Virginia Diez-Gómez

Short and long range lithium motions in powder Li_1+xTi_2−xAl_x(PO₄)₃ (LTAP) NASICON compounds prepared by ceramic (x = 0.2 and 0.4) and sol–gel (x = 0.3 and 0.4) routes are discussed. ND diffraction and MAS-NMR spectroscopy were previously used to investigate structural features of these compounds. In particular, Fourier map differences showed that the amount of Li atoms allocated at M3 increases at the expense of M1 sites when the Li content increases. In this work, PFG-NMR results show that diffusion coefficients rise with the amount of lithium and temperature. The restricted diffusion inside NASICON particles is compared with “free” diffusion processes. At 300 K, diffusion coefficients D_PFG ∼ 5 × 10⁻¹² m² s⁻¹ have been deduced in ceramic x = 0.2 and 0.4 samples, decreasing with diffusion time Δ used in PFG experiments. In sol–gel samples, diffusion coefficients are near those of ceramic samples, but decrease faster with diffusion Δ times, as a consequence of the Li confinement inside sub-micrometric crystallites. The NMR spin-echo signal displays minima at specific q(γgδ) values that are related to the crystallite size. From R_dif ∼ q_m ⁻¹ distances, calculated from the position of minima, and from diffusion coefficients deduced for high Δ values, the mean crystallite size was estimated. Finally, from the temperature dependence of conductivity and diffusion coefficients, the activation energy and charge carriers concentrations were determined.

The crystal structures of the lithium aluminosilicate minerals of the Li₂O–Al₂O₃–SiO₂ (LAS) system (Li_1−xAl_1−xSi_1+xO₄ system for 0.0 ≤ x ≤ 1.0), and bikitaite were determined by X-ray diffraction (XRD) in literature, suggesting several possible lattice models for each of the crystallized phases, because of the intrinsic experimental difficulties of this technique. Here, we correlate powder XRD patterns with Rietveld refinement of cell parameters and magic angle sample spinning multinuclear magnetic resonance (NMR) spectra, including ²⁹Si, ²⁷Al, ⁷Li, and ⁶Li spectroscopy at 7.05 T, 9.4 T, and 20 T. The aim is to select appropriate lattice models from short-range order schemes in the lithium aluminosilicate phases, from natural minerals and synthetic crystals from the crystallization of amorphous gel precursors by a ceramic route and also by hydrothermal high-pressure experiments. Solid solutions were found in α-quartz and α-cristobalite up to x ≥ 0.75, and in β-eucryptite and β-spodumene for 0.0 ≤ x ≤ 1.0, when the ceramic synthesis is at work. The local structures of these intermediate members of the β-eucryptite and β-spodumene solid-solution series have ²⁹Si NMR spectra consistent with the Loewenstein’s rule, i.e., they have short-range order but are strictly non-periodic structures. However, β-eucryptite LiAlSiO₄ end-member has a short-range structure compatible with the long-range order of the P6₄22 symmetry, when the crystallization is produced at hydrothermal conditions. The local structure of α-spodumene LiAlSi₂O₆ is consistent with the C2/c model. α-eucryptite LiAlSiO₄ shows a short-range structure as that suggested by the R-3 lattice model. Petalite LiAlSi₄O₁₀ has a local structure compatible with the P2/a space group. Finally, the ²⁹Si NMR spectra of bikitaite LiAlSi₂O₆·H₂O indicate a short-range structure well-suited with the P1 symmetry. These results are consistent with the Ostwald‘s rule of stages, forming a order-disorder sequence of increasing long-range order from the starting fully disordered solid gels, through crystalline pseudoperiodic structures in non-stoichiometric solid solution crystals that respect the Lowenstein’s rule, up to fully ordered crystals with short-range structures from NMR close to the long-range structures by XRD, as in the stoichiometric compounds found in some natural minerals.

