Spectral studies on Cr ions doped in sodium-lead borophosphate glasses

Electron paramagnetic resonance (EPR), optical absorption and emission spectra of Cr³⁺ ions doped in (30−x) (NaPO₃)₆+30PbO+40B₂O₃+xCr₂O₃ (x=0.5, 2.0, 3.0, 4.0 and 5.0 mol%) glasses have been studied. The EPR spectra exhibit resonance signals with effective g values at g≈4.55 and g≈1.97. The EPR spectra of x=3.0 mol% of Cr₂O₃ in sodium-lead borophosphate glass sample were studied at various temperatures (295-123 K). The intensity of the resonance signals increases with decrease in temperature. The optical absorption spectrum exhibits four bands characteristic of Cr³⁺ ions in octahedral symmetry. From the analysis of the bands, the crystal-field parameter Dq and the Racah interelectronic repulsion parameters B and C have been evaluated. The emission spectrum exhibit one broad band characteristic of Cr³⁺ ions in octahedral symmetry. This band has been assigned to the transition ⁴T_2g (F)→⁴A_2g (F). Correlating EPR and optical data, the molecular bonding coefficient (α) has been evaluated.     

Electron paramagnetic resonance and luminescence of chromium in calcium germanate crystals

The luminescence of Ca₂GeO₄: Cr⁴⁺ single crystals at wavelengths in the range of 1.3 μm upon excitation with a 1-μ m semiconductor laser is investigated in the temperature range up to 573 K. At T<110 K, the Ca₂GeO₄: Cr⁴⁺ crystals are characterized by the electron paramagnetic resonance, which is attributed to the Cr⁴⁺ ions substituted for Ge⁴⁺ ions. The components of the g tensor and its principal axes are determined. It is revealed that the Cr⁴⁺ impurity centers in calcium germanate affect the crystal symmetry to a lesser degree compared to Cr⁴⁺ ions in forsterite. The observed deviation of the temperature dependence of the electron paramagnetic resonance from the Curie law is explained by the transition to the excited state with a low activation energy, as is the case in impurity 3d ions in diamond-like semiconductors. The inference is made that the giant effective degeneracy multiplicity of the excited state is associated with the initiation of soft phonon modes in the crystal upon excitation of the defect.     

EPR observation of phase transitions in calcium cadmium acetate hexahydrate: Using Mn2+ and Cu2+ ions as probe

Results of temperature dependence of EPR spectra of Mn²⁺ and Cu²⁺ ions doped calcium cadmium acetate hexahydrate (CaCd(CH₃COO)₄·6H₂O) have been reported. The investigation has been carried out in the temperature range between room temperature (～ 300 K) and liquid nitrogen temperature. A I-order phase transition at 146 ± 0.5 K has been confirmed. In addition a new II-order phase transition at 128 ± 1 K has been detected for the first time. There is evidence of large amplitude hindered rotations of CH₃ groups which become frozen at ～ 128 K. The incorporation of Cu²⁺ and Mn²⁺ probes at Ca²⁺ and Cd²⁺ sites respectively provide evidence that the phase transitions are caused by the molecular rearrangements of the common coordinating acetate groups between Ca²⁺ and Cd²⁺ sites. In contradiction to the previous reports of a change of symmetry from tetragonal to orthorhombic below 140 K, the symmetry of the host is concluded to remain tetragonal in all the three observed phases between room temperature and liquid nitrogen temperature.     

