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.

Garnet: A Fast Lithium-IonConductor. -Single crystals of the title compound are prepared by sintering mixtures of ZrO2, La2O3, and Li2CO3 using ceramic crucibles in initial steps and later sealed Pt capsules. The samples contain small amounts of Al, probably originating from the crucibles. Li7La3Zr2O12 crystallizes in the cubic space group Ia3d with Z = 8 (single crystal XRD) between 100 and 500 K. Li atoms are located on at least two structural sites that are partially occupied, while other Li atoms in the structure appear to be delocalized. Li NMR spectroscopy indicates fast Li ion diffusion at room temperature. Using solely Pt crucibles during sintering a tetragonal Li7La3Zr2O12 room temp. phase (powder XRD) is obtained which 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+ . A simple crystal chemical model is proposed to explain why cubic Al-containing Li 7 La 3 Zr 2 O 12 is (meta)stable at ambient conditions, and why it is such a good ion conductor. -(GEIGER*, C. A.; ALEKSEEV, E.; LAZIC, B.; FISCH, M.; ARMBRUSTER, T.; LANGNER, R.; FECHTELKORD, M.; KIM, N.; PETTKE, T.; WEPPNER, W.; Inorg. Chem. 50 (2011) 3, 1089-1097, http://dx.

The thermal behavior of Na-exchanged stellerite and stilbite was investigated by in-situ single crystal X-ray diffraction. For comparison with the exchanged forms new data were collected on natural stellerite and stilbite under the same experimental conditions. With the increase of temperature, strong disorder at T and O sites of the tetrahedra of the four-membered ring developed in natural forms. Such disorder was associated with the rupture of TO -T connections and transition from the A to the B phase. Differently from previous studies, stellerite B at T >300°C was found to be monoclinic (space group A2/m). In addition, at 400°C, a new TO -T connection occurred, analogous to that in the B phase of barrerite. Na-stellerite and Na-stilbite were at RT monoclinic, space group F2/m. Upon heating, they also displayed the same structural modifications as observed in natural barrerite and Na-barrerite and adopted space group A2/m. Compared to natural stellerite and stilbite different TO -T connections ruptured leading to a different topology of the B phase. The total volume contraction was-16% at 350°C compared to-8% of pristine materials. The highly-condensed D phase, which does not form in natural stellerite and stilbite, was obtained by heating a Na-stellerite crystal ex-situ at 525°C. The structure corresponded to the D phase of natural barrerite and Na-barrerite. All investigated STI members, after being exchanged with Na, have identical symmetry and demonstrate corresponding behavior upon heating and associated dehydration. Thus, a previously assumed memory effect of the symmetry of the natural parent structure, is not confirmed.

Elastic behavior and pressure-induced structural evolution of synthetic boron-mullite ''Al 5 BO 9 '' (a = 5.678(2) A ˚, b = 15.015(4) A ˚and c = 7.700(3) A ˚, space group Cmc2 1 , Z = 4) were investigated up to 7.4 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase transition or anomalous compressional behavior occurred within the investigated P range. Fitting the P-V data with a truncated second-order (in energy) Birch-Murnaghan Equation-of-State (BM-EoS), using the data weighted by the uncertainties in P and V, we obtained: V 0 = 656.4(3) A ˚3 and K T0 = 165(7) GPa (b V0 = 0.0061(3) GPa -1 ). The evolution of the Eulerian finite strain versus normalized stress (f E -F E plot) leads to an almost horizontal trend, showing that a truncated second-order BM-EoS is appropriate to describe the elastic behavior of ''Al 5 BO 9 '' within the investigated P range. The weighted linear regression through the data points gives: F E (0) = 159(11) GPa. Axial compressibility coefficients yielded: b a = 1.4(2) 9 10 -3 GPa -1 , b b = 3.4(4) 9 10 -3 GPa -1 , and b c = 1.7(3) 9 10 -3 GPa -1 (b a :b b :b c = 1:2.43:1.21). The highest compressibilities observed in this study within (100) can be ascribed to the presence of voids represented by five-membered rings of polyhedra: Al1-Al3-Al4-Al1-Al3, which allow accommodating the effect of pressure by polyhedral tilting. Polyhedral tilting around the voids also explains the higher compressibility along [010] than along [001]. The stiffer crystallographic direction observed here might be controlled by the infinite chains of edge-sharing octahedra running along [100], which act as ''pillars'', making the structure less compressible along the a-axis than along the b-and c-axis. Along [100], compression can only be accommodated by deformation of the edge-sharing octahedra (and/or by compression of the Al-O bond lengths), as no polyhedral tilting can occur. In addition, a comparative elastic analysis among the mullite-type materials is carried out.

This study demonstrates that the compression rate adds a new perspective to phase diagrams of solids. A particular pressure increase rate may trigger unexpected solid-state transformations, producing otherwise inaccessible phases. Our test case is l-serine, characterized by a complex high-pressure behavior with three known polymorphs. However, the critical pressure of each transition, the ranges of coexistence of polymorphs, and the existence of an elusive fourth phase remained open questions, here analyzed and solved using synchrotron powder X-ray diffraction at high pressure, under controlled pressure increase rates. Two parallel paths exist, and the composition of the system depends on the pressure increase rate and the steps during the compression. A slow and continuous compression favors phase IV, whereas phase II can be observed only with a rapid and sharp compression. No direct interconversion occurs between these phases. Moreover, phase III originates only from phase II but never from phase IV. By controlling the strategy of pressure increase, we obtained a powder of phase IV that enabled solving its unknown structure, which resulted as a distorted superstructure of phase I with a tripled a-axis.

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Phase stability, elastic behavior, and pressure-induced structural evolution of synthetic boron-mullite Al 5 BO 9 (a = 5.6780 (7), b = 15.035(6), and c =7.698(3) A, space group Cmc2 1 , Z = 4) were investigated up to 25.6(1) GPa by in situ single-crystal synchrotron X-ray diffraction with a diamond anvil cell (DAC) under hydrostatic conditions. No evidence of phase transition was observed up to 21.7(1) GPa. At 25.6(1) GPa, the refined unit-cell parameters deviated significantly from the compressional trend, and the diffraction peaks appeared broader than at lower pressure. At 26.7(1) GPa, the diffraction pattern was not indexable, suggesting amorphization of the material or a phase transition to a high-pressure polymorph. Fitting the P-V data up to 21.7(1) GPa with a second-order Birch-Murnaghan Equation-of-State, we obtained a bulk modulus K T0 = 164(1) GPa. The axial compressibilities, here described as linearized bulk moduli, are as follows: K T0(a) = 244(9), K T0(b) = 120(4), and K T0(c) = 166(11) GPa (K T0(a) :K T0(b) : K T0(c) = 2.03:1:1.38). The structure refinements allowed a description of the main deformation mechanisms in response to the applied pressure. The stiffer crystallographic direction appears to be controlled by the infinite chains of edge-sharing octahedra running along [100], making the structure less compressible along the a-axis than along the band c-axis.