Ytterbium and erbium doped RbTi1-xMxOPO4 (M = Nb or Ta) crystals.New laser and nonlinear bifunctional materials.L'objectiu d'aquesta tesi va ser millorar la qualitat dels cristalls de RbTi1-xNbxOPO4 dopatsamb Yb3+, obtinguts mitjançant la tècnica Top Seeded Solution Growth-Slow Cooling (TSSG-SC), itambé trobar un altre codopand el qual incrementés la concentració de Yb3+ en els cristalls deRTP, però sense reduir la resposta no lineal dels cristalls de RTP pur. Aquest ió codopandhauria de presentar les mateixes característiques que el Nb5+. Un altre objectiu d'aquesta tesi vaser l'obtenció d'una manera efectiva de fluorescència del Er3+, al voltant de 1.5 &#956;m, la qualdonarà lloc a l'obtenció de radiació làser a aquesta longitud d'ona. Per aconseguir aquest últimobjectiu, es van dopar cristalls de Nb:RTP amb la parella de ions actius Er3+/Yb3+, actuant elYb3+ per sensibilitzar el Er3+.Per aconseguir el primer objectiu proposat, millorar la qualitat i la mida dels cristalls deNb:RTP dopats amb Yb3+, es van optimitzar algunes condicions de creixement, tal com la rampade refredament aplicada durant el procés de creixement, el perfil tèrmic axial de la solució i laqualitat dels gèrmens emprats en les experiències de creixement. Amb aquestes modificacionses van obtenir cristalls de mida i qualitat suficients per tal de preparar mostres, en una de lesquals s'ha obtingut radiació làser al voltant de 1 &#956;m. Amb les mostres obtingudes també s'hapogut determinar la longitud d'ona fonamental per l'acord de fase no crític en la qual es donariala generació de segon harmònic de tipus II en el pla xy, determinant que seria possible obtenirradiació làser autodoblada en la regió espectral del verd en aquest pla a q = 90º i j &#8776; 45º.El segon objectiu, trobar un ió codopand, amb el qual es pogués obtenir aproximadamentla mateixa concentració de Yb3+ que l'obtinguda en cristalls de Nb:RTP, es va assolir utilitzant elTa5+ com a ió codopand. En cristalls volúmics de Ta:RTP es va aconseguir una concentració deYb3+ de 1.6×1020 ions/cm3 i l'eficiència de generació de segon harmònic obtinguda va ser un 80% de la del KTiOPO4 pur. Amb aquests resultats, podem pensar que en els cristalls de Ta:RTPdopats amb Yb3+ també es podria aconseguir radiació làser autodoblada en la regió espectral delverd.Pel que fa a l'augment de l'eficiència de la fluorescència del Er3+, mitjançant el codopatgede cristalls de Nb:RTP amb la parella d'ions actius Er3+/Yb3+, al voltant de 1.5 &#956;m, s'haaconseguit, fins i tot en mostres amb poca concentració de Er3+ (<1020ions/cm3).A més, cal afegir que també s'ha realitzat un estudi comparatiu entre la conductivitatiònica del RTP pur, del RTP crescut en solució de tungstè, Nb:RTP i Nb:RTP dopat amb Yb3+.Totes les mesures s'han realitzat en plaques polides perpendiculars a la direcció cristal logàficac, i dels resultats obtinguts s'han pogut extreure algunes conclusions. Que no hi ha diferènciesentre les conductivitats iòniques de cristall de RTP purs i els crescuts en flux de tungstè, peròen mostres de Nb:RTP la conductivitat augmenta considerablement, tenint una conductivitat del'ordre de l'obtinguda pel KTP (10-7-10-5 S/cm), i que les mostres de Nb:RTP dopades amb Yb3+presenten una conductivitat iònica lleugerament superior (10-8 S/cm) a la que presenten lesmostres de RTP (10-9 S/cm).Ytterbium and erbium doped RbTi1-xMxOPO4 (M = Nb or Ta) crystals.New laser and nonlinear bifunctional materials.The aim of this thesis was