Pressure-induced structural phase transition in fullerides doped with rare-earth metals

Abstract

Rare-earth-metal-doped fullerides with nominal composition of R 3 C 70 (RϭSm, Eu͒ adopt a pseudomonoclinic structure in which C 70 dimers glued with rare-earth ions are involved. High-pressure powder x-ray diffraction experiments revealed that these compounds undergo a reversible first-order structural phase transition at 1.5 GPa, associated with 2.7%-2.9% reduction of the unit cell volume. Structural analyses showed that the rare-earth ions, which are located close to the edge of tetrahedral sites at ambient pressure, move back to the center of the tetrahedral sites. Simultaneously, C 70 molecules are realigned so that the fivefold ͑long͒ axes are perpendicular to the (101 ) or (111 ) f cc plane at high pressure. The derived charge density map indicates that the transition is regarded as a structural change from dimers to three-dimensional polymers of fullerenes. These features are ascribed to the unique bonding nature in rare-earth C 70 compounds.

Isosymmetric Lattice Collapse in Mixed-Valence Rare-Earth Fullerides at High Pressure─Coupling of Lattice and Electronic Degrees of Freedom

“…Notably, the bulk modulus is comparable to that of highly compressible peptide metal–organic frameworks ( K 0 = 21.9(9) GPa) . The computed value of the initial volume compressibility, κ = −d ln V /d P , of the O phase is 0.0411(17) GPa –1 , of the same magnitude as those of other rare-earth fullerides (Sm 2.75 C 60 : κ = 0.033(1) GPa –1 ; Sm 2.78 C 70 κ = 0.038(1) GPa –1 ). , It signifies a highly compressible solid with comparably tight crystal packing to that of the fcc-structured A 3 C 60 alkali fulleridesthe value of the initial volume compressibility lies between those of K 3 C 60 (κ = 0.036(3) GPa –1 ) and Rb 3 C 60 (κ = 0.046(3) GPa –1 ). , Comparable treatment of the structural data of the O′ phase is not possible due to the unavailability of information at low pressures. Instead, a linear fit to the V ( P ) data of the O′ phase in the pressure range 4.2 to 6.0 GPa yields a value of the linear compressibility in this pressure range, κ = 0.0049(1) GPa –1 .…”

“…34 The computed value of the initial volume compressibility, κ = −d ln V/dP, of the O phase is 0.0411(17) GPa −1 , of the same magnitude as those of other rare-earth fullerides (Sm 2.75 C 60 : κ = 0.033(1) GPa −1 ; Sm 2.78 C 70 κ = 0.038(1) GPa −1 ). 16,35 It signifies a highly compressible solid with comparably tight crystal packing to that of the fccstructured A 3 C 60 alkali fullerides�the value of the initial volume compressibility lies between those of K 3 C 60 (κ = 0.036(3) GPa −1 ) and Rb 3 C 60 (κ = 0.046(3) GPa −1 ). 36,37 Comparable treatment of the structural data of the O′ phase is not possible due to the unavailability of information at low pressures.…”

Reversible pressure-induced polymerization of Fe(C5H5)2 doped C70

“…Several different forms of C 70 dimers have been produced, such as C 2h C 140 , C 2v C 140 and C 1 C 140 [15][16][17], and formation of polymeric zigzag chains in initially hexagonally close packed C 70 single crystals is reported by Soldatov et al [18]. We also notice that when the rare-earth metal Sm is intercalated into C 70 [19], the charge transfer between C 70 s and Sm atoms results in Sm-C 70 bonding in a 2D network structure, with a Sm atom as a bridge. Such an interesting physical phenomenon also indicates that charge transfer may play an important role in the polymerization of C 70 and thus requires further investigations.…”

“…In general, the one-dimensional C 70 polymers can only be obtained either by applying simultaneous HPHT conditions or by doping [18,19]. Compared to the case of C 60 polymerization, the various configurations in C 70 polymer phases can be related to the special elliptical shape and the fact that reactive double bonds exist only on the polar caps of the molecules.…”