EPR and magnetic properties of MgO-CaO-SiO2-P2O5-CaF2-Fe2O3 glass-ceramics

Glass-ceramics have been derived from 4.5MgO(45−x)CaO34SiO₂16P₂O₅0.5CaF₂xFe₂O₃ (x=5, 10, 15, 20 wt%) glasses by heat treatment. Room temperature electron paramagnetic resonance (EPR) spectra and temperature-dependent magnetic susceptibility (χ) of the glass-ceramics have been obtained. The EPR absorption line centered at g≈4.3 disappeared at higher concentrations of iron oxide. The intensity and line width of the EPR absorption line centered at g≈2.1 increased as the iron oxide concentration was increased. Temperature-dependent magnetization of samples with low iron oxide content revealed ferrimagnetic as well as paramagnetic contributions. Information about the structural changes involving iron ions, their valence state and the type of magnetic interactions between the Fe ions as a function of composition was obtained using EPR and χ studies.     

Magnon Raman scattering in CoBr2

The one-magnon Raman spectrum of CoBr₂ has been investigated as a function of temperature, and peak frequency, integrated intensity and width parameters obtained. The results obtained for the band energy at low temperature (22.2 ± 0.2 cm^-1 at 5.7.K) are in good agreement with AFMR and neutron scattering results. The one-magnon energy renormalises relatively slowly with increasing temperature and is about 15 cm^-1 at T_N = 19 K, whereas the integrated intensity approaches zero like the magnetization at T_N and the width diverges. A low intensity band at 26.8 ± 1 cm^-1 (7.6K) may be due to two-magnon scattering from spin waves along the c-axis.     

A Mössbauer study of antiferroelectric transition in Fe: NH4H2PO4

Mössbauer studies in ⁵⁷Fe³⁺: NH₄H₂PO₄ (ADP) were conducted over a range of temperatures covering the antiferroelectric transition at 148 K, using both powder and single crystal absorbers. No anomaly was observed in the temperature dependence of chemical isomershift and normalised resonance intensity at the transition point T_c. However, a sudden increase in the line width around T_c was observed, consistent with the expected increase in electric field gradient created by the onset of antiferroelectric ordering. These experiments clearly show that the impurity Fe atoms do not play any role in the soft mode responsible for the spontaneous polarization in ADP.     

EPR and magnetic susceptibility studies of iron ions in ZnO-Fe2O3-SiO2-CaO-P2O5-Na2O glasses

Room temperature electron paramagnetic resonance (EPR) spectra and temperature dependent magnetic susceptibility data have been obtained on bulk x(ZnO,Fe₂O₃)(65−x)SiO₂20(CaO, P₂O₅)15Na₂O (6≤x≤21 mole%) glasses prepared by melt quenching method. EPR spectra of the glasses revealed absorptions centered at g≈2.1 and 4.3. The variations of the intensity and line width of these absorption lines with composition have been interpreted in terms of the variation in the concentration of the Fe²⁺ and Fe³⁺ ions in the glass and the interaction between the iron ions. EPR and magnetic susceptibility data of the glasses reveal that both Fe²⁺ and Fe³⁺ ions are present in the glasses, with their relative concentration being dependent on the glass composition. The studies reveal superexchange type interactions in these glasses, which are strongly dependent on their iron content.     

New infrared integrated band intensities for HC3N and extensive line list for the ν5 and ν6 bending modes

The infrared spectrum of cyanoacetylene (also called propynenitrile) has been investigated from 400 to 4000 cm⁻¹ at a resolution of 0.5 cm⁻¹. Integrated intensities of the main bands and a number of weaker bands have been obtained with an uncertainty better than 5%. Inaccurate values in previous studies have been identified in particular concerning the intensity of the strong ν₅ stretching band at 663.2 cm⁻¹. Former results on the temperature dependence of integrated intensities have also been revisited.Synthetic spectra calculation has been performed for the ν₅ and ν₆ bands on the basis of the best available high resolution data. It has been shown that the GEISA line parameters for HC₃N are not sufficient to reproduce the band intensities and some hot band features observed in our experimental spectra at room temperature. As a first step, the model spectra has been improved by including a number of missing hot subbands and by calculating accurately the hot band relative intensities. Finally, a perfect agreement between calculated and observed spectra was achieved on the basis of a global analysis of HC₃N levels up to 2000 cm⁻¹ combined with the new integrated intensity measurements. A new extensive line list for the ν₅ and ν₆ bending modes of HC₃N has been compiled.     

Raman scattering in FeF3

Peaks in the FeF₃ phonon Raman spectrum are assigned to the A_1g(312 cm^-1) and E_g(105, 187, 450 cm^-1) representations of the D⁶_3d group. Anomalous temperature dependence of the frequency, intensity and line width of the 187 cm^-1 line has been observed.     

