Hydrothermal Synthesis and Characterization of Novel Brackebuschite-Type Transition Metal Vanadates: Ba₂M(VO₄)₂(OH), M = V³⁺, Mn³⁺, and Fe³⁺, with Interesting Jahn–Teller and Spin-Liquid Behavior

A new series of transition metal vanadates, namely, Ba₂M­(VO₄)₂(OH) (M = V³⁺, Mn³⁺, and Fe³⁺), was synthesized as large single crystals hydrothermally in 5 M NaOH solution at 580 °C and 1 kbar. This new series of compounds is structurally reminiscent of the brackebuschite mineral type. The structure of Ba₂V­(VO₄)₂(OH) is monoclinic in space group P2₁/m, a = 7.8783(2) Å, b = 6.1369(1) Å, c = 9.1836(2) Å, β = 113.07(3)°, V = 408.51(2) ų. The other structures are similar and consist of one-dimensional trans edge-shared distorted octahedral chains running along the b-axis. The vanadate groups bridge across edges of their tetrahedra. Structural analysis of the Ba₂Mn­(VO₄)₂­(OH) analogue yielded a new understanding of the Jahn–Teller effect in this structure type. Raman and infrared spectra were investigated to observe the fundamental vanadate and hydroxide vibrational modes. Single-crystal temperature-dependent magnetic studies on Ba₂V­(VO₄)₂(OH) reveal a broad feature over a wide temperature range with maximum at ∼100 K indicating that an energy gap could exist between the antiferromagnetic singlet ground state and excited triplet states, making it potentially of interest for quantum magnetism studies.