not only to enhance the quality and size of Yb dopedRbTi1-xNbxOPO4 (Nb:RTP) crystals obtained by the Top Seeded Solution Growth-Slow Cooling(TSSG-SC) technique, but also to find another codopand which increases the Yb3+ concentrationin RTP without reducing drastically the nonlinear optical response of pure RTP; so a codopandwhich shows the same characteristics of niobium. Another objective of this thesis was to obtainefficient fluorescence of Er3+ at around 1.5 &#956;m which would lead to laser action at thiswavelength, by codoping Nb:RTP with the pair Er3+/Yb3+, acting Yb3+ as a sensitizing ion of Er3+.To achieve the first objective proposed, enhance the quality and size of Yb:Nb:RTP, weoptimize some growth conditions, such as the cooling ramp applied during the crystal growthprocess, the thermal axial gradient of the solution and the quality of the crystals seeds used inthe crystal growths. With those modifications we obtain crystals with size and quality enough toprepare samples from which laser action at around 1 &#956;m has been obtained, for the first time ina crystal of the KTP family, and to determine the fundamental wavelength for non-critical type -II SHG in the xy plane, determining that in this plane, at q = 90º and j &#8776; 45º it will be possibleto obtain self-frequency doubling laser action in the green spectral range. The second objective,find a codopand with which Yb3+ concentration in RTP was in the range of the one obtained inNb:RTP, it was achieved by using tantalum. Firstly, it was determined how the RTPcrystallization region changes with the Ta5+ and Yb3+ introduction, and which was the optimalsolution composition to achieve a higher Yb3+ concentration. With this optimal solutioncomposition bulk crystal of Yb doped RbTi1-xTaxOPO4, grown by TSSG-SC technique, with a Yb3+concentration of 1.6×1020 ions/cm3 were obtained and the SHG efficiency of powdered sampleswas an 80 % of that of pure KTP. So, as the Yb3+ concentration obtained in Ta:RTP is of thesame order of the one obtained in Nb:RTP, the SHG efficiency is not drastically decreased, andthe refractive indexes measured at 632.8 nm do not differ significantly from the ones ofYb:RbTi1-xNbxOPO4, we can assume that in Yb:RbTi1-xNbxOPO4 crystals could also be possible toobtain self-frequency laser action in the green spectral range. About the enhancement of the IRfluorescence of Er3+ by codoping Nb:RTP with Yb3+, bulk crystals of codoped Er3+/Yb3+ Nb:RTPwere grown from solutions containing always the same Yb2O3 mol % and by modifying the Er2O3mol % added in the initial solution. The spectroscopic characterization of these crystals showsthe Er3+ fluorescence at around 1.5 &#956;m by pumping between 900 nm and 980 nm, even in thecrystals containing low Er3+ concentration (<1020ions/cm3).Moreover to these results, a comparative study between the ionic conductivity of pureRTP grown in its self flux, RTP grown in tungsten flux, Nb:RTP and Yb:Nb:RTP had beenperformed, in polished plates perpendiculars to c crystallographic direction, to see how itevolves in the mentioned crystals. The results obtained showed that there are no differencesbetween the ionic conductivity in RTP samples, grown in its self flux or in tungsten flux, but inNb:RTP the ionic conductivity increases considerably. Nb:RTP shows an ionic conductivity of theorder of the KTP one (10-7-10-5 S/cm), but in Yb:Nb:RTP the conductivity is only a little bithigher (10-8 S/cm) of the RTP one (10-9 S/cm).

