The structural stability and physical properties of CrVO₄ under compression were studied by X-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, and ab initio calculations up to 10 GPa. High-pressure X-ray diffraction and Raman measurements show that CrVO₄ undergoes a phase transition from the ambient pressure orthorhombic CrVO₄-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO₄-V phase, which is isomorphic to the wolframite structure. Such a phase transition (CrVO₄-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, is reported.  Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

We explore the potential of Tb- and Yb-doped InVO₄, InTaO₄, and InNbO₄ for applications as phosphors for light-emitting sources. Doping below 0.2% barely change the crystal structure and Raman spectrum but provide optical excitation and emission properties in the visible and near-infrared (NIR) spectral regions. From optical measurements, the energy of the first/second direct band gaps was determined to be 3.7/4.1 eV in InVO₄, 4.7/5.3 in InNbO₄, and 5.6/6.1 eV in InTaO₄. In the last two cases, these band gaps are larger than the fundamental band gap (being indirect gap materials), while for InVO₄, a direct band gap semiconductor, the fundamental band gap is at 3.7 eV. As a consequence, this material shows a strong self-activated photoluminescence centered at 2.2 eV. The other two materials have a weak self-activated signal at 2.2 and 2.9 eV. We provide an explanation for the origin of these signals taking into account the analysis of the polyhedral coordination around the pentavalent cations (V, Nb, and Ta). Finally, the characteristic green (⁵D₄ → ⁷F_J) and NIR (²F_5/2 → ²F_7/2) emissions of Tb³⁺ and Yb³⁺ have been analyzed and explained.

Found in nature or synthesized, materials present amazing properties such as superconductivity, super-hardness, lightweight, or high-energy-density, among others. All these properties can be used in our benefit to improve or develop new applications. Although, many of these properties are not noticeable in the ambient conditions of pressure and temperature. Therefore, only when the materials are exposed to extreme conditions of temperature, pressure, radiation, etc., become notable. For those reasons, it is fundamental to understand their properties and how they are affected by different parameters such as the synthesis process, morphology, doping or external parameters (e.g. pressure, temperature).

High-pressure studies have been shown to be an excellent tool for proving and study the robustness of material properties as well as for the synthesis of new materials. Changes as extreme and spectacular as converting oxygen gas into a superconducting metal or the well-known graphite to diamond conversion among others have been made under high-pressure conditions.

Among all the materials, and due to their interesting properties, in this doctoral thesis we have studied four ternary metal oxide semiconductors (InVO4, CrVO4, InNbO4 and InTaO4) and carbon nanostructure materials (single-walled carbon nanotubes (SWCNTs)) at ambient conditions as well as under high-pressure (static or dynamic compression) using different characterization techniques such as X-ray diffraction (XRD), Raman spectroscopy (RS), optical absorption, transmission electron microscopy (TEM), photoluminescence (PL) and electrical measurements.

InVO4, InNbO4 and InTaO4 are wide metal oxide semiconductors having band-gap energy of 3.62(5), 3.63(5) and 3.79(5) eV, respectively, being InVO4 a direct band-gap semiconductor and, InNbO4 and InTaO4 indirect band-gap semiconductors. These compounds undergo, under pressure, to a structural phase transition from orthorhombic, in the case of InVO4, or monoclinic, in the case of InNbO4 and InTaO4, to another monoclinic system. This structural phase transition triggers interesting phenomena due to the modification of the electronic band structure of the compounds. Phenomena observed under compression include bandgap collapse about 1-1.5 eV depending on the compound, band crossing due to the change to the local maximum on top of the valence band and colour change. Also, the electrical resistivity of the materials is affected by this change in the band structure. All these results are discussed based on our theoretical band structure calculations.

On the other hand, doping these compounds below 0.2% using Tb or Yb rare-earth elements, the crystal structure is barely affected as well as their phonon structure, but the band structure does, giving rise optical excitation and emission properties in the visible and near-infrared (NIR) spectral region. From optical reflectivity measurements, the two first direct transitions are reported at 3.7/4.2 eV in InVO4, 4.7/5.3 eV in InNbO4 and 5.6/6.1 eV in InTaO4. All the compounds present self-activated photoemission signals which are discussed in terms of the distorted polyhedral coordination around V, Nb and Ta atoms. Finally, the characteristic emission of Tb atoms in the green region (5D4→7FJ) and the Yb atoms in the NIR region (2F5/2→2F7/2) are analysed and discussed based on our theoretical calculations.