Detection of partial inverse spinel structure in CdIn2S4 by fourier transform spectroscopy

The lattice vibrational spectrum of a single crystal of CdIn₂S₄ has been recorded at low temperature (4.2 K) by using Fourier transform spectroscopy. The absorption line corresponding to the T_1u IR active mode has been studied in detail. It has been observed that this line is split into two closely spaced lines located at 69.9 and 71.1 cm^?1. This splitting is explained on the basis of the existence of a partially inverse spinel structure.     

Half-field and normal electron spin resonance study of the quasi-two dimensional magnetic crystal Mn(CHOO)2·2H2O

We have studied the normal electron spin resonance at the Larmor frequency as well as the half-field resonance of the quasi two dimensional magnetic crystal Mn(CHOO)₂·2H₂O at 35 GHz and room temperature. There are two groups of Mn²⁺ ions referred as A and B in this compound. The A ions lie in parallel (100) planes and have a strong exchange interaction within the plane. On the other hand, the B ions which lie in the adjacent parallel planes have no exchange interaction in the plane. We have measured the line width of the normal resonance from the A ions as a function of θ and φ where θ is the angle between the d.c. magnetic field H₀ and the a-axis of the crystal, which is very nearly perpendicular to the (100) plane and φ is the azimuthal angle of H₀ in the bc plane with respect to the b-axis. We also measured the intensity of the half-field resonance as a function of φ. We have calculated the second moments of the normal resonance line as a function of θ and φ. There is a satisfactory agreement between the calculated and the observed θ dependence of the line widths. The intensity of the half-field resonance has a maximum at φ = 40° whereas in an ideal 2d magnet, the maximum intensity is expected at φ = 45°. The ESR line width, because of the spindynamics in a 2 dimensional magnetic crystal, has an angular dependence of the form (3 cos²θ ? 1)². The observed dependence of the line width in Mn(COOH)₂·2H₂O seems to follow the same angular dependence when corrected for the contribution from the nearest neighbor B ions.     

Study on vacancy motion in Y2O3-doped CeO2 by 17O NMR technique

The ¹⁷O NMR measurement was made to elucidate the microscopic nature of vacancy motion in Y₂O₃-doped CeO₂. Spin-lattice relaxation rate, T^?1₁, spin-spin relaxation rate, T^?1₂, and resonance intensity were measured at v₀ = 8.13 MHz as a function of temperature [385 < T (K) < 1110] and composition [$\text{0.06 <}\text{Y}{}_2\text{O}{}_3\text{(}\text{m}\text{o}\text{) < 6}$]. The static electric field gradient associated with lattice defects resulted in the composition dependences of the line width and the intensity. In low dopant concentrations, doubly peaked temperature dependence of T^?1₁ was found, while a single and asymmetric peak was observed in high concentrations. T^?1₁ of 4.0 and 6.0 $\text{m}\text{o}$ doped samples were analyzed using a barrier height distribution model for the oxygen vacancy jump. The mean value of the activation energy was found to increase with the Y₂O₃ concentration.     

The NMR study of the mixed compound (CH3NH3)2Cu(Cl0.1Br0.9)4

The mixed compound (CH₃NH₃)₂Cu(Cl_0.1Br_0.9)₄ which has random bonds of ferromagnetic and antiferromagnetic interactions has been investigated by the pulsed NMR of ¹H, ^63,65Cu and ^79,81Br. The temperature dependence of the line width of ¹H showed the existence of the magnetic phase transition at 15 K in the applied field of 4 kOe. The moments of Cu²⁺ have been shown to lie within the c-plane and have some randomness from the field dependence of the NMR spectrum and the spin-echo decay time of the copper and the bromine nuclei.     

Raman scattering in Mg2Sn and Mg2Si

The Raman spectra of single crystal Mg₂Sn and Mg₂Si have been studied at room temperature. In Mg₂Sn we observe a single relatively strong line produced by the degenerate Raman active F_2g modes, seen previously by neutron inelastic scattering. Two weaker peaks are seen, one occuring at the F_2g overtone; the assignment for the other is uncertain. In Mg₂Si we find a strong line having a shift comparable to the value calculated by previous workers.     

