Magnetic Ground State Crossover in a Series of Glaserite Systems with Triangular Magnetic Lattices

The magnetic properties are reported for three members of the glaserite series of compounds, Na<sub>2</sub>BaM­(VO<sub>4</sub>)<sub>2</sub>, M = Mn, Mn<sub>0.6</sub>Co<sub>0.4</sub>, and Co. Large single crystals are grown using a high-temperature hydrothermal synthesis method. This structure type exhibits a triangular magnetic lattice in which M<sup>2+</sup>O<sub>6</sub> octahedra are interconnected with nonmagnetic (VO<sub>4</sub>)<sup>3–</sup> groups. All the structures crystallize at room temperature with rigid trigonal symmetry (space group <i>P</i>3̅<i>m</i>1); however, at lower temperatures both Na<sub>2</sub>BaMn­(VO<sub>4</sub>)<sub>2</sub> and Na<sub>2</sub>BaMn<sub>0.6</sub>Co<sub>0.4</sub>(VO<sub>4</sub>)<sub>2</sub> undergo a structural transition to lower symmetry (monoclinic, <i>C</i>2/<i>c</i>). The bulk magnetic measurements indicate that Mn- and Co-structures are antiferromagnetic and ferromagnetic, respectively. Na<sub>2</sub>BaMn<sub>0.6</sub>Co<sub>0.4</sub>(VO<sub>4</sub>)<sub>2</sub> does not show any long-range ordering down to 0.5 K, although a broad heat capacity anomaly near 1.2 K suggests short-range magnetic order or freezing into a spin-glass-like state related to the chemical disorder and resulting competing magnetic interactions. The magnetic structures of Na<sub>2</sub>BaMn­(VO<sub>4</sub>)<sub>2</sub> and Na<sub>2</sub>BaCo­(VO<sub>4</sub>)<sub>2</sub> were determined using neutron powder diffraction. At zero magnetic field, Na<sub>2</sub>BaMn­(VO<sub>4</sub>)<sub>2</sub> possesses an antiferromagnetic structure with the moments ordered in a Néel-type arrangement and aligned along the C<sub>4</sub> axis of the octahedra. Under applied magnetic field at 0.3 K, the evolution of the magnetic structure toward a fully polarized state is observed. Na<sub>2</sub>BaCo­(VO<sub>4</sub>)<sub>2</sub> represents a ferromagnetic (FM) magnetic structure with Co moments aligned parallel to the <i>c</i>-axis direction. The relationships between these structures and magnetic properties are discussed.