Growth and Characterisation of RbTiOPO4: (Nb,Ln). A New frequency Doubling Crystal.Joan Josep Carvajal MartíFísica I Cristal·lografia de MaterialsDepartament de Química Física i InorgànicaUniversitat Rovira i VirgiliResumDes que Schawlow i Townes varen publicar que el principi del M.A.S.E.R. es podia estendre a la part òptica de l'espectre electromagnètic, i un any i mig més tard Mainman demostrava el primer làser, la importància d'aquest, tant per la societat, s'ha anat incrementant. Avui, els làser formen part de la nostra vida quotidiana.Actualment, l'interès en desenvolupar nous làser per l'emmagatzematge de dades i altres aplicacions s'ha incrementat gràcies a la recerca en làsers de semiconductors de banda ampla, generació d'harmònics per phase-matching en cristalls d'òptica no-lineal, quasi-phase-matching en cristalls, fibres òptiques i altres guies d'ona, i làsers d'up-conversion en cristalls i fibres. Els làsers d'estat sòlid compactes són útils en una amplia gamma d'aplicacions, com l'emmagatzematge de dades en alta densitat, projecció d'imatges en color, impressió làser, medicina, substitució dels làsers d'Ar, biofluorescència, comunicacions submarines, litografia estereogràfica, teràpia fotodinàmica.... Aquests nous làsers es poden aconseguir per processos d'up-conversion, que emeten a una longitud d'ona més curta que la de bombeig. Una alternativa interessant és la conversió de freqüència per processos d'òptica no-lineal. Els cristalls autodobladors de freqüència són cristalls d'òptica no-lineal amb posicions estructurals ocupades per ions actius làser, preferiblement Yb3+, que poden combinar l'emissió làser al voltant d'1 um amb la generació de segon harmònic de la matriu per produir emissió verda per autodoblatge de freqüència. L'objectiu principal d'aquesta Tesi Doctoral ha estat l'obtenció de cristalls de la família del KTiOPO4, coneguts per les seves excel·lents propietats d'òptica no-lineal, dopats amb ions lantànids (Ln). Hem mostrat que és poden obtenir cristalls de RbTiOPO4: (Nb,Ln) de bona qualitat òptica per la tècnica top-seeded solution growth amb un refredament lent de la solució, de mida útil per posteriors caracteritzacions i aplicacions, i amb una concentració de Ln similar a l'obtinguda en altres matrius làser i un codopant, que afavoreix l'entrada d'ions lantànids en la matriu a l'hora que fa de compensador de càrrega, com és el niobi (Nb5+). El refinament de l'estructura ha demostrat que tant el Nb com els lantànids substitueixen únicament al Ti en la xarxa cristal·lina. Mentre que el Nb5+ només substitueix al Ti en la posició Ti (1), degut a un efecte electrostàtic més que no pas estèric, els ions lantànids substitueixen als àtoms de Ti en els octàedres Ti(1)O6 i Ti(2)O6 amb la mateixa probabilitat. Un altre aspecte a destacar és l'habilitat del material per tal d'autocompensar-se elèctricament de la substitució pentavalent del Nb5+ a través de la creació de vacants de rubidi. L'evolució d'aquesta estructura amb la temperatura mostra com el canvi de fase de l'estructura ferroelèctrica depèn del contingut en Nb i que la substitució de Ti4+ pel parell (Nb5+-Rb+) estabilitza la fase d'alta temperatura d'aquests materials. La caracterització òptica de la matriu cristal·lina ha mostrat que l'increment de la birefringència de cristall possibilita l'increment de l'interval de doblatge de longituds d'ona que es pot efectuar amb aquests cristalls. L'àmplia finestra òptica juntament amb l'eficiència de conversió a segon harmònic mostren que aquesta és una bona matriu per hostatjar ions lantànids. Aquestes excel·lents propietats es mantenen fins i tot quan els ions lantànids es troben al cristall. Finalment, la caracterització espectrocòpia del ions actius ha mostrat que es pot obtenir una emissió eficient per part dels dos ions. Er3+b3+ la banda d'emissió ampla i l'elevada vida mitja obtingudes obren la possibilitat d'utilitzar aquests cristalls en aplicacions sintonitzables i de subpicosegons. Totes aquestes observacions ens duen a concloure que amb aquests cristalls es pot obtenir un nou material autodoblador de freqüència.Paraules clau: Òptica No-Lineal, Generació de Segon Harmònic, Autodoblatge de freqüència, Top-Seeded Solution Growth, KTiOPO4, RbTiOPO4, Difacció de raig X, Difracció de neutrons de pols.Growth and characterisation of RbTiOPO4: (Nb, Ln). A New Self-frequencyDoubling Crystal.Joan Josep Carvajal MartíFísica I Cristal·lografia de MaterialsDepartament de Química Física i InorgànicaUniversitat Rovira i VirgiliAbstract Since Schawlow and tornes reported that it should be possible to extend the maser principle to the optical frequency part of the spectrum and only one and a half years later. Mainman demonstrated the first laser, the importance of lasers to both science and society has increased dramatically. Today lasers are part of everyday life. Actually, the interest in developing