Structural relationships among LiNaMg[PO_4]F and Na_2M[PO_4]F (M = Mn-Ni, and Mg), and the magnetic structure of LiNaNi[PO_4]F

摘要

The new compound LiNaMg[PO_4]F has been synthesized by a wet chemical reaction route. Its crystal structure was determined from single-crystal X-ray diffraction data. LiNaMg[PO_4]F crystallizes with the monoclinic pseudomerohedrally twinned LiNaNi[PO_4]F structure, space group P2_1/c, a = 6.772(4), b = 11.154(6), c = 5.021(3) ?, β = 90.00(1)° and Z = 4. The structure contains [MgO_3F]_n chains made up of zigzag edge-sharing MgO_4F_2 octahedra. These chains are interlinked by PO_4 tetrahedra forming 2D-Mg [PO_4]F layers. The alkali metal atoms are well ordered in between these layers over two atomic positions. The use of group-subgroup transformation schemes in the B?rnighausen formalism enabled us to determine precise phase transition mechanisms from LiNaNi[PO_4]F- to Na_2M[PO_4]F-type structures (M = Mn- Ni, and Mg) (see video clip 1 and 2). The crystal and magnetic structure and properties of the parent LiNaNi[PO_4]F phase were also studied by magnetometry and neutron powder diffraction. Despite the rather long interlayer distance, d_(min)(Ni~(+2)-Ni~(+2)) ~ 6.8 ?, the material develops a long-range magnetic order below 5 K. The magnetic structure can be viewed as antiferromagnetically coupled ferromagnetic layers with moments parallel to the b-axis.