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We present the first copper iridium binary metal oxide with the chemical formula Cu2IrO3. The material is synthesized from the parent compound Na2IrO3 by a topotactic reaction where sodium is exchanged with copper under mild conditions. Cu2IrO3 has the same monoclinic space group (C2/c) as Na2IrO3 with a layered honeycomb structure. The parent compound Na2IrO3 is proposed to be relevant to the Kitaev spin liquid on the basis of having Ir4+ with an effective spin of 1/2 on a honeycomb lattice. Remarkably, whereas Na2IrO3 shows a long-range magnetic order at 15 K and fails to become a true spin liquid, Cu2IrO3 remains disordered until 2.7 K, at which point a short-range order develops. Rietveld analysis shows less distortions in the honeycomb structure of Cu2IrO3 with bond angles closer to 120° compared to Na2IrO3. Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places Cu2IrO3 closer to a Kitaev spin liquid than its predecessors.

This work presents an integrated approach to study the crystal chemistry and phonon heat capacity of complex layered oxides. Two quaternary delafossites are synthesized from ternary parent compounds and copper monohalides via a topochemical exchange reaction that preserves the honeycomb ordering of the parent structures. For each compound, Rietveld refinement of the powder X-ray diffraction patterns is examined in both monoclinic C2/ c and rhombohedral R3̅ m space groups. Honeycomb ordering occurs only in the monoclinic space group. Bragg peaks associated with honeycomb ordering acquire an asymmetric broadening known as the Warren line shape that is commonly observed in layered structures with stacking disorder. Detailed TEM analysis confirms honeycomb ordering within each layer in both title compounds and establishes a twinning between the adjacent layers instead of the more conventional shifting or skipping stacking faults. The structural model is then used to calculate phonon dispersions and heat capacity from first principles. In both compounds, the calculated heat capacity accurately describes the experimental data. The integrated approach presented here offers a platform to carefully analyze the phonon heat capacity in complex oxides where the crystal structure can produce magnetic frustration. Isolating phonon contribution from total heat capacity is a necessary and challenging step toward a quantitative study of spin liquid materials with exotic magnetic excitations such as spinons and Majorana fermions. A quantitative understanding of phonon density of states based on crystal chemistry as presented here also paves the way toward higher efficiency thermoelectric materials.

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy-a tendency of spins to align in a certain crystallographic direction known as easy-axis. In layered materials, two limits of easy-axis are in-plane (XY) and out-of-plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin-orbitronic functions. Here, a unique experiment is designed to control the easy-axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl

International audience; This work presents an integrated approach to study the crystal chemistry and phonon heat capacity of complex layered oxides. Two quaternary delafossites are synthesized from ternary parent compounds and copper monohalides via a topochemical exchange reaction that preserves the honeycomb ordering of the parent structures. For each compound, Rietveld refinement of the powder X-ray diffraction patterns is examined in both monoclinic C2/c and rhombohedral R (3) over barm space groups. Honeycomb ordering occurs only in the monoclinic space group. Bragg peaks associated with honeycomb ordering acquire an asymmetric broadening known as the Warren line shape that is commonly observed in layered structures with stacking disorder. Detailed TEM analysis confirms honeycomb ordering within each layer in both title compounds and establishes a twinning between the adjacent layers instead of the more conventional shifting or skipping stacking faults. The structural model is then used to calculate phonon dispersions and heat capacity from first principles. In both compounds, the calculated heat capacity accurately describes the experimental data. The integrated approach presented here offers a platform to carefully analyze the phonon heat capacity in complex oxides where the crystal structure can produce magnetic frustration. Isolating phonon contribution from total heat capacity is a necessary and challenging step toward a quantitative study of spin liquid materials with exotic magnetic excitations such as spinons and Majorana fermions. A quantitative understanding of phonon density of states based on crystal chemistry as presented here also paves the way toward higher efficiency thermoelectric materials.

We present the first copper iridium binary metal oxide with the chemical formula Cu

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy-a tendency of spins to align in a certain crystallographic direction known as easy-axis. In layered materials, two limits of easy-axis are in-plane (XY) and out-of-plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin-orbitronic functions. Here, a unique experiment is designed to control the easy-axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl3-x Brx crystals between CrCl3 with XY and CrBr3 with Ising anisotropy. The easy-axis is controlled between the two limits by varying spin-orbit coupling with the Br content in CrCl3-x Brx . The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.

The chemical shifts of 31 P and 1 H in thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonate) (TmDOTP 5- ) are approximately two orders of magnitude more sensitive to temperature than are water proton and 19 F shifts. In the physiologically relevant pH range, the 31 P and 1 H chemical shifts of TmDOTP 5- are linear functions of temperature between 25 and 47°C. The results indicate that using Tm-DOTP 5- can provide measurements of temperature in vivo that are significantly more accurate than methods based on water and fluorocarbon chemical shifts.

31P NMR spectroscopy was used serially to study rejecting (ACl-->LEW) and nonrejecting (ACl-->ACl) orthotopic liver transplants in rats. Recipients were evaluated on post-transplant days 1, 3, 5, 7, 9, and 11. The relative changes in phosphomonoester (PME), inorganic phosphate (Pi), high-energy phosphates and pH were studied. The earliest significant difference between the rejecting and nonrejecting groups was a decrease in the alpha-NTP peak area on Day 5. This was followed by significant decreases in beta-NTP and pH, and increases in PME and Pi on Day 7. High-resolution 31P NMR spectra of perchloric acid extracts demonstrated the PME increase to be due mainly to elevated phosphoethanolamine. Using the parameter (Pi + PME)/(alpha + beta + gamma-NTP), rejecting livers were distinguished from nonrejecting livers at a moderate stage of rejection.

