Paramagnetic resonance study of the structural phase transition in sodium azide

The electron paramagnetic resonance (EPR) of Mn²⁺ impurities in sodium azide (NaN₃) single crystals has been measured between 178 K and 300 K in order to study the transition between the monoclinic low temperature α phase and the trigonal high temperature β phase of NaN₃. The analysis of the EPR spectra yields the parameters of the zero-field splitting (zfs) in the spin Hamilton operatorD(S _ξ ² ?1/3S(S+1))+E(S _ξ ² ?S _η ² ) associated with any of four different pair states of a Mn²⁺ ion and a cation vacancy. The obtained zfs, which is temperature dependent in α-NaN₃ and constant in β-NaN₃, is consistent with a continuous phase transition atT _c =292 K. Using a point charge model for the crystal potential the zfs parameters are related to the lattice structure in the vicinity of the Mn²⁺ ion. The zfs is not consistent with the temperature dependent atomic displacements in α-NaN₃ obtained previously from an X-ray diffraction study. The critical exponents of the zfs parameters, derived in the temperature range 1 K<T _c ?T<114 K, differ from the critical exponents of atomic displacements in α-NaN₃.     

Effect of hydrostatic pressure and temperature on the EPR spectrum of the Mn2+ ion in Zn(BF4)2 · 6H2O

A study of the effect of hydrostatic pressure and temperature on the EPR spectrum of the Mn²⁺ ion in Zn(BF₄)₂ · 6H₂O is reported. The break in the temperature dependence of the b ₂ ⁰ parameter at 196 K is evidence of the existence of a phase transition accompanied by a change in the thermal expansion coefficient. It is shown that pressure considerably affects the spectral parameters by reducing the axial parameter b ₂ ⁰ and increasing the cubic one, b ₄ ⁰ . At 9 kbar, the b ₂ ⁰ parameter is temperature independent. A comparison of the pressure dependences of the spectra of Zn(BF₄)₂ · 6H₂O, ZnSiF₆ · 6H₂O, ZnTiF₆ · 6H₂O, and MgSiF₂ · 6H₂O crystals suggests equal hydrogen-bond lengths in these compounds. A ligand hyperfine structure has been detected, which originates from the Zeeman interaction with the proton nuclei surrounding Mn²⁺ and manifests itself in the formation of satellites at each EPR line, their separation being proportional to the magnetic field. The nonlinear pressure dependence of the linewidth is related to the structural features of the crystal under study.     

EPR and IR spectral studies on Mn ions in nickel maleate tetrahydrate single crystals

Electron paramagnetic resonance (EPR) studies have been carried out on Mn²⁺ ions doped in nickel maleate tetrahydrate single crystals in the temperature range 103-413 K on X-band frequency. The EPR spectrum at room temperature exhibits a group of five fine structure transitions each splits into six hyperfine components. Angular variation studies reveal the presence of a single site and it is found that Mn²⁺ ions enter the lattice substitutionally. From the observed EPR spectrum, the spin-Hamiltonian parameters have been evaluated. The variation of zero-field splitting parameter (D) with temperature is measured. The observed EPR spectra exhibit a large anisotropy in the widths of Mn²⁺ resonance lines. The widths of Mn²⁺ resonance lines increase with the Zeeman field intensity and these observations have been discussed in detail. The infrared spectrum exhibits bands characteristic of the carboxylic acid salts.     

Temperature and pressure dependences of EPR spectra of Gd ion doped in the EuAl3(BO3)4 monocrystal

The ground state of Gd³⁺ ions substituting for trivalent europium in the EuAl₃(BO₃)₄ single crystal was studied by electron paramagnetic resonance (EPR) over the temperature range of 300-4.2 K and at pressures up to 9 kbar. The EPR spectra were analysed using the spin Hamiltonian of axial symmetry. The following parameters are reported: g=1.981±0.002, b₂⁰=280.18±0.12, b₄⁰=−12.95±0.08 and b₆⁰=0.61±0.12 (at Т=298 K). The distortions of the nearest environment of Gd³⁺ ion were analysed within the framework of the superposition model of crystal field.     

Electron paramagnetic resonance of the Eu2+ ion in SrCl2 at high pressure

This paper reports an EPR study of the effect of hydrostatic pressure (up to 10 kbar) and temperature (300, 77, and 4.2 K) on the spin Hamiltonian parameters of the Eu²⁺ ion in a SrCl₂ cubic crystal. It is found that the b ₄ parameter is related by a power law to the distance from the Cl^?1 ligand (b ₄ ～ R ^?13.5). The pressure and temperature are shown not to be equivalent thermodynamic parameters. Lattice vibrations contribute noticeably to the initial S-ion splitting.     

