SrFe2(PO4)2:: Ab initio structure determination with X-ray powder diffraction data and unusual magnetic properties

Structure of SrFe2(PO4)(2) was solved ab initio from X-ray powder diffraction data (space group P2(1)/c (No. 14); Z = 4; a = 9.3647(2) c, b = 6.8518(l) Angstrom, c = 10.5367(2) Angstrom, and = 109.5140(8)degrees). It has almost linear tetrameric units Fe2-Fe1-Fe1-Fe2 which join with each other through common oxygen atoms creating a complicated two-dimensional network parallel to the bc plane. Specific heat measurements revealed two phase transitions at T-1 = 7.0 K and T-2 = 11.3 K in zero magnetic field. The phase transition at T-2 seems to be a structural phase transition. Magnetization measurements showed that, below T-1, SrFe2(PO4)(2) exhibits weak ferromagnetism and demonstrates clear ferromagnetic hysteresis loops. Above 15 K, Curie-Weiss behavior was observed with an effective magnetic moment of 5.23 muB per Fe2+ ion and Weiss constant of -18.9 K. Weak ferromagnetic properties below T-1 can be explained by canting of antiferromagnetically ordered spins. Several field-induced phase transitions were observed in SrFe2(PO4)(2) at low temperatures.