1. INTRODUCTION
With the discovery of colossal magnetoresistance (CMR) effect in manganites, hole-doping perovskite manganites with unusual electronic transport and magnetic properties have attracted considerable attention. These properties result from an intrinsic interaction among charge, spin, orbital and lattice degrees of freedom which are strongly coupled to each other^[1-6]. Double exchange model combined with John-Teller effect was used to explain part of these properties^[7-9].
    A great deal of work has been carried out on the manganites Ln_1-xA_xMnO₃ since the discovery of colossal magnetoresistance properties in these compounds^[10-11]. Also, many studies have been carried out on the Mn site doping effect (formula AB'_1-xB''_xO₃)^[12-15]. The research work reported by Gayathri et al.^[16] suggests that the substitution of Co for Mn dilutes the DE mechanism and changes the long range FM order of La_0.7Ca_0.3MnO₃ to a cluster glass-type FM order.
    The electronic configuration of the Co³⁺ ions is more complicated due to the possibility of the existence of the Co³⁺ ions in the low spin, the intermediate spin and the high spin states. Most previous studies treat the spin-state from the low-spin state to the thermally excited high-spin state ^[17], since the high-spin (HS) state (HS, t⁴_2ge_g²) is only 10-80 meV higher in energy than the low-spin (LS) state (LS,t⁶_2ge_g⁰)^[18]. Therefore, it is easy to be excited to high spin state with temperature. Potze et al.^[19] reported that the interaction between high spin states of Co³⁺ is expected to be antiferromagnetic. Thus trivalent Co is possible to be in a high spin state at room temperature^[20]. Based on our experimental results in our system we consider it to be predominantly in the high spin state (HS, t⁴_2ge_g²).
    To our knowledge, there is little data being available on coupled A-B-site substitutions in ABO₃^[21]. On the basis of previously published experimental data^[21], we may expect that a coupled substitution involving Ca and Mn would improve the properties of GdCoO₃. So if Ca is doped at Gd-site and Mn doped at Co-site, useful information for understanding the properties of Gd_1-xCa_xCo_1-xMn_xO₃ could be obtained.
    The purpose of the present study is to clarify the transport mechanism in mixture of manganites Gd_1-xCa_xCo_1-xMn_xO₃ (0≤x≤1) and to show how the properties change with the replacement of Gd by Ca and Co by Mn.

2. EXPERIMENTAL
Polycrystalline samples of Gd_1-xCa_xCo_1-xMn_xO₃ (0≤x≤1) were prepared by standard solid-state reaction for x=0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0 from stoichiometric amounts of Gd₂O₃, CaCO₃, Co₃O₄ and MnO₂, the purities of which were at least 99.9%. The Gd₂O₃ powders 99.9% dried for 5 hours at 800⁰C before use. The mixtures were ground by hand in a mortar agate for at least 40 min and pressed into pellets, then preheated the pellets for 8 hours at 800⁰C. Following, the mixtures were ground once again. The ground powders were pressed into pellets and annealed at 1100⁰C in air for 24 h with intermediary grindings and finally cooled down the samples to 500⁰C at 2⁰C/min, then kept the samples in furnace till they reached room temperature.
    The crystal structure and the phase purity of the samples were examined by X-ray diffraction (XRD) using Cu target at room temperature.
    Temperature dependence of magnetization curve was measured by a vibrating sample magnetometer (VSM) in a field of 0.5 T in the temperature range 80-300 K. Electrical resistivity was determined by the standard four-probe DC method in the temperature range 80-300 K.

Fig. 1 Typical X-ray diffraction patterns of Gd_1-xCa_xCo_1-xMn_xO₃ (0.0≤x≤1.0)

