High-Resolution 17O Solid-State NMR as a Unique Probe for Investigating Oxalate Binding Modes in Materials: The Case Study of Calcium Oxalate Biominerals.

Oxalate ligands are found in many classes of materials, including energy storage materials and biominerals. Determining their local environments at the atomic scale is thus paramount to establishing the structure and properties of numerous phases. Here, we show that high-resolution 17O solid-state NMR is a valuable asset for investigating the structure of crystalline oxalate systems. First, an efficient 17O-enrichment procedure of oxalate ligands is demonstrated using mechanochemistry. Then, 17O-enriched oxalates were used for the synthesis of the biologically relevant calcium oxalate monohydrate (COM) phase, enabling the analysis of its structure and heat-induced phase transitions by high-resolution 17O NMR. Studies of the low-temperature COM form (LT-COM), using magnetic fields from 9.4 to 35.2 T, as well as 13C-17O MQ/D-RINEPT and 17O{1H} MQ/REDOR experiments, enabled the 8 inequivalent oxygen sites of the oxalates to be resolved, and tentatively assigned. The structural changes upon heat treatment of COM were also followed by high-resolution 17O NMR, providing new insight into the structures of the high-temperature form (HT-COM) and anhydrous calcium oxalate α-phase (α-COA), including the presence of structural disorder in the latter case. Overall, this work highlights the ease associated with 17O-enrichment of oxalate oxygens, and how it enables high-resolution solid-state NMR, for "NMR crystallography" investigations.

17O solid state NMR as a valuable tool for deciphering reaction mechanisms in mechanochemistry: the case study on the 17O-enrichment of hydrated Ca-pyrophosphate biominerals.

The possibility of enriching in 17O the water molecules within hydrated biominerals belonging to the Ca-pyrophosphate family was investigated, using liquid assisted grinding (LAG) in the presence of 17O-labelled water. Two phases with different hydration levels, namely triclinic calcium pyrophosphate dihydrate (Ca2P2O7·2H2O, denoted t-CPPD) and monoclinic calcium pyrophosphate tetrahydrate (Ca2P2O7·4H2O, denoted m-CPPT ß) were enriched in 17O using a "post-enrichment" strategy, in which the non-labelled precursors were ground under gentle milling conditions in the presence of stoichiometric quantities of 17O-enriched water (introduced here in very small volumes ∼10 µL). Using high-resolution 17O solid-state NMR (ssNMR) analyses at multiple magnetic fields, and dynamic nuclear polarisation (DNP)-enhanced 17O NMR, it was possible to show that the labelled water molecules are mainly located at the core of the crystal structures, but that they can enter the lattice in different ways, namely by dissolution/recrystallisation or by diffusion. Overall, this work sheds light on the importance of high-resolution 17O NMR to help decipher the different roles that water can play as a liquid-assisted grinding agent and as a reagent for 17O-isotopic enrichment.

Molten Salts-Driven Discovery of a Polar Mixed-Anion 3D Framework at the Nanoscale: Zn4 Si2 O7 Cl2 , Charge Transport and Photoelectrocatalytic Water Splitting.

Mixed-anion compounds widen the chemical space of attainable materials compared to single anionic compounds, but the exploration of their structural diversity is limited by common synthetic paths. Especially, oxychlorides rely mainly on layered structures, which suffer from low stability during photo(electro)catalytic processes. Herein we report a strategy to design a new polar 3D tetrahedral framework with composition Zn4 Si2 O7 Cl2 . We use a molten salt medium to enable low temperature crystallization of nanowires of this new compound, by relying on tetrahedral building units present in the melt to build the connectivity of the oxychloride. These units are combined with silicon-based connectors from a non-oxidic Zintl phase to enable precise tuning of the oxygen content. This structure brings high chemical and thermal stability, as well as strongly anisotropic hole mobility along the polar axis. These features, associated with the ability to adjust the transport properties by doping, enable to tune water splitting properties for photoelectrocatalytic H2 evolution and water oxidation. This work then paves the way to a new family of mixed-anion solids.

Monitoring of CaCO3 Nanoscale Structuration through Real-Time Liquid Phase Transmission Electron Microscopy and Hyperpolarized NMR.