compact blue and green laser sources for data storage and display applications has increased due to promising investigations into wide-gap semiconductor diode lasers, harmonic generation by phase matching in non-linear crystals, quasi-phase-matching in bulk, optical fibres and other waveguides, and up conversion lasers in crystals and fibers. Compact all-solid-state laser sources are needed for a variety of applications such as colour projection, high-density optical data storage, laser printing, medicine, Argon-laser replacement, biofluorescence, underwater communications, stereo lithography, photodynamic therapy. A class of these sources is achieved with up-conversion lasers emitting at a shorter wavelength than the pump source. An alternative to up-conversion lasing is frequency conversion by non-linear optical processes such as frequency doubling and sum-frequency mixing. Self-frequency doubling crystals are non-linear optical crystals with suitable sites for laser-active ions, preferably Yb3+, which can combine the 1 um laser emission of the active ions with the second harmonic generation properties of the host to produce green-laser radiation by self-frequency doubling. The aim of this Thesis was to investigate how to obtain crystals of the KTiOPO4 family, which are known for their excellent non-linear optical properties, doped with lanthanide (Ln) ions. This Thesis shows that high-optical quality RbTiOPO4(Nb,Ln) single crystals can be obtained by the top-seeded solution growth technique together with a slow-cooling of the solution, with large volumes useable for later characterisations and technical applications, and with a similar Ln concentration to that of other laser matrices. We introduced a codopant that enhances the concentration of Ln in the crystals at the same time that compensate the electrical charge of the framework. The refinement of the structure demonstrates that Nb and Ln ions only go to the Ti positions. Then, while Nb only substitutes Ti in the Ti(1) crystallographic position due to an electrostatic effect that an steric one, the lanthanide ion substitute Ti in both Ti(1)O6 octahedra with the same probability. Another interesting aspect to notice is the ability of the material to self-compensate the excess of electrical charge introduced by the pentavalent substitution of Nb5+ by the creation of Rubidium vacancies. The evolution of this structure with temperature showed that the transition from the ferroelectric phase to the paraelectric phase depended on the Nb content in the crystals, and that substituting Ti4+ by the (Nb5+-Rb+) pair stabilises the cubic high-temperature phase of these crystals. The optical characterisation of the host showed that the increase of birefringence in the crystals containing Nb causes the limit fundamental wavelength, at which second harmonic generation can be obtained, to shift to lower wavelengths. The large transparency window of the material and the conversion efficiency to second harmonic generation showed that this crystal is a good matrix to host lanthanide ions. These excellent properties are maintained even when the lanthanide ions is hosted in the crystalline matrix. Finally, the spectroscopic characterisation of the active ions showed that efficient emission can be obtained for Er3+ at 1.5 and 1.0 um, respectively. In the case of Yb3+ the broad emission band and the large lifetime obtained open up the possibility to use these crystals in tunable and subpicosecond applications. All these observations seems to converge in the conclusion that with these crystals a new self-frequency doubling material can be obtained.Keywords: Non-linear Optics, Second Harmonic Generation, Self-frequency Doubling, Top-Seeded Solution Growth, KTiOPO4, RbTiOPO4, X-ray Diffraction, Powder Neutron Diffraction.

En los últimos años, las capas delgadas ópticamente activas han adquirido una gran importancia, debido a sus aplicaciones en dispositivos ópticos integrados y dispositivos láser. Un láser de capa delgada requiere capas con una elevada concentración de ión activo y, que a su vez, tengan una elevada sección eficaz de absorción y emisión. Este tipo de láseres presentan como ventajas una mejor refrigeración axial del medio evitándose así problemas como el efecto lente. Los cristales monoclínicos de KRE(WO4)2 con RE=Y, Lu, son materiales utilizados como matrices láser de estado sólido debido a que permiten la substitución parcial o total de uno de los iones de su estructura, por otros iones activos en la emisión láser. Además, presentan una elevada estabilidad química. El objetivo de esta tesis ha sido la obtención de capas de Yb:KYW/KYW y Yb:KLuW/KLuW, de elevada calidad para la posterior realización de experimentos láser. El proceso de crecimiento ha sido optimizado con el principal objetivo de conseguir la máxima concentración de ión activo en las capas epitaxiales. Éstas han sido