*2.3. Raman Spectroscopy*

Two Raman systems were employed in this investigation: One custom-made and one of standard use and commercially available. The former allowed for the set-up of different experimental conditions in terms of the polarization state of light at the incident-on-sample and detection stages, including non-polarized, parallel polarized (p), and cross polarized (s) situations. Adopting the so-called Porto's formalism, these experimental conditions were *Z*(−−)*Z*, *Z*(*YY*)*Z* and *Z*(*YX*)*Z*, respectively; where, in general, *A(BC)D* stands for light propagating in the *A* direction with linear polarization *B*, before the sample, while selective detection is done on the *D* direction with polarization *C* [26].

The commercially available system only featured the non-polarized configuration. It was a Witec alpha300R Confocal-Raman microscope with a 532 nm source of excitation wavelength and 4–5 cm−<sup>1</sup> of spectral resolution. With this equipment, the Raman spectra were collected in the range 100–1200 cm−<sup>1</sup> at room temperature and light incident on the normal component of the sample with a power of 3.4 mW; a Nikon 10 objective was used to focus the incoming light on a 1:5 mm spot. An intensity of approximately 11Wcm−<sup>2</sup> was delivered to the sample. The customized open-air Raman system

consisted of an excitation beam output of a continuous wave diode laser at 638 nm wavelength with a power of 37 mW (Innovative photonic solution). The beam was linearly polarized from variable angle mounting and transmitted through a beam splitter to focus the excitation beam into the sample by an aspherized achromatic lens (NA = 0.5, Edmund optics). The excitation spot diameter measured at the focus point had a ~10 μm radius. The collected Raman scattered light from the sample through the aspheric lens and the beam splitter was focused by two silver coated mirrors and one bi-convex lens into a fiber Raman Stokes probe (InPhotonics) that was connected to a QE65 Raman Pro spectrometer (Ocean optics) for a Raman shift range detection between 250–3000 cm−1. In its use for the characterization of the powders, the light at λ = 638 nm was incident at razing angle with P = 10 mW. The Raman spectra were collected in the range 200–1200 cm−<sup>1</sup> at room temperature with a spectral resolution of 8 cm<sup>−</sup>1. In this case, a laser intensity of approximately 3 kWcm−<sup>2</sup> was delivered to the sample. Due to technical issues, most of the utilized experimental conditions were different from one Raman system to another—it is shown how this did not alter the obtained results, except for the detection mode which in both cases was fixed at the backscattering-detection mode (Figure 2).

**Figure 2.** Scheme of the experimental configurations used for the acquisition of Raman spectra: (**a**) Custom-made featuring both configurations, polarized and non-polarized; (**b**) commercially available featuring only non-polarized measurements. From left to right: RP—Raman probe, F—filter, M—mirror, L—lens, P—polarizer, BS—beam splitter, AL—aspheric lens, PS—powdered sample, S—spectrometer, O—objective.

#### *2.4. UV-Vis Di*ff*use Reflectances and Di*ff*erential Thermal Analysis*

An Ocean Optics USB2000+ UV-VIS Spectrometer and an R400-Angle-Vis Reflection probe were used to collect the diffuse reflectance (DR) spectra of the samples and an Ocean Optics DH-2000-BAL Deuterium-Halogen light source was utilized. Commercially available aluminum oxide (Al2O3) was chosen as the standard reference. Precautions were taken so that the approximations necessary to apply the Kubelka-Munk Theory were accomplished [27–29]. These approximations are, mainly speaking, a preparation of the sample being thick enough so that the measured reflectance does not change with further increasing of this parameter (avoidance of Fresnel reflection) and an averaged size of the particles being smaller than such thickness, but larger relative to the wavelength (scattering independent of the wavelength).

The first of these experimental conditions was fulfilled by using a self-supporting pressed powder rectangular mount (3 × 3 × 3 mm); in all the experiments, an amount of approximately 1 g of powder was deposited. The second requirement was fulfilled by determination of the average size particle in the powders, using a field emission Scanning Electron Microscopy (SEM), with a JEOL JSM 5600-LV microscope (V = 20 kV, at 1500×, Mitaka, Tokyo, Japan). The micrographs were analyzed with *ImageJ* software: The edge length histograms were obtained from statistical analysis of at least 200 particles. Lastly, we followed the recommendation of grinding the powders in an agate mortar for a few minutes

to avoid sample heterogeneity and regular reflection [29]: All samples were ground for 10 min before measurements.

On the other hand, the Curie temperatures for the samples LN-STm, LN + 1%NbP, LN + 2%NbP, and LN + 3%NbP were measured using differential scanning calorimetry (DSC) equipment coupled to thermogravimetry (TGA), SDT Q600 of TA instruments. The calorimeter was calibrated with respect to the copper melting point (1084 ◦C). The samples were analyzed in a wide temperature range between room temperature and 1200 ◦C, at a heating rate of 20 ◦C/min under a nitrogen atmosphere and using alumina containers. The ferroelectric-paraelectric state transition was observed around 1050–1080 ◦C. Subsequently, the samples were analyzed in four cooling cycles from 500 ◦C to 1200 ◦C at the same heating rate, 20 ◦C/min, and the process was seen to be reproducible, indicating that there was no permanent change in the volume of the pseudo-ilmenites.
