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

Abstract

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.

Molten salts: Potential candidates for thermal energy storage applications

“…To lower the freezing temperature and increase thermal stability, LiNO 3 has been added to NaNO 3 ‐KNO 3 94 . In the cited study, 95 caesium nitrate was added to pre‐existing mixtures and succeeded in lowering the fusion temperature to 70°C. The major problem with this is huge cost of the caesium salt, much higher than that of lithium nitrate.…”

“…To lower the freezing temperature and increase thermal stability, LiNO 3 has been added to NaNO 3 -KNO 3 . 94 In the cited study, 95 caesium nitrate was added to pre-existing mixtures and succeeded in lowering the fusion temperature to 70 C. The major problem with this is huge cost of the caesium salt, much higher than that of lithium nitrate. At the Sandia National Laboratory, researchers 96 produced a special quaternary eutectic-mix known as "Sandia Mix" (composed of KNO 3 [40%-52%]-Ca(NO 3 ) 2 [20%-27%]-NaNO 3 [9%-18%]-LiNO 3 [13%-21%]).…”

Phosphorescence in Mn4+-DopedR+/R2+Germanates (R+= Na+or K+,R2+= Sr2+)

“…New compounds featuring unique germanate frameworks in their structures exhibit important physical properties. − More specifically, germanates with simultaneous tetrahedral and octahedral coordination of Ge 4+ are of interest both from the fundamental and practical aspects. These compounds can be used to mimic some high-pressure silicates, in which silicon atoms can adopt an octahedral coordination as well .…”

“…Interestingly, Na 2 Ge 4 O 9 forms a metastable phase, while the lack of the reliable structural data on R = Mg and Ca tetragermantates may indicate their lower thermodynamic stability as compared to the other phases. 32 In this report, we describe the synthesis, structure, and optical properties of three new compounds based on germanate frameworks: tetragermanate Na 0.36 Sr 0.82 Ge 4 O 9 (1), langasite-type Na 2 SrGe 6 O 14 (2), and octagermanate K 2 SrGe 8 O 18 (3), isostructural to K 2 BaGe 8 O 18. 35 The new phases were successfully doped with Mn 4+ (phases 2 and 3) and Pr 3+ (phase 2) to explore their photoluminescence properties.…”

Crystal Growth of New Germanate Framework Structures: Impact of the Presence of Square Planar Copper Species and Mixed Ge/Mn Sites on the Overall Structures of Rb2Cu3Ge5O14, Cs2Cu3Ge5O14, Cs7Cu2Ge11O27F, and [(Cs6F)(Cs3AgF)][Ge12Mn2O32]

“…14 O 32 ],8 [(Rb 6 F)(Rb 4 F)] [Ge14 O 32 ], and [(Rb 6 F)(Rb 3.1 Co 0.9 F 0.96 )]-[Co 3.8 Ge 10.2 O 30 F 2 ] 9. In our continued exploration of the alkali germanate phase space, we investigated the impact of introducing 3d transition metals to the reaction mixture, resulting in the synthesis of Rb 2 Co 1.85 Ge 1.15 O 6 , 10 Cs 2 CoGe 4 O 10 ,11 and CsFeGeO 4 12. In this paper, we discuss the discovery of new germanate compositions that crystallize in new structural modifications when we add copper and manganese to the flux synthesis, which resulted in framework structures containing square planar copper species and a mixed Ge/Mn framework salt-inclusion material.…”