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)     

Estimation of T1 for Co2+ ions from temperature variation of the EPR linewidths for Gd3+ in Ce2Co3(NO3)12.24H2O single crystals

EPR of Gd³⁺ doped in Ce₂M″₃(NO₃)₁₂.24H₂O (M″ = Mg, Zn, Co) single crystals has been studied at various temperatures from room temperature to 77 K using ∼ 9.45 GHz EPR spectrometer. The observation of resolved Gd³⁺ spectra at room temperature in Ce₃Co₂(NO₃)₁₂.24H₂O has been interpreted in terms of a random modulation of the interaction between the Gd³⁺ and the Co²⁺ ions by the rapid spin-lattice relaxation of Co²⁺ ions. It is found that the effective spin-lattice relaxation time T₁ ∝ T⁻ⁿ where n = 1.85 (B∥z axis) and n = 1.75 (B⊥z axis) if 103 < T < 283 K.     

Optical investigations of Mn2+ ion in Cs2CdCl4 and CsCaCl3 single crystals

Summary Polarized absorption, emission and excitation spectra of Cs₂CdCl₄∶Mn²⁺ and emission as well as excitation spectra of CsCaCl₃∶Mn²⁺ single crystals at temperatures down to 77 K are reported. The general features of the spectra are fruitfully analysed within the framework of the strong field scheme of Tanabe and Sugano including Trees' correction. Some finer structure of the crystal field independent levels are observed which are rationalized in terms of vibrational mechanism. A detailed analysis of the spectra indicates octahedral site symmetry around the central metal ion. The authors of this paper have agreed to not receive the proofs for correction.     

EPR of Pb3+ ion in LiBaF3 crystals

Complex EPR spectra of paramagnetic centers Pb³⁺ formed in LiBaF₃:Pb²⁺ crystals under X-ray irradiation are studied in the temperature range of 10–150 K. It is shown that lead ions substitute Ba²⁺ ions in the LiBaF₃ crystal and are in the cubic-octahedral 12-fold environment of the fluorine ions. The hyperfine structure constants describing the observed spectrum are determined and parameters of superhyperfine interaction with the nearest fluorine ions are estimated.     

X-ray investigations of the cubic to tetragonal phase transition in CsCaCl3 at Tc = 95 K

Some new crystallographic data on the cubic to tetragonal phase transition at T_c = 95 K in CsCaCl₃ allow us to infer the space group D¹⁸_4h for the low temperature phase connected with alternate rotations of the CaCl₆ along 〈0 0 1〉 directions. From the relative distortion (c-a)/a we estimate the rotation angle which is the order parameter of the transition, smaller than in other chloride perovskites as CsPbCl₃ and CsSrCl₃.     

Photoinduced phase transition in Ag3AsS3 crystals

It is shown that the illumination of samples leads to the appearance of anomalies in the temperature dependences of the propagation velocity of longitudinal ultrasonic waves along the principal crystallographic directions of proustite (Ag₃AsS₃) at T∼150 K. The features discovered are associated with a photoinduced phase transition caused by restructuring of the cation sublattice of proustite. Fiz. Tverd. Tela (St. Petersburg) 41, 702–704 (April 1999)     

EPR of Yb3+ ions in Ba1−xLaxF2+x mixed crystals

Electron paramagnetic resonance (EPR) spectra of impurity Yb³⁺ ions (about 0.1 at.%) in mixed crystals BaF₂(1-x) plus LaF₃(x) have been investigated for different values of the concentrationx at a frequency of about 9.5 GHz by both continuous-wave (CW) EPR and electron spin echo methods. A spectrum of trigonal symmetry with a complex hyperfine structure is observed in “pure” BaF₂:Yb³⁺ (x=0). Upon admixture of small amounts of LaF₃ (x=0.001), additional EPR lines arise with intensities increasing with the increase ofx up to 0.005. These lines are attributed to trigonal centers including two rare-earth ions and two compensating fluorine ions. A further increase ofx results in a decrease of the total EPR spectrum intensity, and atx≥0.05 the CW resonance becomes practically unobservable. This may be due to the formation of rare-earth ion clusters with paramagnetic Yb³⁺ ions occurring in domains with a disordered structure of surroundings resulting in very broad EPR lines, which cannot be registered by CW EPR. Indeed, very broad (not less than 1 KG) EPR lines were observed by the electron spin echo method for concentrationsx<-0.02.     

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 .     

Fluctuation-induced broadening of Mn2+ EPR lines in the incommensurate phase of Rb2ZnCl4 crystals

The EPR spectra of Mn²⁺ ions in Rb₂ZnCl₄ crystals is investigated in the vicinity of the transition from the paraelectric phase to an incommensurate modulated phase. When these crystals are cooled below the transition temperature T _i =304 K, a splitting of the resonance lines is observed in the singular spectrum. A one-harmonic model is used to discuss the contributions that fluctuations in the amplitude and phase of the incommensurate displacement wave make to the local width of the singular spectra. It is shown that anomalies in the local width of the low-temperature singular peaks observed in the vicinity of T _i are caused by amplitude fluctuations. Fiz. Tverd. Tela (St. Petersburg) 41, 1668–1674 (September 1999)     

Wide-band EPR spectroscopy of YAlO3: Tm3+ single crystals

The YAlO₃: Tm³⁺ single crystal has been studied on a wide-band EPR spectrometer. The EPR spectra of Tm³⁺ ions in the frequency range of 90–160 GHz have been detected for the first time. It has been confirmed that thulium ions substitute the position of Y³⁺ in the crystal lattice. The detected spectra have been described with the use of a spin Hamiltonian with the effective spin S = 1/2. A comparative analysis of the orientation of the magnetic axes of the Tm³⁺ paramagnetic center with earlier data on other rare-earth ions has been performed.     

