¹²⁵Te NMR Probes of Tellurium Oxide Crystals: Shielding-Structure Correlations

The local environments around tellurium atoms in a series of tellurium oxide crystals were probed by ¹²⁵Te solid-state NMR spectroscopy. Crystals with distinct TeO_<i>n</i> units (<i>n</i> from 3 to 6), including Na₂TeO₃, α-TeO₂ and γ-TeO₂, Te₂O­(PO₄)₂, K₃LaTe₂O₉, BaZnTe₂O₇, and CsYTe₃O₈ were studied. The latter four were synthesized through a solid-state process. X-ray diffraction was used to confirm the successful syntheses. The ¹²⁵Te chemical shift was found to exhibit a strong linear correlation with the Te coordination number. The ¹²⁵Te chemical-shift components (δ₁₁, δ₂₂, and δ₃₃) of the TeO₄ units were further correlated to the O–Te–O-bond angles. With the aid of ¹²⁵Te NMR, it is likely that these relations can be used to estimate the coordination states of Te atoms in unknown Te crystals and glasses

Relating ¹³⁹La Quadrupolar Coupling Constants to Polyhedral Distortion in Crystalline Structures

A broad series of crystalline lanthanum oxide-based materials has been investigated through high-field ¹³⁹La solid state nuclear magnetic resonance (ssNMR) spectroscopy and ab initio density functional theory (DFT) calculations. The ¹³⁹La NMR spectra of LaBGeO₅, LaBSiO₅, LaBO₃, LaPO₄·1.8H₂O, La₂(SO₄)₃·9H₂O, and La₂(CO₃)₃·8H₂O are reported for the first time. Both newly reported and literature values of ¹³⁹La quadrupolar coupling constants (<i>C</i>_Q) are related to various quantitative expressions of polyhedral distortion, including sphericity (Σ) and ellipsoid span (ϵ). The compounds were separated into two groups based upon their polyhedral distortion behavior: compounds with the general formula LaMO₃, where M is a trivalent cation; compounds with different general formulas. The ¹³⁹La <i>C</i>_Q of the LaMO₃ family was found to correlate best with ϵ. The ¹³⁹La <i>C</i>_Q of non-LaMO₃ compounds correlates adequately to ϵ but is better described by Σ. The ¹³⁹La isotropic chemical shift (δ_iso^CS) of the non-LaMO₃ compounds is negatively correlated with the lanthanum coordination number; there is insufficient data from the LaMO₃ compounds to draw conclusions relating to chemical shift. DFT calculations of NMR parameters prove to be a sensitive probe of the quality of input geometry, with predicted parameters agreeing with experiment except in cases where the crystal structure is suspect

Self-Assembled Gyroidal Mesoporous Polymer-Derived High Temperature Ceramic Monoliths

Polymer-derived ceramics (PDCs) have enabled the development of nonoxide ceramic coatings and fibers with exceptional thermo-mechanical stability. Here, we report the self-assembly based synthesis of gyroidal (space group Q²³⁰, <i>Ia</i>3̅<i>d</i>) mesoporous silicon oxynitride ceramic monoliths by pyrolysis of blends of commercially available preceramic polysilazane with a structure-directing triblock terpolymer up to temperatures of 1000 °C. Monoliths had pore diameters of 9.4 ± 1.1 nm and surface area of 160 m²/g. The three-dimensionally (3D) ordered periodic pore structure of the as-made hybrids acts to relieve stresses by allowing the escape of gases formed during ceramization. This process in turn enables the retention of smooth monoliths during ceramization under ammonia, a process that both adds nitrogen to the material and removes carbon pyrolysis products. The monoliths are appealing for high-temperature applications such as catalyst supports and microelectromechanical system (MEMS) devices including gas and pressure sensors, as well as strong, stiff, and creep-resistant scaffolds for ordered interpenetrating phase composites

Zero Thermal Expansion in ZrMgMo₃O₁₂: NMR Crystallography Reveals Origins of Thermoelastic Properties

The coefficient of thermal expansion of ZrMgMo₃O₁₂ has been measured and was found to be extremely close to zero over a wide temperature range including room temperature (αl = (1.6 ± 0.2) × 10^–7 K^–1 from 25 to 450 °C by X-ray diffraction (XRD)). ZrMgMo₃O₁₂ belongs to the family of AMgM₃O₁₂ materials, for which coefficients of thermal expansion have previously been reported to range from low-positive to low-negative. However, the low thermal expansion property had not previously been explained because atomic position information was not available for any members of this family of materials. We determined the structure of ZrMgMo₃O₁₂ by nuclear magnetic resonance (NMR) crystallography, using ⁹¹Zr, ²⁵Mg, ⁹⁵Mo, and ¹⁷O magic angle spinning (MAS) and ¹⁷O multiple quantum MAS (MQMAS) NMR in conjunction with XRD and density functional theory calculations. The resulting structure was of sufficient detail that the observed zero thermal expansion could be explained using quantitative measures of the properties of the coordination polyhedra. We also found that ZrMgMo₃O₁₂ shows significant ionic conductivity, a property that is also related to its structure