Collective effects in spin-crossover chains with exchange interaction

The collective properties of spin-crossover chains are studied. Spin-crossover compounds contain ions with a low-spin ground state and low lying high-spin excited states and are of interest for molecular memory applications. Some of them naturally form one-dimensional chains. Elastic interaction and Ising exchange interaction are taken into account. The transfer-matrix approach is used to calculate the partition function, the fraction of ions in the high-spin state, the magnetization, susceptibility, etc., exactly. The high-spin-low-spin degree of freedom leads to collective effects not present in simple spin chains. The ground-state phase diagram is mapped out and compared to the case with Heisenberg exchange interaction. The various phases give rise to characteristic behavior at nonzero temperatures, including sharp crossovers between low- and high-temperature regimes. A Curie-Weiss law for the susceptibility is derived and the paramagnetic Curie temperature is calculated. Possible experiments to determine the exchange coupling are discussed.Comment: 9 pages, 13 color figures, published versio

Local structure of a switchable dielectric Prussian blue analogue

The Prussian blue (cyanide-bridged, ordered double perovskite) analogue potassium imidazolium hexacyanoferrate, (C3N2H5)2KijFeIJCN)6], contains imidazolium cations encapsulated within a metal-cyanide framework. These are free to rotate in the intermediate- and high-temperature phases, but freeze into fixed orientations in the low-temperature phase. The phase transition between intermediate- and low temperature phases thus causes a substantial change in this material's dielectric constant. However, the detailed cation dynamics, and in particular how they differ between intermediate- and high-temperature phases, remain unclear. We report here total neutron scattering measurements on a perdeuterated sample of this material. Reverse Monte Carlo modelling reveals that the intermediate-temperature phase is associated with a stiffening of the metal-cyanide framework compared to either of the other phases. This shows that the dynamics responsible for the phase transitions involve competition between the energetic penalty for bending the metal-cyanide links and the benefit of host–guest hydrogen bonding. Our results demonstrate both that disordered framework materials have important local structure that is not visible to Bragg scattering, and that there is a crucial link between this structure and the dynamics that give rise to exploitable electric properties

Real time contrast enhanced ultrasonography in detection of liver metastases from gastrointestinal cancer

Background: Contrast enhanced ultrasound (CEUS) is an imaging technique which appeared on the market around the year 2000 and proposed for the detection of liver metastases in gastrointestinal cancer patients, a setting in which accurate staging plays a significant role in the choice of treatment. Methods: A total of 109 patients with colorectal (n = 92)or gastric cancer prospectively underwent computed tomography (CT) scan and conventional US evaluation followed by real time CEUS. A diagnosis of metastases was made by CT or, for lesions not visibile at CT, the diagnosis was achieved by histopathology or by a malignant behavior during follow-up. Results: Of 109 patients, 65 were found to have metastases at presentation. CEUS improved sensitivity in metastatic livers from 76.9% of patients (US) to 95.4% (p < 0.01), while CT scan reached 90.8% (p = n.s. vs CEUS, p < 0.01 vs US). CEUS and CT were more sensitive than US also for detection of single lesions (87 with US, 122 with CEUS, 113 with CT). In 15 patients (13.8%), CEUS revealed more metastases than CT, while CT revealed more metastases than CEUS in 9 patients (8.2%) (p = n.s.). Conclusion: CEUS is more sensitive than conventional US in the detection of liver metastases and could be usefully employed in the staging of patients with gastrointestinal cancer. Findings at CEUS and CT appear to be complementary in achieving maximum sensitivity. © 2007 Piscaglia et al; licensee BioMed Central Ltd

Electric-field-induced charge-transfer phase transition: a promising approach toward electrically switchable devices

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb₀․₈Mn-[Fe(CN)₆]₀․₉₃•1.62H₂O and Co₃[W(CN)₈]₂(pyrimidine)₄•6H₂O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices

Matériels associés à l'utilisation des produits explosifs et aux dispositifs d'amorçage

Cet article rédigé par le sous-groupe Matériels associés du GFEE synthétise le travail qu'il a mené. Il a pour vocation d'apporter des compléments d'informations aux utilisateurs, aux responsables sécurité et aux donneurs d'ordre afin d'orienter les prescriptions d'utilisation de ces matériels

Series of M^I[Co(bpy)₃][Mo(CN)₈]·<i>n</i>H₂O (M^I = Li (1), K (2), Rb (3), Cs (4); <i>n</i> = 7−8) exhibiting reversible diamagnetic to paramagnetic transition coupled with dehydration−rehydration process

In this paper we report the synthesis and the structural and magnetic properties of the series of ionic compounds with general formula: MI[Co(bpy)3][Mo(CN)8]·nH2O (MI = Li, n = 8 (1), MI = K, n = 8 (2), MI = Rb, n = 8 (3), MI = Cs, n = 7.5 (4)). Solids 1−4 are characterized by the optical outer-sphere metal-to-metal charge transfer (MMCT) transition from Mo(IV) center to Co(III) center in the visible region and the CoIIIMoIV CoIIMoV spin equilibrium strongly dominated by the CoIIIMoIV form. We show a gentle thermal treatment of diamagnetic compounds 1−4 leading to the dehydrated forms 1a−4a, which reveal a significant increase of paramagnetic contribution (from 0.5 to 2% to 30−40%). The rehydration allows to recover the diamagnetic phases 1b−4b of compositions and properties similar to those of 1−4. The irradiation of the dehydrated form 2a within the MMCT band in the Superconducting Quantum Interference Device (SQUID) cavity at T = 10 K causes further increase of the CoIIMoV contribution giving the metastable phase annealed back to the 2a phase after heating above T = 290 K. The IR, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) spectroscopic data along with the magnetic data are interpreted in terms of strong modification of the CoIIIMoIV CoIIMoV equilibrium occurring in these systems