Combustion synthesized MgAl2O4:Cr phosphors—An EPR and optical study

Magnesium aluminate (MgAl₂O₄) doped with trivalent chromium (Cr³⁺) was synthesized by the combustion method. The prepared sample was characterized by X-ray powder diffraction, Brunauer-Emmet-Teller (BET) adsorption isotherms and diffuse-reflectance UV-vis spectroscopy techniques. Electron paramagnetic resonance (EPR) and photoluminescence (PL) studies have been performed at room temperature and at 110 K. The EPR spectrum exhibit resonance signals at g=5.37, 4.53, 3.82, 2.26 and 1.96 characteristic of Cr³⁺ ions. The luminescence of Cr³⁺-activated MgAl₂O₄ exhibits a red emission peak around 686 nm from the synthesized phosphor particles upon 551 nm excitation. The luminescence is assigned to a transition from the upper ²E_g→⁴A_2g ground state of Cr³⁺ ions. By correlating EPR and optical data the crystal field splitting parameter (D_q), Racah inter-electronic repulsion parameter (B) and the bonding parameters have been evaluated and discussed. The bonding parameters suggests that the ionic nature of Cr³⁺ ions with the ligands and the Cr³⁺ ions are in distorted octrahedral environment.     

EPR measurements on chromium doped GaAs,GaP and InP

EPR measurements have been made on chromium doped GaAs samples at 4.2 K. An n-type sample doped with chromium and silicon was irradiated with 2 MeV electrons to lower the Fermi level. No resonance from substitutional Cr⁺ (3d⁵) was detected, although the Cr_s²⁺ spectrum was observed. The generally accepted assignment of a spectrum to Cr_s⁺ for photoexcited samples must therefore be revised. An isotropic resonance with g = 2 is observed in p-type chromium doped GaAs, GaP and InP but it is still not clear whether this is due to Cr_s⁴⁺ or interstitial Cr_i⁺ (3d⁵).     

Raman light scattering,infrared absorption and DSC studies of the phase transition and vibrational and reorientational dynamics of H2O ligands and ClO4– anions in [Ba(H2O)3](ClO4)2

[Ba(H₂O)₃](ClO₄)₂ between 90 and 300 K possesses two solid phases. One phase transition of the first‐order type at: = 211.3 K (on heating) and = 204.6 K (on cooling) was determined by differential scanning calorimetry. The entropy change value (ΔS ≈ 15 Jmol^–1 K^–1), associated with the observed phase transition, indicates a moderate degree of molecular dynamical disorder. Both, vibrational and reorientational motions of H₂O ligands and ClO₄^– anions, in the high‐temperature and low‐temperature phases, were investigated by Fourier transform far‐infrared and middle‐infrared and Raman light scattering spectroscopies. The temperature dependences of the full‐width at half‐maximum values of the bands associated with ρ_w(H₂O) mode, in both infrared (~570 cm^–1) and Raman light scattering (~535 cm^–1) spectra, suggest that the observed phase transition is not associated with a sudden change of a speed of the H₂O reorientational motions. Ligands reorient fast, with correlation time of the order of several picoseconds, with a mean activation energy value E_a = 5.1 kJ mol^–1 in both high and low temperature phases. On the other hand, measurements of temperature dependences of full‐width at half‐maximum values of the infrared band at ~460 cm^–1, associated with δ_d(OClO)E mode, and Raman band at ~1105 cm^–1, associated with ν_as(ClO)F₂ mode, revealed the existence of a fast ClO₄^– reorientation in phase I and in phase II, with the E_a(I) and E_a(II) values equal to 8.0 and 6.5 kJ mol^–1, respectively. These reorientational motions of ClO₄^– are slightly distorted at the T_C. Fourier transform far‐infrared and middle‐infrared spectra with decreasing of temperature indicated characteristic changes at the vicinity of PT at T_C, which suggested lowering of the crystal structure symmetry. All these experimental facts suggest that the discovered phase transition is associated with small change of H₂O ligands and somewhat major change of ClO₄^– anions reorientational dynamics, and with insignificant change of the crystal structure, too. Copyright © 2012 John Wiley & Sons, Ltd.     

