**3.2.4. Thermal analysis**

The results of simultaneous TG-DTA of investigated fluorapatite specimen are shown in **Fig. 19**. The sample is heated with the rate of 10°C·min−1 up to the temperature of 1425°C. The mass of sample is reduced by 1.25% during TG-DTA when the final temperature is reached. The most important features are the thermal decomposition of CaCO3 and the thermal decompo‐ sition of fluorapatite.

**Fig. 19.** Thermal analysis of investigated specimen of fluorapatite.

**Vibration mode Assignment C6h factor group symmetry Raman shift IR**

**Table 2.** The interpretation of infrared and Raman bands in the spectrum of stoichiometric fluorapatite [164].

an irreducible representation for optically active vibration of [165]:

138 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

site symmetry of phosphate group under the compression [165].

of the PO4

(*ν*3) of planar CO3

**3.2.4. Thermal analysis**

sition of fluorapatite.

Factor group analysis of the hexagonal P63/M space group fluorapatite structure (*Z* = 2) yields

where IR and R denote infrared and Raman activity, respectively. The influence of pressure on the infrared and Raman spectra of fluorapatite was investigated by WILLAMS and KNITTLE [165]. Fluorapatite remains stable under pressures of at least 25 GPa at 300 K. Local environ‐ ment of phosphate groups in fluorapatite becomes progressively less distorted from tetrahe‐ dral symmetry under the compression, as manifested by progressively smaller site-group.

The Davydov (factor group) splitting also decreases under the compression. This decrease is consistent with nondipole effects playing a primary role in the Davydov splitting of apatite; indeed, the magnitude of the Davydov splitting appears to be modulated by increases of the

The spectrum **Fig. 18**(**a**) shows weak peak located in the domain of OH stretching modes (from 3500 to 3600 cm−1) at the wave number of 3535 cm−1. This band belongs to the OH stretching mode in the hydrogen bond F…OH…(F) [166]. According to FREUND and KNOBEL [167], the band at ~744 cm−1 belongs to the vibration of OH…F bond. According to KNUBO‐ VETS [168], the bands in the range from 745 to 720 cm−1 in apatite spectra could also be attrib‐ uted to symmetric valence oscillations of the P-O-P bridge bonds, formed by the condensation

The results of simultaneous TG-DTA of investigated fluorapatite specimen are shown in **Fig. 19**. The sample is heated with the rate of 10°C·min−1 up to the temperature of 1425°C. The mass of sample is reduced by 1.25% during TG-DTA when the final temperature is reached. The most important features are the thermal decomposition of CaCO3 and the thermal decompo‐

3− tetrahedron. The presence of calcite causes that antisymmetric stretching mode

2− ion appears in the infrared spectrum of investigated sample [169].

Ag, E1g 592 – E1u – 601 Ag 608 – E2g 617 –

() ( ) () ( ) () 11 2 12 7 IR 8 11 IR 13 G= + + + + *AR A E R E E R gu g u g* (14)

**[cm−1]**

The content of calcite was verified by thermal analysis. The weight of sample was reduced by 0.26% during the thermal decomposition of calcite. Since the theoretical mass loss of calcite is (100 × 44.09) / 100.086 = 43.97%,24 the content of calcite in the investigated sample of fluora‐ patite is (100 × 0.26) / 43.97 = 0.59%. Although this value is lower than the content of calcite determined by Rietveld analysis, there is still good agreement of both methods. The DTG curve shows that the process starts at the temperature of 565°C and wide of peak is of 135°C. The maximum rate decarbonation is reached at the temperature of 656°C.

At temperatures higher than 900°C, the weight of sample is reduced by the thermal decom‐ position of fluorapatite. The extrapolated beginning of the defluorination process (**Section 8.6**) was determined to be 1199°C. At the temperature of 1425°C, the defluorination process is still not complete. The extrapolation of experimental data25 shows that the thermal decomposi‐ tion is most probably not complete before the temperature of melting point is reached (**Table 7** in **Chapter 1**).
