**3.1.11. Fourier transform infrared and Raman spectroscopy**

Infrared (IR) spectroscopy is one of the most important analytical techniques that can be used for the investigation of any sample in any state. Liquids, solutions, pastes, powders, films, fibers, gases and surfaces can be examined with judicious choice of sampling technique. Infrared spectrometers have been commercially available since the 1940s [117].

Fourier transform infrared (FT-IR or FTIR) spectroscopy is divided into three regions accord‐ ing to the increasing wavelength [118]:


The spectral ranges of near-, mid- and far-infrared spectroscopy are shown in **Fig. 9**(**a**).

**Fig. 9.** Schematic illustration of relationships between the ranges of (a) vibrational spectroscopy and electromagnetic spectrum [118] and (b) spectroscopic transitions underlying several types of vibrational spectroscopy. v0 indicates the laser frequency, while v is the vibrational quantum number. The virtual state is a short-lived distortion of the electron distribution by the electric field of the incident light [119].

The background for Raman spectroscopy was given by the discovery of Raman scattering by Krishna and Raman in 1928. Until approximately 1986 when Fourier transform (FT)–Raman was introduced, physical and structural investigations dominated in literature over relative‐ ly few reports of Raman spectroscopy applied in chemical analysis [119],[120].

Scanning electron microscope can be also used to determine compositional information using characteristic X-ray. The development of instruments for obtaining localized chemical analysis of solid samples, i.e. electron probe microanalyzer (EMPA), occurred at the same time as the

Scanning electron microscopy (SEM) is used for grain interactions and spot analysis [98],[112], electron microprobe microanalysis (EPMA) for the distribution of elements in the matrix, investigation of the effects of impurities on the properties of apatites and investigation of

Infrared (IR) spectroscopy is one of the most important analytical techniques that can be used for the investigation of any sample in any state. Liquids, solutions, pastes, powders, films, fibers, gases and surfaces can be examined with judicious choice of sampling technique.

Fourier transform infrared (FT-IR or FTIR) spectroscopy is divided into three regions accord‐

The spectral ranges of near-, mid- and far-infrared spectroscopy are shown in **Fig. 9**(**a**).

**Fig. 9.** Schematic illustration of relationships between the ranges of (a) vibrational spectroscopy and electromagnetic spectrum [118] and (b) spectroscopic transitions underlying several types of vibrational spectroscopy. v0 indicates the laser frequency, while v is the vibrational quantum number. The virtual state is a short-lived distortion of the electron

The background for Raman spectroscopy was given by the discovery of Raman scattering by Krishna and Raman in 1928. Until approximately 1986 when Fourier transform (FT)–Raman

Infrared spectrometers have been commercially available since the 1940s [117].

development of SEM.

reaction interface [113],[114],[115],[116].

ing to the increasing wavelength [118]:

**3.1.11. Fourier transform infrared and Raman spectroscopy**

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

**1.** Near-IR (NIR) spectroscopy, abbreviated as FT-NIR

**2.** Mid-IR (MIR) spectroscopy, abbreviated as FT-MIR

**3.** Far-IR (FAR) spectroscopy, abbreviated as FT-FAR

distribution by the electric field of the incident light [119].

When monochromatic light with the energy *hν*<sup>0</sup> encounters the matter (gas, solid or liquid), there is a small probability that it will be scattered at the same frequency (**Fig. 9**(**b**)). If the object, e.g. molecule is much smallerthan the wavelength of the light, the scattering is Rayleigh scattering. The "*virtual state*" is not necessarily a true quantum state of the molecule but can be considered a very short-lived distortion of the electron cloud caused by oscillating electric field of the light. Since blue light is more efficiently scattered than red one, Rayleigh scatter‐ ing is responsible for the blue color of sky. The electron cloud of the molecule is also pertur‐ bed by molecular vibrations, and it is possible for the optical and vibration oscillations to interact, leading to Raman scattering. Raman scattering is shown in (**Fig. 9**(**b**)) in which the scattered photon is lower in energy by an amount equal to the vibration transition. Raman spectrum consist of scattered intensity plotted versus energy and each peak corresponds to given Raman shift from the incident light energy *hν*0 [119].

