Syntheses, X-ray Powder Structures, and Preliminary Ion-Exchange Properties of Germanium-Substituted Titanosilicate Pharmacosiderites:  HM₃(AO)₄(BO₄)₃·4H₂O (M = K, Rb, Cs; A = Ti, Ge; B = Si, Ge)

Abstract

This paper describes a continuing research effort that involves synthesizing new tunnel-type materials while attempting to understand their fundamental ion-exchange selectivities through the process of structural elucidation. For this study, we hydrothermally synthesized and characterized two germanium-substituted titanosilicates in the cesium phase and prepared their potassium forms by ion exchange. A mixed Si//Ti/Ge phase, HCs 3 (TiO) 3.5 (GeO) 0.5 (GeO 4 ) 2.5 (SiO 4 ) 0.5 ·4H 2 O, crystallizes in the cubic space group P 4̄3 m with a = 7.9376(1) Å, while the cesium titanogermanate, HCs 3 (TiO) 4 (GeO 4 ) 3 ·4H 2 O, possesses a body-centered supercell belonging to space group I 23, a = 15.9604(3) Å. Differences in symmetry between the two cesium compounds can be explained in terms of entropy and site mixing in the Si/Ti/Ge compound. Upon ion exchange with potassium, the resulting phases, HK 3 (TiO) 3.5 (GeO) 0.5 (GeO 4 ) 2.5 (SiO 4 ) 0.5 ·4H 2 O and HK 3 (TiO) 4 (GeO 4 ) 3 ·4H 2 O, distorted to the tetragonal space group P 4̄ b 2, with a = b = 11.1571(2), c = 7.9165(2) Å, and a = b = 11.215(1), c = 7.9705(2) Å, respectively. For the first time, we have observed tetragonal distortions with alkali cation forms of the pharmacosiderite analogues. As compared to HK 3 (TiO) 4 (SiO 4 ) 3 ·4H 2 O, these potassium germanium-substituted phases show remarkable increases in strontium and cesium selectivity, which proves very beneficial for nuclear waste remediation applications. An increase in selectivity can be explained in terms of their inherent structures and bond strengths associated with the charge-neutralizing cations and framework oxygens.