Even though, being a prototype structure of a family of compounds denoted as CrVO4-type materials, there is still scarce information on the behaviour under pressure of the CrVO4 compound. Here, it is also studied CrVO4 having an orthorhombic structure under pressure up to 10 GPa. Crystal structure, phonon band structure, optical and electrical properties are analysed showing a structural phase transition similar to that in InVO4 with an increase in the vanadium atoms coordination from 4 to 6. This phase transition triggers also a band-gap collapse of 1.1 eV, a change in the phonon structure and a sharp decrease in the resistivity of the material. All these results are discussed in terms of our theoretical calculations and comparison with its isostructural partner InVO4.

To conclude, we study the effects of the dynamic pressure of 0.5 Mbar (50 GPa) on SWCNTs which is way beyond the limit of their structural stability in quest of new forms of carbon nanostructures. Thus, no nanotubes survived to this pressure. The recovered material is composed of two types of material which are classified in a multi-layer graphene phase (MLG) with high defect concentration and multi-phase material which dominates the sample. Even the reached conditions during the shock-compression were favourable for the diamond formation, we were unable to find traces of diamond-like carbon in the very inhomogeneous sample. The crystal size of both materials has been estimated at 13 nm for disordered carbon and 30 nm for MLG phase. The dispersion of the Raman modes was also studied using several lasers and the observations were supported by TEM analysis.

Molybdates are inorganic materials with great potential in white phosphors application, being an alternative to traditional lighting sources. In this study, we report the synthesis and characterization of Mg_1-xSr_xMoO₄ (x = 0, 0.25, 0.50, 0.75, and 1) powders with white light-emitting properties. Using X-ray diffraction, the formation of the monoclinic β-MgMoO₄ phase was observed for x = 0 and the formation of the tetragonal scheelite phase of SrMoO4 was observed for x = 1. The formation of a heterostructure composed of both phases was found for compositions with x = 0.25, 0.50 and 0.75. Scanning electron microscopy images showed that the Mg_1-xSr_xMoO₄ particles exhibit a spherical morphology formed by several primary nanoparticles. Raman scattering spectroscopy enabled the accurate identification of the Raman modes for different compositions and their assignment to either the SrMoO₄ or β-MgMoO₄ modes. The bandgap energies were determined to fluctuate between 4.25 eV and 4.44 eV, being influenced by the degree of structural disorder. The photoluminescence emission spectra of the nanoparticles showed neutral- and cool-white emission with high-quality white light (CRI > 80%). The samples synthesized with x ≤ 0.50 are potential materials for the application in LED lamps (6500 K) and pure white-light sources (5500 K).

We present an experimental study of the high-pressure, high-temperature behaviour of cerium up to ~22 GPa and 820 K using angle-dispersive x-ray diffraction and external resistive heating. Studies above 820 K were prevented by chemical reactions between the samples and the diamond anvils of the pressure cells. We unambiguously measure the stability region of the orthorhombic oC4 phase and find it reaches its apex at 7.1 GPa and 650 K. We locate the α-cF4–oC4–tI2 triple point at 6.1 GPa and 640 K, 1 GPa below the location of the apex of the oC4 phase, and 1–2 GPa lower than previously reported. We find the α-cF4 → tI2 phase boundary to have a positive gradient of 280 K (GPa)⁻¹, less steep than the 670 K (GPa)⁻¹ reported previously, and find the oC4 → tI2 phase boundary to lie at higher temperatures than previously found. We also find variations as large as 2–3 GPa in the transition pressures at which the oC4 → tI2 transition takes place at a given temperature, the reasons for which remain unclear. Finally, we find no evidence that the α-cF4 → tI2 is not second order at all temperatures up to 820 K.

