Twenty new alkali rare earth thiosilicates and thiogermanates with the general formula ALnTS4 (A = alkali metal, Ln = lanthanide, and T = Si, Ge) were grown as X-ray diffraction-quality single crystals from molten alkali chloride fluxes. These include KNdSiS4, KPrSiS4, RbLnSiS4 (Ln = Ce, Pr, Nd, Gd, Tb, Dy, and Ho), RbLaGeS4, CsLnSiS4 (Ln = La, Pr, and Nd), and CsLnGeS4 (La, Ce, Pr, Nd, Eu, Gd, and Tb). Herein, we discuss the use of a molten chloride flux growth approach for the preparation of the title compounds and their structure determination via single-crystal X-ray diffraction. In addition, we comment on the magnetic properties of RbNdSiS4, CsNdSiS4, CsNdGeS4, and CsGdGeS4, which were found to be paramagnetic for T = 2–300 K and exhibited negative Weiss temperatures with no obvious antiferromagnetic transition down to 2 K. The optical properties of CsLaGeS4 and CsNdTS4(T = Si, Ge) were measured by UV–vis spectroscopy. Second harmonic generation measurements performed on CsLaGeS4 confirmed the crystallization of the compound in the noncentrosymmetric orthorhombic space group, P212121; CsLaGeS4 was found to be SHG-active with nearly half the intensity of α-SiO2 upon irradiation with a Nd:YAG 1064 nm laser, and a semiconductor exhibiting a band gap of 3.60 eV based on UV–vis diffuse reflectance measurements.

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.

Single crystals of new cesium cobalt silicates and germanates exhibiting three-dimensional, ion-exchangeable crystal structures were grown from a mixed CsCl–CsF flux, and their electronic and magnetic properties were studied using DFT calculations.

Single crystals of nine new quaternary and penternary rare earth containing thiophosphates, Cs2KLn­(PS4)2, Rb3‑x Na x Ln­(PS4)2 (Ln = La, Nd), Rb2NaNd­(PS4)2, Cs3La­(PS4)2, Rb3Gd­(PS4)2, and Cs5NaLn2(PS4)4 (Ln = Gd, Tb), were grown in alkali halide eutectic fluxes. All title compounds were structurally characterized by single crystal X-ray diffraction and were found to crystallize in pseudo-one-dimensional structure types, reminiscent of those found for other known alkali rare earth thiophosphates, in a variety of space groups including monoclinic P21/c, P21/m, P21, and orthorhombic Ccca. The crystal structures of the reported compounds are characterized by the formation of complex one-dimensional [(Ln­(PS4)2)3–] and [(Ln2(PS4)4)6–] anionic chains that run along the crystallographic a-axis in Cs2KLn­(PS4)2, Rb3‑x Na x Ln­(PS4)2 (Ln = La, Nd) and Rb2NaNd­(PS4)2, b-axis in Cs3La­(PS4)2 and Rb3Gd­(PS4)2, and c-axis in Cs5NaLn2(PS4)4 (Ln = Gd, Tb). Cs2KLn­(PS4)2, Rb3‑x Na x Ln­(PS4)2 (Ln = La, Nd), Rb2NaNd­(PS4)2, Rb3Gd­(PS4)2, and Cs3La­(PS4)2 feature two crystallographically unique thiophosphate tetrahedra that face- and edge-share with the LnS x polyhedra forming infinite one-dimensional zigzag-like chains. This unusual face-sharing or face-grafting coordination mode of the thiophosphate group is however absent in Cs5NaLn2(PS4)4 (Ln = Gd, Tb), which exhibit only one unique phosphorus site. The magnetic susceptibility and the field-dependent magnetization were measured for Rb2NaNd­(PS4)2 and Cs2KNd­(PS4)2 which were found to be paramagnetic over the entire 2–300 K temperature range. Optical properties of Cs2KNd­(PS4)2 and Rb2.65Na0.35La­(PS4)2 were measured by UV–vis diffuse reflectance spectrometry which revealed numerous weak absorption bands in the 300–900 nm range characteristic of the f–f electronic transitions in Nd3+; Rb2.65Na0.35La­(PS4)2 was found to be a semiconductor with a direct band gap of 3.60 eV.