Theory-Guided Inelastic Neutron Scattering of Crystalline Alkaline Aluminate Salts Bearing Principal Motifs of Solution-State Species

Abstract

Aluminate salts precipitated from caustic alkaline solutions exhibit a correlation between the anionic speciation and the identity of the alkali cation in the precipitate, with the aluminate ions occurring either in monomeric (Al(OH) 4 − ) or dimeric (Al 2 O(OH) 6 2− ) forms. The origin of this correlation is poorly understood as are the roles that oligomeric aluminate species play in determining the solution structure, prenucleation clusters, and precipitation pathways. Characterization of aluminate solution speciation with vibrational spectroscopy results in spectra that are difficult to interpret because the ions access a diverse and dynamic configurational space. To investigate the Al(OH) 4 − and Al 2 O(OH) 6 2− anions within a well-defined crystal lattice, inelastic neutron scattering (INS) and Raman spectroscopic data were collected and simulated by density functional theory for K 2 [Al 2 O(OH) 6 ], Rb 2 [Al 2 O(OH) 6 ], and Cs[Al(OH) 4 ]•2H 2 O. These structures capture archetypal solution aluminate species: the first two salts contain dimeric Al 2 O(OH) 62− anions, while the third contains the monomeric Al(OH) 4 − anion. Comparisons were made to the INS and Raman spectra of sodium aluminate solutions frozen in a glassy state. In contrast to solution systems, the crystal lattice of the salts results in well-defined vibrations and associated resolved bands in the INS spectra. The use of a theory-guided analysis of the INS of this solid alkaline aluminate series revealed that differences were related to the nature of the hydrogen-bonding network and showed that INS is a sensitive probe of the degree of completeness and strength of the bond network in hydrogen-bonded materials. Results suggest that the ionic size may explain cation-specific differences in crystallization pathways in alkaline aluminate salts.

Solution and Interface Structure and Dynamics in Geochemistry: Gateway to Link Elementary Processes to Mineral Nucleation and Growth

“…61 The structuredirecting role of M + cations in the solid compound made from these solutions appears to be dependent on the M + ionic radii. 63 For instance, the presence of small Li + and Na + cations results in compact structures with extensively bridged tetrahedral aluminate networks or octahedral aluminates. The medium-sized K + and Rb + cations engaged in more ionic interactions with dimeric Al 2 O(OH) 6 2− complexes result in solids with no coordinating water in the precipitated potassium and rubidium aluminate phases.…”

“…If one would keep the same concentration of the constituted elements ( m M , total and m Al,total ) except the identity of cations in these solutions, the influence of cation chemistry has a major impact on the extent of aluminate oligomerization and imposes control over which of the M + -aluminate coordination compounds are present in the precipitating crystalline solid. In fact, crystalline aluminate hydroxy (an)­hydrates have been used often to infer aluminum coordination changes (from tetrahedral Al in solution to octahedral Al coordination in some solid phase(s)) and to approximate the extent oligomerization reactions in the solution state. − As summarized in Figure , the presence of K + or Rb + in solution promotes direct crystallization of dimeric Al 2 O­(OH) 6 2– complexes, whereas the presence of larger Cs + promotes crystallization of monomeric Al­(OH) 4 – complexes . The presence of Li + , in contrast, allows the formation of Li-Al layered double hydroxides that comprise edge-sharing Al­(OH) 6 3– octahedra with Li + in the vacancy sites of the octahedral sheet .…”

“…Interestingly, sodium aluminate hydroxy hydrates are found to carry either solely monomeric Al­(OH) 6 3– octahedral complexes in one structure or solely oxolated Al 2 O 3 (OH) 2 2– layers in another structure, with tetrahedral Al each coordinated to three bridging oxygens and one terminal hydroxy in the latter . The structure-directing role of M + cations in the solid compound made from these solutions appears to be dependent on the M + ionic radii . For instance, the presence of small Li + and Na + cations results in compact structures with extensively bridged tetrahedral aluminate networks or octahedral aluminates.…”

Sodium site occupancy and phosphate speciation in natrophosphate are invariant to changes in NaF and Na3PO4 concentration

“…[4][5][6] One strategy to analyze sub ensembles of complex electrolyte species such as polyatomic ion aggregates is to unravel the spectroscopic contributions of distinct individual species through the use of model salt hydrates that preserve relevant chemical motifs at the molecular level. 7,8 Our group has pioneered this approach for alkali aluminate salts relevant to nuclear waste processing, [9][10][11][12] and this work expands the strategy of using model salt hydrates to study polyatomic ion aggregates of relevance to geochemistry and radioactive waste to a new ternary electrolyte compositions by investigating the crystal structure of natrophosphate (Na 7 FPO 4 •19H 2 O).…”