Broad Electron Paramagnetic Resonance Signal Research Articles

Gamma Irradiation Effect on Polymeric Chains of Epoxy Adhesive.

The study of materials for space exploration is one of the most interesting targets of international space agencies. An essential tool for realizing light junctions is epoxy adhesive (EA), which provides an elastic and robust material with a complex mesh of polymeric chains and crosslinks. In this work, a study of the structural and chemical modification of a commercial two-part flexible EA (3M™ Scotch-Weld™ EC-2216 B/A Gray), induced by 60Co gamma radiation, is presented. Combining different spectroscopic techniques, such as the spectroscopic Fourier transform infrared spectroscopy (FTIR), the THz time-domain spectroscopy (TDS), and the electron paramagnetic resonance (EPR), a characterization of the EA response in different regions of the electromagnetic spectrum is performed, providing valuable information about the structural and chemical properties of the polymers before and after irradiation. A simultaneous dissociation of polymeric chain and crosslinking formation is observed.The polymer is not subject to structural modification at an absorbed dose of 10 kGy, in which only transient free radicals are observed. Differently, between 100 and 500 kGy, a gradual chemical degradation of the samples is observed together with a broad and long-living EPR signal appearance. This study also provides a microscopic characterization of the material useful for the mechanism evaluation of system degradation.

Electron Paramagnetic Resonance Studies of Irradiated Grape Snails (Helix pomatia) and Investigation of Biophysical Parameters.

A study of grape snails (Helix pomatia) using the electron paramagnetic resonance (EPR) spectroscopy method, where shells were exposed to ionizing gamma radiation, indicated that the effect of radiation up to certain doses results in the emergence of magnetic properties in the organism. The identification of the EPR spectra of the body and shell parts of the control and irradiated grape snails separately showed that more iron oxide magnetic nanoparticles are generated in the body part of the grape snail compared to the shells. A linear increase in free radical signals (g = 2.0023) in the body and shell parts of grape snails, and a non-monotonic change in the broad EPR signal (g = 2.32) characterizing iron oxide magnetic nanoparticles was determined depending on the dose of ionizing gamma radiation. Additionally, the obtained results showed that grape snails can be used as bioindicators for examining the ecological state of the environment. At the same time, the radionuclide composition of the body and shell parts of the grape snails and their specific activities were determined by CANBERRA gamma spectroscopy. The FTIR spectra of mucin, a liquid secreted by snails, were recorded.

Temperature-induced phase transition and tunable luminescence properties of Ce3+-Mn2+-Zr4+ tri-doped LaPO4 phosphor

Tunable blue-white-orange emitting LaPO4:Ce3+/Mn2+/Zr4+ (LPCMZ) phosphors have been synthesized by co-precipitation followed by calcination. The X-ray diffraction (XRD) shows the phase transition from hexagonal to monoclinic between 600 and 700 °C, accompanied by the increase of average nanocrystallites size from 8 to 64 nm. The Raman measurements revealed the vibrational modes associated with the LPCMZ crystalline phases, where the band positions and the full width at half maximum values depend on the structural parameters and nanocrystals size. For low-temperature calcination of 500 °C, scanning electron microscopy (SEM) revealed nanoclusters composed of thinner nanoneedles, which developed into a rod-like self-assembly shape for higher calcination temperatures at around 900 °C. Electron paramagnetic resonance (EPR) spectroscopy reveals a broad isotropic EPR signal, assigned to agglomerated/clustered Mn2+ ions, which are dispersed only at high temperatures above 900 °C. The photoluminescence spectra recorded under UV-excitation of Ce3+ ions showed the Mn2+ green/red (546, 630 nm) emissions due to an energy transfer (ET) between Ce3+ and Mn2+. Depending on the calcination temperature, the Mn2+ emission color can be finely adjusted from blue to white and orange.