Calcium carbonate (CaCO3) is one of the most significant biominerals in nature. Living organisms are able to control its biomineralization by means of an organic matrix to tailor a myriad of hybrid functional materials. The soluble organic components are often proteins rich in acidic amino-acids such as l-aspartic acid. While several studies have demonstrated the influence of amino acids on the crystallization of calcium carbonate, nanoscopic insight of their impact on CaCO3 mineralization, in particular at the early stages, is still lacking. Herein, we implement liquid phase-transmission electron microscopy (LP-TEM) in order to visualize in real-time and at the nanoscale the prenucleation stages of CaCO3 formation. We observe that l-aspartic acid favors the formation of individual and aggregated prenucleation clusters which are found stable for several minutes before the transformation into amorphous nanoparticles. Combination with hyperpolarized solid state nuclear magnetic resonance (DNP NMR) and density functional theory (DFT) calculations allow shedding light on the underlying mechanism at the prenucleation stage. The promoting nature of l-aspartic acid with respect to prenucleation clusters is explained by specific interactions with both Ca2+ and carbonates and the stabilization of the Ca2+-CO32-/HCO3- ion pairs favoring the formation and stabilization of the CaCO3 transient precursors. The study of prenucleation stages of mineral formation by the combination of in situ LP-TEM, advanced analytical techniques (including hyperpolarized solid-state NMR), and numerical modeling allows the real-time monitoring of prenucleation species formation and evolution and the comprehension of their relative stability.

Leucine on Silica: A Combined Experimental and Modeling Study of a System Relevant for Origins of Life, and the Role of Water Coadsorption.

Leucine on silica constitutes an interesting system from the point of view of origins of life studies since leucine coadsorbed on SiO2 together with glutamic acid can give rise to rather long linear polypeptides upon activation. It is also an ideal system to test methods of molecular characterization of biomolecules deposited on mineral surfaces since it combines a small-scale model of peptides and proteins, which are among the most important components of biodevices, with one of the most widely used inorganic materials. We have deposited l-leucine on a high surface fumed silica in the submonolayer range and characterized it by a multipronged approach including macroscopic information (thermogravimetry, X-ray diffraction), in situ spectroscopic methods (IR, multinuclear solid-state NMR including single-pulse and CP-MAS, 2-D HETCOR), and molecular modeling by density functional theory (DFT), including calculation of NMR parameters. Specific information can be obtained on the adsorption and interaction mechanism. Leucine is rather strongly adsorbed without any covalent bonds, through the formation of a specific lattice of H-bonds that often involve coadsorbed water molecules. Its state is indeed strongly dependent on the drying procedure: insufficient drying results in liquid-like surroundings for the leucine functional groups, while vacuum drying only retains a limited number of waters (of the order of 5 per leucine molecule). The most stable models have zwitterionic leucine interacting directly with surface silanols through their ammonium group, while the carboxylate interacts through bridging waters. Experimental NMR chemical shifts are satisfactorily predicted for these models, and leucine can be viewed as a probe for specific groups of surface sites known as silanol nests.

Electron Precise Sodium Carbaboride Nanocrystals from Molten Salts: Single Sources to Boron Carbides.

Boron-rich solids exhibit specific crystal structures and unique properties, which are only very scarcely addressed in nanoparticles. In this work, we address the original inorganic structural chemistry and reactivity of boron-rich nanoparticles, by reporting the first occurrence of sodium carbaboride nanocrystals based on the NaB5C crystal structure. To design these sub-10 nm nano-objects, we use liquid-phase synthesis in molten salts at 900 °C. By combining a set of characterization tools including powder X-ray powder diffraction, transmission electron microscopy, solid-state nuclear magnetic resonance coupled to DFT modeling, and X-ray photoelectron spectroscopy, we demonstrate that these nanocrystals deviate from the ideal stoichiometry reported for the bulk compound. We suggest that the carbon and sodium contents compensate each other to ensure that the octahedral cluster-based framework is stabilized by fulfilling an electron counting rule. These nanocrystals encompass substituted octahedral covalent structural building units not reported in the related bulk compound. They then shed new light on the ability of nanoparticles to host wide solid solution ranges in covalent solids and then to yield new solids. We finally show that these nanocrystals are efficient single sources of boron and carbon to form a nanostructured boron carbide, thus paving the way to new nanostructured materials.

A 43 Ca nuclear magnetic resonance perspective on octacalcium phosphate and its hybrid derivatives.

