## 3 Results and Discussion
### Structural Characterization  
Figures 2 and 3 show X-ray diffraction patterns of the Ba(Co\({}_{0,7}\)Zn\({}_{0,3}\))\({}_{1/3}\)Nb\({}_{2/3}\)O\({}_{3}\) (BCZN) ceramics after sintering at 1450 \({}^{\circ}\)C for 2 h (sample \(SI\)) and after annealing at 1300 \({}^{\circ}\)C (sample \(SI\)a) for 30 h. In order to identify the surface composition, the analyses were carried out at the upper surface for both specimens and at depths of 90 and 120 \(\mu\)m for sintered and annealed ceramics, respectively. It can be seen that the surface XRD pattern (Figure 2a) can be interpreted as a mixture of Ba(Co\({}_{0,7}\)Zn\({}_{0,3}\))\({}_{1/3}\)Nb\({}_{2/3}\)O\({}_{3}\) (BCZN) and Ba(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) phases. The complex perovskite ceramic Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) is isostructural with Ba\({}_{8}\)Ni\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) and Ba\({}_{8}\)Zn\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\)[11]. The Co:Zn ratio in Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) may be variable. At a depth of 90 \(\mu\)m below the surface (Figure 2b), all reflections can be attributed to the cubic structure of Ba(Co\({}_{0,7}\)Zn\({}_{0,3}\))\({}_{1/3}\)Nb\({}_{2/3}\)O\({}_{3}\), whereas no peaks attributable to Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) were observed. These results clearly show that the Ba(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) phase is only formed on the surface of the BCZN ceramics and that it represents a secondary phase. The appearance of this phase is probably due to the volatilization of some Zn and Co elements during the sintering steps [12]. For  
Figure 1: Shrinkage curve of a BCZN sample as a function of temperature. annealed ceramics (Figure 3), all diffraction peaks in Figure 3a (on the surface) can be attributed to the Ba(Co\({}_{0.7}\)Zn\({}_{0.3}\))\({}_{1/3}\)Nb\({}_{2/3}\)O\({}_{3}\) and Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) phases. However, the relative intensities of the main peaks of the two phases are reversed as compared to those observed for the sintered materials, suggesting that the amount of Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) increased on the surface of the BCZN ceramic after the annealing process. On the other hand, the patterns obtained at a depth of 120 microns below the surface (Figure 3b), are similar to those obtained at a depth of 90 \(\upmu\)m after sintering (Figure 2b). The latter findings are consistent with previous works indicating that the core of the specimens consists only of the Ba(Co\({}_{0.7}\)Zn\({}_{0.3}\))\({}_{1/3}\)Nb\({}_{2/3}\)O\({}_{3}\) phase and that it transforms into Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) at a high temperature on the surface [10]. The X-ray patterns also show a superlattice peak at 20 = 17.74\({}^{\circ}\) (marked by an open circle), as shown in the small insert in Figure 3b. This reflection highlights the 1:2 cation ordering within the crystal [7]. The effect of the cation ordering on the dielectric proprieties of BCZN will be discussed in the following section in paragraph 3.4. The insert shows also the reflection peak at 20 = 29.59\({}^{\circ}\) (marked by a sold star) arising from the Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) phase. This indicates that the secondary phase of Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\) is located at a depths of over 120 \(\upmu\)m.  
Figure 3: X-ray diffractograms of a BCZN ceramic (\(S1a\)): (**a**) at the upper surface and (**b**) at a depth of 120 \(\upmu\)m below the surface. \(\bullet\): Ba(Co\({}_{0.7}\)Zn\({}_{0.3}\))\({}_{1/3}\)Nb\({}_{2/3}\)O\({}_{3}\); \(\bullet\): Ba\({}_{8}\)(Co,Zn)\({}_{1}\)Nb\({}_{6}\)O\({}_{24}\). Inset: The region limited between 15\({}^{\circ}\) and 35\({}^{\circ}\) 20 shows a superlattice peak for 1:2 ordering which is denoted by an open circle.