Low-temperature behavior of YBaCo(2)O(5.5): Coexistence of two spin-state ordered phases


The low temperature antiferromagnetic-antiferromagnetic phase transition in polycrystalline YBaCo(2)O(5.5) cobaltite was studied by neutron powder diffraction. On the basis of symmetry arguments in combination with Rietveld refinement, the coexistence of two phases with different type of spin-state ordering between the diamagnetic (t(2g)(6)e(g)(0), S=0) and paramagnetic (t(2g)(4)e(g)(2), S=2) sixfold coordinated Co(3+) cations was revealed below the transformation temperature, T(L)similar to 195 K. One of the phases having Pmma (2a(p)x2a(p)x2a(p)) symmetry is characteristic of the high-temperature (T>T(L)) homogeneous state, whilst another with Bmmm (2a(p)x2a(p)x4a(p)) symmetry involves another type of spin-state ordered superstructure. The conjugated magnetic structures possess k=c(*)/2 and k=0 wave vectors, respectively. The structural changes are driven by competition between the elastic and exchange energies.






Khalyavin, DD; Argyriou, DN; Amann, U; Yaremchenko, AA; Kharton, VV

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