Modulation of radiative defects in MgAl2O4 nanocrystals probed using NMR, ESR, and PL spectroscopies

We have investigated the influence of combustion fuels on radiative defects of MgAl2O4 spinel. Monoethanolamine (MEA), glycine, and urea fuels were employed to synthesize three samples of MgAl2O4 nanocrystals, choice consequence of which has been observed on cation inversion and various intrinsic defects. Synchrotron powder x-ray diffraction patterns were refined by the Rietveld method. All samples exhibited a spinel structure, with a minor secondary phase of MgO in the sample synthesized by urea fuel. 27Al nuclear magnetic resonance (NMR) inferred the distribution of Al cations among tetrahedral and octahedral sites with structural distortion of interstices. Rietveld, NMR, and diffuse reflectance spectra analyses revealed the presence of antisite defects, cation, and anion vacancies, which may be anticipated to induce F(Mg) defect centers through charge compensation. An electron spin resonance (ESR) study was carried out to investigate the paramagnetic defect centers. Two prominent broad ESR signals, attributed to F+ and V− defect centers, originated from the delocalization of paramagnetic spins and their interaction with nearby cations. Photoluminescence studies further confirmed the existence of F(Mg) defect centers, VO-related defects, and MgAl′ and VMg″ defects in all samples. It was found that the nature of the defects did not change, but the concentration of defects varied significantly with the type of fuel employed and excitation wavelength. The MgAl2O4 samples synthesized using glycine, urea, and MEA fuels were found to be rich in F(Mg) defect centers, VO-related defects, and MgAl′, VMg″, VO-related defects, respectively. The tailoring of defect-assisted photoluminescence by varying fuel types in MgAl2O4 spinel can be exploited in various luminescence applications.

Synthesis, spectroscopic characterization, and theoretical studies on the substitution reaction of chromium(III) picolinate

Chromium (III) picolinate, [Cr(pic)3], was synthesized and characterized by solid electron paramagnetic resonance (EPR) in both room and frozen temperatures. Broad EPR signals of the mer-[Cr(pic)3] complex were found for the solid with g values of 3.45 and 3.47 at room temperature and frozen at 100 K, respectively. This result indicates that the mer-[Cr(pic)3] system has a rhombic character and a large zero-field splitting D value. In addition, its purity was confirmed by elementary analysis, atomic absorption spectroscopy (AAS), HPLC, UV–Vis, ATR-FTIR, Raman spectroscopy, and ESI-mass spectrometry. DFT calculations reveal that the most likely pathway for the acid-catalyzed [Cr(pic)3] aquation is via a two-step mechanism, which is in excellent agreement with the experimental result. Furthermore, the importance of outer sphere solvation was investigated with the addition of up to three explicit water molecules in the second coordination sphere. The enthalpies of activation (ΔH‡) for water-water exchange on the D pathway of [Cr(pic)(H2O)4]2+…nH2O complexes are found to decrease for n = 0–3 (64, 58, 56, and 53 kJ mol−1, respectively).

Conjuring of defect-induced short and long-range ferromagnetism ordering in Ba(1−x)NdxTi0.99Co0.01O3

In this letter, we report on the conjuring of defect-induced magnetism of Ba(1−x)NdxTi0.99Co0.01O3, x = 0.00 (BTC), 0.25% (BNTC-1), 0.50% (BNTC-2)and 0.75% (BNTC-3) ferroelectric ceramics synthesized by sol-gel combustion method. XRD and Raman spectral analysis are exploited to emphasize the chemical homogeneity and structural sensitivity, which revealing an unpolluted phase with a structural transition of Tetragonal to Pseudo-cubic on Nd doping. Electron paramagnetic resonance (EPR) spectra have facilitated the aboriginal role of defects present in the samples through g factor of 1.997 and 1.968. A typical broad EPR signal with g-factor 2.413 for BNTC-1 ensures the ferromagnetic resonance (FMR) signal. It unveiled that the magnetic micro-domain state arises due to the existence of complex defects. Thus, inducing a magnetic saturation in M-H curve of BNTC-1 at high field strength, unlike the short-range ferromagnetism (FM) with diamagnetic ordering has observed in the other samples. These results indict that the short range and long range FM ordering ascribed to the crucial role of defects.

Reconciling Structural and Spectroscopic Fingerprints of the Oxygen-Evolving Complex of Photosystem II: A Computational Study of the S2 State.