Electron paramagnetic resonance (EPR) and spin-lattice relaxation of trigonal Co2+ centers in ZnS single crystals

Single crystals with a ZnS structure are of great practical value, in which connection they are studied in detail by different methods including the spectroscopic. In particular, such crystals activated by ion impurities are investigated in detail by the EPR method. However, the spin-lattice relaxation (SLR) of such systems has hardly been studied. Papers studying the SLR of cubic Mn²⁺ centers [1] and trigonal Nd³⁺ and Yb³⁺ [2] centers in ZnS did only appear recently.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 141–143, August, 1978.The authors are grateful to A. I. Ryskin for providing the single crystals for the investigation and to R. Yu. Abdulsabirov for assistance in performing the experiments.     

Gd3+, Eu2+ in SrFCl and BaFCl single crystals: EPR results

Electron Paramagnetic Resonance results are reported on Gd³⁺, Eu²⁺ incorporated into BaFCl and SrFCl crystals.     

EPR of Co+2 in Mg(CH3COO)2·4H2O

EPR spectra of the ⁵⁹Co⁺² ion in oriented crystals of Mg(CH₃COO)₂·4H₂O have been measured at 9.4 GHz and a temperature of 4.2 K. The data for each of the two metal ion sites per unit cell are well-described by a spin Hamiltonian for $\text{S =}\text{1}\text{2}\text{, I =}\text{7}\text{2}$ with g_x = 6.13, g_y = 389, g_z = 2.49 and A_x = 0.0193, A_y' = 0.075, A_z' = 0.0032 cm^?1. Only the x-axes of the g- and A-tensors coincide. Orientation of the principal directions relative to the crystal axes has been established for the g- and A-tensors at both sites. They are consistent with the weakly ferromagnetic canted antiferromagnetism found in Co(CH₃COO)₂·4H₂O below T_N ?85mK.     

EPR investigation of local paramagnetic centers in perovskite-like crystals

In ScF₃ single crystals (pure and doped) as well as in Rb₂KScF₆ and Rb₂KDyF₆ crystals with a perovskite-like structure, point nanodefects (vacancy in place of trivalent cations) have been found and studied. Electron paramagnetic resonance has been used to investigate local paramagnetic centers that are not detected using X-ray diffraction. The angular dependence of the spectra indicates a local distortion of the cubic symmetry of the crystals. An additional hyperfine structure in the observed spectra is due to the delocalization of electrons over six F^? ions forming the first coordination polyhedron around the vacancy. The crystals studied are characterized by a high electron mobility and a high electron velocity, which depends on the impurity. The high mobility of electrons of the cation center can be indirectly responsible for the structural phase transition occurring in the ScF₃ crystal under uniaxial pressure.     

The effect of off-center Cu2+ ions on the phase transition in lead germanate crystals

The properties of Pb₅Ge₃O₁₁: Cu²⁺ crystals near the temperature of the ferroelectric phase transition are discussed in terms of a phenomenological approach. Assuming a quadratic interaction with the order parameter, the effect of Cu²⁺ is considered a result of static distortions, which result from the off-center position of the copper impurity ions in the lead germanate structure. In this approximation, Cu²⁺ jumps between off-center positions do not affect the dynamic properties of the crystal matrix near T _C.     

EPR of Cr2O3 in the phase transition region

The temperature dependence of the Cr₂O₃ EPR spectra is obtained. The linewidth of the EPR is defined by the phonon's modulation of the exchange interactions at T >T_N + 4–5 K though at T_N < T < T_N + 4 K the critical broadening of the EPR in Cr₂O₃ is determined by fluctuation effects.     

Polarized absorption spectra of Co2+ in Na2Cd3Cl8 single crystals

Summary The polarized optical absorption spectra of Na₂Cd₃Cl₈: Co²⁺ in the range of 15 000 to 40 000 cm⁻¹ down to 15 K are reported. The Co²⁺ ion is found to occupy the Cd²⁺ sites in octahedral geometry and the spectra are interpreted satisfactorily in terms of a cubic ligand field model including spin-orbit coupling. The observed crystal field spectra are well reproduced withB=745 cm⁻¹,C=3410 cm⁻¹,Dq=700 cm⁻¹ and ζ (spin-orbit interaction) =520 cm⁻¹. No spectral evidence for tetragonal distortion is observed.     

Covalency and EPR hyperfine structure constant of Eu2+ in crystals

The covalent reduction of the EPR hyperfine constant of Eu²⁺ in different crystals is presented as esulting from the exchange polarization mechanism. Free-ion values of hyperfine constants of Eu²⁺ isotope are calculated.