Abnormal Raman spectra in Er‐doped BaTiO3 ceramics

Greatly enhanced and abnormal Raman spectra were discovered in the nominal (Ba_{1 − x}Er_x)Ti_{1 − x/4}O₃ (x = 0.01) (BET) ceramic for the first time and investigated in relation to the site occupations of Er³⁺ ions. BaTiO₃ doped with Ti‐site Er³⁺ mainly exhibited the common Raman phonon modes of the tetragonal BaTiO₃. Er³⁺ ions substituted for Ba sites are responsible for the abnormal Raman spectra, but the formation of defect complexes will decrease spectral intensity. A large increase in intensity showed a hundredfold selectivity for Ba‐site Er³⁺ ions over Ti‐site Er³⁺ ions. A strong EPR signal at g = 1.974 associated with ionized Ba vacancy defects appeared in BET, and the defect chemistry study indicated that the real formula of BET is expressed by (Ba_{1 − x}Er_3x/4)(Ti_{1 − x/4}Er_x/4)O₃. These abnormal Raman signals were verified to originate from a fluorescent effect corresponding to ⁴S_3/2→⁴I_15/2 transition of Ba‐site Er³⁺ ions. The fluorescent signals were so intense that they overwhelmed the traditional Raman spectra of BaTiO₃. The significance is that the abnormal Raman spectra may act as a probe for the Ba‐site Er³⁺ occupation in BaTiO₃ co‐doped with Er³⁺ and other dopants. A new broad EPR signal at g = 2.23 was discovered, which originated from Er³⁺ Kramers ions. Copyright © 2014 John Wiley & Sons, Ltd.     

EPR Study of Sc<Subscript>2</Subscript>SiO<Subscript>5</Subscript>:Nd<Superscript>143</Superscript> Isotopically Pure Impurity Crystals

The Sc₂SiO₅ single crystals doped with 0.001 at.% of the ¹⁴³Nd³⁺ ion were studied by continuous-wave and pulse electron paramagnetic resonance methods. The g-tensors and hyperfine structure tensors for two magnetically non-equivalent Nd ions were obtained. The spin–spin and spin–lattice relaxation times were measured at 9.82 GHz in the temperature range from 4 to 10 K. It was established that three relaxation processes contribute to the spin–lattice relaxation processes. There are one-phonon spin–phonon interaction, two-phonon Raman interaction and two-phonon Orbach–Aminov relaxation processes. It was established that spin–spin relaxation time is of the same magnitude for neodymium ion doped in Sc₂SiO₅ and in Y₂SiO₅.     

Electrical properties of FeCr2S4 and CoCr2S4

Resistivity and magnetoresistance are measured as functions of temperature (77–450 K) on several FeCr₂S₄ and CoCr₂S₄ single crystals (as-grown, annealed in vacuum, annealed in S-atmosphere, and doped) and found to be very sensitive to these treatments. These results are discussed on the basis of the model consisting of donor level due to S-deficiencies in addition to Fe²⁺ and Cr²⁺ levels.     

Destruction of the short‐range disorder due to erbium doping in Pb0.8La0.2TiO3films

We show the destruction of a displacement of Ti in the short‐range structure by observing the disappearance of emission and Raman signals when the Er³⁺ concentration exceeds 7 mol% in sol–gel‐derived Pb_0.8La_0.2TiO₃ polycrystalline films. It is believed that there always exists disorder due to displacement of B ions in the skeleton of BO₆ in perovskite ABO₃ materials. This disorder due to the displacement of Ti ions breaks the center of symmetry to activate emission of rare‐earth ions such as Er³⁺ and Raman modes of perovskites. We found that the breaking of symmetry can be diminished by introducing more Er³⁺ ions. Copyright © 2009 John Wiley & Sons, Ltd.     

Cr3+−Cr3+ excited state Raman scattering from molecular vibrations in binuclear Cs3Cr2Cl9 induced by exchange striction

On increasing the temperature, several new bands appear at higher energy from their parent vibrational modes in the Raman spectrum of the molecular complex Cr₂Cl^3?₉. The parent and new bands have intensities that follow the thermal population factors of various ⁴A_2g⁴A_2g Cr³⁺ pair states. This behaviour is attributed to exchange striction which produces a change in vibrational energy with excited state for those normal modes with large net axial Cr³⁺?Cr³⁺ displacements.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle

