An EPR study of a triplet state center in (NH4)2SO4

Heating ammonium sulphate to near its dissociation temperature and then quenching to room temperature produces a stable triplet state center that is identified as NH₂⁺. The symmetry of the EPR spectra indicates that the center is located in two non equivalent sites with each of these sites having slightly different EPR parameters and two measurably differents orientations. Thus the center is apparently occupying the positions of the NH₄⁺ groups in the perfect lattice. A study is made of this center both at room temperature and also well below the ferroelectric phase transition of (NH₄)₂SO₄. It is found that above and below the transition temperature (?50°C) there is very little temperature dependence of the EPR spectra but at the phase transition there is an abrupt change in the spectra. The number of detectable centers doubles below the phase transition indicating that the inversion symmetry of the NH₂⁺ sites is eliminated at the transition temperature.     

Phase transition in Cu2+-doped RbH2 PO4 as observed by EPR

The electron paramagnetic resonance (EPR) spectra of Cu²⁺-doped RbH₂ PO₄ at elevated temperatures indicate a phase transition at 358 K. The EPR-silent state at this temperature is attributed to a so-called polymeric phase transition. After the transition when the temperature is lowered to 293 K, the EPR signal does not appear; therefore, the transition is irreversible. This result seems to be in agreement with the other observations. The EPR spectra for the sample indicate the presence of two sites for Cu²⁺, and the values of EPR parameters are in accord with the literature on Cu²⁺-doped single crystals. Any other phase transitions could not to be observed at low temperatures down to 113 K.     

Synthesis,TGA and EPR studies of phase transitions in Cu2+ doped (NH4)4ThF8 and (NH4)3ThF7

The paper presents the EPR evidence for the occurrence of phase transitions in (NH₄)₄ThF₈ and (NH₄)₃ThF₇. In both cases the Cu²⁺ probe occupies a NH⁴⁺ site and predominantly experiences a dynamic coordination, either due to dynamic Jahn-Teller effect, or due to fluxional behaviour of surrounding (ThF₈)^4- units in (NH₄)₄ThF₈. It is proposed that the structural phase transition in this compound at 214 K is associated with the transition of the dynamic coordination of Cu^2- into a static one, probably due to freezing of motion of (ThF₈)^4- units below this temperature. In (NH₄)₃ThF₇ the dynamic features of the Cu²⁺ EPR spectrum are absent and characteristics of a local orthorhombic symmetry are seen down to 77 K. However, in the high temperature range a change from orthorhombic to axial symmetry is observed at 524 K, possibly due to a phase transition.     

Static and dynamic effects in the EPR spectra of [A(H2O)6]BF6 systems

EPR studies of [Mg(H₂O)₆]BF₆ (B=Si, Ge) crystals are performed by using Mn²⁺ paramagnetic probes. The temperature dependence of the EPR spectra shows that the crystals undergo a ferroelastic phase transition to a monoclinic phase. In both systems the ferroelastic phase transition is preceded by intermediate states. The spin Hamiltonian parameters determined in the low and high temperature phases of the systems are useful to precise some aspects of the local atomic displacements involved in the intermediate states. The nature of this state is analyzed in the framework of different models as well.     

First order phase transition of Li3 ThF7 at 281 K: A comparative study between EPR and Raman scattering

Abstract

Electron Paramagnetic Resonance (EPR) and Raman Spectroscopy were used to study the low temperature phase transition of pulled Li₃ThF₇ single crystals, occurring at 281 K. In both cases, the room temperature spectra were very broad, owing to the statistical disorder and high ionic mobility of the lithium ions, which occupy only 3/4 of their structural sites. The results are compatible with a first order ferroelastic transition from the room temperature orthorhombic D_2h ²² phase to a monoclinic C_2h ^(x) one. The symmetry rules are well respected assuming a model with four lithium ions in average per chemical formula. The EPR spectra show also the appearing of additional ferroelastic domains.     

