Pseudocubic Crystal Structure and Phase Transition in Doped Ye’elimite

Sodalites are tridimensional alumino-silicate materials containing cages where loosely bonded anions are located. Ye’elimite, Ca₄[Al₆O₁₂]­SO₄, is outstanding as an aluminate sodalite with a flexible framework accepting several type of dopants with important structural consequences. Moreover, ye’elimite is also important from an applied perspective as it is the most relevant phase in calcium sulfoaluminate cements. The crystal structure of stoichiometric ye’elimite has recently been unraveled, but the structure of dopant-containing ye’elimite, which is present in cements, is not well studied. Here, we report the pseudocubic crystal structure of doped ye’elimite, Ca_3.8Na_0.2Al_5.6­Fe_0.2Si_0.2O₁₂SO₄, from high-resolution synchrotron powder diffraction data. The powder pattern is indexed with a cubic cell, and a structural model is reported based on the <i>I</i>4̅3<i>m</i> space group. However, this compound displays diffraction peak narrowing on heating. Furthermore, some high-angle split peaks become a single peak on heating, and a phase transition is measured at 525 °C. Therefore, it is concluded that the crystal structure at room temperature has a lower symmetry, although it can be described as cubic. The structural study at 800 °C suggests a truly cubic structure, and we speculate that this phase transition, on heating, is likely related to the dynamical disordering of the sulfate anions. Finally it is concluded that the high temperature cubic state was not quenchable to ambient, even when the tested chemical substituents are introduced into the structure

In Situ Bragg Coherent Diffraction Imaging Study of a Cement Phase Microcrystal during Hydration

Results of Bragg coherent diffraction imaging (BCDI) confirm that ion migration and consumption occur during hydration of calcium monoaluminate (CA). The chemical phase transformation promotes the hydration process and the formation of new hydrates. There is a potential for the formation of hydrates near where the active ions accumulate. BCDI has been used to study the in situ hydration process of CA over a 3 day period. The evolution of three-dimensional (3D) Bragg diffraction electron density, the “Bragg density”, and strain fields present on the nanoscale within the crystal was measured and visualized. Initial Bragg densities and strains in CA crystal derived from sintering evolve into various degrees during hydration. The variation of Bragg density within the crystal is attributed to the change of the degree of crystal ordering, which could occur through ion transfer during hydration. The observed strain, coming from the interfacial mismatch effect between high Bragg density and low Bragg density parts in the crystal, remained throughout the experiment. The first Bragg density change during the hydration process is due to a big loss of Bragg density as seen in the image amplitude but not its phase. This work provides new evidence supporting the through-solution reaction mechanism of CA

Hydration Reactions and Mechanical Strength Developments of Iron-Rich Sulfobelite Eco-cements

Belite calcium sulfoaluminate (BCSA) cements are low-CO₂ building materials. However, their hydration behavior and its effect on mechanical properties have still to be clarified. Here, we report a full multitechnique study of the hydration behavior up to 120 days of nonactivated and activated BCSA laboratory-prepared clinkers, with β- or α_H-belite as main phase, respectively. The effects of the amount of gypsum added were also studied. The hydration and crystallization processes are reported and discussed in detail. Finally, shrinkage/expansion data are also given. The optimum amount of gypsum was close to 10 wt %. Our study has demonstrated that β-belite reacts at a higher pace than α′_H-belite, irrespective of the gypsum content. The hydration mechanism of belite determines the development of the mechanical strengths. These are much higher for activated BCSA cement, ∼65 MPa at 120 days, against ∼20 MPa for nonactivated BCSA cement, with the latter having larger amounts of stratlingite

Chemistry and Mass Density of Aluminum Hydroxide Gel in Eco-Cements by Ptychographic X‑ray Computed Tomography

Eco-cements are a desirable alternative to ordinary Portland cements because of their lower CO₂ footprints. Some ye’elimite-based eco-cements are attracting a lot of interest. Understanding the reasons for the mechanical performances requires the characterization of features such as mass density of the hydrated component phases, including the amorphous gel, on the submicrometer scale, which is challenging. Here we use ptychographic X-ray computed tomography to provide 3D mass density and attenuation coefficient distributions of eco-cement pastes with an isotropic resolution close to 100 nm allowing to distinguish between component phases with very similar contrast. In combination with laboratory techniques such as the Rietveld method, ²⁷Al MAS NMR, and electron microscopies, we report compositions and densities of key components. The ettringite and gel volume distributions have been mapped out in the segmented tomograms. Moreover, we discriminate between an aluminum hydroxide gel and calcium aluminum monosulfate, which have close electron density values. Specifically, the composition and mass density of two aluminum hydroxide gel agglomerates have been determined: (CaO)_0.04Al­(OH)₃·2.3H₂O with 1.48(3) g·cm^–3 and (CaO)_0.12Al­(OH)₃ with 2.05(3) g·cm^–3, which was a long-standing challenge

