Abstract
The compound Tm 2BaNiO 5, which has been described as belonging to the Nd 2BaNiO 5 structural type, space group Immm Z=2, is actually a dimorphic compound. The low temperature (LT) phase is isomorphous to Yb 2BaNiO 5, presenting the Sm 2BaCuO 5 structural type, space group Pnma Z=4 with Room Temperature (RT) lattice parameters a=12.2003(2) A ̊ , b=5.65845(6) A ̊ , c=6.9745(1) A ̊ . At variance with the Yb case, a structural change is observed wh en 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) A ̊ , b=5.7214(1) A ̊ , c=11.2456(2) A ̊ 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.
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