Phase transitions in the Na3M2,(PO4)2F3 family (M = Al3+, V3+, Cr3+, Fe3+, Ga3+):: Synthesis, thermal, structural, and magnetic studies

The investigated compounds Na3M2(PO4)(2)F-3 (M = Al3+, V3+, Cr3+, Fe3+, Ga3+) have been prepared hy hydrothermal synthesis or by solid state synthesis and characterized by thermal analyses (TGA, DTA, DSC), powder and single crystal X-ray works, or neutron diffraction. Several other ways of characterization have also been used such as diffraction by TEM, IR, magnetism, and Mossbauer spectrometry, These compounds present two crystallographic transitions which involve three modifications: alpha, beta, and alpha. The room temperature(beta) phases can be split into two groups: the first (beta(1)), with M = Al3+, Cr3+, Ga3+, crystallizes in the space group P4(2)/mbc (a = 12.406(2) Angstrom, c = 10.411(2) Angstrom, Z=8 for M = Al3+) whereas the second group (beta(3)) with M = V3+, Fe3+ crystallizes in the space group P4(2)/mnm (a = 9.047(2) Angstrom, c = 10.705(2) Angstrom, Z = 4 for M = V3+). On the other hand all the high temperature (alpha) phases adopt the same space group I4/mmm (a = 6.206(1) Angstrom, c = 10.418(4) Angstrom, Z = 2 for M = Al3+). Mossbauer spectrometry and powder neutron diffraction (paramagnetic state) allowed us to evidence a low temperature (gamma) phase on the Fe compound which is characterized by an orthorhombic symmetry (a =1 2.756(1) Angstrom, b = 12.803(1) Angstrom, c = 10.602(2) Angstrom, Z = 8, space group Pbam). All these compounds adopt an identical three-dimensional [M-2(PO4)(2)F-3](infinity)(3-) network built up by shaping four corners between PO4 tetrahedra and M2O8F3 bioctahedra tone fluorine apex bridges two octahedra), Na+ ions are statistically distributed inside the resulting channels. Susceptibility measurements reveal an antiferromagnetic behavior for each of the paramagnetic compounds (M = V3+, Cr3+, Fe3+). (C) 1999 Academic Press.