Structure and Properties of Li⁺(Cryptand [2.1.1])e^-, an Electride with a 1D “Spin-Ladder-like” Cavity-Channel Geometry

The title electride crystallizes in the orthorhombic space group Pbcn with a = 10.060(4) Å, b = 23.134(8) Å, c = 8.380(4) Å, and z = 4. A second powdered phase of unknown structure, but with rather different properties, forms when rapidly precipitated “seed” powder is used. The crystalline phase contains electron-trapping cavities, each of approximate diameter 4.3 Å, connected in zigzag fashion along the c axis by rather open channels of minimum diameter 2.4 Å (center-to-center distance 7.9 Å). Each cavity is also connected to next-neighbor cavities, 8.2 Å away along c, by channels of diameter 1.5 Å. Inter-chain channels are <1.0 Å in diameter so that the cavity-channel structure is well-described as “ladder-like”. Replacing the trapped electron by Na^- in the corresponding sodide, Li⁺ (cryptand [2.1.1])Na^-, results in an orthorhombic structure (space group Pna2₁). Although the anionic sites in the sodide are only slightly bigger than those in the electride, the channel structure is markedly different, forming a 3-dimensional network. The magnetic susceptibilities of both phases of the electride above 14 K are well-fit by the linear chain Heisenberg antiferromagnetic model with values of J/k_B of −54 (crystals) and −17.8 K (powder). Both types show marked deviations below 14 K toward greater spin-pairing. This behavior was verified by EPR measurements of intensity vs temperature. The electride was characterized further by two probe pressed pellet conductivity, variable temperature ⁷Li MAS-NMR, and DSC.