A synthesis and crystal chemical study of the fast ion conductor Li(7-3x)Ga(x)La3 Zr2O12 with x = 0.08 to 0.84.

Fast-conducting phase-pure cubic Ga-bearing Li7La3Zr2O12 was obtained using solid-state synthesis methods with 0.08 to 0.52 Ga(3+) pfu in the garnet. An upper limit of 0.72 Ga(3+) pfu in garnet was obtained, but the synthesis was accompanied by small amounts of La2Zr2O12 and LiGaO3. The synthetic products were characterized by X-ray powder diffraction, electron microprobe and SEM analyses, ICP-OES measurements, and (71)Ga MAS NMR spectroscopy. The unit-cell parameter, a0, of the various garnets does not vary significantly as a function of Ga(3+) content, with a value of about 12.984(4) Å. Full chemical analyses for the solid solutions were obtained giving: Li7.08Ga0.06La2.93Zr2.02O12, Li6.50Ga0.15La2.96Zr2.05O12, Li6.48Ga0.23La2.93Zr2.04O12, Li5.93Ga0.36La2.94Zr2.01O12, Li5.38Ga0.53La2.96Zr1.99O12, Li4.82Ga0.60La2.96Zr2.00O12, and Li4.53Ga0.72La2.94Zr1.98O12. The NMR spectra are interpreted as indicating that Ga(3+) mainly occurs in a distorted 4-fold coordinated environment that probably corresponds to the general 96h crystallographic site of garnet.

Synthesis and crystal chemistry of the fast Li-ion conductor Li7La3Zr2O12 doped with Fe.

Nominal Li7La3Zr2O12 (LLZO) garnet, doped with (57)Fe2O3, was synthesized by sintering oxides and carbonates at T = 1100 °C in air. X-ray powder diffraction measurements show that Li(7-3x)Fe(3+)(x)La3Zr2O12 with x = 0.19 crystallizes in the cubic space group Ia-3d, with a0 = 12.986(4) Å at room temperature. SEM and electron microprobe measurements were made to obtain compositional information and check for the presence of phases other than garnet. Inductively coupled plasma optical emission spectroscopy measurements were made to determine the Li content. (57)Fe Mössbauer spectra obtained at 295 and 80 K show that about 96% of the total iron occurs as Fe(3+) and 4% as Fe(2+). Roughly two-thirds of the Fe(3+) cations are assigned to the tetrahedral site (24d) and roughly one-quarter to a highly distorted site (possibly at 96h) in the garnet structure. Smaller amounts of Fe(3+) and Fe(2+), around 5% each, occur at other crystallographic sites. On the basis of published (27)Al MAS NMR results and analysis of the (57)Fe Mössbauer spectra, it appears that at low concentrations Al(3+) and Fe(3+) substitute in Li7La3Zr2O12 in a similar manner. The aliovalent substitution Al(3+)/Fe(3+) ↔ 3Li(+) in LLZO stabilizes the cubic phase and also probably promotes its high Li-ion conductivity.

Nitrogen Incorporation in Potassic and Micro- and Meso-Porous Minerals: Potential Biogeochemical Records and Targets for Mars Sampling.

We measured the N concentrations and isotopic compositions of 44 samples of terrestrial potassic and micro- and meso-porous minerals and a small number of whole-rocks to determine the extent to which N is incorporated and stored during weathering and low-temperature hydrothermal alteration in Mars surface/near-surface environments. The selection of these minerals and other materials was partly guided by the study of altered volcanic glass from Antarctica and Iceland, in which the incorporation of N as NH4+ in phyllosilicates is indicated by correlated concentrations of N and the LILEs (i.e., K, Ba, Rb, Cs), with scatter likely related to the presence of exchanged, occluded/trapped, or encapsulated organic/inorganic N occurring within structural cavities (e.g., in zeolites). The phyllosilicates, zeolites, and sulfates analyzed in this study contain between 0 and 99,120 ppm N and have δ15Nair values of -34‰ to +65‰. Most of these minerals, and the few siliceous hydrothermal deposits that were analyzed, have δ15N consistent with the incorporation of biologically processed N during low-temperature hydrothermal or weathering processes. Secondary ion mass spectrometry on altered hyaloclastites demonstrates the residency of N in smectites and zeolites, and silica. We suggest that geological materials known on Earth to incorporate and store N and known to be abundant at, or near, the surface of Mars should be considered targets for upcoming Mars sample return with the intent to identify any signs of ancient or modern life.

