X-ray and neutron diffraction studies of coupled structural phase   transitions in DyBaCo$_{2}$O$_{5.5}$
  A structural transition at $T\approx 322$ K from the $Pmmm$ to $Pmma$ phase is found to coincide with an anomaly of resistivity. Another structural phase transition doubling the lattice parameter $c$, which has been postulated earlier to accompany a low-temperature magnetic transition in TbBaCo$_{2}$O$_{5.5}$, is observed in a single crystal DbBaCo$_{2}$O$_{5.5}$ by means of the X-ray and neutron diffraction. The low temperature phase does not belong to the space group $Pcca$ that has been chosen earlier as the highest subgroup of the $Pmma$. The transition is of the first order with the temperature hysteresis, between $T\approx 100$ and $T\approx 200$ K, which probably explains anomalous magnetic properties in this temperature range. 
Yu.P. Chernenkov, V.P. Plakhty, A.G. Gukasov, S.N. Barilo, S.V.   Shiryaev, G.L. Bychkov, V. Hinkov, V.I. Fedorov, V.A. Chekanov