STRUCTURAL CHARACTERIZATION OF R2BANIO5 (R=TM AND YB) - POLYMORPHISM FOR R=TM

The compound Tm2BaNiO5, which has been described as belonging to the Nd2BaNiO5 structural type, space group Immm Z = 2, is actually a dimorphic compound. The low temperature (LT) phase is isomorphous to Yb2BaNiO5, presenting the Sm2BaCuO5 structural type, space group Pnma Z = 4 with Room Temperature (RT) lattice parameters a = 12.2003(2) angstrom, b = 5.6584(6) angstrom, c = 6.9745(1) angstrom. At variance with the Yb case, a structural change is observed when the synthesis temperature increases. The brown coloured Pnma specimen changes to a green coloured compound which belong to the space group Immm Z = 2 with parameters a = 3.75128(4) angstrom, b = 5.7214(1) angstrom, c = 11.2456(2) angstrom at RT. We have characterized the new LT structure and confirmed the known high temperature (HT) structure by using powder X-ray and neutron diffraction techniques. For the HT-Immm phase we have found antiferromagnetic ordering from magnetic susceptibility and neutron diffraction data. However, for the LT-Pnma phase we have not found any magnetic ordering down to 1.5K.