Kaliophilite is a feldspathoid mineral found in two Italian magmatic provinces and represents one of the 12 known phases with composition close to KAlSiO₄. Despite its apparently simple formula, the structure of this mineral revealed extremely complex and resisted structure solution for more than a century. Samples from the Vesuvius–Monte Somma and Alban Hills volcanic areas were analyzed through a multi-technique approach, and finally the crystal structure of kaliophilite was solved using 3D electron diffraction and refined against X-ray diffraction data of a twinned crystal. Results were also ascertained by the Rietveld method using synchrotron powder intensities. It was found that kaliophilite crystallizes in space groupP3 with unit-cell parametersa= 27.0597 (16),c= 8.5587 (6) Å,V= 5427.3 (7) ųandZ= 54. The kaliophilite framework is a variant of the tridymite topology, with alternating SiO₄and AlO₄tetrahedra forming sheets of six-membered rings (6³nets), which are connected along [001] by sharing the apical oxygen atoms. Considering the up (U) and down (D) orientations of the linking vertex, kaliophilite is the first framework that contains three different ring topologies: nine (1-3-5) (UDUDUD) rings, six (1-2-3) (UUUDDD) rings and twelve (1-2-4) (UUDUDD) rings. This results in a relatively open (19.9 tetrahedra nm⁻³) channel system with multiple connections between the double six-ring cavities. Such a framework requires a surprisingly large unit cell, 27 times larger than the cell of kalsilite, the simplest phase with the same composition. The occurrence of some Na for K substitution (3–10%) may be related to the characteristic structural features of kaliophilite. Micro-twinning, pseudo-symmetries and anisotropichkl-dependent peak broadening were also detected, and they may account for the elusive character of the kaliophilite crystal structure.

Ab initio molecular dynamics (MD) simulations and NMR GIPAW (gauge including projector augmented wave) calculations have been used to analyze the coordination and mobility of Li ions in LiTi₂(PO₄)₃ (rhombohedral), LiZr₂(PO₄)₃ (triclinic), and LiZr₂(PO₄)₃ (rhombohedral) phases. Significant discrepancies are observed between static calculations of ⁷Li quadrupolar parameters and experimental values. The dynamical origin of this disagreement is demonstrated by incorporating in the calculations thermal vibrations and local motion of atoms with MD simulations. For LiTi₂(PO₄)₃, the quadrupolar constant associated with Li ions grows with temperature because the local symmetry of the system decreases, whereas for the Zr phases, the quadrupolar constant decreases because thermal vibrations reduce the anisotropy of the interaction. Finally, for both Zr phases, MD yields Li distributions that compare well with disorder reported from diffraction studies.

Ti/Ge distribution in rhombohedral LiTi_2-xGe_x(PO₄)₃ NASICON series has been analyzed by ³¹P Magic-Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy and first principles Density Functional Theory (DFT) calculations. Nuclear Magnetic Resonance results an excellent probe to follow Ti/Ge disorder, as it is sensitive to the atomic scale environment without long-range periodicity requirements. In samples considered here, PO₄ units are surrounded by four Ti/Ge octahedra, then, five different components ascribed to P(OTi)₄, P(OTi)₃(OGe), P(OTi)₂(OGe)₂, P(OTi)(OGe)₃ and P(OGe)₄ environments are expected in ³¹P MAS-NMR spectra of R-3c NASICON samples. However, ³¹P MAS-NMR spectra of analyzed series display a higher number of signals, suggesting that although overall symmetry remains R-3c, partial substitution causes a local decrement in symmetry. With the aid of first principles DFT calculations, ten detected ³¹P NMR signals have been assigned to different Ti_4-nGe_n arrangements in the R3 subgroup symmetry. In this assignment, the influence of octahedra of the same or different R₂(PO₄)₃ structural units has been considered. The influence of bond distances, angles and atom charges on ³¹P NMR chemical shieldings has been discussed. Simulation of the LiTi_2-xGe_x(PO₄)₃ series suggests that detection of ten P environments is mainly due to the existence of two oxygen types, O1 and O2, whose charges are differently affected by Ge and Ti occupation of octahedra. From quantitative analysis of detected components, a random Ti/Ge distribution has been deduced in next nearest neighbor (NNN) sites that surround tetrahedral PO₄ units. This random distribution was supported by XRD data displaying the Vegard’s law.