Synthesis of 3d metallic single-molecule magnets

Hydrogen-terminated diamond exhibits a high surface conductivity (SC) that is commonly attributed to the direct action of hydrogen-related acceptors. We give experimental evidence that hydrogen is only a necessary requirement for SC; exposure to air is also essential. We propose a mechanism in which a redox reaction in an adsorbed water layer provides the electron sink for the subsurface hole accumulation layer. The model explains the experimental findings including the fact that hydrogenated diamond is unique among all semiconductors in this respect.

Origin of Surface Conductivity in Diamond

Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.

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High-pressure phases of group-IV, III–V, and II–VI compounds

We herein present the synthesis, crystal structure, and magnetic properties of a new heterometallic chain of MnIII and NiII ions, [Mn2(saltmen)2Ni(pao)2(py)2](ClO4)2 (1) (saltmen2- = N,N'-(1,1,2,2-tetramethylethylene) bis(salicylideneiminate) and pao- = pyridine-2-aldoximate). The crystal structure of 1 was investigated by X-ray crystallographic analysis: compound 1 crystallized in monoclinic, space group C2/c (No. 15) with a = 21.140(3) A, b = 15.975(1) A, c = 18.6212(4) A, beta = 98.0586(4) degrees , V = 6226.5(7) A3, and Z = 4. This compound consists of two fragments, the out-of-plane dimer [Mn2(saltmen)2]2+ as a coordination acceptor building block and the neutral mononuclear unit [Ni(pao)2(py)2] as a coordination donor building block, forming an alternating chain having the repeating unit [-Mn-(O)2-Mn-ON-Ni-NO-]n. In the crystal structure, each chain is well separated with a minimum intermetallic distance between Mn and Ni ions of 10.39 A and with the absence of interchain pi overlaps between organic ligands. These features ensure a good magnetic isolation of the chains. The dc and ac magnetic measurements were performed on both the polycrystalline sample and the aligned single crystals of 1. Above 30 K, the magnetic susceptibility of this one-dimensional compound was successfully described in a mean field approximation as an assembly of trimers (Mn...Ni...Mn) with a NiII...MnIII antiferromagnetic interaction (J = -21 K) connected through a ferromagnetic MnIII...MnIII interaction (J'). However, the mean field theory fails to describe the magnetic behavior below 30 K emphasizing the one-dimensional magnetic character of the title compound. Between 5 and 15 K, the susceptibility in the chain direction was fitted to a one-dimensional Ising model leading to the same value of J'. Hysteresis loops are observed below 3.5 K, indicating a magnet-type behavior. In the same range of temperature, combined ac and dc measurements show a slow relaxation of the magnetization. This result indicates the presence of a metastable state without magnetic long-range order. This material is the first experimental design of a heterometallic chain with ST = 3 magnetic units showing a "single-chain magnet" behavior predicted in 1963 by R. J. Glauber for an Ising one-dimensional system. This work opens new perspectives for one-dimensional systems to obtain high temperature metastable magnets by combining high spin magnetic units, strong interunit interactions, and uniaxial anisotropy.

Evidence for Single-Chain Magnet Behavior in a MnIII−NiII Chain Designed with High Spin Magnetic Units: A Route to High Temperature Metastable Magnets

Supramolecular coordination compounds bear exceptional advantages over their organic counterparts. They are available in one-pot reactions and in high yields and display physical properties that are generally inaccessible with organic species. Moreover, their weak, reversible, noncovalent bonding interactions facilitate error checking and self-correction. This Review emphasizes the achievements in supramolecular coordination chemistry initiated by serendipity and their materialization based on rational design. The recognition of similarities in the synthesis of different supramolecular assemblies allows prediction of potential results in related cases. Supramolecular synthesis obeys guidelines comparable to the "lead sheet" used by small jazz ensembles for improvisation and therefore more often leads to unpredicted results. The combination of detailed symmetry considerations with the basic rules of coordination chemistry has only recently allowed for the design of rational strategies for the construction of a variety of nanosized systems with specified size and shape.

Supramolecular coordination chemistry: the synergistic effect of serendipity and rational design.

