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 Xray powder diffraction data (space group P21/c (No. 14); Z = 4; a = 9.3647(2) A, b = 6.8518(1) A, c = 10.5367(2) A, and(beta)= 109.5140(8)). 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 T1 = 7.0 K and T2 = 11.3 K in zero magnetic field. The phase transition at T2 seems to be a structural phase transition. Magnetization measurements showed that, below T1, SrFe2(PO4)2 exhibits weak ferromagnetism and demonstrates clear ferromagnetic hysteresis loops. Above 15 K, CurieWeiss behavior was observed with an effective magnetic moment of 5.23 (mu)B per Fe2+ ion and Weiss constant of -18.9 K. Weak ferromagnetic properties below T1 can be explained by canting of antiferromagnetically ordered spins. Several field-induced phase transitions were observed in SrFe2(PO4)2 at low temperatures.