Spin transition and thermal expansion in the layered cobaltite GdBaCo$_2$O$_{5.5}$

The electrical conductivity and thermal expansion coefficient of GdBaCo$_2$O$_{5.5}$ samples have been measured in order to reveal the mechanism of the metal-insulator transition in cation-ordered cobaltites RBaCo$_2$O$_{5.5}$ (where R is a rare-earth element) and its relation to the change in the spin state of cobalt ions. It has been established that the unit cell volume considerably increases upon the transition to the metallic state at $T_{MI}\approx$ 360–365 K and that the thermal expansion exhibits anomalies (which are two orders of magnitude weaker) due to the ferromagnetic and antiferromagnetic orderings. The data obtained confirm that the spin transition in Co$^{3+}$ ions actually proceeds simultaneously with the metal-insulator transition and excludes the possibility of stepwise spin transitions occurring at lower temperatures.