Interstitial defect | EPFL Graph Search

In materials science, an interstitial defect is a type of point crystallographic defect where an atom of the same or of a different type, occupies an interstitial site in the crystal structure. When the atom is of the same type as those already present they are known as a self-interstitial defect. Alternatively, small atoms in some crystals may occupy interstitial sites, such as hydrogen in palladium. Interstitials can be produced by bombarding a crystal with elementary particles having energy above the displacement threshold for that crystal, but they may also exist in small concentrations in thermodynamic equilibrium. The presence of interstitial defects can modify the physical and chemical properties of a material. The idea of interstitial compounds was started in the late 1930s and they are often called Hagg phases after Hägg. Transition metals generally crystallise in either the hexagonal close packed or face centered cubic structures, both of which can be considered to be made up of layers of hexagonally close packed atoms. In both of these very similar lattices there are two sorts of interstice, or hole: Two tetrahedral holes per metal atom, i.e. the hole is between four metal atoms One octahedral hole per metal atom, i.e. the hole is between six metal atoms It was suggested by early workers that: the metal lattice was relatively unaffected by the interstitial atom the electrical conductivity was comparable to that of the pure metal there was a range of composition the type of interstice occupied was determined by the size of the atom These were not viewed as compounds, but rather as solutions, of say carbon, in the metal lattice, with a limiting upper “concentration” of the smaller atom that was determined by the number of interstices available. A more detailed knowledge of the structures of metals, and binary and ternary phases of metals and non metals shows that: generally at low concentrations of the small atom, the phase can be described as a solution, and this approximates to the historical description of an interstitial compound above.

Effect of ZnO on the reactivity of cementitious systems

Andrea Eloisa Teixeira Pita

One of the biggest driving forces in cement research today is the mitigation of the CO2 emissions caused by its production. A potential solution to reduce the environmental impact is to replace cement with supplementary cementitious materials (SCMs). This replacement results in low strength at early ages because they are slow to react. The incorporation of minor elements has shown the potential to improve cement reactivity. A few percent of ZnO in C3S causes a signifi- cant increase in reactivity. This effect was related to the incorporation of zinc into the C-S-H struc- ture, which increases its needle length. This research investigates the effect of ZnO in more realis- tic systems such as alite, C3A-polyclinker, and C4AF-polyclinker. The positive effects observed in C3S are translated in the alite system, but differences were found in the polyclinkers, which have a prolonged induction period. This resulted due concentration of zinc in the interstitial phases and a small amount of zinc in alite phase. An amorphous phase was identified in the C3A polyclinker; it is believed to react rapidly with water, releasing Zn ions into the solution, which delays the reaction of alite. The addition of more gypsum controls the reaction of this phase, and thus the induction period is shortened.This work explores the possibilities of retaining Zn in alite to enhance the reactivity of more realis- tic cementitious systems. The cooling rate was critical, as a slow cooling rate promoted more Zn in the alite, less amorphous, and hence, an enhancement in the hydration of C3A-polyclinker doped with ZnO. The recalcination of this system did not affect the hydration of alite but increased the amount of crystalline C3A, which reacted faster. Thus, the amorphous phase was related to an aluminate phase. Moreover, a C4AF-polyclinker and a system with a lower amount of interstitial phase were investigated. The results showed a higher Zn concentration in alite but also extended induction periods. This was associated with the free ZnO and the amorphous content. Even when the hydration was delayed, the height of the alite peak increased with Zn doping, probably due to the incorporation of Zn into the C-S-H observed in pure C3S.In addition, the interaction between sulfates and zinc in the alite system doped with ZnO was in- vestigated. Severe retardation was observed, and the reason is proposed to be the formation of amorphous Zn compound. The presence of aluminum, ettringite formation, or an amorphous layer were discarded as the cause of the retardation.

