SYNTHESES, X-RAY-POWDER STRUCTURES, AND PRELIMINARY ION-EXCHANGE PROPERTIES OF GERMANIUM-SUBSTITUTED TITANOSILICATE PHARMACOSIDERITES - HM3(AO)(4)(BO4)(3)CENTER-DOT-4H(2)O (M = K, RB, CS, A = TI, GE, B = SI, GE)

This paper describes a continuing research effort that involves synthe sizing new tunnel-type materials while attempting to understand their fundamental ion-exchange selectivities through the process of structur al elucidation. For this study, we hydrothermally synthesized and char acterized two germanium-substituted titanosilicates in the cesium phas e and prepared their potassium forms by ion exchange. A mixed Si/Ti/Ge phase, HCs3(TiO)(3.5)(GeO)(0.5)(GeO4)(2.5)(SiO4)(0.5). 4H(2)O, crysta llizes in the cubic space group P (4) over bar 3m with a = 7.9376(1) A ngstrom, while the cesium titanogermanate, HCs3(TiO)(4)(GeO4)(3) . 4H( 2)O, possesses a body-centered supercell belonging to space group I23, a = 15.9604(3) Angstrom. Differences in symmetry between the two cesi um compounds can be explained in terms of entropy and site mixing in t he Si/Ti/Ge compound. Upon ion exchange with potassium, the resulting phases, HK3(TiO)(3.5)(GeO)(0.5)(GeO4)(2.5)(SiO4)(0.5). 4H(2)O and HK3( TiO)(4)(GeO4)(3) . 4H(2)O, distorted to the tetragonal space group P ( 4) over bar b2, with a = b = 11.1571(2), c = 7.916(2) Angstrom, and a = b = 11.215(1), c = 7.9705(2) Angstrom, respectively. For the first t ime, we have observed tetragonal distortions with alkali cation forms of the pharmacosiderite analogues. As compared to HK3(TiO)4(SiO4)3 . 4 H(2)O, these potassium germanium-substituted phases show remarkable in creases in strontium and cesium selectivity, which proves very benefic ial for nuclear waste remediation applications. An increase in selecti vity can be explained in terms of their inherent structures and bond s trengths associated with the charge-neutralizing cations and framework oxygens.