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) Å3. 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.一类新型过渡金属钒酸盐，通式为Ba₂M(VO₄)₂(OH)（其中M = V³+、Mn³+、Fe³+），通过水热法在5 mol/L氢氧化钠溶液中、580 ℃及1千巴压力下合成得到大尺寸单晶。该系列化合物的晶体结构与羟钒铅矿（brackebuschite）矿物结构类型具有相似性。其中Ba₂V(VO₄)₂(OH)的晶体结构属于单斜晶系，空间群为P2₁/m，晶胞参数为a = 7.8783(2) Å、b = 6.1369(1) Å、c = 9.1836(2) Å，β = 113.07(3)°，晶胞体积V = 408.51(2) ų。其余同系列化合物的晶体结构与之相似，均由沿b轴延伸的一维反式棱共享扭曲八面体链构成。钒酸根基团通过其四面体的棱进行桥联。通过对Ba₂Mn(VO₄)₂(OH)同构物的结构解析，人们对该结构类型中的姜-泰勒效应（Jahn–Teller effect）有了全新的认识。通过拉曼（Raman）光谱与红外（infrared）光谱测试，对该体系中钒酸根基团与氢氧根基团的基本振动模式进行了表征。对Ba₂V(VO₄)₂(OH)开展的单晶变温磁性研究显示，其在宽温度区间内呈现出宽化的磁性特征，峰值位于约100 K，这表明该体系的反铁磁单重基态与激发三重态之间存在能隙，使其有望成为量子磁学研究的潜在候选体系。