The self-trapped-electron center in ${\mathrm{PbCl}}_{2}$ crystals, i.e., the ${\mathrm{Pb}}_{2}^{3+}$ molecular ion resulting from the trapping of a conduction electron by a pair of ${\mathrm{Pb}}^{2+}$ lattice cations along the a direction, exhibits in the 10--78 K temperature range a strong temperature dependence in both g and hyperfine tensors of the ESR spectrum. Such an unusual large variation, which is not observed for the isoelectronic ${\mathrm{Tl}}_{2}^{+}$ and ${\mathrm{Pb}}_{2}^{3+}$ impurity centers in alkali chlorides, seems to be related to the anisotropic thermal expansion of the layered ${\mathrm{PbCl}}_{2}$ lattice.

Temperature variation of the ESR parameters of the self-trapped-electron center in PbCl2.

Experimental values of the $g$ and hyperfine components of ${{X}_{2}}^{\ensuremath{-}}\ensuremath{-}{V}_{K}$ centers and ${{Y}_{2}}^{\ensuremath{-}}\ensuremath{-}{V}_{K}$-type centers ($X, Y=\mathrm{F},\phantom{\rule{0ex}{0ex}}\mathrm{C}\mathrm{l},\phantom{\rule{0ex}{0ex}}\mathrm{B}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{I}$) in a large number of pure ($\mathrm{AX}$) and doped ($AX:{Y}^{\ensuremath{-}}$) alkali halides have been determined through a careful analysis of their electron-paramagnetic-resonance (EPR) spectra. Expressions for the $g$ components have been derived. For an accurate quantitative analysis, it is necessary to take into account the overlap terms in the calculation of $\ensuremath{\delta}$, the matrix element of the orbital angular momentum $\stackrel{\ensuremath{\rightarrow}}{1}$ between the $^{2}\ensuremath{\Sigma}_{u}^{+}$ ground state and the $^{2}\ensuremath{\Pi}_{u}$ state. A calculation for ${\mathrm{Cl}}_{2}^{\ensuremath{-}}$ yields $\ensuremath{\delta}=0.78$. Representative experimental values for ${\mathrm{Cl}}_{2}^{\ensuremath{-}}$ and ${\mathrm{Br}}_{2}^{\ensuremath{-}}$ are $\ensuremath{\delta}=0.73 \mathrm{and} 0.70$, respectively. The $^{2}\ensuremath{\Sigma}_{u}^{+}\ensuremath{-}^{2}\ensuremath{\Pi}_{u}$ energy differences thus determined decrease going from the Rb salt to the Li salt. This behavior, in general, parallels that of the $^{2}\ensuremath{\Sigma}_{u}^{+}\ensuremath{-}^{2}\ensuremath{\Sigma}_{g}^{+}$ and $^{2}\ensuremath{\Sigma}_{u}^{+}\ensuremath{-}^{2}\ensuremath{\Pi}_{g}$ energy differences which are known from the optical-absorption measurements. The crystal field splitting of the $^{2}\ensuremath{\Pi}_{u}$ state is determined from the orthorhombic character of the $g$ tensor. Expressions for the hyperfine components have been derived which include the first- and second-order correction terms originating from the combined effect of the spin-orbit and hyperfine operators. From the analysis of the combined behavior of the orthorhombic character of the $g$ and hyperfine (hf) components, it can be concluded that ${A}_{\ensuremath{\parallel}}g0$ and ${A}_{\ensuremath{\perp}}g0$ for ${\mathrm{Cl}}_{2}^{\ensuremath{-}}$, ${\mathrm{Br}}_{2}^{\ensuremath{-}}$, and ${\mathrm{I}}_{2}^{\ensuremath{-}}$, and ${A}_{\ensuremath{\parallel}}g0$ and ${A}_{\ensuremath{\perp}}l0$ for ${\mathrm{F}}_{2}^{\ensuremath{-}}$. The analysis of the hf components then shows that the isotropic part of the hf interaction decreases monotonically going from the Rb to the Li salt, while the anisotropic part remains approximately constant. EPR results for ${V}_{\mathrm{KA}}$, ${V}_{\mathrm{KAA}}$, and ${V}_{F}$ centers are also given and they are compared with the ${V}_{K}$-center data.

g and Hyperfine Components of V K Centers

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H2 gas flow rates, keeping a constant [CH4]:[H2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic atomic species, is directly related to the concentration of the extended lattice defects.