Structure, Atomistic Simulations, and Phase Transition of Stoichiometric Yeelimite

Yeelimite, Ca₄[Al₆O₁₂]­SO₄, is outstanding as an aluminate sodalite, being the framework of these type of materials flexible and dependent on ion sizes and anion ordering/disordering. On the other hand, yeelimite is also important from an applied perspective as it is the most important phase in calcium sulfoaluminate cements. However, its crystal structure is not well studied. Here, we characterize the room temperature crystal structure of stoichiometric yeelimite through joint Rietveld refinement using neutron and X-ray powder diffraction data coupled with chemical soft-constraints. Our structural study shows that yeelimite has a lower symmetry than that of the previously reported tetragonal system, which we establish to likely be the acentric orthorhombic space group <i>Pcc</i>2, with a √2a × √2a × a superstructure based on the cubic sodalite structure. Final unit cell values were <i>a</i> = 13.0356(7) Å, <i>b</i> = 13.0350(7) Å, and <i>c</i> = 9.1677(2) Å. We determine several structures using density functional theory calculations, with the lowest energy structure being <i>Pcc</i>2 in agreement with our experimental result. Yeelimite undergoes a reversible phase transition to a higher-symmetry phase which has been characterized to occur at 470 °C by thermodiffractometry. The higher-symmetry phase is likely cubic or pseudocubic possessing an incommensurate superstructure, as suggested by our theoretical calculations which show a phase transition from an orthorhombic to a tetragonal structure. Our theoretical study also predicts a pressure-induced phase transition to a cubic structure of space group <i>I</i><u>4</u>3<i>m</i>. Finally, we show that our reported crystal structure of yeelimite enables better mineralogical phase analysis of commercial calcium sulfoaluminate cements, as shown by R_F values for this phase, 6.9% and 4.8% for the previously published orthorhombic structure and for the one reported in this study, respectively

High Proton Conductivity in a Flexible, Cross-Linked, Ultramicroporous Magnesium Tetraphosphonate Hybrid Framework

Multifunctional materials, especially those combining two or more properties of interest, are attracting immense attention due to their potential applications. MOFs, metal organic frameworks, can be regarded as multifunctional materials if they show another useful property in addition to the adsorption behavior. Here, we report a new multifunctional light hybrid, MgH₆ODTMP·2H₂O­(DMF)_0.5 (<b>1</b>), which has been synthesized using the tetraphosphonic acid H₈ODTMP, octamethylenediamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis­(methylenephosphonic acid), by high-throughput methodology. Its crystal structure, solved by Patterson-function direct methods from synchrotron powder X-ray diffraction, was characterized by a 3D pillared open framework containing cross-linked 1D channels filled with water and DMF. Upon H₂O and DMF removal and subsequent rehydration, MgH₆ODTMP·2H₂O (<b>2</b>) and MgH₆ODTMP·6H₂O (<b>3</b>) can be formed. These processes take place through crystalline–quasi-amorphous–crystalline transformations, during which the integrity of the framework is maintained. A water adsorption study, at constant temperature, showed that this magnesium tetraphosphonate hybrid reversibly equilibrates its lattice water content as a function of the water partial pressure. Combination of the structural study and gas adsorption characterization (N₂, CO₂, and CH₄) indicates an ultramicroporous framework. High-pressure CO₂ adsorption data are also reported. Finally, impedance data indicates that <b>3</b> has high proton conductivity σ = 1.6 × 10^–3 S cm^–1 at <i>T</i> = 292 K at ∼100% relative humidity with an activation energy of 0.31 eV

High Proton Conductivity in a Flexible, Cross-Linked, Ultramicroporous Magnesium Tetraphosphonate Hybrid Framework

