Negative thermal expansion and local dynamics inCu2OandAg2O

Abstract

High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure ͑EXAFS͒ measurements have been performed on the iso-structural framework crystals Cu 2 O and Ag 2 O as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion ͑NTE͒ of the lattice parameter over extended temperature intervals ͑from 9 to 240 K for Cu 2 O, up to 470 K for Ag 2 O͒ and anisotropic thermal displacements of M atoms ͑M = Cu,Ag͒. EXAFS measures a positive expansion of the nearest-neighbors M-O pair distance and a perpendicular to parallel anisotropy of relative motion, much stronger than the anisotropy of the absolute M motion. The M-O bond is much stiffer against stretching than against bending. According to EXAFS, out of the 12 MM next-nearest-neighbor pairs, the 6 connected via a bridging oxygen undergo negative expansion, while the 6 lacking the bridging oxygen undergo positive expansion. These results show a rather complex local behavior, which, while confirming the connection of NTE to strong perpendicular vibrations, is inconsistent with rigid unit modes models and suggests a more flexible model based on rigid M-O rods.

Unusual molecular material formed through irreversible transformation and revealed by 4D electron microscopy

“…In this study, we demonstrate that a fraction of Fe(pyrazine)-Pt(CN) 4 nanoparticles undergo an irreversible phase transformation upon photothermal excitation, i.e. the combined laser photon excitation and the resulting temperature jump.…”

“…On a time scale of tens of seconds, the particle shrinks, looses its distinct diffraction contrast and its electron diffraction pattern becomes quite broad. What at first seems to be a damage of the nanoparticle, eventually appears to be an irreversible phase transformation originating from the well-known crystal structure of Fe(pyrazine)Pt(CN) 4 . 11,15 The appearance of the nanoparticle and its diffraction pattern remains unchanged after the phase transformation, even under further light irradiation.…”

“…1D and E). [11][12][13] Our previous 4D electron microscopy work on this compound has demonstrated the reversible thermoswitching, between lowand high-spin phases of single Fe(pyrazine)Pt(CN) 4 nanoparticles excited by short pulses of laser light. 14 The unit cell expansion was mapped in real-and reciprocal-space.…”

“…16. First, two aqueous solutions of reactants were prepared: (a) 0.2 mmol of Fe(BF 4 ) 2 ÁH 2 O + 0.8 mmol of pyrazine (in excess) dissolved in 4 mL of deionized water, and (b) 0.2 mmol of K 2 Pt(CN) 4 dissolved in 4 mL of deionized water. Then, two starting micro-emulsions were prepared by the slow addition of these aqueous solutions to 44.5 mmol of dioctyl sodium sulfosuccinate (NaAOT) previously dissolved in 88 mL of n-heptane.…”

“…Here, we employ 4D electron microscopy, 9,10 with in situ photothermal excitation and timeresolved single-nanoparticle imaging and diffraction capabilities, to directly visualize NTE of molecular framework nanoparticles. The starting material is the spin-crossover compound Fe II (pyrazine)Pt II (CN) 4 in the form of plate-like nanocrystals ( Fig. 1A-C), which exhibits a cooperative first-order phase transition from a diamagnetic low-spin state to a paramagnetic high-spin state around 250 K ( Fig.…”