Fe2+ spin transition in [Fe(trz)(Htrz)2](BF4) as evidenced by a variable temperature EPR study

The EPR of Fe³⁺ ions has been used for the first time to evidence a low-spin (S=0) to high-spin (S=2) transition of Fe²⁺ ions in an octahedral ferrous complex [Fe(trz)(Htrz)₂](BF₄). The temperature dependence of the intensity of the Fe³⁺ EPR line atg=4.3 reveals a spin transition which occurs for the Fe²⁺ ions, with hysteresis. The transition temperatures areT _c↑=374 K in the warming mode andT _c↓=345 K in the cooling mode. The analysis of the EPR spectral data indicates the presence of a structural phase transition accompanying the spin transition.     

EPR of the ferroelectric phase transition in Li2Ge7O15:Cr3+ crystals

A Cr³⁺ EPR study of lithium heptagermanate crystals, Li₂Ge₇O₁₅ (LHG), close to the phase transition is reported. Orientation dependences of the spectra in the paraelectric phase of LHG have been measured. An anomalous broadening of the resonant lines accompanied by a crossover in their shape was found in the vicinity of the transition point. Doublet splitting of the EPR lines was observed to occur below T _C as a result of emergence of two structurally nonequivalent positions for Cr³⁺ ions. Fiz. Tverd. Tela (St. Petersburg) 40, 1102–1105 (June 1998)     

Temperature dependence of Mn2+ EPR in Sn2P2S6 near the phase transition

The X-band EPR spectrum of Mn²⁺ in Sn₂P₂S₆ was studied in the temperature rangeT=223–363 K. At room temperature the spin-Hamiltonian constants areg=2.00±0.01,B ₂ ⁰ =(163±3)·10^?4 cm^?1,B ₂ ² =(159±3)·10^?4 cm^?1,A=?(75±1)·10^?4 cm^?1. The effect of the invariance in temperature of the resonance magnetic fields in the narrow temperature rangeT=337–340 K and the model of the paramagnetic centre are discussed. According to EPR data a phase transition occurs atT=337 K. This transition from the paraelectric phase to the ferroelectric one is accompanied by a dramatic change in value of the spin-Hamiltonian constantB ₂ ⁰ .     

EPR study of the ferroelectric phase transition in chromium-doped DMAAS

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr³⁺-doped dimethylammonium aluminium sulphate hexahydrate are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K from the ferroelastic to the ferroelectric phase. The spin-Hamiltonian parameters have been determined for both phases. The spin-Hamiltonian parameters in the ferroelectric phase are:g=1.980±0.003,b ₂ ⁰ =(1140±15)·10^?4 cm^?1,b ₂ ² =(214±10)·10^?4 cm^?1. Remarkable EPR line width changes confirm the order-disorder character of the ferroelectric phase transition on a microscopic level and demonstrate that the dimethylammonium reorientation freezing-out is the prime reason for this transition.     

The effect of clustering of VO2+ ions in sub- and supercritical water. An in situ EPR study

Temperature variations of the EPR spectra of VO²⁺ ions in sub- and supercritical water under isothermal and temperature gradient conditions are investigated using an in situ EPR. Broadening of the hyperfine structure at increasing temperature and the appearance of an unresolved broad low-intensity line (ΔH _pp ≈ 300 Oe) in the supercritical state are observed in the absence of temperature gradients, indicating an increase of exchange interaction between VO²⁺ ions in supercritical water. An exchange-narrowed anisotropic absorption line is observed under the temperature-gradient conditions in the subcritical water near the transition to a supercritical state. The shape of this line is close to that observed in the solid salt VOSO₄ · 3H₂O. It is shown that in situ EPR allows us to investigate the effects of changing the local environment of paramagnetic ions, which precedes the well-known process of clustering and formation of amorphous oxide particles in sub- and supercritical conditions.     