Just like Rayleigh scattering, Raman scattering depends on the polarizability of scattering molecules. IR band, on the other hand, arises from a change in the dipole moment. In many cases, the transitions that are allowed in Raman are forbidden in IR, so these techniques are often complementary (please compare **Fig. 18**(**a**) and (**b**)). In polarizable molecules, incident light can excite the vibrational modes, leading to scattered light diminished in energy by the amount of vibrational transition energies (same as in fluorescence). Scattered light underthese conditions reveals the satellite lines below the Rayleigh scattering peak at the incident frequency–Stokes lines (Stokes part of spectrum). If there is enough energy, it is also possi‐ ble to see anti-Stokes lines. Since anti-Stokes lines are usually weaker than Stokes lines, only the Stokes part of spectrum is usually measured [121].

The method combining Raman spectrometer with microscopic tools, typically an optical microscope, is known as micro-Raman spectroscopy (μRS) or also Raman microscopy. The μRS is nondestructive and noncontact method forthe characterization of organic and inorganic materials [122].

Infrared [97],[98],[123],[112],[124],[125],[126], Raman [97],[98] and micro-Raman spectroscop‐ ies [125] were often used to identify and investigate the structure and extent of substitution and to optimize the synthesis conditions of minerals from the supergroup of apatite. Since carbonate ions exhibit clear vibrational signature in infrared spectrum, infrared spectrosco‐ py is widely used to investigate the structure and to evaluate the carbonate/phosphate ratio (*r*c/p) and the amount of carbonate ions in carbonate-apatites [127]:

$$\text{CO}\_3^{2-} \left[ \text{wt.} \% \right] = 28.62 \text{ } r\_{c/p} + 0.0843 \tag{1}$$

The example of infrared and Raman spectrum of fluorapatite is described in **Section 3.2.3**.

Infrared spectra of phosphate minerals in the pyromorphite series are described byADLER [128]. In the pyromorphite series, the equilibrium internuclear X-O distance in XO4 3− ion (PO4 3−, AsO4 3− and VO4 3−) is primarily a function of the ionic radium of X atom. Since Pb, in this case, is always the dominant externally coordinated cation, for various members, there is no significant change in the interaction between the molecular vibration and the external environment. Bradger's equation [128],[129],

$$k\_{\rm o} = 1.86 \cdot 10^{\circ} \,/\left(R - d\_{\rm y}\right)^{\circ} \tag{2}$$

although specifically applicable to internuclear distances in diatomic molecules, reflects generally the inverse relationship between the force constant *k*0 and the internuclear dis‐ tance *R*. Symbol *d*ij denotes the constant the values of which depend on the nature of bond‐ ed atoms. The molecular vibration frequency *v* is dependent on the restoring forces, measured in terms of *k*0, between participating atoms as well as on the masses of these atoms. The relationship may be expressed approximately by the equation:

$$
\omega = \frac{1}{2}\pi c \sqrt{\frac{k}{\mu}}\tag{3}
$$

where the vibration frequency *v* is a function of the force constant *k* and the reduced mass *u* of vibrating atoms, all other terms being invariant.

The spectral frequency differences between pyromorphite, mimetite and vanadinite are explicable and to a considerable degree predictable in terms of these parameters. On com‐ plete substitution of As or V for P the effect of reduced force constants is reinforced by increases in mass, thereby shifting *ν*3 and *ν*1 to lower frequencies. Because of opposing mass and forceconstant effects and perhaps also because of dissimilarities in orbital configurations, the relative positions of absorption bands are less predictable for mimetite and vanadinite than for pyromorphite and mimetite. The theoretical frequency trends are depicted in **Fig. 10** [128].

**Fig. 10.** Theoretical effect of change in mass and ionic radius on infrared vibration frequency of tetrahedral XO4 3− ions, where *X* = P5+, As5+ of V5+ [128].
