Topotactic structural conversion and hydration-dependent thermal expansion in robust LnMIII(CN)6·nH2O and flexible ALnFeII(CN)6·nH2O frameworks (A = Li, Na, K; Ln = La–Lu, Y; M = Co, Fe; 0 ≤ n ≤ 5)

Abstract

The structures of the A_xLnM(CN)₆·nH₂O (A = Li, Na, K; Ln = La–Lu, Y; M = Co, Fe; x = 0, 1; 0 ≤ n ≤ 5) cyanide frameworks, their thermal expansion behaviour, and their transformations upon dehydration are explored using X-ray and neutron single crystal diffraction and X-ray powder diffraction. Modification from positive to negative thermal expansion in the LnCo(CN)₆·nH₂O phases is achieved by removal of the guest water molecules. Most notable is the unprecedented flexibility demonstrated by the “coiling” of KLnFe(CN)₆·nH₂O frameworks upon their dehydration, wherein the lanthanoid coordination geometry reversibly converts from a 9-coordinate tri-capped trigonal prism to a 6-coordinate octahedron via a single-crystal-to-single-crystal process, accompanied by a large (14–16%) decrease in unit cell volume.