Тепловой баланс помещения с электрической кабельной системой отопления

Solvothermal oxidation of metallic gallium in monoethanolamine for 72 h at 240 °C yields a crystalline sample of γ-Ga₂O₃ (∼30 nm crystallites). While Rietveld refinement (cubic spinel structure, <i>Fd</i>3̅<i>m</i>; <i>a</i> = 8.23760(9) Å) reveals that Ga occupies two pairs of octahedral and tetrahedral sites (ideal spinel and nonspinel), it provides no information about their local distribution, which cannot be statistical owing to the short Ga–Ga contacts produced if neighboring ideal spinel and nonspinel sites are simultaneously occupied. To create an atomistic model to reconcile this situation, a 6 × 6 × 6 supercell of the crystal structure is constructed and refined against neutron total scattering data using a reverse Monte Carlo (RMC) approach. This accounts well for the local as well as long-range structure and reveals significant local distortion in the octahedral sites that resembles the structure of thermodynamically stable β-Ga₂O₃. ⁷¹Ga solid-state NMR results reveal a octahedral:tetrahedral Ga ratio that is consistent with the model obtained from RMC. Nanocrystalline samples of γ-Ga₂O₃ are produced by either a short solvothermal reaction (240 °C for 11 h in diethanolamine; ∼15 nm crystallites) or by precipitation from an ethanolic solution of gallium nitrate (∼5 nm crystallites). For these samples, the Bragg scattering profile is broadened by their smaller crystallite size, consistent with transmission electron microscopy results, and analysis of the relative Bragg peak intensities provides evidence that a greater proportion of tetrahedral versus octahedral sites are filled. In contrast, neutron total scattering shows the same average Ga–O distance with decreasing particle size, consistent with ⁷¹Ga solid-state NMR results that indicate that all samples contain the same overall proportion of octahedral:tetrahedral Ga. It is postulated that increased occupation of tetrahedral sites within the smaller crystallites is balanced by an increased proportion of octahedral surface Ga sites, owing to termination by bound solvent or hydroxide

Investigating Relationships between the Crystal Structure and ³¹P Isotropic Chemical Shifts in Calcined Aluminophosphates

Solid-state NMR spectra have historically been assigned using simple relationships between NMR parameters, e.g., the isotropic chemical shift, and aspects of the local structure of the material in question, e.g., bond angles or lengths. Density functional theory (DFT) calculations have effectively superseded these relationships in many cases, owing to the accuracy of the NMR parameters typically able to be calculated. However, the computational time required for DFT calculations may still be prohibitive, particularly for very large systems, where structure-spectrum relationships must still be used to interpret the NMR spectra. Here we show that, for calcined aluminophosphates (AlPOs), structure-spectrum relationships relying on either the mean P–O–Al angle or the mean P–O distance, both suggested in previous literature, provide a poor prediction of the ³¹P isotropic shielding, σ_iso, calculated by DFT. However, a relationship dependent on both parameters yields predicted σ_iso in excellent agreement with DFT, with a mean error of ∼1.6 ppm. The predictive ability of the relationship is not improved by introducing further parameters (many used in previous work) describing the local structure, suggesting that the two-parameter relationship is close to an optimum balance between accuracy and overparameterisation. The ability to predict accurately the outcome of DFT-level calculations will be of particular interest in cases where the actual calculations would be impractical or even impossible with current computational hardware, or where many such calculations are required quickly

Unusual Phase Behavior in the Piezoelectric Perovskite System, Li_<i>x</i>Na_1–<i>x</i>NbO₃

The system Li_<i>x</i>Na_1–<i>x</i>NbO₃ has been studied by using a combination of X-ray and neutron powder diffraction and ²³Na solid-state NMR spectroscopy. For <i>x</i> = 0.05 we confirm a single polar orthorhombic phase. For 0.08 ≤ <i>x</i> ≤ 0.20 phase mixtures of this orthorhombic phase, together with a rhombohedral phase, isostructural with the low-temperature ferroelectric polymorph of NaNbO₃, are observed. The relative fractions of these two phases are shown to be critically dependent on synthetic conditions: the rhombohedral phase is favored by higher annealing temperatures and rapid cooling. We also observe that the orthorhombic phase transforms slowly to the rhombohedral phase on standing in air at ambient temperature. For 0.25 ≤ <i>x</i> ≤ 0.90 two rhombohedral phases coexist, one Na-rich and the other Li-rich. In this region the phase behavior is independent of reaction conditions