43 Ca nuclear magnetic resonance (NMR) spectroscopy has been extensively applied to the detailed study of octacalcium phosphate (OCP), Ca8 (HPO4 )2 (PO4 )4 .5H2 O, and hybrid derivatives involving intercalated metabolic acids (viz., citrate, succinate, formate, and adipate). Such phases are of importance in the development of a better understanding of bone structure. High-resolution 43 Ca magic angle spinning (MAS) experiments, including double-rotation (DOR) 43 Ca NMR, as well as 43 Ca{1 H} rotational echo DOR (REDOR) and 31 P{43 Ca} REAPDOR NMR spectra, were recorded on a 43 Ca-labeled OCP phase at very high magnetic field (20 T), and complemented by ab initio calculations of NMR parameters using the Gauge-Including Projector Augmented Wave-density functional theory (GIPAW-DFT) method. This enabled a partial assignment of the eight inequivalent Ca2+ sites of OCP. Natural-abundance 43 Ca MAS NMR spectra were then recorded for the hybrid organic-inorganic derivatives, revealing changes in the 43 Ca lineshape. In the case of the citrate derivative, these could be interpreted on the basis of computational models of the structure. Overall, this study highlights the advantages of combining high-resolution 43 Ca NMR experiments and computational modeling for studying complex hybrid biomaterials.

17 O solid-state NMR at ultrahigh magnetic field of 35.2 T: Resolution of inequivalent oxygen sites in different phases of MOF MIL-53(Al).

MIL-53(Al) is a member of the most extensively studied metal-organic framework (MOF) families owing to its "flexible" framework and superior stability. 17 O solid-state NMR (SSNMR) spectroscopy is an ideal site-specific characterization tool as it probes local oxygen environments. Because oxygen local structure is often altered during phase change, 17 O SSNMR can be used to follow phase transitions. However, 17 O is a challenging nucleus to study via SSNMR due to its low sensitivity and resolution arising from the very low natural abundance of 17 O isotope and its quadrupolar nature. In this work, we describe that by using 17 O isotopic enrichment and performing 17 O SSNMR experiments at an ultrahigh magnetic field of 35.2 T, all chemically and crystallographically inequivalent oxygen sites in two representative MIL-53(Al) (as-made and water adsorbed) phases can be completely resolved. The number of signals in each phase is consistent with that predicted from the space group refined from powder X-ray diffraction data. The 17 O 1D magic-angle spinning (MAS) and 2D triple-quantum MAS (3QMAS) spectra at 35.2 T furnish fine information about the host-guest interactions and the structural changes associated with phase transition. The ability to completely resolve multiple chemically and crystallographically inequivalent oxygen sites in MOFs at very high magnetic field, as illustrated in this work, significantly enhances the potential for using the NMR crystallography approach to determine crystal structures of new MOFs and verify the structures of existing MOFs obtained from refining powder X-ray diffraction data.

Looking into the dynamics of molecular crystals of ibuprofen and terephthalic acid using 17 O and 2 H nuclear magnetic resonance analyses.

Oxygen-17 and deuterium are two quadrupolar nuclei that are of interest for studying the structure and dynamics of materials by solid-state nuclear magnetic resonance (NMR). Here, 17 O and 2 H NMR analyses of crystalline ibuprofen and terephthalic acid are reported. First, improved 17 O-labelling protocols of these molecules are described using mechanochemistry. Then, dynamics occurring around the carboxylic groups of ibuprofen are studied considering variable temperature 17 O and 2 H NMR data, as well as computational modelling (including molecular dynamics simulations). More specifically, motions related to the concerted double proton jump and the 180° flip of the H-bonded (-COOH)2 unit in the crystal structure were looked into, and it was found that the merging of the C=O and C-OH 17 O resonances at high temperatures cannot be explained by the sole presence of one of these motions. Lastly, preliminary experiments were performed with a 2 H-17 O diplexer connected to the probe. Such configurations can allow, among others, 2 H and 17 O NMR spectra to be recorded at different temperatures without needing to tune or to change probe configurations. Overall, this work offers a few leads which could be of use in future studies of other materials using 17 O and 2 H NMR.

Higher Magnetic Fields, Finer MOF Structural Information: 17O Solid-State NMR at 35.2 T.