    In Table 2, the ionic radii of the cations involved in this study are shown, according to shannon^[24]. From geometrical view the increase in radius on A site by substitution of Ca²⁺ for Gd³⁺ is less than decrease in radius on B site by substitution of Mn⁴⁺ for Co³⁺ (see Table 2), so the lattice parameters should be decreased by the coupled substitutions of A sites with that of B sites. In fact,the substitutions of Ca for Gd and Mn for Co cause an increase of the lattice parameters for the series with x=0.0-0.5, and a decrease of the lattice parameters can be observed for the series with x=0.5-1.0 (see Table 1). The reason is described as following. In perovskite oxides, the spin states of the d electrons in the Mn and Co ions form according to the strong Hund’s coupling. The vacant state with opposite spin lies higher in energy due to exchange interactions. Both occupied and unoccupied spin states are furtherly split into t_2g and e_g orbitals by the octahedral crystal field. Increasing energy, these states are t_2g­ , e_g­ , t_2g¯ and e_g¯ . The electronic configurations are for Co³⁺, for Co⁴⁺, for Mn³⁺ and for Mn⁴⁺, respectively. In a solid, these states form bands. For Co³⁺ ions, the t_2g­ and e_g­ bands are fully occupied, the t_2g¯ is partly occupied, and the e_g¯ are empty. For Mn⁴⁺ ions, the t_2g­ bands are fully occupied, and the e_g¯ are empty. The t_2g¯ of Co³⁺ band, which is occupied by one electron, and the e_g­ of Mn⁴⁺ band, which can accommodate a maximum of two electrons per ion, respectively, play a crucial role like the e_g­ bands of Fe and Mn in the La_1-xCa_xFe_1-xMn_xO₃ system^[25]. Because the Co⁴⁺ ion is easier to found than Fe⁴⁺ ion in Co-containing perovskites^[26], the system of Gd_1-xCa_xCo_1-xMn_xO₃ (0.0≤x≤1.0) is expected to have a similar band structure as that of La_1-xCa_xFe_1-xMn_xO₃^[25] (see Fig. 2). The Co t_2g¯ band overlaps with the Mn e_g­ band at a certain extent as shown in Fig. 2. In a mixed system of Co and Mn, the widths and energies of their t_2g¯ and e_g­ bands dictate the electron distribution of the Co and Mn ions. The energy diagram clearly shows that t_2g¯ electrons of Co³⁺ can hop to e_g­ orbitals of Mn⁴⁺ easily which can produce an amount of Co⁴⁺ and Mn³⁺ (see Fig. 2). Therefore, an increase of the lattice parameters for the series with x=0.0-0.5 can be explained by change of oxidation state of Mn ion from +4 to +3 and Co ion from +3 to +4 and ionic radius of Mn³⁺ bigger than that of Co³⁺ or Co⁴⁺ (see Table 2). It also can be seen from Table 1 that in the Ca and Mn dominant range with x = 0.5-1.0, the substitutions by coupling on A site and B site cause the lattice parameters decrease. The reason is that doped Mn partly decreases to low oxidation state, +3, based on the principle of neutrality and the Mn⁴⁺ concentration increases and the radius of Mn⁴⁺ is smaller than that of Mn³⁺ ion (see table 2). Therefore, the lattice parameters decrease with continuously doped Ca and Mn ions with x = 0.5-1.0.

Fig. 2 Band structure of Fe and Mn in La_1-xCa_xFe_1-xMn_xO₃ and Co and Mn in Gd_1-xCa_xCo_1-xMn_xO₃

3.3 Electrical resistivity
The temperature dependence of the electrical resistivity of the samples is shown in Fig. 5. Insulating behavior is observed for all samples in the studied temperature range. No I–M transition in the series Gd_1-xCa_xCo_1-xMn_xO₃ (0.0≤x≤1.0) is observed in the studied temperature range. The electrical resistivity of the Gd_1-xCa_xCo_1-xMn_xO₃ (0.0≤x≤1.0) solid solution slowly decreases with increasing x when the temperature is above about 100K. As some samples’ electrical resistivity is too high to measure on our apparatus at low temperatures, we can not obtain the values of electrical resistivities below 100 K. It can be seen from Fig. 5 and the inset of Fig. 5 that the electrical resistivity of GdCoO₃ compound is the biggest in the series Gd_1-xCa_xCo_1-xMn_xO₃ (0.0≤x≤1.0) and of all other samples decrease with Ca and Mn doped (except x = 0.3 at low temperature (below 157K)). The electrical resistivity is more than 2 order of magnitude higher at room temperature for x = 0.0 (GdCoO₃) than x = 1.0 (CaMnO₃).
    In order to get more insight into the electrical transport process, the electrical resistivity data were fitted to various electrical transport models. Fig. 6 shows the electrical transport behaviors for x =0.1, 0.3, 0.5, 0.7 and 1.0 samples. It can be seen from Fig. 6 that two steps transport of small polaron model is confirmed for x = 0.1, 0.3 and 0.5 (see Fig. 6a) and that adiabatic polaron is demonstrated for x = 0.7 and 1.0 (see Fig. 6b). For the specimens of x = 0.0 and 0.9, the electrical transport behaviors shows adiabatic polaron at low temperature and non-adiabatic polaron at high temperature, respectively.

Fig. 5 The temperature dependence of electrical resistivity for polycrystalline samples of the solid solution Gd_1-xCa _x Co_1-x Mn_xO₃ (0.0≤x≤1.0) as a function of temperature


Fig. 6 (a) log r / T vs. T^-1/4, (b) log r / T vs. T^-1 plots for selected samples

4. CONCLUSION
We have investigated the structure, electrical transport and magnetic properties of the series Gd_1-xCa_xCo_1-xMn_xO₃ (0.0≤x≤1.0) compounds. The lattice parameters a, b, c increase firstly with x≤0.5 and following decrease with x≥0.5. The magnetic transition temperatures of all the samples are below 80.0 K. The values of θ_CW of all samples are negative. The electrical behaviors of all specimens demonstrate insulator and the electrical resistivity decreases with Mn and Ca ionic doped. The detailing analysis of electrical transport shows that the electrical transport process of some samples are controlled by two steps transport of small polaron model, some by adiabatic small polaron hopping model in the whole studied temperature range and others of the electrical transport can be described by adiabatic small polaron hopping model at low temperature and by non-adiabatic hopping model at high temperature, respectively.

ACKNOWLEDGEMENT  This work was supported by Foundation for Doctor, Education Department of Hebei Province (B2002102), P. R. China.