caracterizadas estructural y espectroscópicamente, mostrando resultados prometedores para la emisión láser. La última parte de esta tesis se ha dedicado al estudio de los experimentos láser, en los que hemos conseguido obtener radiación láser con una longitud de onda alrededor de los 1030 nm con una elevada pendiente de eficiencia, la cual en algunos casos ha excedido las publicadas previamente para los correspondientes monocristales dopados con iterbio.Keywords: Top Seeded Solution Growth, Liquid Phase Epitaxy, KRE(WO4)2, Materiales láser. / In the past few years, optically active thin layers have attracted much attention due to the possibility of using it in the integrated optics as well as thin disk laser technology. The thin disk laser concept need layers hundreds of microns thick highly doped with active ions and high absorption and emission cross section. The lasers based on thin films have the advantage of high beam quality with high efficiency, making possible to obtain high power with low thermal lensing.The crystals of the monoclinic tungstates, such as KRE(WO4)2 with RE=Y, Lu, are attractive materials to be used as solid state host doped with lanthanide ions as ytterbium due to the possibility of obtaining highly doped active media.The aim of this thesis is to investigate how to obtain thin layers of Yb:KYW/KYW and Yb:KLuW/KLuW, with quality enough for laser experiments. The growth process has been optimized in order to obtain crystalline layers with high ytterbium concentration. Structural and spectroscopic properties of these layers have been studied, suggesting that laser emission can be achieved.The last part of thesis is dedicated to study the laser experiments. We have achieved laser emission around 1030 nm with high slope efficiency, which in some cases exceeded the reported for the ytterbium doped bulk crystalsKeywords: Top Seeded Solution Growth, Liquid Phase Epitaxy, KRE(WO4)2, Laser materials

Multi-component crystallisation is investigated as a route to controlling crystalline forms of selected materials that possess pharmaceutical properties. This includes investigating the use of co-crystallisation methodology to selectively crystallise metastable polymorphs and solvated forms of these materials. This differs from the conventional use of co-crystallisation, as the aim of this aspect of the investigation is not to obtain a molecular complex of the two components, but instead for them to crystallise independently, while one component perturbs the solution environment to direct the crystallisation of the second component towards a different, often metastable, polymorph (or solvate). This co-crystallisation methodology is used as a route to crystallising new or elusive polymorphs (or solvates) of the active pharmaceutical ingredients paracetamol, piroxicam, gallic acid monohydrate and piracetam. It is also demonstrated that the use of this method can lead to crystal forms with otherwise unobtainable structural features. Co-crystallisation is also investigated as a route to controlling the ionisation state of piroxicam in the formation of molecular complexes. Molecular complexes were formed with a number of mono-substituted benzoic acids as well as with nitrogen-heterocycles and strong acids. In the molecular complexes formed, piroxicam was found to adopt the non-ionised, zwitterionic, anionic or cationic form, depending on the co-former used. Attempts are made to rationalise the occurrence of each ionisation state by consideration of the relative pKa values of piroxicam and the co-formers. The hydrogen bonded supramolecular synthons in these molecular complexes are also investigated. Co-crystallisation is also used as a route to obtaining molecular complexes of paracetamol and its derivative, 4-acetamidobenzoic acid, with nitrogen-heterocycles as co-formers. Molecular complexes of the two, with similar co-formers, are compared in terms of their hydrogen bonded supramolecular synthons. Despite having otherwise similar structural features, the phenolic hydroxyl group in paracetamol and carboxylic acid group in 4-acetamidobenzoic acid result in the formation of very different synthons and in some cases different component ratios. The susceptibility of 4-acetamidobenzoic acid to deprotonation is found to play a major role in the differences observed. Molecular complexes of paracetamol with co-formers containing multiple carboxylic acid groups are also investigated, with a view towards further crystal engineering approaches for molecular complexes of paracetamol. Piracetam complexes with carboxylic acids are investigated in a similar manner. The potential for transfer of a range of these multi-component crystallisations into a non-evaporative environment, with a view to implementing continuous crystallisation approaches, is also investigated. This transfer is found to be challenging for the systems investigated.

