**2. Materials and methods**

## **2.1 Powder synthesis**

A solid state method was adopted to prepare strontium fluorobritholites compounds Sr8La2�xNdx(PO4)4(SiO4)2F2 with 0 ≤ x ≤ 2 [36]. The starting reagents: strontium fluoride SrF2 (99.99%. Merck)), strontium carbonate SrCO3 (≥99.00% Fluka), silica SiO2 (Prolabo), lanthanum and neodymium oxide (La2O3�Nd2O3) (99.99% Merck) and strontium diphosphate (Sr2P2O7) were used. The reaction equation (1) is the following:

$$\begin{aligned} &3\mathbf{\hat{S}r}\mathbf{C}\mathbf{O}\_3 + 2\mathbf{S}\mathbf{r}\_2\mathbf{P}\_2\mathbf{O}\_7 + \mathbf{S}\mathbf{r}\mathbf{F}\_2 + 2\mathbf{S}\mathbf{i}\mathbf{O}\_2 + \frac{2-\mathbf{x}}{2}\mathbf{L}\mathbf{a}\_2\mathbf{O}\_3 + \frac{\mathbf{x}}{2}\mathbf{N}\mathbf{d}\_2\mathbf{O}\_3\\ &\rightarrow \mathbf{S}\mathbf{r}\_8\mathbf{L}\mathbf{a}\_{2-\mathbf{x}}\mathbf{N}\mathbf{d}\_{\mathbf{x}}(\mathbf{P}\mathbf{O}\_4)\_4(\mathbf{S}\mathbf{i}\mathbf{O}\_4)\_2\mathbf{F}\_2 + \mathbf{3}\mathbf{C}\mathbf{O}\_2 \uparrow \text{with } \mathbf{0} \le \mathbf{x} \le 2. \end{aligned} \tag{1}$$

Sr2P2O7 was synthesized by the following reaction at 900°C:

$$2\text{SrCO}\_3 + 2(\text{NH}\_4)\_2\text{HPO}\_4 \rightarrow \text{Sr}\_2\text{P}\_2\text{O}\_7 + 2\text{CO}\_2 + 3\text{H}\_2\text{O} + 4\text{NH}\_3 \tag{2}$$

SrCO3 (>96% Riedel de Haen), Gd2O3 (>99.5% Prolabo), Nd2O3 (>99.5% Prolabo) SiO2 (>99.5% Alfa), SrF2 (>99.5% Prolabo) and (NH4)2HPO4 (>99% Acros Organics) were used as raw materials. For each composition the molar ratio (Sr + La + Nd)/ (P + Si) and the obtained quantity of each composition should be respectively 1.67 and 1.5 � <sup>10</sup>�<sup>3</sup> moles. Before synthesis, each quantity of lanthanum and neodymium

*Ionic Conductivity of Strontium Fluoroapatites Co-doped with Lanthanides DOI: http://dx.doi.org/10.5772/intechopen.102410*


**Table 1.**

*Masses in grams of lanthanum and neodymium oxides used in the synthesis of Sr8La2*�*xNdx(PO4)4(SiO4)2F2 with 0* ≤ *x* ≤ *2.*

**Figure 1.** *Thermal cycle used for strontium fluorobritholite sintering.*

oxides given in **Table 1** was furnaced at 1000°C for 12 h to avoid the formation of Ln-hydroxide. Then, the solid mixture was milled and homogenized in an agate mortar for about 30 min, and then cold pressed under 100 MPa into pellets (30 and 3 mm). During sintering, the pellets were sintered in the temperature range 1250–1450°C in a carbolyte type furnace with controlled argon atmosphere. The temperature varied with 50°C for each value of x. The sintering cycle is shown in **Figure 1**. The heating and cooling rate was of 10°C min�<sup>1</sup> . In the following sections, the samples will be named SrLa2�xNdxF where x is the substituted Nd rate.

### **2.2 Analysis and characterization**

A PANalytical X'pert Pro diffractometer with a KαCu anode (λ = 1.54 Å) operating 40 kV and 40 mA was the apparatus used for the XRD patterns recording. The scans range was between 10 and 70° (2θ) with a step size of 0.02°. The crystallite size of the powder Dhkl was calculated using the (300) and (002) reflections following Debye Sheerer equation [37]:

$$\mathbf{D} = \frac{\mathbf{K}\lambda}{\mathfrak{J}^{1/2}\mathbf{cos}\mathfrak{G}}\tag{3}$$

Needs to remember that λ is the X-ray wavelength of the monochromatic X-ray beam. For the apatitic crystallites K is a constant equal to 0.9. β1/2 is the full width at half maximum of the selected reflection and θ is the Bragg's diffraction angle.

The Fourier transformed infrared (FTIR)-attenuated total reflection (ATR) spectra were performed at room temperature on a Perkin Elmer spectrometer in the spectral range 4000–400 cm�<sup>1</sup> .

The chemical analysis of Sr., P, Si, La and Nd ions in the synthesized samples was determined via an inductively coupled plasma atomic emission spectroscopy (ICP-AES) (JY-Horiba Ultima-C spectrometer). The samples were thus previously mixed with 99.9% lithium metaborate, fused at 1000°C for 25 min and dissolved in HCl (0.6 M). The fluoride content in the synthesized samples was measured by a specific ion-selective electrode.

The complex impedance measurements were performed on pellets sintered at between 1250 and 1450°C for 24 h. Their densities varied from 72 to 83% of the theoretical density as a function the sintering temperature. The two faces of the pellets were coated with a silver paint and then two platinum wires electrodes linked them to a Hewlett-Packard 4192-A impedance analyzer. The measurements were recorded with the temperatures variation from 450 to 780°C and frequencies from 10 Hz to 13 MHz.