The catalytic cycle of photosynthetic water oxidation occurs at the Mn4CaO5 oxygen-evolving complex (OEC) of photosystem II. Extensive spectroscopic data have been collected on the intermediates, especially the S2 (Kok) state, although the proton and electron inventories (Mn oxidation states) are still uncertain. The "high oxidation" paradigm assigns S2 Mn oxidation level (III, IV, IV, IV) or (IV, IV, IV, III), whereas a "low oxidation" paradigm posits two additional electrons. Here, we investigate the geometric (X-ray diffraction, extended X-ray absorption fine structure) and spectroscopic (electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR)) properties of the S2 state using quantum chemical density functional theory calculations, focusing on the neglected low paradigm. Two interconvertible electronic spin configurations are predicted as ground states, producing multiline ( S = 1/2) and broad ( S = 5/2) EPR signals in the low paradigm oxidation state (III, IV, III, III) and with W2 as OH- and O5 as OH-. They have "open" ( S = 5/2) and "closed" ( S = 1/2) Mn3CaO4-cubane geometries. Other energetically accessible isomers with ground spin states 1/2, 7/2, 9/2, or 11/2 can be obtained through perturbations of hydrogen-bonding networks (e.g., H+ from His337 to O3 or W2), consistent with experimental observations. Conformers with the low oxidation state configuration (III, IV, IV, II) also become energetically accessible when the protonation states are O5 (OH-), W2 (H2O), and neutral His337. The configuration with (III, IV, III, III) agrees well with earlier low-temperature EPR and ENDOR interpretations, whereas the MnII-containing configuration agrees partially with recent ENDOR data. However, the low oxidation paradigm does not yield isotropic ligand hyperfine interactions in good agreement with observed values. We conclude that the low Mn oxidation state proposal for the OEC can closely fit most of the available structural and electronic data for S2 at accessible energies.

Electron Spin Resonance Study of ZnxCo1−xFe2−xAlxO4 Nanoparticles

Zn- and Al-substituted cobalt ferrite ZnxCo1−xFe2−xAlxO4 (0 $\leq x \leq $ 0.9) with particle size in the range of 6–13 nm were synthesized by glycol-thermal reaction. The compounds were investigated by electron spin resonance (ESR) measurements. CoFe2O4 (x = 0) shows a broad ESR signal associated with magnetic dipole-dipole interactions among particles which is highly sensitive to the Zn and Al ion concentrations. Narrower ESR signals in Zn- and Al-doped samples are attributed to the weakening of super-exchange interactions. ESR and magnetization measurements reveal a reduction in magnetization with increasing particle size. This is explained on the basis of the redistribution of Zn2+ ions in both tetrahedral (A) and octahedral (B) sites in fine powders.

Synthesis and characterization of conjugated and nanostructured poly(propargyl alcohol) polymers

New types of π-conjugated and colored poly(propargyl alcohol) polymers (poly-PGA) were prepared by the polymerization of propargyl alcohol (PGA) monomer in different media under the action of high energy photons, γ-rays, without the use of catalysts. The polymerization conditions depend on the used solvents (water, chloroform and dimethylformamide) and gases (O2 and N2). A singlet and broad electron paramagnetic resonance signal was observed in poly-PGA polymers, indicating the presence of free electrons and the delocalization of electrons through the polymer π-backbone. The synthesized polymers have FTIR bands (C=C) in the range of 1607–1652 cm−1 and an absorption broadband in the spectrum range of 305–316 nm. The poly-PGA polymers which synthesized in chloroform and DMF exhibited amorphous structures as approved by XRD results. In addition, both TEM and DLS measurements showed the formation of nanostructured polymers in the shape of nano-spheres, nano-stars and nano-networks depending on polymerization conditions. Radiation polymerization of the monomer in chloroform produced a polychlorinated polymer as demonstrated by EDX analysis. The polymers have optical band gaps in the range of 2.85–3.50 eV and conductivity in the range of 2.45 × 10−6 to 9.43 × 10−7 depending on the polymerization conditions and the media used.

Electron Spin Resonance Investigations on Perovskite Solar Cell Materials Deposited on Glass Substrate

ABSRTACTIn this article, we report low-temperature electron spin resonance (ESR) investigations carried out on solution processed three-layer inverted solar cell structures: PC61BM/CH3NH3PbI3/PEDOT:PSS/Glass, where PC61BM and PEDOT:PSS act as electron and hole transport layers, respectively. ESR measurements were conducted on ex-situ light (1 Sun) illuminated samples. We find two distinct ESR spectra. First ESR spectra resembles a typical powder pattern, associated with gx = gy = 4.2; gz = 9.2, found to be originated from Fe3+ extrinsic impurity located in the glass substrate. Second ESR spectra contains a broad (peak-to-peak line width ∼ 10 G) and intense ESR signal appearing at g = 2.008; and a weak, partly overlapped, but much narrower (peak-to-peak line width ∼ 4 G) ESR signal at g = 2.0022. Both sets of ESR spectra degrade in intensity upon light illumination. The latter two signals were found to stem from light-induced silicon dangling bonds and oxygen vacancies, respectively. Our controlled measurements confirm that these centers were generated during UV-ozone treatment of the glass substrate –a necessary step to be performed before PEDOT:PSS is spin coated. This work forms a significant step in understanding the light-induced- as well as extrinsic defects in perovskite solar cell materials.

