a minor proportion of Ti-poor a-Al 1−x Ti x (x < 0.15) phase [20], however, inspection of Fig. 1 reveals that for the H cycled NaAlH 4 + 0.1ScCl 3 system, there is no discernible deviation in the 2Å region, suggesting that the formation of a-Al 1−x Sc x has not occurred.…”

“…The broad peaks indicate either nanoscopic or amorphous Al 1−x TM x phases, dependent on the observation of further broad crystalline reflections at lower d-spacings. As discussed in [20], the broad peaks from PM and H cycled VCl 3 and FeCl 3 enhanced NaAlH 4 are the primary halo from amorphous a-Al 72 V 28 , and a-Al 88.5 Fe 11.5 respectively. For the H cycled PM NaAlH 4 + 0.1VCl 3 system, we observe both crystalline and amorphous Al 1−x V x phases co-exisiting in a composite nanoscopic morphology embedded on the NaAlH 4 surface.…”

“…A cAl 90 V 10 solid solution (producing the low d-spacing shoulders on Al reflections in Fig. 1) is embedded within the a-Al 72 V 28 matrix [20]. In the discussion below, we describe how the broad peaks from Cr, Mn and Co are not primary halos from amorphous phases, rather, they represent extremely broad reflections from nanoscopic bcc Al 1−x TM x solid solutions.…”

“…2 shows the (1 1 1) reflection of crystalline fcc Al 1−x TM x (x < 0.25) solid solutions formed for TM = Sc, Ti, V, Cr, Mn, Zr and Pd in twice H cycled PM NaAlH 4 + 0.1TMCl n , with their associated unit cell dimension. We observe crystalline fcc solid solutions that typically contain significantly more TM atoms than the maximum TM solubility according to the equilibrium phase diagram, such as Al 87.6 Sc 12.4 (see analysis below), with equilibrium solubility of 0.08 at.% Sc [21], Al 85 Ti 15 [10] (equilibrium solubility of 2 at.% [22]), Al 90 V 10 [20] (equilibrium solubility of 0.2 at.% [23]), Al 97.73 Cr 2.27 [24] (equilibrium solubility of 0.04 at.% [25]), Al 95.67 Mn 4.33 [26] (equilibrium solubility of 0.65 at.% [27]), Al 94.5 Zr 5.5 [28] (equilibrium solubility of 0.08 at.% [28]), and Al 90 Pd 10 [29] (equilibrium solubility of 0.2 at.% [30]). For fcc Al 1−x TM x solid solutions with extended solubility, knowledge of the concentration dependent (0 < x < 25 at.%) lattice parameters is crucial to determine the correct Al:TM composition, as discussed extensively for the Ti case in [10], and for the concentration dependent lattice parameters provided for Cr, Mn and Zr in [24,26,28] respectively.…”