Shape and nanocrystal transformations: Cation exchange reactions that change the composition of a nanocrystal (NC) under mild conditions usually preserve the sublattice of the larger anions. Li et al. found that the shape of roxbyite (Cu _1.8 S) nanocrystals, which have a distorted, hexagonal, close-packed sulfide anion sublattice, affected the outcome of exchange reactions with cobalt ions. Flat nanoplates retained the anion lattice and formed cobalt sulfide, but tall nanorods transformed into Co ₉ S ₈ nanocrystals with a cubic, close-packed structure. Facile crystal plane sliding in the taller nanocrystals appears to have driven the different layer stacking of sulfide anions. Science , abh2741, this issue p. 332

Ultrafast control of ferroelectricity and magnetism by light is essential for future development in multiple functioning devices. Here, we demonstrate that the intense and ultrafast photo-modulation of the electric dipole can be realized by photocarrier injection into a multiferroic BiFeO₃ thin film using optical pump-probe and second harmonic generation measurements. Results of ultrafast electron diffraction with <100 fs time resolution and theoretical study reveal that the localized photocarrier strongly couples with the lattice structure and becomes the origin for the observed sudden change in the electric dipole. In addition, the subsequent structural dynamics involve a strong oscillation with a frequency of ~3.3 THz despite a poor structural symmetry change. Based on a theoretical calculation, this oscillation can be attributed to an unexpectedly softened new phonon mode generated by mixing essential two phonon modes governing the multiferroic (ferroelectric and antiferromagnetic) nature of BiFeO₃ in the ground state due to strong coupling with a localized photocarrier. The comprehensive study shows that injection of the localized photocarrier strongly coupled with the lattice vibration mode can simultaneously realize the ultrafast switching of electric dipoles and magnetic interaction at once, even at room temperature, without modifying the long-range lattice structure.

The crystal structure of the spin dimer magnet NaCu₂VP₂O₁₀was determined using single-crystal X-ray diffraction and electron diffraction. NaCu₂VP₂O₁₀displayed a non-centrosymmetric orthorhombicC222₁structure witha= 6.13860 (10) Å,b= 14.4846 (3) Å andc= 8.2392 (2) Å. The layered structure comprised CuO₄plaquettes, VO₆octahedra and PO₄tetrahedra. A pair of CuO₄plaquettes formed Cu₂O₆structural dimers through edge sharing. The Cu–Cu network formed a distorted puckered-layer structure with pseudo-one-dimensional characteristics. Maximum magnetic susceptibility was observed at ∼60 K and NaCu₂VP₂O₁₀became non-magnetic upon further cooling. The spin gap between the spin-singlet non-magnetic ground state and triplet excited state was estimated to be 43.4 K. Thus, NaCu₂VP₂O₁₀was assumed to be an alternating chain system with a singlet ground state of dimer origin. The V⁵⁺ions in the VO₆octahedra showed large off-centre displacements along the [110] direction in the primitive perovskite structure, which were attributed to the pseudo-Jahn–Teller distortion ofd⁰transition metals.

X-ray fluorescence holography (XFH) measurements were carried out for multiferroic $\mathrm{Pb}\left({\mathrm{Fe}}_{1/2}{\mathrm{Nb}}_{1/2}\right){\mathrm{O}}_3$ (PFN) single crystal to investigate variations in the local structures around Nb ions with decreasing temperature from 300 to 100 K, in combination with single-crystal x-ray diffraction (XRD) experiments. We find that the Nb–Pb correlation is more significantly changed by cooling than the average structure derived from the single-crystal XRD experiments, indicating a mobile feature of Nb ions. Also, we analyze previously obtained XFH results around Fe ions in detail for comparison with those around Nb ions. In contrast to the case of Nb ions, the variation in the Fe–Pb correlation with decreasing temperature is well explained by the average structure from 300 K down to the Néel temperature of about 150 K. On the other hand, a remarkable positional fluctuation between Fe and Pb ions is observed at 100 K, which is evaluated to be 90% larger than that between Nb and Pb ions. This observation strongly indicates the presence of spin-lattice coupling in PFN. The present study clearly elucidates the differences of local environment around the Nb ions from that around the Fe ions located at the same $B$-site.