We report the direct observation by x-ray diffraction of a photoinduced paraelectric-to-ferroelectric structural phase transition using monochromatic 100-picosecond synchrotron pulses. It occurs in tetrathiafulvalene-p-chloranil, a charge-transfer molecular material in which electronic and structural changes are strongly coupled. An optical 300-femtosecond laser pulse switches the material from a neutral to an ionic state on a 500-picosecond time scale and, by virtue of intrinsic cooperativity, generates self-organized long-range structural order. The x-ray data indicate a macroscopic ferroelectric reorganization after the laser irradiation. Refinement of the structures before and after laser irradiation indicates structural changes at the molecular level.

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Neutron diffraction studies have been carried out to shed light on the unprecedented order-disorder phase transition (ca. 155 K) observed in the mixed-valence iron(II)-iron(III) formate framework compound [NH(2)(CH(3))(2)](n)[Fe(III)Fe(II)(HCOO)(6)](n). The crystal structure at 220 K was first determined from Laue diffraction data, then a second refinement at 175 K and the crystal structure determination in the low temperature phase at 45 K were done with data from the monochromatic high resolution single crystal diffractometer D19. The 45 K nuclear structure reveals that the phase transition is associated with the order-disorder of the dimethylammonium counterion that is weakly anchored in the cavities of the [Fe(III)Fe(II)(HCOO)(6)](n) framework. In the low-temperature phase, a change in space group from P31c to R3c occurs, involving a tripling of the c-axis due to the ordering of the dimethylammonium counterion. The occurrence of this nuclear phase transition is associated with an electric transition, from paraelectric to antiferroelectric. A combination of powder and single crystal neutron diffraction measurements below the magnetic order transition (ca. 37 K) has been used to determine unequivocally the magnetic structure of this Néel N-Type ferrimagnet, proving that the ferrimagnetic behavior is due to a noncompensation of the different Fe(II) and Fe(III) magnetic moments.

We report on high correlation and ordering of multi-electron transfer at neutral-ionic transformation in quasi–one-dimensional prototype crystal of tetrathiafulvalene-chloranil (TTF-CA). We present experimental evidences that the ordering of lattice-relaxed charge-transfer strings, which are nano-scale objects, occurs under static thermal equilibrium condition and non-equilibrium one achieved by femto-second laser irradiation. We suggest that the transformation is a profound example of self-organized, cooperative electron transfer with tendency for phase separation driven by energetics of inter-chain interaction and ferroelectric ordering.

We have determined the crystal structure of ammonia monohydrate phase II (AMH II) employing a combination of ab initio computational structure prediction and structure solution from neutron powder diffraction data using direct space methods. Neutron powder diffraction data were collected from perdeuterated AMH II using the D2B high-resolution diffractometer at the Institut Laue-Langevin. AMH II crystallizes in space-group Pbca with 16 formula units in a unit-cell of dimensions a = 18.8285(4) A, b = 6.9415(2) A, c = 6.8449(2) A, and V = 894.61(3) A3 [rho(calc)(deuterated) = 1187.56(4) kg m(-3)] at 502 MPa, 180 K. The structure is characterized by sheets of tessellated pentagons formed by orientationally ordered O-D...O, O-D...N, and N-D...O hydrogen-bonds; these sheets are stacked along the a-axis and connected by N-D...O hydrogen bonds alone. With the exception of the simple body-centered-cubic high-pressure phases of ammonia monohydrate and ammonia dihydrate, this is the first complex molecular structure of any of the high-pressure stoichiometric ammonia hydrates to be determined. The powder structure solution is complemented by an ab initio structure prediction using density functional theory which gives an almost identical hydrogen bonding network.

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Direct structural evidence of the periodic ordering of neutral and ionic planes is reported for 2,6dimethyltetrathiafulvalene-p-chloranil, detected by high-resolution x-ray diffraction experiments. Simultaneous to the neutral-ionic layered ordering, theoretically predicted a long time ago to occur in stages, an antiparallel dimerization ordering takes place, leading to a ''ferrielectric'' structure. Both intramolecular deformations and intermolecular contacts, related to the charge-transfer ordering and to the dimerization process, are discussed and compared with results obtained for the prototype compound of the neutral to ionic transition, tetrathiafulvalene-p-chloranil. Thus essential insights are provided for an understanding of the mechanism of this unusual electronic-structural instability, and important physical features are discussed such as the interstack interactions, the coupled order parameters, and the interplay between the staging and condensation mechanisms.