We have studied the electronic properties at ambient pressure and under high pressure of InVO4, InNbO4, and InTaO4 powders, three candidate materials for hydrogen production by means of photocatalytic water splitting using solar energy. A combination of optical absorption and resistivity measurements and band structure calculations have allowed us to determine that these materials are wide band-gap semiconductors with a band-gap energy of 3.62(5), 3.63(5), and 3.79(5) eV for InVO4, InNbO4, and InTaO4, respectively. The last two compounds are indirect band-gap materials, and InVO4 is a direct band-gap material. The pressure dependence of the band-gap energy and the electrical resistivity have been determined too. In the three compounds, the band gap opens under compression until reaching a critical pressure, where a phase transition occurs. The structural transition triggers a band-gap collapse larger than 1.2 eV in the three materials, being the abrupt decrease in the band-gap energy related to an increase in the pentavalent cation coordination number. The phase transitions also cause changes in the electrical resistivity, which can be correlated with changes induced by pressure in the band structure. An explanation to the reported results is provided based upon ab initio calculations. The conclusions attained are of significance for technological applications of the studied oxides.

Resistively heated diamond-anvil cells have been used together with synchrotron x-ray diffraction to investigate the phase diagram of calcium up to 50 GPa and 800 K. The phase boundaries between the Ca-I (fcc), Ca-II (bcc), and Ca-III (simple cubic, sc) phases have been determined at these pressure-temperature conditions, and the ambient temperature equation of state has been generated. The equation of state parameters at ambient temperature have been determined from the experimental compression curve of the observed phases by using third-order Birch-Murnaghan and Vinet equations. A thermal equation of state was also determined for Ca-I and Ca-II by combining the room-temperature Birch-Murnaghan equation of state with a Berman-type thermal expansion model.

Single-walled carbon nanotubes (SWCNTs) have been dynamically (shock) compressed to 0.5 Mbar, above the limit of their structural integrity. Two distinct types of material are identified by high-resolution transmission electron microscopy (HRTEM) and multi-wavelength Raman spectroscopy in the sample recovered after shock: multi-layer graphene (MLG) and a two-phase material composed of nano-clustered graphene and amorphous carbon whereas no diamond-like carbon or carbon nano-onions are found. Peak decomposition of the Raman spectra was used to estimate the coherent scatterers (clusters) size in MLG at 36 nm from the D- to G-band intensity ratio dependence on the photon excitation energy. Botella et al. (article no. 1700315) propose the peak fitting model for decomposition of the Raman spectra of highly disordered carbon material containing graphene nano-clusters and stress the importance of accounting for heptagonal- and pentagonal-ring defects in graphene layers for the analysis of such spectra. The cover image shows HRTEM images and the correspondent Raman spectra of the two types of material along with peak decomposition of the two-phase material with the peaks assigned to heptagons (a) and pentagons (b). Particulars of the SWCNTs transformation to other structural forms of carbon at high pressure/temperature are discussed

Single-walled carbon nanotubes (SWCNTs) have been dynamically (shock) compressed to 0.5 Mbar, above the limit of their structural integrity. Two distinct types of material are identified by high-resolution transmission electron microscopy (HRTEM) and multi-wavelength Raman spectroscopy in the sample recovered after shock: multi-layer graphene (MLG) and a two-phase material composed of nano-clustered graphene and amorphous carbon whereas no diamond-like carbon or carbon nano-onions are found. Peak decomposition of the Raman spectra was used to estimate the coherent scatterers (clusters) size in MLG at 36 nm from the D- to G-band intensity ratio dependence on the photon excitation energy. Botella et al. (article no. 1700315) propose the peak fitting model for decomposition of the Raman spectra of highly disordered carbon material containing graphene nano-clusters and stress the importance of accounting for heptagonal- and pentagonal-ring defects in graphene layers for the analysis of such spectra. The cover image shows HRTEM images and the correspondent Raman spectra of the two types of material along with peak decomposition of the two-phase material with the peaks assigned to heptagons (a) and pentagons (b). Particulars of the SWCNTs transformation to other structural forms of carbon at high pressure/temperature are discussed