STRUCTURAL AND MAGNETIC PHASE-TRANSITIONS OF THE 2D FERROMAGNETIC FLUORIDE TL2CUF4

A new transition phase has been observed in the copper complex fluorid e, Tl2CuF4, with the K2NiF4-derived structure. A Rietveld refinement o f the room-temperature X-ray powder pattern in the I4/mmm space group [a = 4.206(2) Angstrom; c = 13.673(5) Angstrom] has confirmed the anti ferrodistortive arrangement of the e(g) orbitals of CU2+. Around 200 K a structural phase transition to a low-temperature monoclinic phase h as been evidenced by DSC and X-ray diffraction [space group I2/m, a = 4.182(2) Angstrom,b = 4.183(2) Angstrom, c = 13.492(6) Angstrom; beta = 94.564(4)degrees]. Tl2CuF4 exhibits ferromagnetic properties below T -c = 10 K. The dependence of the magnetization on the applied held is characterized at 1.7 K by a coercive held of 2.5 x 10(-2) T and a rema nent magnetization of 0.6 mu(B). The phase sequence has been compared with those found in La2Cu4-type compounds. (C) 1996 Academce Press, In c.