A Variable Temperature X- and W-Band EPR Study of Fe-Doped SiCN Ceramics Annealed at 1000, 1100, and 1285 °C: Dangling Bonds, Ferromagnetism and Superparamagnetism

Polymer-derived SiCN ceramics, annealed (also referred to as pyrolyzed) at 1000, 1100, and 1285 °C, and doped with Fe(III) acetylacetonate, are investigated by electron paramagnetic resonance (EPR) from 4 to 120 K at X-band (9.425 GHz). In addition, the SiCN ceramic, annealed at 1100 °C, was studied by EPR at 300 K at W-band (93.96 GHz). There was observed a significant increase in EPR linewidth due to dangling bonds (g = 2.001) below 20 K at X-band. The low-field X-band FMR line (g ≈ 12) indicated the presence of ferromagnetic Fe5Si3 crystallites. There were found two EPR lines due to carbon-related dangling bonds: (1) those present as defects on the surface of the free-carbon phase (as sp2 carbon-related dangling bonds with g = 2.0011) and (2) those present within the bulk of carbon phase (as sp3 carbon-related dangling bonds with g = 2.0033). On the other hand, the intense low-field EPR signal observed at X-band was not observed at W-band. As well, there was observed splitting of the single broad EPR signal observed at g = 2.05 at X-band into two signals at W-band at g = 1.99 and g = 2.06, due to two different Fe-containing superparamagnetic nanocrystallites. Two new EPR signals, not observed at X-band, were observed at W-band, namely at g = 2.28 and g = 3.00, which are also due to g∥ of these superparamagnetic nanocrystallites.

Energetics of the S2 State Spin Isomers of the Oxygen-Evolving Complex of Photosystem II.

The S2 redox intermediate of the oxygen-evolving complex in photosystem II is present as two spin isomers. The S = 1/2 isomer gives rise to a multiline electron paramagnetic resonance (EPR) signal at g = 2.0, whereas the S = 5/2 isomer exhibits a broad EPR signal at g = 4.1. The electronic structures of these isomers are known, but their role in the catalytic cycle of water oxidation remains unclear. We show that formation of the S = 1/2 state from the S = 5/2 state is exergonic at temperatures above 160 K. However, the S = 1/2 isomer decays to S1 more slowly than the S = 5/2 isomer. These differences support the hypotheses that the S3 state is formed via the S2 state S = 5/2 isomer and that the stabilized S2 state S = 1/2 isomer plays a role in minimizing S2QA- decay under light-limiting conditions.

Dielectric properties and defect chemistry of (Ba1−xLax)(Ti1−xLux)O3 ceramics

The single-phase (Ba1−xLax)(Ti1−xLux)O3 (0.02 ≤ x ≤ 0.05) ceramics (BLTL) were prepared at 1400 °C using a solid-state reaction method. The structure, microstructure, point defects, and dielectric properties of BLTL were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, electron paramagnetic resonance (EPR), and dielectric measurements. The fine-grained BLTL (∼0.9 μm) is tetragonal for x ≤ 0.03 and cubic for x ≥ 0.04. The x = 0.05 ceramic with Y5S dielectric specification (ε′RT ∼1800) exhibits a lowest dielectric loss (tan δ = 0.01) as compared to (Ba1−xLax)(Ti1−xRx)O3 with x = 0.05 (R = Tb, Dy). A regularity is discovered that the dielectric loss decreases with lanthanide contraction when BaTiO3 is co-doped with La and R (R is a rare-earth ion) under the same preparation conditions. Lu3+ exhibits a weak amphoteric behavior in BLTL. The defect chemistry, the low dielectric loss, the origin of EPR signals of vacancy point defects and an unexpected, broad and intense EPR signal at g = 2.2, are discussed. An approach to the completely self-compensated LaBa•−LuTi' mode with the increase of x is the main reason of the low dielectric loss.