The influence of chromium impurity centers on the critical properties of a weakly polar ferroelectric Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript>

The Cr³⁺ EPR spectra of Li₂Ge₇O₁₅ (LGO) crystals are analyzed in the temperature range of the ferroelectric phase transition. The temperature dependence of the local order parameter is determined from the measured splittings of the EPR lines in the polar phase. The experimental critical exponent of the order parameter β=0.31 in the range from the phase transition temperature T _C to (T _C -T) ～ 40 K corresponds to the critical exponent of the three-dimensional Ising model. Analysis of the available data demonstrates that, away from the phase transition temperature T _C , the macroscopic and local properties of LGO crystals are characterized by a crossover from the fluctuation behavior to the classical behavior described in terms of the mean-field theory. The temperature dependence of the local order parameter for LGO: Cr crystals does not exhibit a crossover from the Ising behavior (β=0.31) to the classical behavior (β=0.5). This is explained by the defect nature of Cr³⁺ impurity centers, which weaken the spatial correlations in the LGO host crystal. The specific features of the critical properties of LGO: Cr³⁺ crystals are discussed within a microscopic model of structural phase transitions.     

EPR study of copper ions in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The EPR spectra of Cu²⁺ ions (² D _5/2) located at two structurally nonequivalent positions Cu1 and Cu2 in crystals of lithium heptagermanate Li₂Ge₇O₁₅ are recorded. The angular dependences of the EPR spectrum are measured in the paraelectric phase of the Li₂Ge₇O₁₅ compound (T = 300 K). The components of the g factor and the hyperfine interaction tensor A are determined, and the orientation of the magnetic axes with respect to the crystallographic basis is established. The EPR spectra are recorded in the temperature range in the vicinity of the temperature T _C = 283 K of the transition from the paraelectric phase to the ferroelectric phase. The position symmetry of the Cu1 and Cu2 centers is determined at temperatures above and below the phase transition temperature T _C . The localization of paramagnetic centers in the structure is discussed, An analysis of the results obtained demonstrates that the Cu1 and Cu2 centers in the Li₂Ge₇O₁₅ crystal lattice replace lithium ions located at two structurally nonequivalent positions with the symmetries described at temperatures above T _C by the triclinic C _i and monoclinic C ₂ point groups, respectively.     

Temperature dependence of soliton density in the incommensurate phase of [N(CH3)4]2ZnCl4 crystals

The successive phase transitions of [N(CH₃)₄]₂ZnCl₄ have been investigated by EPR in a Mn²⁺ doped-sample. The temperature change in the resonance line shape near the commensurate-incommensurate transition temperature was analysed by a one-dimensional phase soliton model, so that the soliton density, modulation amplitude and initial phase were determined. The discommensuration phenomenon was also observed and discussed in terms of the domain wall resulting from the soliton.     

120 K superconductivity and the zero-field absorption in the BiCaSrCu2Ox system

Onset transition temperature (123±1) K was observed in the BiCaSrCu₂O_x bulk materials prepared by means of conventional ceramic technique. The resistance and AC susceptibility measurements show that there are two highT _c phases. EPR measurement shows that belowT _c (R=0), an intense line at zero field was observed. It is either associated with the glass feature or the spin triplet transition of Cu²⁺–Cu²⁺ pair via exchange interaction.The Project Supported by National Natural Science Foundation of China     

EPR study of the high-TC superconductor YBa2Cu3O7−δ

EPR spectra of superconducting YBa₂Cu₃O_7-δ and insulating Y₂BaCuO₅ have been examined. The intensity of Cu²⁺ ions in YBa₂Cu₃O_7-δ is about 1/3000 of that in Y₂BaCuO₅ at room temperature. Owing to the Meissner effect in the superconducting state, the EPR signal of Cu²⁺ ions in YBa₂Cu₃O_7-δ disappears; moreover the EPR signal of DPPH deposited on the YBa₂Cu₃O_7-δ specimen is shifted. An additional EPR line is found for YBa₂(Cu_2.94Cr_0.06)O_7-δ at temperatures below 113K; this can be related to the phenomenon of superconductivity.     

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.     

