Novel Gallium Phosphate Framework Encapsulating Trinuclear Mn<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>O<sub>8</sub> Cluster:  Hydrothermal Synthesis and Characterization of Mn<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>Ga<sub>4</sub>(PO<sub>4</sub>)<sub>6</sub> HsuKuei-Fang WangSue-Lein 2000 A new manganese gallium phosphate, Mn<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>Ga<sub>4</sub>(PO<sub>4</sub>)<sub>6</sub>, has been synthesized under hydrothermal conditions at 150 °C and characterized by single-crystal X-ray diffraction, thermogravimetric analysis, magnetic susceptibility, and electron paramagnetic resonance (EPR) spectroscopy. It crystallized in the monoclinic space group, <i>P</i>2<sub>1</sub>/<i>n</i>, with <i>a</i> = 8.9468(4) Å, <i>b</i> = 10.1481(5) Å, <i>c</i> = 13.5540(7) Å, β = 108.249(1)°, and <i>Z</i> = 2. The compound is unusual in that it is not only the first nonorganically templated MnGaPO phase but also the first instance where edge-shared trinuclear manganese−oxygen clusters are encapsulated in a metal phosphate lattice. The trimer involves a central Mn(H<sub>2</sub>O)<sub>4</sub>O<sub>2</sub> octahedron, which links to two Mn (H<sub>2</sub>O)<sub>2</sub>O<sub>4</sub> octahedra at trans edges. The Mn<sub>3</sub>(H<sub>2</sub>O)<sub>6</sub>O<sub>8</sub> clusters reside in tunnels built from GaO<sub>5</sub> trigonal bipyramids and PO<sub>4</sub> tetrahedra. Our magnetic study revealed that superexchange interactions occurred between the neighboring Mn<sup>II</sup> centers. A good fit of the magnetic susceptibility data for the isolated trimers was obtained by using a derived expression based on Van Vleck's equation. Unlike all existing linear trinuclear Mn<sup>II</sup> complexes, the χ<sub>Μ</sub><i>T</i> product in the range 8−4 K remains at a constant value corresponding to one spin <i>S</i> = <sup>5</sup>/<sub>2</sub> per three Mn<sup>II</sup> centers. The Curie behavior at such low temperatures has been confirmed by EPR data. According to the thermogravimetric analysis/differential thermal analysis (TGA/DTA) results, the title compound is thermally stable up to ca. 200 °C.