Structure and Ionic-Transport Properties of Lithium-Containing Garnets Li 3 Ln 3 Te 2 O 12 (Ln ) Y, Pr, Nd, Sm-Lu) Michael P. O’Callaghan, ² Danny R. Lynham, ² Edmund J. Cussen,* ,² and George Z. Chen ‡ The School of Chemistry and The School of Chemical, EnVironmental and Mining Engineering, The UniVersity of Nottingham, Nottingham, United Kingdom NG7 2RD ReceiVed April 28, 2006. ReVised Manuscript ReceiVed July 23, 2006 Lithium-containing compounds of the formula Li 3 Ln 3 Te 2 O 12 (Ln ) Y, Pr, Nd, Sm-Lu) have been prepared by solid-state ceramic methods at temperatures up to 900 °C. Rietveld refinement against X-ray and neutron powder diffraction data show that these phases adopt the garnet structure (space group Ia3 hd) with lattice parameters in the range 12.15970(14) Å (Li 3 Lu 3 Te 2 O 12 ) to 12.61596(7) Å (Li 3 Pr 3 Te 2 O 12 ). The Ln 3+ and Te 6+ cations occupy the 8-fold and octahedrally coordinated sites, and Li + is accommodated exclusively in the tetrahedral sites commonly occupied in the garnet structure. Neutron diffraction data collected from Li 3 Nd 3 Te 2 O 12 at 300 and 600 °C show that the lithium coordination does not change over this temperature range. Impedance spectroscopy measurements indicate that Li 3 Nd 3 Te 2 O 12 shows minimal Li + mobility with an activation energy of 1.22(15) eV, resulting in a maximum observed conductivity of σ ≈ 1 × 10 -5 S cm -1 at 600 °C. Introduction The search for higher-density power sources for use in portable electronic devices has led to the emergence of Li- ion batteries as the leading technology in this area. 1 This is due to a number of attractive features of lithium batteries, such as high energy density, high cycling stability, and ease of miniaturization. However, further applications 2 for this technology could be realized by increasing the gravimetric power density and moving away from the polymer electro- lytes, 3 which currently have applications in this role. Replac- ing current technology with an all-solid-state battery could lead to substantial improvements in high-temperature opera- tion, mechanical strength, toxicity, and environmental impact on disposal. In crystalline solids, the highest Li + conductivi- ties of ∼1 × 10 -3 S cm -1 at room temperature have been realized in a number of systems, 4,5 but problems of (electro)- chemical stability mean that these high-conductivity phases are not suitable for use in rechargeable lithium batteries. Recent research has identified a series of Ta 5+ - and Nb 5+ - based garnets as promising lithium-ion conductors. 6,7 The conductivity of these garnet phases rivals those of the most conductive crystalline phases and, most importantly, these garnets are stable to metallic lithium, moisture, air, and common electrode materials. 8 The well-known garnet structure has the general formula A 3 B 2 C 3 O 12 , where A, B, and C refer to 8-coordinate, octahedral, and tetrahedral cation sites, respectively, as illustrated in Figure 1. The identification of facile Li + mobility in Li 5 La 3 M 2 O 12 (M ) Nb, Ta) has led to the study of a number of related phases, 7,9 although the structure of these compounds has been controversial despite a number of X-ray diffraction studies. 10-12 The 8-fold coordinated site is occupied by La 3+ ; the M 5+ cation resides on the cen- trosymmetric octahedral site, but the space group and position of the lithium cations were not conclusively identified by X-ray diffraction experiments. We have recently employed neutron diffraction to show that Li 5 La 3 M 2 O 12 (M ) Nb, Ta) crystallizes in the space group Ia3 hd and that the lithium occupies a range of sites. 13 The tetrahedral site commonly occupied in the garnet structure, 24d, is partially occupied (ca. 80%) by Li + , with the remainder of the lithium distributed with considerable disorder in heavily distorted octahedral coordination at the 48g site with ca. 40% occupancy. The tetrahedral and octahedral sites are linked by a shared face and the simultaneous occupation of these adjacent sites leads to short Li-Li distance, as shown in Figure 1. The 48g sites are linked to one another by shared edges, and the distortion of the oxide octahedra provides a large aperture for Li + migration that, taken in conjunction with the observed Li + disorder on the 48g site, suggests that * To whom correspondence should be addressed. E-mail: edmund.cussen@nottingham.ac.uk. Fax: 44-115-951-3563. ² The School of Chemistry, The University of Nottingham. ‡ The School of Chemical, Environmental and Mining Engineering, The University of Nottingham. (1) Tarascon, J.-M.; Armand, M. Nature 2001, 414, 359-367. (2) Bruce, P. G. Chem. Commun. 1997, 1817-1824. (3) Christie, A. M.; Lilley, S. J.; Staunton, E.; Andreev, Y. G.; Bruce, P. G. Nature 2005, 433, 50-53. (4) Robertson, A. D.; West, A. R.; Ritchie, A. G. Solid State Ionics 1997, 104,1-11. 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