Incommensurate Phases of the MgSiF6·6H2O Crystals: EPR and Group-Theoretical Studies

The group-theoretical study of the structural phase transition to incommensurate state of MgSiF₆·6H₂O crystals, revealed by the electron paramagnetic resonance (EPR) method, as well as analysis of the EPR results, are presented. The consideration of temperature dependences of Mn²⁺ admixture ion EPR spectrum symmetry and parameters leads to the conclusion that at T _i1 = 370 ± 0.3 K they undergo second-order structural phase transition to incommensurately modulated state, the order parameter of this transition may be the angle of [Mg(H₂O)₆]²⁺ octahedra rotation around crystal C ₃ axis. At temperature decreasing below T _i1 the gradual transformation of plane-wave modulation of lattice displacements into soliton mode occurs, which is interrupted by the first-order phase transition at T _i2 = 343 ± 0.3 K accompanied by abrupt decrease in modulation amplitude. At T _c = 298.5 ± 0.3 K the first-order improper ferroelastic phase transition into monoclinic phase occurs. The group-theoretical analysis of the phase transition at T _i1 in the investigated crystals, carried out for the first time, has shown that the existence of the incommensurately modulated phase is conditioned by the fundamental reasons (presence of Lifshitz invariant). The conclusions of this analysis on the nature of order parameter, the structural motifs of incommensurate phase and the possible character of temperature evolution of the structure are in agreement with the EPR investigation data.     

EPR of cadmium ferric voltaite above the ferrimagnetic transition

A single isotropic EPR line of Fe³⁺ in synthetic cadmium ferric voltaite, (NH₄)₂Cd₅Fe₃Al(SO₄)₁₂ · 18H₂O, was observed in a wide temperature range from 295 to 1.57°K. The ferrimagnetic transition temperature of CdFe voltaite was determined to be ～ 0.7°K using the temperature dependence of the g-factor and the line width. The cubic crystal field parameter, a, for Fe³⁺ in CdFe voltaite is extracted from the EPR line width measurement using the exchange-narrowed line width model of Anderson and Weiss. The parameter a for Fe³⁺ in CdFe voltaite at 4.2°K is 157 × 10^-4cm^-1 which is consistent with the corresponding values for Fe³⁺ in other cubic structures.     

Dielectric-spectroscopic and a.c. conductivity investigations on copper doped layered Na1.8K0.2Ti3O7 ceramics

Dielectric studies on copper doped derivatives of polycrystalline layered mixed alkali trititanate Na_1.8K_0.2Ti₃O₇ ceramics indicate that the losses are of mixed type and decrease on copper doping. However, the temperature dependent permittivity plots are characteristic of the diffuse nature of a possible ferroelectric phase transition and hence give indication of relaxor ferroelectric behaviour. From the EPR spectra, recorded at room temperature, it can be seen that the substitution of copper occurs at Ti⁴⁺ as preferred site with a divalent oxidation state (Cu²⁺) for all compositions. Also, copper doping enhances the transition temperature, which is indicative of the stabilization of the existing ferroelectric phase up higher temperatures. Besides bolstering electron hopping conduction, acceptor doping restrains the interlayer ionic conduction. Moreover, electron hopping (polaron) conduction is dominant over the lower temperature region, while interlayer ionic conduction prevails in the higher temperature region.     

Static and dynamic disordered states in VO2+-doped BaCl2·2H2O crystals

From the EPR investigation, two different crystalline states were found in the VO²⁺ ion-doped BaCl₂·H₂O crystal. One is a dynamic disordered state, where the trapped VO⁺² ion exhibits hindered rotational motion in a wide temperature region. This state is transformed to a static disordered state even at ambient temperature. The orientation of the doped ion reveals random distribution in the lattice sites. Such polymorphism does not come from phase transition of the host crystal, but from the existence of the metastable state of the VO²⁺ ion trapped in the lattice. The absence of the phase transition of pure BaCl₂·2H₂O was confirmed by the measurement of ¹³⁷Ba NQR and by differential thermal analysis.