Mn doped TiO₂ nanoparticles are synthesized by sol–gel method. Incorporation of Mn shifts the diffraction peak of TiO₂ to lower angle. The position and width of the Raman peak and photoluminescence intensity of the doped nanoparticles varies with oxygen vacancy and Mn doping level. The electron spin resonance spectra of the Mn doped TiO₂ show peaks at g = 1.99 and 4.39, characteristic of Mn²⁺ state. Reduction in the emission intensity, on Mn doping, is owing to the increase of nonradiative oxygen vacancy centers. Mn doped TiO₂, with 2% Mn, shows ferromagnetic ordering at low applied field. Paramagnetic contribution increases as Mn loading increases to 4% and 6%. Temperature dependent magnetic measurement shows a small kink in the ZFC curve at about 40 K, characteristic of Mn₃O₄. The ferromagnetic ordering is possibly due to the interaction of the neighboring Mn²⁺ ions via oxygen vacancy (F⁺ center). Increase in Mn concentration increases the fraction of Mn₃O₄ phase and thereby increases the paramagnetic ordering.     

Softening of charge density wave excitations at the superstructure transition in 2H-TaSe2

Raman scattering measurements performed between 5 K and 300 K on 2H-TaSe₂ reveal new modes which are assigned to the modes of the charge density wave, observed in light scattering due to the Fermi surface induced distortion. The mode at 49 cm^?1 of E_2g symmetry softens (with concurrent line-width broadening) towards 122 K, the transition temperature from the incommensurate distorted to the undistorted phase. The mode at 82 cm^?1 of A_1g symmetry appears to be connected with the transition at 90 K from the commensurate to the incommensurate superstructure. The mode at 24.5 cm^?1 of E_2g shows no temperature dependence and is clearly due to the rigid-layer vibration.     

27Al Magic Angle Spinning Nuclear Magnetic Resonance Study of Al1−x Cr x K(SO4)2·12H2O (x = 0, 0.07, and 0.2)

The physical properties of Al_1?x Cr _x K(SO₄)₂·12H₂O (x = 0, 0.07, and 0.2) were studied as a function of temperature using magic angle spinning nuclear magnetic resonance for ²⁷Al. On the basis of the physical properties of pure AlK(SO₄)₂·12H₂O, the effects of partially replacing Al³⁺ with Cr³⁺ ions were examined. Molecular motion changed with the concentration of Cr³⁺ ions. The relaxation process near 320 K was found to undergo molecular motion as described by the Bloembergen–Purcell–Pound theory. The activation energies, phase transition temperatures, and spin–lattice relaxation times in the rotating frame T _1ρ changed with the concentration of paramagnetic ions.     

Phase transitions in RbCaF3. I: Optical studies

Phase transitions in RbCaF₃ have been investigated by optical birefringence measurements and Raman scattering experiments. A near discontinuity in the onset of spontaneous birefringence at 196 K shows that the cubic to tetragonal phase change is “slightly” first order. The Raman spectra of the tetragonal phase support a D¹⁸_4h structure in which two phonons of A_1g and E_g symmetry soften as the 196 K transition is approached from lower temperatures. A very slow transition to a lower symmetry structure was observed at about 42 K in the Raman measurements.     

Transition of synthetic chromium oxide gel to crystalline chromium oxide: a hot‐stage Raman spectroscopic study

Chromium oxide gel material was synthesised and appeared to be amorphous in X‐ray diffraction study. The changes in the structure of the synthetic chromium oxide gel were investigated using hot‐stage Raman spectroscopy based upon the results of thermogravimetric analysis. The thermally decomposed product of the synthetic chromium oxide gel in nitrogen atmosphere was confirmed to be crystalline Cr₂O₃ as determined by the hot‐stage Raman spectra. Two bands were observed at 849 and 735 cm⁻¹ in the Raman spectrum at 25 °C, which were attributed to the symmetric stretching modes of O Cr^III OH and O Cr^III O. With temperature increase, the intensity of the band at 849 cm⁻¹ decreased, while that of the band at 735 cm⁻¹ increased. These changes in intensity are attributed to the loss of OH groups and formation of O Cr^III O units in the structure. A strongly hydrogen‐bonded water H O H bending band was found at 1704 cm⁻¹ in the Raman spectrum of the chromium oxide gel; however, this band shifted to around 1590 cm⁻¹ due to destruction of the hydrogen bonds upon thermal treatment. Six new Raman bands were observed at 578, 540, 513, 390, 342 and 303 cm⁻¹ attributed to the thermal decomposed product Cr₂O₃. The use of the hot‐stage Raman spectroscopy enabled low‐temperature phase changes brought about through dehydration and dehydroxylation to be studied. Copyright © 2010 John Wiley & Sons, Ltd.     