Synthesis and characterization of optical nanocrystals and nanostructures. An approach to transparent laser nanoceramicsMontserrat Galceran MestresEls materials nanocristal·lins són materials policristal·lins amb una mida de partícula en el rang dels nanòmetres. Presenten diferents propietats físiques, òptiques, electròniques, químiques i estructurals perquè tenen una proporció més elevada d'àtoms a la superfície que els materials en volum. La síntesi de nanocristalls és la primera etapa per fabricar materials nanoceràmics per a aplicacions làser. Són atractius per a làsers en estat sòlid perquè ofereixen avantatges com ara: baix cost, fabricació senzilla i bones propietats mecàniques i òptiques.Aquesta tesi tracta la síntesi i caracterització de nous nanomaterials òptics (KRE(WO4)2, RE2O3, KTiOPO4) dopats amb ions actius com les terres rares (RE), erbi, iterbi, holmi i europi com a primera etapa per sintetitzar noves ceràmiques làser Ho:Lu2O3. Per avaluar les qualitats d'aquests materials com a possibles làser en estat sòlid o materials d'òptica nolinear, s'ha dut a terme una caracterització estructural, òptica i espectroscòpica aquests nanomaterials (nanocristalls, nanoestructures i nanoceràmiques).Synthesis and characterization of optical nanocrystals and nanostructures. An approach to transparent laser nanoceramicsMontserrat Galceran MestresNanocrystalline materials are polycrystalline materials with a particle size in the nanometer range that have different physical, optical, electronic, chemical and structural properties because they have a larger fraction of surface atoms than larger-scale materials. The synthesis of nanocrystals is the first step in manufacturing nanoceramic laser materials; they are very attractive to solid state laser due to several advantages such as low cost, easy fabrication and good mechanical and optical properties.This thesis deals with the synthesis and characterization of new optical nanomaterials (KRE(WO4)2, RE2O3, KTiOPO4) doped with active rare earth (RE) ions as erbium, ytterbium, holmium and europium as a first step to synthesize a new Ho:Lu2O3 laser ceramics. An extensively structural, optical and spectroscopic characterization of the nanomaterials (nanocrystals, nanostructures and nanoceramics) was performed in order to evaluate these materials as a promising solid state laser or nonlinear optical materials.

La sincristalización orgánica constituye un campo de investigación relativamente nuevo, en el que entran en juego interacciones muy diversas, y variaciones en las propiedades de los materiales a menudo muy sensibles a las condiciones de temperatura, presión y sobre todo a la composición. Por lo tanto, cualquier estudio sobre el tema debe comprender muchas líneas de enfoque: control de pureza de los materiales de partida, síntesis de las aleaciones, estudio de los equilibrios de las fases que se originan, conocer las estructuras cristalinas y cómo se modifican en el proceso de substitución molecular..., e incluso podemos llegar a una posible aplicación de estos materiales en el mundo industrial.Este dominio de estudio tan amplio se está llevando a cabo simultánea y conjuntamente en diferentes laboratorios que constituyen el grupo REALM (Red Europea sobre las Aleaciones Moleculares), cuyo objetivo es el estudio y comprensión de la formación, los límites de estabilidad, la previsión y la valorización de las aleaciones moleculares.El trabajo que hemos realizado en la presente tesis se enmarca en este objetivo general, pero concretándolo a un estudio de la sincristalización dentro de los derivados paradisubstituídos del benceno, con cloro, bromo y iodo como substituyentes. En esta memoria presentamos los resultados de diez sistemas binarios, formados entre cinco paradisubstituídos bencénicos, todos ellos con grupo espacial y número de moléculas idéntico. Para cada uno de dichos sistemas binarios presentamos, en primer lugar, el diagrama de fases experimental; a continuación realizamos un estudio cristalográfico de las aleaciones moleculares, a través de la variación de los parámetros cristalinos con la composición, y de un tensor de deformación composicional. Finalmente, se realiza un análisis global de la sincristalización en esta serie de derivados paradisubstituídos del benceno, que se inicia con la evaluación cristalográfica de las aleaciones moleculares, a partir de los "parámetros cristalinos de exceso". A continuación, realizamos una síntesis de los diversos criterios de previsión de la sincristalización, tal y como han sido establecidos por nuestro equipo. A pesar de que su aplicación en esta serie no explica completamente los grados de miscibilidad observados, la correlación del grado de isomorfismo cristalino, con los parámetros termodinámicos de exceso calculados para estas aleaciones moleculares, muestra un comportamiento general de esta serie frente a la sincristalización. Es este un punto original e importante del presente trabajo, que abre nuevas perspectivas de futuro en la comprensión del fenómeno de la sincristalización entre materiales orgánicos, a partir de la correlación entre las aportaciones cristalográficas y termodinámicas.