Applications of NMR Crystallography to Problems in Biomineralization: Refinement of the Crystal Structure and ³¹P Solid-State NMR Spectral Assignment of Octacalcium Phosphate

By combining X-ray crystallography, first-principles density functional theory calculations, and solid-state nuclear magnetic resonance spectroscopy, we have refined the crystal structure of octacalcium phosphate (OCP), reassigned its ³¹P NMR spectrum, and identified an extended hydrogen-bonding network that we propose is critical to the structural stability of OCP. Analogous water networks may be related to the critical role of the hydration state in determining the mechanical properties of bone, as OCP has long been proposed as a precursor phase in bone mineral formation. The approach that we have taken in this paper is broadly applicable to the characterization of crystalline materials in general, but particularly to those incorporating hydrogen that cannot be fully characterized using diffraction techniques

Structural Study of La_1–<i>x</i>Y_<i>x</i>ScO₃, Combining Neutron Diffraction, Solid-State NMR, and First-Principles DFT Calculations

The solid solution La_1–<i>x</i>Y_<i>x</i>ScO₃ (<i>x</i> = 0, 0.2, 0.4, 0.6, 0.8, and 1) has been successfully synthesized using conventional solid-state techniques. Detailed structural characterization has been undertaken using high-resolution neutron powder diffraction and multinuclear (⁴⁵Sc, ¹³⁹La, ⁸⁹Y, and ¹⁷O) solid-state NMR and is supported by first-principles density functional theory calculations. Diffraction data indicate that a reduction in both the unit cell parameters and unit cell volume is observed with increasing <i>x</i>, and an orthorhombic perovskite structure (space group <i>Pbnm</i>) is retained across the series. ⁴⁵Sc multiple-quantum (MQ) MAS NMR spectra proved to be highly sensitive to subtle structural changes and, in particular, cation substitutions. NMR spectra of La_1–<i>x</i>Y_<i>x</i>ScO₃ exhibited significant broadening, resulting from distributions of both quadrupolar and chemical shift parameters, owing to the disordered nature of the material. In contrast to previous single-crystal studies, which reveal small deficiencies at both the lanthanide and oxygen sites, the powder samples studied herein are found to be stoichiometric

Exploiting the Chemical Shielding Anisotropy to Probe Structure and Disorder in Ceramics: ⁸⁹Y MAS NMR and First-Principles Calculations

The local structure and cation disorder in Y₂(Sn,Ti)₂O₇ pyrochlores, materials proposed for the encapsulation of lanthanide- and actinide-bearing radioactive waste, is investigated using ⁸⁹Y (<i>I</i> = 1/2) NMR spectroscopy and, in particular, measurement of the ⁸⁹Y anisotropic shielding. Although known to be a good probe of the local environment, information on the anisotropy of the shielding interaction is removed under magic angle spinning (MAS). Here, we consider the feasibility of experimental measurement of the ⁸⁹Y anisotropic shielding interaction using two-dimensional CSA-amplified PASS experiments, implemented for ⁸⁹Y for the first time. Despite the challenges associated with the study of low-γ nuclei, and those resulting from long T₁ relaxation times, the successful implementation of these experiments is demonstrated for the end member pyrochlores, Y₂Sn₂O₇ and Y₂Ti₂O₇. The accuracy and robustness of the measurement to various experimental parameters is also considered, before the approach is then applied to the disordered materials in the solid solution. The anisotropies extracted for each of the sideband manifolds are compared to those obtained using periodic first-principles calculations, and provide strong support for the assignment of the spectral resonances. The value of the span, Ω, is shown to be a sensitive probe of the next nearest neighbor (NNN) environment, i.e., the number of Sn and Ti on the six surrounding “B” (i.e., six-coordinate) sites, and also provides information on the local geometry directly, through a correlation with the average Y–O_8b distance (where 8b indicates the Wyckoff position of the oxygen)

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted