Polar Materials with Isolated V⁴⁺ S = 1/2 Triangles: NaSr₂V₃O₃(Ge₄O₁₃)Cl and KSr₂V₃O₃(Ge₄O₁₃)Cl

Crystals of ASr₂V₃O₃(Ge₄O₁₃)Cl, A = Na, K, were synthesized from high-temperature hydrothermal brines, and their structure and magnetic properties were investigated. These materials present a unique combination of a salt inclusion lattice, a polar crystal structure, and isolated V⁴⁺ (S = 1/2) trimer magnetic clusters. The structures consist of a trimeric V₃O₁₃ unit based on V⁴⁺ (S = 1/2), having rigorous 3-fold symmetry with a short V-V separation of 3.325(3) Å. The trinuclear V⁴⁺ units are formed by three edge shared VO₆ octahedra sharing a central μ₃-oxygen atom, which also imparts a polar sense on the structure. The V₃O₁₃ units are isolated from one another by tetranuclear Ge₄O₁₃ units, which are similarly arranged in a polar fashion, providing a unique opportunity to study the magnetic behavior of this triangular d¹ system as a discrete unit. Magnetization measurements indicate spin-1/2 per V atom at high temperature, and spin-1/2 per V₃ trimer at low temperature, where two V moments in each triangle are antiferromagnetically aligned and the third remains paramagnetic. The crossover between these two behaviors occurs between 20 and 100 K and is well-described by a model incorporating strong antiferromagnetic intra-trimer interactions and weak but nonzero inter-trimer interactions. More broadly, the study highlights the ability to obtain new materials with interesting structure-property relationships via chemistry involving unconventional solvents and reaction conditions.