The spectroscopic study of oxygen, a vital element in materials, physical, and life sciences, is of tremendous fundamental and practical importance. 17O solid-state NMR (SSNMR) spectroscopy has evolved into an ideal site-specific characterization tool, furnishing valuable information on the local geometric and bonding environments about chemically distinct and, in some favorable cases, crystallographically inequivalent oxygen sites. However, 17O is a challenging nucleus to study via SSNMR, as it suffers from low sensitivity and resolution, owing to the quadrupolar interaction and low 17O natural abundance. Herein, we report a significant advance in 17O SSNMR spectroscopy. 17O isotopic enrichment and the use of an ultrahigh 35.2 T magnetic field have unlocked the identification of many inequivalent carboxylate oxygen sites in the as-made and activated phases of the metal-organic framework (MOF) α-Mg3(HCOO)6. The subtle 17O spectral differences between the as-made and activated phases yield detailed information about host-guest interactions, including insight into nonconventional O···H-C hydrogen bonding. Such weak interactions often play key roles in the applications of MOFs, such as gas adsorption and biomedicine, and are usually difficult to study via other characterization routes. The power of performing 17O SSNMR experiments at an ultrahigh magnetic field of 35.2 T for MOF characterization is further demonstrated by examining activation of the MIL-53(Al) MOF. The sensitivity and resolution enhanced at 35.2 T allows partially and fully activated MIL-53(Al) to be unambiguously distinguished and also permits several oxygen environments in the partially activated phase to be tentatively identified. This demonstration of the very high resolution of 17O SSNMR recorded at the highest magnetic field accessible to chemists to date illustrates how a broad variety of scientists can now study oxygen-containing materials and obtain previously inaccessible fine structural information.

Unveiling the Structure and Reactivity of Fatty-Acid Based (Nano)materials Thanks to Efficient and Scalable 17O and 18O-Isotopic Labeling Schemes.

Fatty acids are ubiquitous in biological systems and widely used in materials science, including for the formulation of drugs and the surface-functionalization of nanoparticles. However, important questions regarding the structure and reactivity of these molecules are still to be elucidated, including their mode of binding to certain metal cations or materials surfaces. In this context, we have developed novel, efficient, user-friendly, and cost-effective synthetic protocols based on ball-milling, for the 17O and 18O isotopic labeling of two key fatty acids which are widely used in (nano)materials science, namely stearic and oleic acid. Labeled molecules were analyzed by 1H and 13C solution NMR, IR spectroscopy, and mass spectrometry (ESI-TOF and LC-MS), as well as 17O solid state NMR (for the 17O labeled species). In both cases, the labeling procedures were scaled-up to produce up to gram quantities of 17O- or 18O-enriched molecules in just half-a-day, with very good synthetic yields (all ≥84%) and enrichment levels (up to an average of 46% per carboxylic oxygen). The 17O-labeled oleic acid was then used for the synthesis of a metal soap (Zn-oleate) and the surface-functionalization of ZnO nanoparticles (NPs), which were characterized for the first time by high-resolution 17O NMR (at 14.1 and 35.2 T). This allowed very detailed insight into (i) the coordination mode of the oleate ligand in Zn-oleate to be achieved (including information on Zn···O distances) and (ii) the mode of attachment of oleic-acid at the surface of ZnO (including novel information on its photoreactivity upon UV-irradiation). Overall, this work demonstrates the high interest of these fatty acid-enrichment protocols for understanding the structure and reactivity of a variety of functional (nano)materials systems using high resolution analyses like 17O NMR.

Enhanced sieving from exfoliated MoS2 membranes via covalent functionalization.

Nanolaminate membranes made of two-dimensional materials such as graphene oxide are promising candidates for molecular sieving via size-limited diffusion in the two-dimensional capillaries, but high hydrophilicity makes these membranes unstable in water. Here, we report a nanolaminate membrane based on covalently functionalized molybdenum disulfide (MoS2) nanosheets. The functionalized MoS2 membranes demonstrate >90% and ~87% rejection for micropollutants and NaCl, respectively, when operating under reverse osmotic conditions. The sieving performance and water flux of the functionalized MoS2 membranes are attributed both to control of the capillary widths of the nanolaminates and to control of the surface chemistry of the nanosheets. We identify small hydrophobic functional groups, such as the methyl group, as the most promising for water purification. Methyl- functionalized nanosheets show high water permeation rates as confirmed by our molecular dynamic simulations, while maintaining high NaCl rejection. Control of the surface chemistry and the interlayer spacing therefore offers opportunities to tune the selectivity of the membranes while enhancing their stability.

Synthesis in Molten Salts and Characterization of Li6B18(Li2O)x Nanoparticles.