Temperature dependence of the ground state splitting of Mn2+ in some scheelite type crystals

The EPR spectrum of Mn²⁺ in CaMoO₄, CdMoO₄, CaWO₄ and SrWO₄ has been investigated in the temperature range from 4.2–380 K. Results are given for the Spin-Hamiltonian parametersb ₂ ⁰ andb ₄ ⁰ . They are completed by data forb ₂ ⁰ of Mn²⁺ in SrMoO₄ [1] and discussed with respect to resonant vibrations.     

An EPR Study of Cu2+ Doped Sodium Hydrogen Oxalate Monohydrate Single Crystal

The EPR spectra of Cu²⁺ in Sodium hydrogen oxalate monohydrate, NaHC₂O₄.H₂O(SHOMH hereafter) single crystal was studied at room temperature. The angular variation of EPR spectra showed that the Cu²⁺ ion in SHOMH single crystal substitutes with Na⁺ monovalent cation together with a monovalent vacancy to compensate oxygen in the crystal. Since the crystal symmetry is triclinic, only one site is observed in the EPR spectra in three perpendicular axis. The spin Hamiltonian parameters were obtained, and the ground state wave function of Cu²⁺ ion in the lattice was constructed.     

Electron paramagnetic resonance and optical spectroscopy of K3Na(CrO4)2 ferroelastics activated by MnO 4 2− molecular impurity ions

Crystals of a proper ferroelastic K₃Na(CrO₄)₂ containing molecular impurity ions MnO ₄ ^2? are studied using electron paramagnetic resonance (EPR) and optical spectroscopy. The EPR spectrum of the Mn⁶⁺ ion contained in the molecular impurity ion MnO ₄ ^2? is identified at low temperatures (T ≤ 20 K). The intensity of this spectrum decreases unusually fast as the temperature increases. A broad IR luminescence band with a vibronic structure well resolved at a temperature of 8 K is revealed. Theoretical treatment of the Mn⁶⁺ ion involved in the molecular impurity ions MnO ₄ ^2? of the K₃Na(CrO₄)₂ ferroelastic crystal suggests that an important role in this case is played by the pseudo-Jahn-Teller. The pseudo-Jahn-Teller effect offers an explanation for the appearance of a fine structure in the vibronic replicas in the luminescence spectrum, on the one hand, and accounts for the fast decrease in the intensity of the EPR spectrum of K₃Na(CrO₄)₂: MnO ₄ ^2? with increasing temperature, on the other.     

The migration of Mn2+ ions in Cd1−xMnxS nanocrystals: Thermal annealing control

This work reports evidence of the induced migration of Mn²⁺ ions in Cd_(1?x)Mn_xS nanocrystals (NCs) by selecting a specific thermal treatment for each sample. The growth and characterization of these magnetic dots were investigated by atomic force microscopy (AFM), optical absorption (OA), and electronic paramagnetic resonance (EPR) techniques. The comparison of experimental and simulated EPR spectra confirms the incorporation of Mn²⁺ ions both in the core and at the dot surface regions. The thermal treatment of a magnetic sample, via selected annealing temperature and/or time, affects the fine and hyperfine interaction constants which modify the shape and the intensity of the EPR transition spectrum. The identification of these changes has allowed tracing the magnetic ion migration from core to surface regions of a dot as well as inferring the local density of the magnetic impurity ions.     

Local structure and the electron paramagnetic resonance parameters for the Cr3+ ions at K+-vacancy site in Cr3+:KZnF3 laser crystal

The quantitative relationship between the electron paramagnetic resonance (EPR) parameters D,g_∥,g_⊥ and the local structure parameters of Cr³⁺ ion in KZnF₃ crystals is established. The local structure for Cr³⁺ paramagnetic center in KZnF₃:Cr³⁺ crystal has been determined from EPR parameters of Cr³⁺ ion. This work shows that the trigonal crystal field of Cr³⁺ ion in KZnF₃ crystals comes from following two origins: (1) the nearest-neighbor K⁺ vacancy caused by the charge compensation in the [1 1 1]-axis direction; and (2) the lattice distortions of the nearest-neighbor fluorine coordination caused by the K⁺ vacancy and the differences in mass, charge, and radius between Cr³⁺ ion and Zn²⁺ ion. The unified calculation of the EPR zero-field splitting and g factors, taking into account the K⁺ vacancy and the lattice distortions, has been carried out on the basis of the complete diagonalization procedure and the superposition crystal-field model, all calculation results are in excellent agreement with the experimental data. Although the main source of the trigonal crystal field comes from the K⁺ vacancy caused by the charge compensation, the contribution of the lattice distortion cannot be neglected.     

