Thermochemistry of Barium Hollandites

Abstract

Barium hollandites, a family of framework titanates that can potentially be used for the immobilization of short‐lived fission products (especially 137Cs) in radioactive wastes, have been investigated by high‐temperature oxide melt solution calorimetry using 2PbO·B2O3 solvent at 702°C. The enthalpies of formation from constituent oxides show increasing energetic stability of the hollandite phase as Ti4+ is substituted by Mg2+, Al3+, and Fe3+, in that order. In general, the thermodynamic stability increases with decreasing average cation radius in the β sites, and when the tolerance factor approaches one. The Al‐ and Fe‐hollandites are more stable than phase assemblages containing BaTiO3 perovskite and Al/Fe/Ti oxides, whereas Mg‐hollandite is less stable than the corresponding assemblage of BaTiO3 perovskite, MgTiO3 ilmenite, and TiO2. This instability makes Mg‐hollandite a less suitable host for fission products. Hollandite phase formation during metal citrate combustion synthesis depends more on thermodynamic stability and phase chemistry than on the annealing temperature.

Heat capacities, standard entropies and Gibbs energies of Sr-, Rb- and Cs-substituted barium aluminotitanate hollandites

“…We have recently measured the enthalpies of formation for two series of Ba hollandites from constituent oxides and/or competing ternary phases using high-temperature oxide melt solution calorimetry [11,23]. Heats of formation of barium hollandites with Al 3+ , Fe 2+ , or Mg 2+ substituted at the B site indicate that while Al and Fe hollandites are thermodynamically stable phases, Mg hollandite is not an appropriate waste form candidate [23]. More recently, the enthalpies of formation of barium aluminotitanate hollandites with Cs + , Rb + , or Sr 2+ substituted at the A site (Ba 1.…”

Thermodynamic assessment of the hollandite high‐level radioactive waste form

“…The structure is composed of edge and corner sharing TiO 6 and MO 6 octahedra that form a framework consisting of tunnels parallel to the c ‐axis or b ‐axis for tetragonal or monoclinic hollandites, respectively . The atom positions located within the tunnel sites can be occupied by A‐site cations such as Cs +1 and Ba +2 , which is beneficial as both 137 Cs and its decay product 137 Ba can remain immobilized in the hollandite structure …”

“…Aubin‐Chevaldonnet et al used a solid‐state reaction to form oxide pellets that were calcined and sintered at 1473 K for 30 hours in air. Costa et al prepared hollandite samples by first mixing, heating, and evaporating citrate solutions before ultimately forming and heat treating pellets at 1523 K for 3 hours. Database calculations were conducted at each of these final heat treatment temperatures for comparison with the phase equilibria reported in these studies.…”

“…Similarly, the reported 1473 K temperature at which the cooling rate of melt processed samples started slowing was adopted as defining the equilibrium state and, as such, calculations for comparison with melt processed sample results were conducted at this temperature. Xu et al, Aubin‐Chevaldonnet et al, and Costa et al synthesized hollandites with the additives Ga, Al, Cr, and Fe, hence the database was developed to include the oxides of these additives.…”

“…Wu et al derived a standard enthalpy of formation using drop solution calorimetry for a hollandite phase with the composition Ba 1.18 Cs 0.21 Al 2.44 Ti 5.53 O 16. Costa et al employed the same approach for the Ba 1.24 Al 2.48 Ti 5.52 O 16 and Ba 1.24 Fe 2.48 Ti 5.52 O 16 compositions. Xu et al and Wen et al used density functional theory (DFT) to calculate formation enthalpies at 0 K from the Ba to Cs endmember of two‐thirds A‐site occupied hollandites containing Al and Ga.…”