In the practice of solid state chemistry, structural phase transitions are fairly common events. Nonetheless, their understanding, in terms of both: A rationalization of the observed changes in symmetry pattern and; An understanding of the mechanisms allowing for a particular transformation, are outstanding problems. The thermodynamic classification of phase transitions distinguishes between first and second order transitions, on the basis of the discontinuous behavior of quantities related to first or second derivatives of the free energy, respectively. Small atomic displacements are typically associated with second order phase transitions, and latent heat changes amount to a few calories per gram only. Additionally, the symmetries of the phases surrounding the transition are typically in the relation of a group and a subgroup. Reconstructive phase transitions, on the contrary, involve breaking of (large) parts of the bond scaffolding of the initial structure, and exhibit drastic changes at the transition point, with large latent heat and hysteresis effects. The corresponding atomic displacements can be in the order of the lattice parameters, and no group-subgroup is found, between the symmetry of the phases. These type of transitions have generally a strong first-order character. Landau theory accounts for continuous, second-order phase transitions. As a phenomenological theory, it does not establish the existence of a phase transition, which remains an experimental fact. It only bridges microscopic characteristics, like space-group symmetries and structural changes, or phonon softening effects, with measurable macroscopic quantities. Therein, distortions are carried by an order parameter, which fully specifies the form of the analytical variational free energy. The latter is continuous and derivable with respect to temperature, pressure and atomic displacement, at the transition point. First order, non-continuous phase transitions are still within the scope of Landau theory in the mentioned special case of the existence of a group-to-(isotropic) subgroup relationship. In the majority of cases, however, and for the most interesting phase transitions (for basic and applied research), such a relationship is missing, making the choice of an order parameter less straightforward. Most of the allotropic transformations of the elements, many intermetallic systems, and numerous insulating systems belongs to this class. This class also includes most interesting and fundamental electronic effects, like metallization in perovskites or spinel oxides for example. This very simple fact of a missing symmetry condition has helped reinforcing the opinion of first-order phase transitions being a world apart, and possibly contributed to discouraging a firm theory to develop, able to account for their transformation mechanisms and the change of physical properties across phase transition. The thermodynamic distinction between first and second order phase transitions is too narrow, as, in case of first order phase transitions, it embraces both weakly discontinuous transition and reconstructive ones, where bonds are being strongly modified. Especially, a mean to qualify the distance between two structures (geometric, with respect to symmetry, a.s.o.), is missing. Clearly, a group-subgroup relationship may, and typically does imply shortest shifting distances, as a tiny atomic displacement can already do for a symmetry lowering. Naively, missing such a relation means no constraints, and apparently no means to conclude at a connection of two structures in general, let alone a full mechanistic analysis. First order phase transitions proceed by nucleation and subsequent growth of the new phase from the initial one. Different from (second-order) continuous phase transitions, they do imply coexistence of the transforming motifs. The discontinuity in some order parameter between the two phases is driven by lowering of the free energy as the new phase forms. However, close to the transition, the original phase remains metastable, and a fluctuation is needed to cause the formation of the new phase to set in. Such a process responds to thermal changes, and depending on the height of the nucleation barrier, its rate may be slower or faster. In the former case, large deviations from equilibrium may be required to achieve transformation to the stable phase, which means that large hysteresis effects will be observed in the course of transformation. The intent of this work consists in giving a face to the intermediate configurations appearing in first order phase transitions, in solid-solid reconstructive processes. Apart of a mechanistic elucidation, consisting in answering the question “Which atomic displacements bring structural motif A into structural motif B ?”, another purpose of this work is a rather pedagogical one, that is, showing that first-order phase transitions can be understood in detail, not only in principle but in fact. The core of the examples illustrated in this work is concerned with phase transformations where pressure represents the thermodynamic controlling parameter. Pressure is extensively used in chemical synthesis, as a mean to achieve novel properties, optical or mechanical just to mention a few. Additionally, reports on novel high-pressure polymorphs are regularly appearing. In this sense, pressure is a relevant parameter for approaching fundamental questions in solid state chemistry.