Multifunctional materials, especially those combining two or more properties of interest, are attracting immense attention due to their potential applications. MOFs, metal organic frameworks, can be regarded as multifunctional materials if they show another useful property in addition to the adsorption behavior. Here, we report a new multifunctional light hybrid, MgH₆ODTMP·2H₂O­(DMF)_0.5 (<b>1</b>), which has been synthesized using the tetraphosphonic acid H₈ODTMP, octamethylenediamine-<i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetrakis­(methylenephosphonic acid), by high-throughput methodology. Its crystal structure, solved by Patterson-function direct methods from synchrotron powder X-ray diffraction, was characterized by a 3D pillared open framework containing cross-linked 1D channels filled with water and DMF. Upon H₂O and DMF removal and subsequent rehydration, MgH₆ODTMP·2H₂O (<b>2</b>) and MgH₆ODTMP·6H₂O (<b>3</b>) can be formed. These processes take place through crystalline–quasi-amorphous–crystalline transformations, during which the integrity of the framework is maintained. A water adsorption study, at constant temperature, showed that this magnesium tetraphosphonate hybrid reversibly equilibrates its lattice water content as a function of the water partial pressure. Combination of the structural study and gas adsorption characterization (N₂, CO₂, and CH₄) indicates an ultramicroporous framework. High-pressure CO₂ adsorption data are also reported. Finally, impedance data indicates that <b>3</b> has high proton conductivity σ = 1.6 × 10^–3 S cm^–1 at <i>T</i> = 292 K at ∼100% relative humidity with an activation energy of 0.31 eV

Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids

Two new families of divalent metal hybrid derivatives from the aromatic tetraphosphonic acids 1,4- and 1,3-<i>bis</i>(aminomethyl)­benzene-<i>N</i>,<i>N</i>′-<i>bis</i>(methylenephosphonic acid), (H₂O₃PCH₂)₂–N–CH₂C₆H₄CH₂–N­(CH₂PO₃H₂)₂ (designated herein as <b><i>p</i>-H₈L</b> and <b><i>m</i>-H₈L</b>) have been synthesized by crystallization at room temperature and hydrothermal conditions. The crystal structures of M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂(H₂O)₂]·2H₂O (M = Mg, Co, and Zn), <b>M–(<i>p</i>-H₆L)</b>, and M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂]·<i>n</i>H₂O (M = Ca, Mg, Co, and Zn and <i>n</i> = 1–1.5), <b><b>M–(<i>m</i>-H₆L)</b></b>, were solved ab initio by synchrotron powder diffraction data using the direct methods and subsequently refined using the Rietveld method. The crystal structure of the isostructural <b><b>M–(<i>p</i>-H₆L)</b></b> is constituted by organic–inorganic monodimensional chains where the phosphonate moiety acts as a bidentate chelating ligand bridging two metal octahedra. <b><b>M–(<i>m</i>-H₆L)</b></b> compounds exhibit a 3D pillared open-framework with small 1D channels filled with water molecules. These channels are formed by the pillaring action of the organic ligand connecting adjacent layers through the phosphonate oxygens. Thermogravimetric and X-ray thermodiffraction analyses of <b><b>M–(<i>p</i>-H₆L)</b></b> showed that the integrity of their crystalline structures is maintained up to 470 K, without significant reduction of water content, while thermal decomposition takes place above 580 K. The utility of <b><b>M–(<i>p</i>-H₆L)</b></b> (M = Mg and Zn) hybrid materials in corrosion protection was investigated in acidic aqueous solutions. In addition, the impedance data indicate both families of compounds display similar proton conductivities (σ ∼ 9.4 × 10^–5 S·cm^–1, at 98% RH and 297 K), although different proton transfer mechanisms are involved

Structural Variability in Multifunctional Metal Xylenediaminetetraphosphonate Hybrids