Lithium borides have been synthesized exclusively through classical solid-state chemistry processes that lead to bulk materials. Indeed, due to the lack of reactivity of the solid boron precursors usually employed and to the high covalent connectivity in such solids, high temperatures and long reaction times are necessary to obtain lithium borides. These conditions result in extensive crystal growth. Here we present the synthesis of nanoparticles of a lithium boride bearing tunnel-like cavities templated by neutral Li2O species, which have been reported to be labile. To reach this goal, a liquid-phase synthesis in inorganic molten salts has been developed. The Li6B18(Li2O)x nanoparticles have been characterized by scanning and transmission electronic microscopy (SEM and TEM), X-ray diffraction (XRD), and Raman spectroscopy. We provide an in-depth structural characterization by using 1H, 7Li, and 11B solid-state nuclear magnetic resonance (NMR) coupled with DFT modeling to provide the first assignment of 7Li and 11B solid-state NMR signals in lithium borides. We then assess the nanoparticle morphology oriented along the direction of the cavities. This feature shows similarities with structurally related hexagonal tungsten bronzes and could therefore affect the electrochemical and ion exchange properties.

Recent directions in the solid-state NMR study of synthetic and natural calcium phosphates.

Materials containing a calcium phosphate component have been the subject of much interest to NMR spectroscopists, especially in view of understanding the structure and properties of mineralized tissues like bone and teeth, and of developing synthetic biomaterials for bone regeneration. Here, we present a selection of recent developments in their structural characterization using advanced solid state NMR experiments, highlighting the level of insight which can now be accessed.

In-Situ Synthesis and Characterization of Nanocomposites in the Si-Ti-N and Si-Ti-C Systems.

The pyrolysis (1000 °C) of a liquid poly(vinylmethyl-co-methyl)silazane modified by tetrakis(dimethylamido)titanium in flowing ammonia, nitrogen and argon followed by the annealing (1000-1800 °C) of as-pyrolyzed ceramic powders have been investigated in detail. We first provide a comprehensive mechanistic study of the polymer-to-ceramic conversion based on TG experiments coupled with in-situ mass spectrometry and ex-situ solid-state NMR and FTIR spectroscopies of both the chemically modified polymer and the pyrolysis intermediates. The pyrolysis leads to X-ray amorphous materials with chemical bonding and ceramic yields controlled by the nature of the atmosphere. Then, the structural evolution of the amorphous network of ammonia-, nitrogen- and argon-treated ceramics has been studied above 1000 °C under nitrogen and argon by X-ray diffraction and electron microscopy. HRTEM images coupled with XRD confirm the formation of nanocomposites after annealing at 1400 °C. Their unique nanostructural feature appears to be the result of both the molecular origin of the materials and the nature of the atmosphere used during pyrolysis. Samples are composed of an amorphous Si-based ceramic matrix in which TiNxCy nanocrystals (x + y = 1) are homogeneously formed "in situ" in the matrix during the process and evolve toward fully crystallized compounds as TiN/Si3N4, TiNxCy (x + y = 1)/SiC and TiC/SiC nanocomposites after annealing to 1800 °C as a function of the atmosphere.

High-Pressure Melting Curve of Zintl Sodium Silicide Na4Si4 by In Situ Electrical Measurements.

The inorganic chemistry of the Na-Si system at high pressure is fascinating, with a large number of interesting compounds accessible in the industrial pressure scale, below 10 GPa. In particular, Na4Si4 is stable in this whole pressure range and thus plays an important role in understanding the thermodynamics and kinetics underlying materials synthesis at high pressures and high temperatures. In the present work, the melting curve of the Zintl compound Na4Si4 made of Na+ and Si44- tetrahedral cluster ions is studied at high pressures up to 5 GPa, by using in situ electrical measurements. During melting, the insulating Na4Si4 solid transforms into an ionic conductive liquid that can be probed through the conductance of the whole high-pressure cell, i.e., the system constituted of the sample, the heater, and the high-pressure assembly. Na4Si4 melts congruently in the studied pressure range, and its melting point increases with pressure with a positive slope dTm/dp of 20(4) K/GPa.

Nature of Hexagonal Silicon Forming via High-Pressure Synthesis: Nanostructured Hexagonal 4H Polytype.