A machining process is one of the most common techniques used to remove material in order to create a final product. Most studies on mechanisms of cutting are performed under the assumption that the studied material is isotropic, homogeneous and continuous. One important feature of material- its anisotropyis linked to its crystallographic nature, which is usually ignored in machining studies. A crystallographic orientation of a workpiece material exerts a great influence on the chip-formation mechanism. Thus, there is a need for developing fundamental understanding of material's behaviour and material removal processes. While the effect of crystallographic orientation on cutting-force variation is extensively reported in the literature, the development of the single crystal machining models is somewhat limited.

A z-cut congruent lithium niobate crystal (LiNbO3) has been used in this thesis, as a platform for the surface second harmonic generation (SHG) studies and for the designs of polygonal microcavities for nonlinear applications. Reflection second harmonic generation (RSHG) experiments were performed on LiNbO3 to reveal the interfacial layer symmetry as the crystal is rotated around the z axis. RSHG was also used, unsuccessfully as a non-destructive tool to map the domain-inverted area in the poled LiNbO3 crystals. But nevertheless, the polarity of the direction of the y-axis of the crystal was determined from RSHG data and the data shows that this direction also inverts, during domain inversion. RSHG was used unsuccessfully to monitor the relaxation of the internal field within the domain inverted area of the poled LiNbO3. A general operational principle of optical microcavities was discussed, in which a detailed theory governing the operational modes of a resonating hexagonal microcavity, made from bulk LiNbO3 crystal was reviewed for nonlinear device applications. A model for a total internal reflection (TIR) technique for the QPM method in a double-resonating hexagonal mi- crocavity was formulated. The TIR-QPM model was based on finding a suitable hexagonal dimension in which, both the fundamental and SHG signal resonate simultaneously while at the same time allowing QPM to occur via TIR. The TIR-QPM model and the FDTD simulation were used to demonstrate the potential capability of the double-resonating hexagonal microcavity for efficient SHG. The model to achieve a nonlinear microcavity by periodically poling ring/disk resonator Ti:LiNbO3 ridge waveguide was introduced.

The aim of this thesis is to investigate the influence of ultraviolet laser light on single crystal lithium niobate surfaces with a focus on useful functionalisation. The investigations split naturally into three areas: effects on hydrophilicity by low intensity laser irradiation, effects on ferroelectric domain inversion by focussed laser irradiation, and modelling of the heating and subsequent lithium diffusion during focussed laser irradiation. It was seen that irradiation of ultraviolet laser light with a photon energy greater than the bad gap of lithium niobate resulted in an increase to the hydrophilicity, or wettability, of the surface. The magnitude of the change was seen to be dependent upon both the conditions of laser intensity and exposure time and upon the environment in which exposures took place with a greater change seen with greater exposure time and/or intensity and with a greater environmental humidity. Under vacuum no change to the hydrophilicity was seen. It was shown that spatial structuring of the surface wettability could be achieved at the sub-micron level. Irradiation of focused continuous wave ultraviolet laser light was seen to have two contrasting effects upon ferroelectric domain inversion. It was first seen that illumination of focussed laser light of high enough intensity could directly invert the ferroelectric polarity in the exposed region. The effect was characterised by chemical etching in hydrofluoric acid and by piezoresponse force microscopy and was seen to be most effective on the -z face of the crystal. An explanation was proposed whereby photo-excited charges travel within a pyro-electric field, creating a space-charge field which locally inverts the spontaneous polarisation. The anisotropy in behaviour between the +z and -z faces was explained by the different mobilities of electrons and holes. It was also seen that if the crystal was electrically poled after illumination of the +z face, domain inversion was inhibited in the illuminated region. The region of inhibited domain inversion was seen to be greater than the region of direct poling and created domains with high quality surfaces. This was demonstrated to be a highly effective method for topographical surface microstructuring when followed by chemical etching. The heating of lithium niobate by focussed continuous wave ultraviolet laser light was modelled. It was found that the temperature distributions created were highly localised to the beam spot and were independent of beam scan speeds for practically achievable speeds. The temperature dependence of the thermal diffusivity was included and seen to greatly increase the gradients of the temperature distributions at high temperatures. The diffusion of lithium ions due to the steep temperature gradients and, hence, ionic diffusivity gradient was modelled and it was found that lithium ions will diffuse from the surface, into the bulk of the crystal. This will leave a high refractive index surface layer through the dependence upon lithium concentration. The depth of the modelled lithium concentration profiles after the passage of the beam were found not to be sufficient to cause optical waveguiding.