J Am Ceram Soc. 2020;103:6531–6542. wileyonlinelibrary.com/journal/jace | 6531 © 2020 The American Ceramic Society 1 | INTRODUCTION The US development of nuclear weapons and nuclear energy has created, and will continue to create a significant quantity of radioactive waste that requires durable, effective, and ef- ficient waste form in order to safely sequester radionuclides from the biosphere. 1 While developed waste forms have been largely successful, there remains an interest in discovering novel materials for some of the more problematic compo- nents, and for finding means for increasing the efficiency of processing and increasing waste loading. Furthermore, on the way to finding new waste form materials, we can broaden our chemical understanding that will aid in the discovery, devel- opment, and design of future materials for a variety of waste disposal-related applications. 2,3 Crystalline ceramic wasteforms, particularly SYNROC 4 and titanate/alumina-based ceramics, have received a lot of attention due to their ability to incorporate a broad spectrum of chemical species within available lattice sites, potentially higher waste loadings, and resistance to hydrothermal leach- ing. 5-7 Among the ceramic materials studied are hollan- dite-type structures, generally A x M 8 O 16 . These are composed Received: 21 February 2020 | Revised: 12 June 2020 | Accepted: 13 June 2020 DOI: 10.1111/jace.17327 ARTICLE Structure and stability of alkali gallates structurally reminiscent of hollandite Christian A. Juillerat 1,2 | Vancho Kocevski 2,3 | Vladislav V. Klepov 1 | Jake W. Amoroso 2,4 | Theodore M. Besmann 2,3 | Hans-Conrad zur Loye 1,2,4 1 Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC, USA 2 Center for Hierarchical Wasteform Materials (CHWM), University of South Carolina, Columbia, SC, USA 3 Nuclear Engineering Program, University of South Carolina, Columbia, SC, USA 4 Savannah River National Laboratory, Aiken, SC, USA Correspondence Hans-Conrad zur Loye, Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC, USA. Email: zurloye@mailbox.sc.edu Present address Vancho Kocevski, MST-8, Los Alamos National Laboratory, Los Alamos, NM, USA Funding information Basic Energy Sciences, Grant/Award Number: SC0016574; University of South Carolina; Savannah River National Laboratory; US Department of Energy, Grant/Award Number: DE-AC09- 08SR22470 Abstract Single crystals of CsGa 7 O 11 , RbGa 7 O 11 , and RbGa 4 In 5 O 14 were grown from alkali halide melts and their structures were characterized by single crystal and powder X-ray diffraction. CsGa 7 O 11 and RbGa 7 O 11 adopt the same structure type, remi- niscent of the hollandite structure type, as it contains nearly rectangular channels made up of two dimers of edge-sharing GaO 6 octahedra, and two corner-sharing octahedron/tetrahedron pairs. The structure of RbGa 4 In 5 O 14 is more complex and is comprised of indium octahedra, gallium trigonal bipyramids, and gallium tetra- hedra, and contains similar sized tunnels as CsGa 7 O 11 and RbGa 7 O 11 . CsGa 7 O 11 and RbGa 4 In 5 O 14 were further characterized by TGA, ion exchange experiments, and DFT studies revealing that both structures are thermodynamically stable up to 850°C; however, CsGa 7 O 11 decomposes to GaO(OH) xH 2 O when heated in warm aqueous solutions. CsGa 7 O 11 undergoes ion exchange in both an aqueous solution of RbCl and a RbNO 3 melt, as predicted by DFT studies, where the ion exchange is more extensive in the RbNO 3 melt. KEYWORDS crystal growth, ion-exchange, single crystals