Superoxide- and NO-Dependent Mechanisms of the Reprogramming of Bone Marrow Cells by Tumor Cells

Electron paramagnetic resonance (EPR) experiments in vitro; spin trapping of the reactive oxygen/nitrogen species (superoxide radicals and nitric oxide, NO); gel zymography measurements in the tumor tissues, in the healthy and tumor-affected bone marrow (BM) samples of rats are carried out. The superoxide and NO generation rates are derived; matrix metalloproteinases (MMP-2 and MMP-9) concentrations are measured. Their changes after the incubation of BM samples with Guerin carcinoma cells at 37 °C are defined. It is shown that the impact of tumor cells on BM manifests in the metabolic disorder, increased concentrations of active forms of MMP-2 and MMP-9, increased production of superoxide and NO radicals. Correlation between the appearance and intensity of the broad EPR signal at g = 2.2–2.4 with the concentrations of active forms of MMP-2 and MMP-9, NO and superoxide radicals’ rates is observed. The obtained spatial and temporal changes of the measured parameters demonstrate the usefulness of the potential application of EPR imaging to study the mechanisms of tumor invasion. The EPR signal may indicate the presence of distant metastases, may become a part of diagnostics and used for the estimation of the therapeutic treatments in the pre-clinical studies. It is proposed that labile iron pool is responsible for the appearance of the EPR signal in tumor and BM samples.

Spectroscopic characterization of CuOx/TiO2–ZrO2 catalysts prepared by a-step sol–gel method

Copper–titania–zirconia mixed oxides with TiO2/ZrO2 1:1 atomic ratio and copper content of 1, 3 and 5wt% were prepared by one-step sol–gel method. X-ray diffraction and gas-volumetric analysis showed poor crystallinity and high specific surface area (around 250m2/g) of the catalysts calcined at 400°C. The nature of the copper species in the mixed oxides was characterized by temperature-programmed reduction in hydrogen (TPR), low-temperature electron paramagnetic resonance (EPR) and diffuse reflectance (DR) UV–Vis spectroscopies. These techniques suggested the presence of different copper species including distorted octahedral Cu2+ ions (isolated or quasi-isolated) in the solid solution bulk, characterized by a TPR peak above 400°C (labeled as T1), electronic d–d transitions around 800nm and an anisotropic EPR signal with two hyperfine interactions in the g|| region. The symmetry of this site changed from rhombic (S1) to axial (S3) while increasing copper loading. The inclusion of copper ions in the TiO2–ZrO2 matrix was also testified by changes in the mixed-oxide edge position. Two families of supported copper oxide were found. CuO “bulk-like” showed relatively high reduction temperature (T2, around 250°C), while highly dispersed CuOx nanoparticles/clusters resulted in TPR peaks around 170 and 200°C (T4 and T3). These species were responsible for the UV–Vis adsorption above 400nm and for the broad and asymmetric axial EPR signal with g|| in the range from 2.14 to 2.29 (S2 and S2′). Finally, 2-propanol decomposition was employed as a test reaction to probe the catalyst's surface properties. The results suggested a marked effect of copper loading on the reactivity pattern, confirming the presence of CuO/CuOx particles on the surface of the TiO2–ZrO2 support.

Defect-induced ferromagnetism of mechanical-milled BaTi0.98Mn0.02O3 nanoparticles.

Room-temperature ferromagnetism was found in BaTi0.98Mn0.02O3 nanoparticles fabricated by solid-state reaction and mechanical-ball milling. The nature of ferromagnetism was systematically investigated by means of X-ray diffraction (XRD), electron spin resonance (ESR), superconducting quantum interference device (SQUID), and X-ray absorption (XAS). A weak ferromagnetism was found in the defect-rich samples with the milling time (t(m)) longer than 6 hrs (i.e., t(m) > 6 hrs), while defect-poor ones with t(m) = 0-6 hrs exhibited mainly the paramagnetic properties. Detailed analyses of ESR and XAFS data indicated the shift in the oxidation state of Mn from 4+ to 3+ after milling, and both Mn3+ and Mn4+ were incorporated into the Ti sites of the BaTiO3-tetragonal host lattice. With the results obtained, we believe that exchange interactions due to Mn(4+)-Mn4+ and/or Mn(3+)-Mn3+ dipole pairs are responsible for broad ESR signals in the Lorentzian shape and the paramagnetic behaviors of the samples. Meanwhile, the weak ferromagnetic interaction is generated from various point defects such as Ti-site cation vacancies or Ti(3+)-Ti3+ super-exchange interaction pairs.

