Synthesis, structure and characterization of new compounds HA5M6(OH)4F4(HPO4)8 (A = K, Rb; M = Fe, Ga)

New fluorophosphate compounds H A 5 M 6 (OH) 4 F 4 (HPO 4 ) 8 ( A  = K, Rb; M  = Fe, Ga) were synthesized by a fluorine-rich hydrothermal method . These compounds crystallize in the monoclinic system with a P2 1 /n space group, exhibiting an open framework structure built by [PO 3 (OH)] groups, [MO 4 (OH)F], and [MO 4 F 2 ] octahedra, where ten-membered-ring channels, eight-membered-ring channels, six-membered-ring channels, and the linear M 3+ trimers are seen. The characterizations were carried out by powder XRD , EDS , FT-IR spectra, and magnetic measurements. The results confirmed that H A 5 Fe 6 (OH) 4 F 4 (HPO 4 ) 8 ( A  = K, Rb) show a quite similar magnetic behavior, possessing an antiferromagnetic ordering at 14.7 K and 15.6 K, respectively. The changes of the magnetic entropy for such antiferromagnetic orderings can be estimated as ∼10.7 J mol −1  K −1 and ∼13.4 J mol −1  K −1 .