Room-Temperature Magnetic Bistability in Organic Radical Crystals
08 Oct 1999-Science (American Association for the Advancement of Science)-Vol. 286, Iss: 5438, pp 261-262
TL;DR: A large first-order magnetic phase transition in an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl, is described and may have applications in thermal sensors, switching units, and information storage media based on organic radical crystals.
Abstract: A large first-order magnetic phase transition in an organic radical, 1,3,5-trithia-2,4,6-triazapentalenyl, is described. The transition occurs with a wide thermal hysteresis loop over the temperature range 230 to 305 kelvin. The high-temperature phase is paramagnetic, and its structure consists of a uniform one-dimensional stacking of the radical. The low-temperature phase is diamagnetic because of strong dimerization along the stacking direction. The results may have applications in thermal sensors, switching units, and information storage media based on organic radical crystals.
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References
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TL;DR: In this paper, a new method for obtaining optimized parameters for semi-empirical methods has been developed and applied to the modified neglect of diatomic overlap (MNDO) method.
Abstract: A new method for obtaining optimized parameters for semiempirical methods has been developed and applied to the modified neglect of diatomic overlap (MNDO) method. The method uses derivatives of calculated values for properties with respect to adjustable parameters to obtain the optimized values of parameters. The large increase in speed is a result of using a simple series expression for calculated values of properties rather than employing full semiempirical calculations. With this optimization procedure, the rate-determining step for parameterizing elements changes from the mechanics of parameterization to the assembling of experimental reference data.
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TL;DR: In this article, the transition temperature of transition metal compounds can be fine tuned using an approach based on the concept of a molecular alloy, and it is possible to design a compound for which room temperature falls in the middle of the thermal hysteresis loop.
Abstract: Some 3dn (4 ≤ n ≤ 7) transition metal compounds exhibit a cooperative transition between a low-spin (LS) and a high-spin (HS) state. This transition is abrupt and occurs with a thermal hysteresis, which confers a memory effect on the system. The intersite interactions and thus the cooperativity are magnified in polymeric compounds such as [Fe(Rtrz)3]A2·nH2O in which the Fe2+ ions are triply bridged by 4-R-substituted-1,2,4-triazole molecules. Moreover, in these compounds, the spin transition is accompanied by a well-pronounced change of color between violet in the LS state and white in the HS state. The transition temperatures of these materials can be fine tuned, using an approach based on the concept of a molecular alloy. In particular, it is possible to design a compound for which room temperature falls in the middle of the thermal hysteresis loop. These materials have many potential applications, for example, as temperature sensors, as active elements of various types of displays, and in information storage and retrieval.
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TL;DR: In this article, the paramagnetic resonance spectrum of copper acetate is anomalous in that it resembles that of an ion of spin 1, and its intensity decreases as the temperature is lowered.
Abstract: The paramagnetic resonance spectrum of copper acetate is anomalous in that it resembles that of an ion of spin 1, and its intensity decreases as the temperature is lowered. The latter is correlated with the decreasing susceptibility found by Guha (1951). The following hypotheses are suggested: (1) the crystalline field acting on each copper ion is similar to that in other salts such as the Tutton salts; (2) isolated pairs of copper ions interact strongly through exchange forces, each pair forming a lower singlet state and an upper triplet state, the latter only being paramagnetic. On this basis both the fine structure and the hyperfine structure of the spectrum have a simple explanation, and the theory also predicts a small initial splitting of the triplet state of the same order as that found experimentally. The unit cell of the crystal contains two differently oriented pairs of ions, and, using an empirical value for the exchange parameter, fair agreement with the susceptibility measurements of Guha is obtained.
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Abstract: We analyze a series of recent experiments in which it has been found that pressure induces changes in charge or spin of iron [i.e., Fe(III) reduces to Fe(II) and in Fe(II) there are spin changes involving either decrease or increase of multiplicity]. In addition to considering the volume difference of the two states at zero pressure, we find it important to include effects both of the difference in compressibility of the two states, and the change in compressibility of a given state with pressure. The theory casts the problem in terms of the changes in Coulomb energy, closed shell repulsions, and both covalent bonding energy and crystal field energy accompanying the change in electronic state. In addition, interactions between converted iron atoms are included by a form of mean field theory and the effects are shown to be significant. Not only is the theory discussed analytically, but also a simple graphical solution is shown which makes it possible to examine readily the qualitative effects of the various parameters. Repulsive interactions spread the conversion over a larger pressure range and may thus explain why so many compounds exhibit rather broad transitions. Attractive and repulsive interactions can lead to cooperative effects. They should account for the reversible yet discontinuous jumps in conversion as a function of temperature previously observed in several phenanthroline and dipyridyl compounds. The possibility of hysteresis is also indicated. Major anomalies in the combined temperature and pressure variation of the conversion are accounted for by including the temperature dependence of the free energy of interaction.
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373 citations