EPR-Determined Anisotropy of the <Emphasis Type="Italic">g</Emphasis>-Factor and Magnetostriction of a Cu<Subscript>2</Subscript>MnBO<Subscript>5</Subscript> Single Crystal with a Ludwigite Structure

Electron paramagnetic resonance (EPR) and magnetostriction of the Cu₂MnBO₅ single crystal have been studied. The EPR spectrum consists of a single Lorentzian line due to the exchange-coupled system of spins of Cu²⁺ and Mn³⁺ ions. It has been established experimentally that the g-factor in the paramagnetic region is strongly anisotropic and anomalously small, which is not typical of the exchange-coupled system of spins of Cu²⁺ and Mn³⁺ ions. At a temperature of 150 K, the g-factors along the crystallographic a, b, and c axes are 2.04, 1.96, and 1.87, respectively. Such small effective g-factor values can be due to the effect of the anisotropic Dzyaloshinskii–Moriya exchange interaction between the spins of Cu²⁺ and Mn³⁺ ions directed along the a axis. The presence of two Cu²⁺ and Mn³⁺ Jahn–Teller ions occupying four nonequivalent positions in the crystal is responsible for the absence of the inversion center. It is found that the behavior of the magnetostriction of Cu₂MnBO₅ is not typical of transition-metal crystals but is closer to the behavior of crystals containing rare-earth ions.     

Theoretical investigation of the local lattice structure of Mn 2+ ion doped in tetragonal K 2ZnF 4 crystal

The complete energy matrices for a d⁵configuration ion in a tetragonal ligand-field has been constructed on the basis of the complete set of basis of d⁵configuration (252 dimension), and the relationship between the low-symmetry EPR parameters b₂ ⁰ ,b₄ ⁰ and the local distortion parameters has been established based on the complete energy matrices. As an application, we have studied the EPR parameters and the local lattice structure of Mn²⁺ ion doped in tetragonal K₂ZnF₄ system. The calculation indicated that the local lattice structure around a tetragonal Mn²⁺ ion center has an expansion distortion. Simultaneously, the local lattice structure parameters R₁ =2.0727 ?, R₂ =2.0801 ? at room temperature (295 K) and R₁ ^′ = 2.0439 ?, R₂ ^′ =2.05478 ? at low temperature (4.2 K) are determined.     

A Low Temperature (10 K) High-Frequency (208 GHz) EPR Study of the Non-Kramers Ion Mn3+ in a MnMo6Se8 Single Crystal

A high-frequency (208 GHz) electron paramagnetic resonance (EPR) study on Mn³⁺ (3d⁴, S = 2) ions embedded in a MnMo₆Se₈ single crystal has been performed at 10 K. The experimental spectra reveal the presence of only one set of EPR lines from Mn³⁺ ions, whose magnetic axes are oriented along the crystal axes. The spin-Hamiltonian parameters are evaluated by the method of least-squares, fitting all the observed line positions simultaneously, for the three orthogonal orientations of the external magnetic field. The symmetry of the spin Hamiltonian at the site of the Mn³⁺ ions has been deduced from the EPR spectra.     

Radiation stabilization of U5+ in CaO matrix and its thermal stability: Electron paramagnetic resonance and thermally stimulated luminescence studies

Electron paramagnetic resonance (EPR) evidence is presented for the radiation stabilization of pentavalent uranium in CaO matrix. From the theoretical predictions ofg value for U⁵⁺ in axial symmetries, it was concluded that U⁵⁺ at Ca²⁺ site is associated with a second neighbour charge compensating Ca²⁺ vacancy. EPR measurements also revealed the presence of Mn²⁺, Mn⁴⁺ and Cu²⁺ impurities in the samples. The thermal stability of U⁵⁺ was investigated using EPR and thermally stimulated luminescence (TSL) techniques. The TSL and EPR studies on gamma irradiated uranium doped calcium oxide samples had shown that the intense glow peak at 540 K is associated with the reduction in the intensity of EPR signal of U⁵⁺ ion around this temperature. This peak is associated with the process U⁵⁺+hole→U^6+*→U⁶⁺+hv. The activation energy for this process was determined to be 1.4eV.     