Two-dimensional critical behaviour in antiferrodistortive Al ur6(CIO4)3 and Ga ur6(ClO4)3

Critical behaviour with dimensionality d = 2 has been observed for the 300 K antiferrodistortive phase transition in Al ur₆(ClO₄)₃ and Ga ur₆(ClO₄)₃ by means of the temperature dependence of the ESR parameter D. The systems exhibited d = 2 behaviour in the static critical behaviour for T<T_c?40 K for T>T_c + 40 K. From the ESR data including line width measurements the local order parameter relaxation rate ω₁ has been obtained for various temperatures above T_c, with a lowest value of ω₁ = 150 MHz at T_c + 15 K     

Studies of thermal decomposition of (NH4)2Fe(SO4)2·6H2O

Thermal decomposition products of the Mohr salt (NH₄)₂Fe(SO₄)₂·6H₂O have been studied and identified using the Mössbauer effect, X-ray diffraction, infrared spectroscopy, and the gravimetric and thermal differential methods. It has been found that the Mohr salt heated for 96 hr. in air at 520K changes to a single substance identified as NH₄Fe(SO₄)₂ with a single Mössbauer line (width 0.30 mm/sec; isomeric shift 0.30 mm/sec). When the Mohr salt is heated for 1 hr. in air at 770 K it changes to Fe₂(SO₄)₃ with a single Mössbauer line (width 0.33 mm/sec; isomeric shift 0.31 mm/sec) strikingly similar to line of NH₄Fe(SO₄)₂.     

Electron spin resonance study of bulk and nanosized La0.875Sr0.125MnO3

The structural, transport and electron spin resonance properties of bulk and nanosized La_0.875Sr_0.125MnO₃ prepared by a sol-gel method have been investigated. The bulk sample has an orthorhombic structure and a ferromagnetic insulating ground state. The ESR spectra indicate the coexistence of the ferromagnetic insulating and ferromagnetic metallic phases below T_C. In addition to a sharp peak in the vicinity of T_C, another sharp peak close to is clearly observed in the intensity of the spectra, which may be correlated with the structural transition and orbital ordering at this temperature. For the nanosized sample, a drastically different behavior is found. With a rhombohedral structure down to 70 K, the nanosized sample shows a ferromagnetic metallic ground state. The ESR studies reveal the coexistence of the paramagnetic and ferromagnetic resonance signals. The resonance intensity shows a broad peak around 200 K, which may be due to the wide ferromagnetic transition in the nanoparticle.     

The soft mode properties in Raman spectra of improper ferroelastics Hg2Cl2 and Hg2Br2

The temperature dependence of the frequency and intensity of the soft mode line appearing in the Raman spectra of Hg₂Cl₂, Hg₂Br₂ below the critical point T_c was studied. A proportionality between the scattering cross section of the soft mode and the square of its frequency v_sm was established. A strong stress-induced increase of the intensity and frequency of the soft mode line was observed near T_c, the relative shift of v_sm being ～30% at ～0.5 kg/mm² stress and T_c?T=5°K.     

Detailed infrared spectroscopic study of thermal transitions in new hybrid [(CH3)2CHNH3]4Cd3Cl10 crystal

Phase transitions of tetra(isopropylammonium)decachlorotricadmate(II) [(CH₃)₂CHNH₃]₄Cd₃Cl₁₀ crystal have been studied by infrared, far infrared and Raman measurements in wide temperature range, between 11 K and 388 K. The temperature changes of wavenumber, center of gravity, width and intensity of the bands were analyzed to clarify cationic and anionic contributions to the phase transitions mechanism. The results of investigation showed earlier by differential scanning calorimetry (DSC), thermal expansion and dielectric measurements clearly confirmed the sequence of phase transitions at T₁=353 K, T₂=294 K and T₃=260 K. The current results derived from DSC and infrared measurements revealed additional phase transition at T₄=120 K.     

F NMR and EPR study of fluorinated buckminsterfullerene C60F58

Solid state ¹⁹F NMR in the temperature range from 96 to 366 K and room temperature EPR studies of fluorinated buckminsterfullerene C₆₀F₅₈ have been carried out. The temperature dependence of the line width and the spin-lattice relaxation time show hindered molecular motion with the activation energy of ΔE_a=1.9 kcal/mol. Neither phase transition nor random rotation of C₆₀F₅₈ have been obtained. The spin-lattice relaxation rate is strongly affected by the presence of paramagnetic centers, namely, dangling C-C bonds yielding localized unpaired electrons. Such broken bonds are caused by C-C bond rupture in a cage-opened structure of hyperfluorinated species.     

Deep radiative transitions in AlxGa1−xAs:Sn

Deep radiative transitions in LPE-grown Al_xGa_1?xAs:Sn (0?x?0.28) have been studied at temperatures T between 78 and 297 K using photoluminescence. Each sample has a broad, featureless, saturable spectrum with a peak energy which has an x-dependence similar to that of the L-conduction band minima. The asymmetry of the line shape is similar for all x, and the T dependence of the spectral width fits an electron-lattice interaction type equation. Thermal quenching is observed and studied by measuring the photoluminescent decay time as a function of temperature. A configuration coordinate model is proposed to explain the observed spectral data.     

High resolution spectroscopy of the ν4 band of methane

The fundamental ν₄ band of methane in the 1292–1306 cm^?1 region has been investigated at high resolution using a tunable diode laser. Line center frequency separations and relative line intensities have been measured. A comparison is presented with recent theoretical calculations showing a satisfactory agreement both in line frequency assignment and in line intensity.