Polymorphism and Molten Nitrate Salt-Assisted Single Crystal to Single Crystal Ion Exchange in the Cesium Ferrogermanate Zeotype: CsFeGeO4

Two polymorphs of a new cesium ferrogermanate zeotype, CsFeGeO4, were synthesized using the molten CsCl-CsF flux growth approach at 900 °C. The orthorhombic polymorph, referred to as (1), crystallizes in the centrosymmetric nonpolar Pbcm space group. The compound exhibits a three-dimensional porous framework structure composed of disordered (Fe/Ge)O4 corner-sharing tetrahedra that generate large eight-sided channels running down the b-axis. These channels are occupied by Cs ions that provide charge balance to the anionic framework. Minor modifications in the reaction conditions lead to the synthesis of a monoclinic polymorph of CsFeGeO4, referred to as (2), crystallizing in the noncentrosymmetric polar space group P21 and exhibiting an identical framework structure to (1), albeit featuring ordered FeO4 and GeO4 tetrahedra. Solid state synthesis of CsFeGeO4 produces a polycrystalline mixture of (1) and (2), referred to as (6). Polarization-electric field (P-E) measurements of (6) indicate that the material is not ferroelectric. Powder second harmonic generation (SHG) measurements of (2) and (6) revealed them to be SHG active with intensities of 1.5 and 0.2 times that of α-SiO2, respectively. The temperature dependent magnetic susceptibility of (2) exhibits a downturn at T = 2.6 K, indicative of antiferromagnetic ordering. First-principles calculations in the form of density functional theory showed that (1) and (2) differ in stability by only 1.3 meV/atom, with (2) being the thermodynamically stabilized phase. Additional calculations for (1), using molten nitrate as reference, predicted the formation of energetically favorable phases, KFeGeO4 (3) and RbFeGeO4 (4). They were subsequently prepared via a molten nitrate salt bath treatment of (1) to replace Cs with K and Rb, affording (3) and (4) as single-crystal to single-crystal ion exchange products. Structure determination and property measurements for a pyroxene phase, CsFeGe2O6, referred to as (5), are also reported. This compound crystallized as a side product in the flux synthesis of CsFeGeO4.