Anti-ferrodistortive Nanodomains in PMN Relaxor

Temperature dependent studies of the 1/2(hk0) superlattice reflections \alpha spots by synchrotron x-ray scattering measurements were performed in (PMN) and (PMN-xPT) with Ti doping x<0.32 single crystals. Separation of the \alpha spots from the underlying diffuse scattering background allowed studying them as separate entities for the first time. Structure factor calculations have shown that alpha spots constitute the presence of a new kind of anti-ferrodistortive nanoregions (AFR) in the form of fluctuations produced by anti-parallel short-range correlated Pb^2+ displacements. AFR appear to be different and unrelated to the chemical nanodomains (CND) and ferroelectric polar nanoregions (PNR). Simultaneous presence of AFR and PNR can explain relaxor behavior as a result of competition between randomly occurring ferroelectric and anti-ferroelectric fluctuations. Temperature dependence of the \alpha spots in PMN showed a direct correlation with the freezing phase transition near Tf~220 K.Comment: 10 pages, 7 figures, Conference-Fundamental Physics of Ferroelectrics 200

Synchrotron x-ray Studies of Superlattice Ordering in Pb(Mg1/3Nb2/3)O3 Single Crystals Doped with PbTiO3

The temperature dependence of the superlattice reflections: a) F spots and b) alpha spots in a lead magnesium niobate (PMN) single crystals containing 0% and 6% of PbTiO3 (PT) has been studied using synchrotron x-ray scattering techniques. (No superlattice reflections were found in PMN doped with 32% PT). Analysis of the temperature dependence of the alpha spots suggests the existence of the correlated anti-parallel atomic displacements that form nanoregions different from the chemical nanodomains. While the correlation length is temperature independent, the magnitude of these displacements increases on cooling below the freezing temperature Tf. Intensities of the alpha spots above this temperature become indistinguishable from the background. Our results show that value of Tf for each composition is very close to the one obtained from a Vogel-Fulcher fit to the frequency dependence of the dielectric constant maximum Tm. The relation of these correlated anti-ferrodistortive fluctuations to polar ferroelectric nanodomains and relaxor behavior needs further study.Comment: 10 pages, 7 figures, Conference-Fundamental Physics of Ferroelectrics 200

Condensation of the atomic relaxation vibrations in lead-magnesium-niobate at T=T∗T=T^*

We present neutron diffraction, dielectric permittivity and photoconductivity measurements, evidencing that lead-magnesium niobate experiences a diffuse phase transformation between the spherical glass and quadrupole glass phases, in the temperature interval between 400 K and 500 K, with the quadrupole phase possessing extremely high magnitudes of dielectric permittivity. Our analysis shows that the integral diffuse scattering intensity may serve as an order parameter for this transformation. Our experimental dielectric permittivity data support this choice. These data are important for the aplications desiring giant dielectric responses, in a wide temperature intervals and not related to electron's excitations.Comment: 6 figure

Electrochemical deposition of zeolitic imidazolate framework electrode coatings for supercapacitor electrodes

Zn and Co electrodes have been successfully coated with five different zeolitic imidazolate frameworks ZIFs (ZIF-4, ZIF-7, ZIF-8, ZIF-14 and ZIF-67) via the anodic dissolution method. Careful control of the reaction conditions allows for electrode coating growth; in contrast to previous reports of electrochemical ZIF growth, which have not succeeded in obtaining ZIF electrode coatings. Coating crystallinity is also shown to be heavily dependent upon reaction conditions, with amorphous rather than crystalline material generated at shorter reaction times and lower linker concentrations. Electrochemical applications for ZIF-coated electrodes are highlighted with the observation of an areal capacitance of 10.45 mF cm−2 at 0.01 V s−1 for additive-free ZIF-67 coated Co electrodes. This is superior to many reported metal organic framework (MOF)/graphene composites and to capacitance values previously reported for additive-free MOFs

Determining crystal structures through crowdsourcing and coursework

We show here that computer game players can build high-quality crystal structures. Introduction of a new feature into the computer game Foldit allows players to build and real-space refine structures into electron density maps. To assess the usefulness of this feature, we held a crystallographic model-building competition between trained crystallographers, undergraduate students, Foldit players and automatic model-building algorithms. After removal of disordered residues, a team of Foldit players achieved the most accurate structure. Analysing the target protein of the competition, YPL067C, uncovered a new family of histidine triad proteins apparently involved in the prevention of amyloid toxicity. From this study, we conclude that crystallographers can utilize crowdsourcing to interpret electron density information and to produce structure solutions of the highest quality

A COMPARISON OF EXPERIMENT AND THE THEORY OF CONTINUOUS ORDERING

La transformée de Fourier de l'Energie Libre d'Helmholtz F(k), pour des fluctuations de composition de vecteur d'onde k, a été obtenue dans le cas de plusieurs alliages ordonnés au-dessus de Tc en effectuant des mesures absolues, à haute température, de l'intensité due à l'ordre local le long des rangées de l'espace réciproque. Les monocristaux étudiés étaient les suivants : Cu3Au, Cu-23 atomes % Au, Cu-18,5 atomes % Au, et CoPt3. Les résultats sont comparés à la théorie des transformations continues et il apparaît que l'approximation harmonique linéaire est insuffisante. En particulier l'énergie due au gradient dépend fortement de k. Ceci apparaît aussi à partir des mesures des temps de relaxation effectuées suivant différentes directions cristallographiques : ceux-ci varient moins que prévu avec la direction quand on néglige la variation de l'énergie due au gradient. Il est possible de mesurer l'énergie d'activation de l'interdiffusion en volume dans les alliages à basse température en effectuant des mesures de l'intensité absolue due à l'ordre local en fonction de la température et en mesurant ensuite les variations de cette intensité avec le temps lorsque la température varie. Les résultats sont donnés pour les alliages déjà cités et pour l'alliage Fe-29,2 atomes % Al. Cette recherche a été financée par la U.S. National Science Foundation grâce à un contrat n° DMR 73-07549 A01 et par le Centre de Recherche des Matériaux de la Northwestern University subventionné par la National Science Foundation sous contrat n° DMR 760157.The Fourier transform of the Helmholtz Free Energy F(k), for composition fluctuations of wave vector k, has been obtained for several ordering alloys above Tc by measuring the absolute X-ray diffuse scattering intensity due to local order, at high temperatures, along lines in reciprocal space. The single crystals examined were Cu3Au, Cu-23 at pct Au, Cu-18.5 at pct Au, and CoPt3. The values obtained are compared to the theory of continuous transformations and it is found that the linear theory is satisfactory but the nearest-neighbour approximation is inadequate. In particular, the gradient energy is a strong function of k. This is also indicated from measured relaxation times in different crystallographic directions ; these vary less with direction than predicted when the dependence of the gradient energy on k is ignored. It is possible to measure the activation energy for interdiffusion in bulk alloy crystals at low temperatures by measuring the absolute intensity due to local order us. temperature and then measuring changes in this intensity with time when the temperature is varied. Values are given for the above alloys and for Fe-29.2 at pct Al