Crystal chemistry and stability of "Li7La3Zr2O12" garnet: a fast lithium-ion conductor.

Recent research has shown that certain Li-oxide garnets with high mechanical, thermal, chemical, and electrochemical stability are excellent fast Li-ion conductors. However, the detailed crystal chemistry of Li-oxide garnets is not well understood, nor is the relationship between crystal chemistry and conduction behavior. An investigation was undertaken to understand the crystal chemical and structural properties, as well as the stability relations, of Li(7)La(3)Zr(2)O(12) garnet, which is the best conducting Li-oxide garnet discovered to date. Two different sintering methods produced Li-oxide garnet but with slightly different compositions and different grain sizes. The first sintering method, involving ceramic crucibles in initial synthesis steps and later sealed Pt capsules, produced single crystals up to roughly 100 µm in size. Electron microprobe and laser ablation inductively coupled plasma mass spectrometry (ICP-MS) measurements show small amounts of Al in the garnet, probably originating from the crucibles. The crystal structure of this phase was determined using X-ray single-crystal diffraction every 100 K from 100 K up to 500 K. The crystals are cubic with space group Ia3̅d at all temperatures. The atomic displacement parameters and Li-site occupancies were measured. Li atoms could be located on at least two structural sites that are partially occupied, while other Li atoms in the structure appear to be delocalized. (27)Al NMR spectra show two main resonances that are interpreted as indicating that minor Al occurs on the two different Li sites. Li NMR spectra show a single narrow resonance at 1.2-1.3 ppm indicating fast Li-ion diffusion at room temperature. The chemical shift value indicates that the Li atoms spend most of their time at the tetrahedrally coordinated C (24d) site. The second synthesis method, using solely Pt crucibles during sintering, produced fine-grained Li(7)La(3)Zr(2)O(12) crystals. This material was studied by X-ray powder diffraction at different temperatures between 25 and 200 °C. This phase is tetragonal at room temperature and undergoes a phase transition to a cubic phase between 100 and 150 °C. Cubic "Li(7)La(3)Zr(2)O(12)" may be stabilized at ambient conditions relative to its slightly less conducting tetragonal modification via small amounts of Al(3+). Several crystal chemical properties appear to promote the high Li-ion conductivity in cubic Al-containing Li(7)La(3)Zr(2)O(12). They are (i) isotropic three-dimensional Li-diffusion pathways, (ii) closely spaced Li sites and Li delocalization that allow for easy and fast Li diffusion, and (iii) low occupancies at the Li sites, which may also be enhanced by the heterovalent substitution Al(3+) ⇔ 3Li.

DFT Study of the Role of Al3+ in the Fast Ion-Conductor Li7-3x Al3+x La3Zr2O12 Garnet.

We investigate theoretically the site occupancy of Al3+ in the fast-ion-conducting cubic-garnet Li7-3x Al3+x La3Zr2O12 (Ia-3d) using density functional theory. By comparing calculated and measured 27Al NMR chemical shifts an analysis shows that Al3+ prefers the tetrahedrally coordinated 24d site and a distorted 4-fold coordinated 96h site. The site energies for Al3+ ions, which are slightly displaced from the exact crystallographic sites (i.e., 24d and 96h), are similar leading to a distribution of slightly different local oxygen coordination environments. Thus, broad 27Al NMR resonances result reflecting the distribution of different isotropic chemical shifts and quadrupole coupling constants. From an energetic point of view, there is evidence that Al3+ could also occupy the 48g site with its almost regular octahedral coordination sphere. Although this has been reported by neutron powder diffraction, the NMR chemical shift calculated for such an Al3+ site has not been observed experimentally.