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li7 is a natural and stable lithium isotope that is used in pressurized water reactors (PWRs) to regulate the pH level of the coolant. It is also used for the production of the medical research radioisotope Be-7 and in thermonuclear weapons.

li7 is synthesized by reaction of LLZO with Fe3+ in an aqueous solution. It is then characterized by NMR spectroscopy.

Structural Modifications

The tetragonal low-temperature phase in space group (SG) I41/acd has a fully ordered Li+ arrangement, whereas the cubic high-temperature modification in SG Ia3d shows a disordered Li+ distribution and low ion conductivity of 10-6 S cm-1 at room temperature (RT). It can be stabilized by a substitution of Zr4+ with Bi5+.

Structural Details

A polyhedral rendering of the tetragonal I43d structure shows that La3+ occupies the 8-fold coordinated 24d site and Zr4+ is bonded to an octahedrally coordinated 16c site. Moreover, there are two tetrahedral Li1 (12a) sites and a distorted octahedral Li2 (12b) site with different bond lengths, polyhedral distortion, and volumes.

Analytical studies of equivalent isotropic atomic displacement parameters show that Li+ ions generally have the largest atomic displacements, followed by Zr4+ and La3+. However, at lower substitutional levels at the Li2 (12b) site, the atomic displacement parameters are quite distinct and show anisotropic characteristics.

Li ion dynamics, able to average homonuclear dipole-dipole couplings, are observed only for samples with large amounts of Bi incorporated. This can be seen in the motional narrowing of the rigid-lattice line width, nrl, on the order of a few kilohertz. This is in contrast to the samples with xint 0.60, which are not sensitive to this narrowing effect, and only exhibit a change in nrl at 273 K.

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