Electron paramagnetic resonance and optical absorption studies of manganese ions doped in polyvinyl(alcohol) complexed with polyethylene glycol polymer films

Electron paramagnetic resonance (EPR), optical absorption, and infrared spectral studies have been carried out on Mn²⁺ ions doped in poly(vinyl alcohol) (PVA) complexed with polyethylene glycol (PEG) films prepared by solution cast technique. The EPR spectra of 0.25?mol% Mn²⁺ ions doped polymer complex (PVA+PEG) at room temperature exhibit sextet hyperfine structure (hfs), centered at g????1.99. The spin?CHamiltonian parameter values indicate that the ground state of Mn²⁺ ion is d⁵ and the site symmetry around Mn²⁺ ions in tetragonally distorted octahedral site. The spin concentration participating in the resonance is measured as a function of temperature and it is observed that it obeys Boltzmann??s law. The paramagnetic susceptibility (??) is calculated from the EPR data at various temperatures (93?C333?K) and it obeys the Curie?CWeiss law. The optical absorption spectra exhibits two bands which are assigned to ⁶A_1g (S)??⁴A_1g (G) or ⁴E_g (G) and ⁶A_1g (S)??⁴T_2g (G) transitions. The infrared spectrum exhibits few bands due to the presence of O?CH, C?CH, and C=C groups.     

Double-electromagnetically induced transparency in a Y-type atomic system

We present secondary phase identification studies on Cr doped ZnO nanoparticles prepared by the sol-gel method. X-ray diffraction analysis confirms the formation of chromium oxides and there is found to be an increase of lattice parameter with thermal annealing. Scanning electron microscopic studies show the increase in the crystalline nature and particle size. Optical absorption measurements of the as prepared sample exhibit a strong band at 356 nm due to the free exciton absorption of the ZnO nanoparticles. An absorption band at 277 nm is due to the ³T₁ → ³T₂ transition in Cr⁴⁺ ions which appears only for the annealed samples. Photoluminescence studies show that deep level emission is completely suppressed after Cr₂O₃ formation/thermal annealing. Raman and FTIR spectra reveal formation of the Cr₂O₃ phase. Thermal annealing leads to the increase of crystalline nature which gives an enhancement to the Raman modes.     

Structure and phase composition of thin a-C:H films modified by Ag and Ti

The structure and phase composition of thin a-C:H and a-C:H〈M〉 films (M = Ag, Ti, or Ag + Ti) have been studied by Raman and X-ray photoelectron spectroscopy. The a-C:H〈M〉 films were prepared by ion-plasma magnetron sputtering of a combined target of graphite and metal in an Ar–CH₄ gas mixture. The Raman spectra of these films indicate that their structure is amorphous. The a-C:H〈Ag + Ti〉 films have a more graphitized structure in comparison with pure a-C:H films and films containing only one metal. It is established that carbon in the a-C:H〈Ag + Ti〉 films is in the sp ², sp ³, and C=O states, which are characteristic of the a-C:H, a-C:H〈Ag〉, and a-C:H〈Ti〉 films. In addition, there are also ether (–C–O–C–) or epoxy (?C?O–) carbon groups in the a-C:H〈Ag + Ti〉 films. It has been revealed that silver atoms in the a-C:H〈Ag〉 and a-C:H〈Ag + Ti〉 films form no chemical bonds with carbon, oxygen, and titanium. Titanium in the a-C:H〈Ti〉 and a-C:H〈Ag + Ti〉 films exists in the form of titanium IV oxide (TiO₂).