Mn2+ paramagnetic resonance and structural phase transitions in Rb2CdCl4

Structural phase transition in Rb₂CdCl₄: Mn²⁺ single crystal has been found near 133 K by EPR, X-ray and optical methods. Octahedral tilt system in the low temperature phase corresponds to the symmetry change D¹⁷_4h → D¹⁸_2h or D¹⁷_4h → C⁶_2h.     

Ferromagnetic superstructure of an La0.85Sr0.15MnO3 manganite single crystal

The elastic thermal-neutron scattering patterns of a La_0.85Sr_0.15MnO₃ manganite orthorhombic single crystal are investigated in the temperature range 4.2–300 K. It is found that, in addition to the known ferromagnetic ordering (T _C=240 K), this compound exhibits a ferromagnetic superstructure with the (010)2π/b wave vector (in the Pnma setting of the space group D _2h ¹⁶ ). The ferromagnetic superstructure is observed in the studied crystal at temperatures ranging from 4.2 to 200 K. It is shown that the formation of the ferromagnetic superstructure in this compound is directly associated with a 1/8-type charge ordering of Mn³⁺ and Mn⁴⁺ ions.     

Variable temperature EPR spectra of Copper (II) ions in kainite crystals

X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in KMgClSO₄·3H₂O single crystals have been performed at low temperature (123 K). The angular variation of the EPR spectra reveals the presence of two Cu²⁺ sites, which have different orientations. The spin-Hamiltonian parameters of this six-coordinated cupric ion have been evaluated from the EPR spectra at 123 K. The forbidden lines due to Δm_I=±1 transitions are observed in between allowed transitions. The temperature variation EPR studies have also been performed both for a single crystal and a polycrystalline sample. The ground state wavefunction of Cu²⁺ ions has been estimated and is found to be an admixture of d_3z²−r² and d_x²−y². The temperature variation of the EPR spectra reveals that Cu²⁺ ions exhibit dynamic Jahn-Teller effect. From the polycrystalline EPR data, the temperature dependent magnetic susceptibilities are evaluated and discussed.     

An anomalous thermal expansion in the perovskite system,Gd1−xSrxMnO3 (0 ≤ x ≤ 0.3)

The thermal expansion behavior of sintered samples of Gd_1−xSr_xMnO₃ (X = 0.0–0.4) was studied. The sintered bodies in this system showed negative thermal expansion over a wide temperature range. The detailed crystal structure refinements with respect to temperature showed that the volume of the orthorhombic perovskite lattice monotonically increased with temperature, however, in addition to this, the release of distortion from the Jahn-Teller effect of Mn³⁺ ion occurred over a wide temperature range, which brought the negative expansion of the a-axis, although the b- and c-axes increased with temperature. The anomalous thermal expansion is explained by the sum of the effects of the shrinkage of the a-axis and absorption of the b- and c-axes' expansion by the pores in the sintered body.     

Theoretical investigation of zero field splitting parameters for Mn centers in ammonium tartrate

Zero-field splitting (ZFS) parameters D and E for Mn²⁺ centers in ammonium tartrate single crystal are calculated with perturbation formulae using the superposition model. The theoretically calculated ZFS parameters for Mn²⁺ at site I and site II of ammonium ion are compared with the experimental values obtained by electron paramagnetic resonance (EPR) at room temperature. The superposition model gives the ZFS parameters similar to those from experiment. The energy band positions of optical absorption spectrum of Mn²⁺in ammonium tartrate are calculated using the CFA package and crystal field parameters from superposition model. These are in good agreement with experimental energy band positions.     

EPR study of a new low-temperature phase transition in Zn(ClO4)2·6H2O

From a temperature variation EPR study of Mn²⁺ doped single crystals of Zn(ClO₄)₂·6H₂O phase transition has been detected at T₂~290 K. The phase relationships in this crystal are as follows. Phase I transforms atT₂~346K to Phase II, which in turn transforms to Phase III at T₂ ~ 290K. The latter exists down to at least 220 K. The space group symmetry of crystal may be the same, i.e. Pmn2₁ both above and below T₂. The water-perchlorate sublattice symmetry below T₂ is found to be lower than the P6₃mc symmetry determined previously by X-ray measurements. The onset of a monoclinic or lower symmetry distortion of the water octahedron around a metal ion which starts just below T₂, is reflected through the observed temperature dependence of the rhombic distortion parameter E. It is felt that during this phase transition a change in the degree of configurational disorder associated with the perchlorate tetrahedra takes place, which in turn modifies the hydrogen bonded interaction in the crystal and consequently results in the onset of temperature dependent displacements of the mean positions of the oxygens of the water molecules.