EPFL2022

Structural and Dynamical Properties of Potassium Dodecahydro-monocarba-closo-dodecaborate: KCB11H12

Mirjana Dimitrievska, Wei Zhou

MCB11H12 (M: Li, Na) dodecahydro-monocarba-closo-dodecaborate salt compounds are known to have stellar superionic Li+ and Na+ conductivities in their high-temperature disordered phases, making them potentially appealing electrolytes in all-solid-state batteries. Nonetheless, it is of keen interest to search for other related materials with similar conductivities while at the same time exhibiting even lower (more device-relevant) disordering temperatures, a key challenge for this class of materials. With this in mind, the unknown structural and dynamical properties of the heavier KCB11H12 congener were investigated in detail by X-ray powder diffraction, differential scanning calorimetry, neutron vibrational spectroscopy, nuclear magnetic resonance, quasielastic neutron scattering, and AC impedance measurements. This salt indeed undergoes an entropy-driven, reversible, order-disorder transformation and with a lower onset temperature (348 K upon heating and 340 K upon cooling) in comparison to the lighter LiCB11H12 and NaCB11H12 analogues. The K+ cations in both the low-T ordered monoclinic (P2(1)/c) and high-T disordered cubic (Fm (3) over barm) structures occupy octahedral interstices formed by CB11H12- anions. In the low-T structure, the anions orient themselves so as to avoid close proximity between their highly electropositive C-H vertices and the neighboring K+ cations. In the high-T structure, the anions are orientationally disordered, although to best avoid the K+ cations, the anions likely orient themselves so that their C-H axes are aligned in one of eight possible directions along the body diagonals of the cubic unit cell. Across the transition, anion reorientational jump rates change from 6.2 x 10(6) s(-1) in the low-T phase (332 K) to 2.6 x 10(10) s(-1) in the high-T phase (341 K). In tandem, K+ conductivity increases by about 30-fold across the transition, yielding a high-T phase value of 3.2 x 10(-4 )S cm(-1 )at 361 K. However, this is still about 1 to 2 orders of magnitude lower than that observed for LiCB(11)H(12 )and NaCB11H12, suggesting that the relatively larger K+ cation is much more sterically hindered than Li+ and Na+ from diffusing through the anion lattice via the network of smaller interstitial sites.

AMER CHEMICAL SOC2020

Structural disorder and magnetic correlations driven by oxygen doping in Nd2NiO4+delta (delta similar to 0.11)

We investigated the influence of oxygen overstoichiometry on apical oxygen disorder and magnetic correlations in Nd2NiO4+delta (delta similar to 0.11) in the temperature range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron single-crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystallizes in a tetragonal commensurate structure with a P4(2)/ncm space group with excess oxygen atoms occupying the 4b (3/4 1/4 1/4) interstitial sites, coordinated by four apical oxygen atoms. Large and anisotropic thermal displacement parameters are found for equatorial and apical oxygen atoms, which are strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase. Maximum entropy analysis of the neutron single-crystal diffraction data uncovered anharmonic contributions to the displacement parameters of the apical oxygen atoms, toward the nearest vacant 4b interstitial site, related to the phonon-assisted oxygen diffusion mechanism. Macroscopic magnetization measurements and neutron powder diffraction studies reveal long-range antiferromagnetic ordering of the Ni sublattice at T-N similar to 53K with a weak ferromagnetic component along the c axis, while the long-range magnetic ordering of the Nd sublattice occurs below 10 K. Temperature-dependent neutron diffraction patterns show the appearance of a commensurate magnetic order at TN with the propagation vector k = (100) and the emergence of an additional incommensurate phase below 30 K, while both phases coexist at 2 K. The commensurate magnetic structure is best described by the P4(2)/nc'm' Shubnikov space group. Refined magnetic moments of the Ni and Nd sites at 2 K are 1.144(76) and 1.632(52) mu(B), respectively. A possible origin of the incommensurate phase is discussed and a tentative magnetic phase diagram is proposed.

AMER PHYSICAL SOC2019