Effects of ultraviolet photon irradiation on the transition metal impurities in LaAlO3

By measuring electron spin resonance (ESR) spectra at the X-band frequency and absorption spectra from the visible to ultraviolet region at room temperature, it was confirmed that perovskite single crystal LaAlO3 contains Cr and Fe as impurities. When LaAlO3 is exposed to photons with energies higher than 4.5 eV, the intensities of ESR signals due to Cr3+ and Fe3+ decrease, which indicates that electrons released by the photon irradiation are captured by Cr3+ and Fe3+. Concurrently with this, a broad optical absorption band at about 3.0 eV and two new broad and weak ESR signals appear. The former is attributable to a combination of a hole and a La3+ (or Al3+) vacancy, and the two weak ESR signals are assigned to the O− center and F+ center.

Effect of Rare Earth Element Codoping on the Mn Red Emission in Ca and Sr Thiogallates: Focusing on Its Mechanism through Electron Spin Resonance Study of Single Crystals

The weak Mn2+ involved red emission in CaGa2S4 can be greatly enhanced by codoping with a suitable rare earth element (REE). To clarify the involved physical mechanism, the actual Mn sites in the host crystal are investigated through electron spin resonance (ESR) measurements of single crystals. Three different patterns of ESR signals with different angular dependences are observed by rotating the magnetic field around the three crystal axes. As a result, three independent sites occupied by the Mn2+ ions are confirmed. Based on the experimental finding that the hyper fine constants (HFS) for the three sites are nearly the same, the Mn2+ ion sites may be ascribed to the Ca or Ga sites. To finally solve this ascription issue, ESR measurements of Eu2+ in CaGa2S4 are carried out, where the Eu2+ ions have been shown to exclusively substitute at Ca sites. From the similar angular variation of the ESR signals of Eu2+ and Mn2+, the Mn substitutional sites are finally determined to be the three Ca ones. A broad ESR signal appearing in the vicinity of the Lande g factor (g = 2.0) suggests the possible presence of Mn clusters. We have to investigate to see whether these may be the actual emission centers.

Characterization of Indian serpentine by X-ray diffraction, photoacoustic spectroscopy and electron paramagnetic resonance spectroscopy

A variety of serpentines collected from Cuddapah region of India was investigated through X-ray diffraction (XRD), photoacoustic spectroscopy (PAS) and electron paramagnetic resonance (EPR) studies. XRD studies indicated the presence of only one phase in one of the serpentines (sample 1) whereas three other serpentines (samples 2, 3 and 4) were either biphasic or triphasic. The deconvoluted PAS spectra showed prominent peaks around 350, 374, 440, 510, 630 and 850 nm. According to the ligand field theory, these peaks were attributed to the presence of isolated Fe 3+ and Fe 2+ ions. EPR studies revealed the presence of Mn 2+ in octahedral site ( g = 2.004, D = 80 G, A Mn = 95 G) and spin counting suggested that it was present at trace level. Apart from this, two more signals at g = 4.2 and g = 2.00 were observed and were assigned to Fe 3+ ions at tetrahedral and octahedral sites. An unusually broad EPR signal was observed at room temperature in samples 2, 3 and 4, which was attributed to the presence of iron clusters or MgFe 2O 4.

High Field Magnetism of the Spinel Oxide CoAl2O4

We report the results of magnetization and electron spin resonance (ESR) measurements on sintered samples of the spinel oxide CoAl2O4 in magnetic fields up to 53 T. In CoAl2O4, Co2+ (S=3/2) ions make a diamond lattice, indicating naively that no geometrical frustration takes place. This spinel oxide, however, is thought to be in the vicinity of a quantum melting point of the antiferromagnetically ordered state because of no sharp peak in the temperature dependence of specific heat. Magnetization curve at 4.2 K is almost linear in magnetic fields and a broad ESR signal was observed on a straight resonance line with g=2.26, implying an antiferromagnetic long range order or a well developed antiferromagnetic short range order with very small anisotropy.