The rotating-frame imaging (RFI) method has proven useful for measuring the spatial distributions of NMR-visible nuclides without the aid of static magnetic field gradients. However, traditional RFI has several disadvantages, including relatively poor time efficiency and high RF power deposition in the sample. A B1-gradient imaging technique, rapid rotating-frame imaging using an RF pulse train (RIPT), is discussed which can reduce both the image data-acquisition time and the RF power deposition by several orders of magnitude compared to RFI. The method is most readily applied to samples having only one resonance line, but a chemical-shift-selective version is also described which permits imaging of arbitrary lines in multipeak spectra. It is shown that RIPT images can be produced even when sample relaxation times are only a few milliseconds. Furthermore, by adding a suitable preparation period to the sequence, the RIPT signal intensity can be weighted by the relaxation time constant. Finally, postprocessing techniques are described which correct some of the image distortions common to most B1-gradient imaging techniques.

The reconstruction of two-dimensional spatial images with 0.5-mm resolution using radio frequency gradients generated by an NMR coil system rotated about the sample is demonstrated for 23Na. The method retains chemical shift, should be capable of mapping T1 and T2 information, and might offer sensitivity advantages for nuclides with short T2. © 1990 Academic Press, Inc.

NMR thermometry has previously suffered from poor thermal resolution owing to the relatively weak dependence of chemical shift on temperature in diamagnetic molecules. In contrast, the shifts of nuclear spins near a paramagnetic center exhibit strong temperature dependencies. The chemical shifts of the thulium 1,4,7, 10-tetraazacyclododecane-1,4,7,10-tetrakis(methylene phosphonate) complex (TmDOTP5-) have been studied as a function of temperature, pH, and Ca2+ concentration over ranges which may be encountered in vivo. The results demonstrate that the 1H and 31P shifts in TmDOTP5- are highly sensitive to temperature and may be used for NMR thermometry with excellent accuracy and resolution. A new technique is also described which permits simultaneous measurements of temperature and pH changes from the shifts of multiple TmDOTP5- spectral lines.

The concentration dependence of the205Tl chemical shifts of Tl+ and of (CH3)2Tl+ ions was determined in several solvents with NO3− and ClO4− counterions. In general, increased ion pairing caused a low-frequency shift of the205Tl resonance, with the exceptions of (CH3)2TlNO3 inn-butylamine and TlNO3 in N,N-dimethylformamide (DMF) and in hexamethylphosphorotriamide (HMPA). In HMPA,205Tl linewidths of both Tl+ and (CH3)2Tl+ increased dramatically with dilution below 0.1M. Analysis of the data allowed ion-pair formation constants and205Tl chemical shifts for the ion-paired cation and for the free (solvated) cation to be estimated for some of the solvents.

W ith the widespread use of surface coils (I) for noninvasive NMR studies of living systems, it has become increasingly important to clearly delineate the spatial volume from which data are acquired. The B, profile of the surface coil itself lim its this volume to some extent, but the boundar ies of the sensitive region are “fuzzy” in simple pulse experiments. Two basic approaches have been used successfully to improve signal localization in surface coil studies. One of these emp loys phasecycled rf pulse sequences to selectively excite and detect spins residing at a certain distance from the coil. Such so-called “depth pulse” sequences have been proposed by Bendall and co-workers (2-3, Shaka and co-workers (6, 7), and Tycko and Pines (8). As an alternative to the excitation and detection of spins in only certain regions of the sample, Hoult (9) described a method of rotating-frame imaging (RFI) in which all sample spins are examined simultaneously. This is accomplished using a two-dimensional time-domain experiment in which an incremented pulse width forms the evolution period t, and the response of the entire sample is detected during tZ. Subsequent 2D Ff processing separates signals along a displacement axis according to their nutation frequencies F, which, in turn, depend on their distances from the coil

NMR pulse sequences have been investigated which selectively suppress undesired signal components from multiple-exponential spin-spin and spin-lattice relaxation curves. Suppression is achieved by exploiting differences in the relaxation times of sample components. Special attention is devoted to discussion of practical details necessary for implementation of these methods, as well as their relative advantages and disadvantages. The techniques are illustrated using multiple-compartment aqueous phantoms and are then employed in measurements of proton T1 and T2 values for freshly isolated guinea pig lenses immersed in aqueous buffer solutions to prevent dehydration and maintain viability. These methods obviate many of the difficulties inherent in obtaining accurate relaxation times for minor components of multiple-exponential curves and should be useful in many applications of NMR to the study of intact tissue, both in vitro and in vitro. © 1986 Academic Press, Inc.

While most NMR pulse sequences designed to measure nuclear spin relaxation times yield true two-dimensional time-domain data, it is standard practice to Fourier transform these in only the t2 dimension and to extract the relaxation time from a time-domain fit in the t1 dimension. As an alternative to this procedure, the consequences of a full two-dimensional frequency-domain analysis are examined. It is shown that exponential time-domain relaxation data produce excellent Lorentzian lines in the frequency domain. Accurate relaxation times may be determined from the full width at half height using LW = 1π · Tn, where n = 1 or 2. An advantage of the frequency-domain fit is that it requires only two parameters instead of three, as often used in conventional analyses. In addition, new pulse sequences which allow the simultaneous determination of both T1 and T2 in a single 2D experiment are demonstrated for single-line spectra. In this case, only three parameters are necessary to determine both relaxation times, as compared with five when conventional procedures are employed.

La localisation spatiale des signaux RMN utilisant un gradient de champ RF peut etre decrit comme une formation d'image a cadre tournant