Nitrogen and hydrogen in thick diamond films grown by microwave plasma enhanced chemical vapor deposition at variable H2 flow rates

$〈111〉$-oriented F${\mathrm{Cl}}^{\ensuremath{-}}$ molecule ions occupying one negative ion vacancy are produced in ${\mathrm{F}}^{\ensuremath{-}}$-doped KCl by x or $\ensuremath{\gamma}$ irradiation at 77\ifmmode^\circ\else\textdegree\fi{}K. At temperatures above 150\ifmmode^\circ\else\textdegree\fi{}K these F${\mathrm{Cl}}^{\ensuremath{-}}$ centers decay by dissociating into a substitutional ${\mathrm{Cl}}^{\ensuremath{-}}$ ion and a F\ifmmode^\circ\else\textdegree\fi{} atom which can migrate interstitially. If the crystal also contains substitutional ${\mathrm{Br}}^{\ensuremath{-}}$ or ${\mathrm{I}}^{\ensuremath{-}}$ ions, the F\ifmmode^\circ\else\textdegree\fi{} atoms can be retrapped to form $〈111〉$-oriented F${\mathrm{Br}}^{\ensuremath{-}}$ and F${\mathrm{I}}^{\ensuremath{-}}$. The ESR spectra of these centers have also been found and analyzed in the other alkali chlorides together with a $〈111〉$-oriented F${\mathrm{Br}}^{\ensuremath{-}}$ in ${\mathrm{F}}^{\ensuremath{-}}$-doped KBr. This analysis and comparison with the ${{X}_{2}}^{\ensuremath{-}}$ centers indicates that for the fluorine hyperfine components in ${\mathrm{F}}_{2}^{\ensuremath{-}}$ and $\mathrm{F}{X}^{\ensuremath{-}}$, ${A}_{\mathrm{II}}(\mathrm{F})g0$ and ${A}_{\ensuremath{\perp}}(\mathrm{F})l0$, while for ${{X}_{2}}^{\ensuremath{-}}$ and $\mathrm{F}{X}^{\ensuremath{-}}$, ${A}_{\mathrm{II}}(X)g0$ and ${A}_{\ensuremath{\perp}}(X)g0$ ($X=\mathrm{C}\mathrm{l},\phantom{\rule{0ex}{0ex}}\mathrm{B}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{I}$). The main optical absorption band of F${\mathrm{Cl}}^{\ensuremath{-}}$ is situated at 300 m\ensuremath{\mu}, while the approximate positions for the F${\mathrm{Br}}^{\ensuremath{-}}$ and F${\mathrm{I}}^{\ensuremath{-}}$ transitions are at 294 m\ensuremath{\mu} and 275 m\ensuremath{\mu}, respectively.

〈 111 〉 -Oriented F Cl − , F Br − , and F I − Centers in Mixed Alkali Halides

Two thallium atom defects of tetragonal symmetry around $〈100〉$ are produced by x irradiation above 230 K in KCl: TlCl. Their electron-spin-resonance spectra are characterized by comparable and large $g$ shifts but quite different hyperfine parameters. A relativistic many-body calculation of the ${\mathrm{Tl}}^{0}$ atom hyperfine interaction including the effect of a tetragonal crystal field permits a quantitative analysis of the spin-Hamiltonian parameters. It is found in particular that the hyperfine data of the two ${\mathrm{Tl}}^{0}$ defects can be explained by assuming the presence of one or two perturbing positive charges, respectively, and a corresponding 35 and 45% delocalization of the $6{p}^{1}$ electron on the surrounding lattice. This analysis together with x-ray production, optical-excitation, and pulse-anneal data and the fact that both ${\mathrm{Tl}}^{0}$ defects can only be produced at temperatures where negative-ion vacancies are mobile (g230 K) allows one to propose precise defect structures: In one defect the ${\mathrm{Tl}}^{0}$ atom (on a positive-ion site) is associated with a single negative-ion vacancy in a nearest-neighbor position along $〈100〉$; in the other one, the ${\mathrm{Tl}}^{0}$ is flanked by two nearest-neighbor negative-ion vacancies along $〈100〉$. The data also provide strong evidence for the existence of a defect consisting of a ${\mathrm{Tl}}^{+}$ ion and a nearest-neighbor anion vacancy.

Dirk Schoemaker

Papers

Temperature variation of the ESR parameters of the self-trapped-electron center in PbCl2.

g and Hyperfine Components of V K Centers

Nitrogen and hydrogen in thick diamond films grown by microwave plasma enhanced chemical vapor deposition at variable H2 flow rates

〈 111 〉 -Oriented F Cl − , F Br − , and F I − Centers in Mixed Alkali Halides

Electron-spin-resonance study of Tl atom defects in KCl and relativistic many-body analysis of the hyperfine structure