| Layered cobalt oxides Ln Ba Co2O5+δ(Ln for rare earth) have attracted a great deal of interest due to plenty of fascinating physical properties since the beginning of 1990 s. The Ln Ba Co2O5+δ compounds possess a layered perovskite structure which consists of square-lattice layers [Co O2]-[Ba O]-[Co O2]-[Ln Oδ] ranged consecutively along the c axis, and provide two crystallographic sites for the cobalt cations with two different environments(pyramindal Co O5 and octahedral Co O6). The variability of δ determines the well-ordered crystal structure of the compound where the oxygen ions form filled and empty chains in the [Ln Oδ] layers along the b axis. In this paper, Ln Ba Co2O5+δ polycrystalline samples with different rare earth were synthesised by using solid state reaction method. Samples were investigated by x-ray powder diffraction and XRD patterns indicate that all the samples have formed single phase. δ of Ln Ba Co2O5+δ(Ln=Nd, Gd) were tuned by annealing with the different temperature, time and in different atmospheres. The oxygen content of samples was determined by the optimized iodometric titration. The dcmagnetization and resistivity of polycrystalline Ln Ba Co2O5+δ(Ln=Nd, Gd) were measured with PPMS and were investigated in detail.The samples synthesized by the solid state reaction were proved to be single phase, wihich means Ln Ba Co2O5+δ with different Ln could be prepared by the solid state reaction with the right temperature and sintering time in air. The proper annealing temperature in air is in the range of 1000~1150℃, which differs with the lanthanoid. To synthesise perfect Ln Ba Co2O5+δ samples, repeated grinding and firing are necessary. During oxygen determination with the iodometric titration, the air was removed by N2 flow, sodium thiosulfate should been standardized by KIO3. The oxygen contents of Ln Ba Co2O5+δ were determined accurately, δ=0.81 for La Ba Co2O5+δ, δ=0.64 for Nd Ba Co2O5+δ while δ=0.48 for Gd Ba Co2O5+δ, etc. In this paper we modified the oxygen content of Nd Ba Co2O5+δ and Gd Ba Co2O5+δ, δ=0.71 for annealing Gd Ba Co2O5+δ with 1000℃ in p(O2)≈4MPa atmosphere, δ=0.32 for annealing Nd Ba Co2O5+δ in H2&N2 atmosphere and so on. Tuning the oxygen content of Gd Ba Co2O5+δ(Ln=Nd, Gd) indicates that δ could vary in a wide range, while comparison with oxygen contents of different Ln compounds means that δ is also limited by the ionic radius of Ln3+.The magnetization versus temperature curves of Gd Ba Co2O5+δ indicate the successive PM-FM and FM-AFM transition which were discovered in the majority of Ln Ba Co2O5+δ by many authors. Variation of δ affects the magnetic behaviors of Ln Ba Co2O5+δ dramatically. Curie-Weiss fitting of Ln Ba Co2O5.48’s χ(T) above TC indicates a 1:1 mixture of low-spin and intermediate-spin states of Co3+ and there is no spin-state transition in the intriguing range of temperature. TC of all samples keeps stable with different δ, which indicates Co—O—Co superexchange interation being primarily responsible for FM state. The balance of FM and AF ordering is quite delicate and affected easily by the magnetic field, lower TN of FM-AFM transition under higher H indicates the AFM state comes from the weak AF coupling of FM ladders formed by magnetic Co ions which are separated by nonmagnetic layers is weaker ordering. Another feature of Ln Ba Co2O5+δ is an obvious thermo-magnetic irreversibility, but the magnetization doesn’t behavior like as the spin glass. Comparing with Gd3+, maybe bigger Nd3+ changes the bond distance and angle of Co—O—Co decreasing the superchange interation, leads different magnetization curves of samples.The resistivity of Ln Ba Co2O5+δ doesn’t behave as a conventional insulator or semiconductor, and there is metal-insulator like transiton above TC for Ln Ba Co2O5+δ whose oxygen content is close to δ=0.5. The resistivity versus temperature curves of Ln Ba Co2O5+δ exhibit the VRH mode, this behavior indicates that the samples are disordered systems where the charge carriers move by hopping between localized electronic states whose energies are close. |
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