Hexagonal Si allotropes are expected to enhance light absorption in the visible range as compared to common cubic Si with diamond structure. Therefore, synthesis of these materials is crucial for the development of Si-based optoelectronics. In this work, we combine in situ high-pressure high-temperature synthesis and vacuum heating to obtain hexagonal Si. High pressure is one of the most promising routes to stabilize these allotropes. It allows one to obtain large-volume nanostructured ingots by a sequence of direct solid-solid transformations, ensuring high-purity samples for detailed characterization. Thanks to our synthesis approach, we provide the first evidence of a polycrystalline bulk sample of hexagonal Si. Exhaustive structural analysis, combining fine-powder X-ray and electron diffraction, afforded resolution of the crystal structure. We demonstrate that hexagonal Si obtained by high-pressure synthesis correspond to Si-4H polytype (ABCB stacking) in contrast with Si-2H (AB stacking) proposed previously. This result agrees with prior calculations that predicted a higher stability of the 4H form over 2H form. Further physical characterization, combining experimental data and ab initio calculations, have shown a good agreement with the established structure. Strong photoluminescence emission was observed in the visible region for which we foresee optimistic perspectives for the use of this material in Si-based photovoltaics.

Synthesis of Carbon-Nitrogen-Phosphorous Materials with an Unprecedented High Amount of Phosphorous toward an Efficient Fire-Retardant Material.

Phosphorus incorporation into carbon can greatly modify its chemical, electronic, and thermal stability properties. To date this has been limited to low levels of phosphorus. Now a simple, large-scale synthesis of carbon-nitrogen-phosphorus (CNP) materials is reported with tunable elemental composition, leading to excellent thermal stability to oxidation and fire-retardant properties. The synthesis consists of using monomers that are liquid at high temperatures as the reaction precursors. The molten-state stage leads to good monomer miscibility and enhanced reactivity at high temperatures and formation of CNP materials with up to 32 wt % phosphorus incorporation. The CNP composition and fire-retardant properties can be tuned by modifying the starting monomers ratio and the final calcination temperature. The CNP materials demonstrate great resistance to oxidation and excellent fire-retardant properties, with up to 90 % of the materials preserved upon heating to 800 °C in air.

Nanocomposites through the Chemistry of Single-Source Precursors: Understanding the Role of Chemistry behind the Design of Monolith-Type Nanostructured Titanium Nitride/Silicon Nitride.

Monolith-type titanium nitride/silicon nitride nanocomposites, denoted as TiN/Si3 N4 , have been prepared by a reaction of polysilazanes with a titanium amide precursor, warm pressing of the resultant polytitanosilazanes, and subsequent pyrolysis of the green bodies at 1400 °C. Initially, a series of polytitanosilazanes was synthesized and the role of the chemistry behind their synthesis was studied in detail by using solid-state NMR spectroscopy, elemental analysis, and molecular-weight measurements. The intimate relationship between the chemistry and the processability of these precursors is discussed. Polytitanosilazanes display the appropriate requirements for facile processing in solution and as a melt, but they must be treated with liquid ammonia to be adapted for solid-state processing, that is, warm-pressing, to design dense and mechanically stable structures after pyrolysis. We provide a comprehensive mechanistic study of the nanocomposite conversion based on solid-state NMR spectroscopy coupled with thermogravimetric experiments. HRTEM images coupled with XRD and Raman spectroscopy confirmed the unique nanostructural features of the nanocomposites, which appear to be a result of the molecular origin of the materials. The as-obtained samples are composed of an amorphous Si3 N4 matrix, in which TiN nanocrystals are homogeneously formed in situ in the matrix during the process. The hardness and Young moduli were measured and are discussed.

Molecular-Level Processing of Si-(B)-C Materials with Tailored Nano/Microstructures.

The design of Si-(B)-C materials is investigated, with detailed insight into the precursor chemistry and processing, the precursor-to-ceramic transformation, and the ceramic microstructural evolution at high temperatures. In the early stage of the process, the reaction between allylhydridopolycarbosilane (AHPCS) and borane dimethyl sulfide is achieved. This is investigated in detail through solid-state NMR and FTIR spectroscopy and elemental analyses for Si/B ratios ranging from 200 to 30. Boron-based bridges linking AHPCS monomeric fragments act as crosslinking units, extending the processability range of AHPCS and suppressing the distillation of oligomeric fragments during the low-temperature pyrolysis regime. Polymers with low boron contents display appropriate requirements for facile processing in solution, leading to the design of monoliths with hierarchical porosity, significant pore volume, and high specific surface area after pyrolysis. Polymers with high boron contents are more appropriate for the preparation of dense ceramics through direct solid shaping and pyrolysis. We provide a comprehensive study of the thermal decomposition mechanisms, and a subsequent detailed study of the high-temperature behavior of the ceramics produced at 1000 °C. The nanostructure and microstructure of the final SiC-based ceramics are intimately linked to the boron content of the polymers. B4 C/C/SiC nanocomposites can be obtained from the polymer with the highest boron content.