The crystal and magnetic structure of Li-aegirine LiFe~(3+)Si_2O_6: a temperature-dependent study

摘要

Single crystals of Li-aegirine LiFe~(3+)Si_2O_6 were synthesized at 1573 K and 3 GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223 K. LiFe~(3+)Si_2O_6 is monoclinic, space group C2/c, a = 9.6641(2) A, b = 8.6612(3) A, c = 5.2924(2) A, β = 110.12(1)°at 300 K as refined from powder neutron data. At 229 K Li-aegirine undergoes a phase transition from C2/c to P2_1/c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h + k ≠ 2n. In the low-temperature form two non-equivalent Si-sites with = 1.622 A and = 1.624 A at 100 K are present. The bridging angles of the SiO_4 tetrahedra O3-O3-O3 are 192.55(8)°and 160.02(9)°at 100 K in the two independent tetrahedral chains in space group P2_1/c, whereas it is 180.83(9)°at 300 K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100 K four Li-O bond lengths lie within 2.072(4)-2.172(3) A, the fifth Li-O bond length is 2.356(4) A, whereas the Li-O3 A bond lengths amount to 2.796(4) A. From ~(57)Fe Mossbauer spectroscopic measurements between 80 and 500 K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5 K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe~(3+)Si_2O_6. Distinct preordering phenomena can be observed up to 35 K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T_N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a-c plane.