Two new families of divalent metal hybrid derivatives from the aromatic tetraphosphonic acids 1,4- and 1,3-<i>bis</i>(aminomethyl)­benzene-<i>N</i>,<i>N</i>′-<i>bis</i>(methylenephosphonic acid), (H₂O₃PCH₂)₂–N–CH₂C₆H₄CH₂–N­(CH₂PO₃H₂)₂ (designated herein as <b><i>p</i>-H₈L</b> and <b><i>m</i>-H₈L</b>) have been synthesized by crystallization at room temperature and hydrothermal conditions. The crystal structures of M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂(H₂O)₂]·2H₂O (M = Mg, Co, and Zn), <b>M–(<i>p</i>-H₆L)</b>, and M­[(HO₃PCH₂)₂N­(H)­CH₂C₆H₄CH₂N­(H)­(CH₂PO₃H)₂]·<i>n</i>H₂O (M = Ca, Mg, Co, and Zn and <i>n</i> = 1–1.5), <b><b>M–(<i>m</i>-H₆L)</b></b>, were solved ab initio by synchrotron powder diffraction data using the direct methods and subsequently refined using the Rietveld method. The crystal structure of the isostructural <b><b>M–(<i>p</i>-H₆L)</b></b> is constituted by organic–inorganic monodimensional chains where the phosphonate moiety acts as a bidentate chelating ligand bridging two metal octahedra. <b><b>M–(<i>m</i>-H₆L)</b></b> compounds exhibit a 3D pillared open-framework with small 1D channels filled with water molecules. These channels are formed by the pillaring action of the organic ligand connecting adjacent layers through the phosphonate oxygens. Thermogravimetric and X-ray thermodiffraction analyses of <b><b>M–(<i>p</i>-H₆L)</b></b> showed that the integrity of their crystalline structures is maintained up to 470 K, without significant reduction of water content, while thermal decomposition takes place above 580 K. The utility of <b><b>M–(<i>p</i>-H₆L)</b></b> (M = Mg and Zn) hybrid materials in corrosion protection was investigated in acidic aqueous solutions. In addition, the impedance data indicate both families of compounds display similar proton conductivities (σ ∼ 9.4 × 10^–5 S·cm^–1, at 98% RH and 297 K), although different proton transfer mechanisms are involved

Guest Molecule-Responsive Functional Calcium Phosphonate Frameworks for Tuned Proton Conductivity

We report the synthesis, structural characterization, and functionality (framework interconversions together with proton conductivity) of an open-framework hybrid that combines Ca²⁺ ions and the rigid polyfunctional ligand 5-(dihydroxyphosphoryl)­isophthalic acid (<b>PiPhtA</b>). Ca₂[(HO₃PC₆H₃COOH)₂]₂[(HO₃PC₆H₃(COO)₂H)­(H₂O)₂]·5H₂O (<b>Ca-PiPhtA-I</b>) is obtained by slow crystallization at ambient conditions from acidic (pH ≈ 3) aqueous solutions. It possesses a high water content (both Ca coordinated and in the lattice), and importantly, it exhibits water-filled 1D channels. At 75 °C, <b>Ca-PiPhtA-I</b> is partially dehydrated and exhibits a crystalline diffraction pattern that can be indexed in a monoclinic cell with parameters close to the pristine phase. Rietveld refinement was carried out for the sample heated at 75 °C, <b>Ca-PiPhtA-II</b>, using synchrotron powder X-ray diffraction data, which revealed the molecular formula Ca₂[(HO₃PC₆H₃COOH)₂]₂[(HO₃PC₆H₃(COO)₂H)­(H₂O)₂]. All connectivity modes of the “parent” <b>Ca-PiPhtA-I</b> framework are retained in <b>Ca-PiPhtA-II</b>. Upon <b>Ca-PiPhtA-I</b> exposure to ammonia vapors (28% aqueous NH₃) a new derivative is obtained (<b>Ca-PiPhtA-NH</b>_<b>3</b>) containing 7 NH₃ and 16 H₂O molecules according to elemental and thermal analyses. <b>Ca-PiPhtA-NH</b>_<b>3</b> exhibits a complex X-ray diffraction pattern with peaks at 15.3 and 13.0 Å that suggest partial breaking and transformation of the parent pillared structure. Although detailed structural identification of <b>Ca-PiPhtA-NH</b>_<b>3</b> was not possible, due in part to nonequilibrium adsorption conditions and the lack of crystallinity, FT-IR spectra and DTA-TG analysis indicate profound structural changes compared to the pristine <b>Ca-PiPhtA-I</b>. At 98% RH and <i>T</i> = 24 °C, proton conductivity, σ, for <b>Ca-PiPhtA-I</b> is 5.7 × 10^–4 S·cm^–1. It increases to 1.3 × 10^–3 S·cm^–1 upon activation by preheating the sample at 40 °C for 2 h followed by water equilibration at room temperature under controlled conditions. <b>Ca-PiPhtA-NH</b>_<b>3</b> exhibits the highest proton conductivity, 6.6 × 10^–3 S·cm^–1, measured at 98% RH and <i>T</i> = 24 °C. Activation energies (<i>E</i>_a) for proton transfer in the above-mentioned frameworks range between 0.23 and 0.4 eV, typical of a Grothuss mechanism of proton conduction. These results underline the importance of internal H-bonding networks that, in turn, determine conductivity properties of hybrid materials. It is highlighted that new proton transfer pathways may be created by means of cavity “derivatization” with selected guest molecules resulting in improved proton conductivity