B. S. Red’kin

Evolution of the spectral response of amorphous europium molybdate under annealing

The phase transformations occurring in amorphous europium molybdate Eu2(MoO4)3 during annealing at atmospheric pressure are studied using optical spectroscopy and x-ray diffractometry. It is established that the metastable β phase is formed at a temperature of ∼550°C, whereas the transition to the stable equilibrium α phase takes place at higher temperatures T ≥ 700°C. The spectral characteristics of the α phase, which differ substantially from those of the amorphous state and the β phase, are measured for the first time.

Effect of solid-phase amorphization on the spectral characteristics of europium-doped gadolinium molybdate

A method is proposed for detecting spectral characteristics of optically inactive molybdates of rare-earth elements by their doping with rare-earth ions whose luminescence lies in the transparency region of all structural modifications of the sample. Gadolinium molybdate is chosen as the object of investigations, while europium ions are used as an optically active and structurally sensitive admixture. It is shown that after the action of a high pressure under which gadolinium molybdate passes to the amorphous state, the spectral characteristics of Gd1.99Eu0.01(MoO4)3 (GMO:Eu) change radically; namely, considerable line broadening is observed in the luminescence spectra and the luminescence excitation spectra, while the long-wave threshold of optical absorption is shifted considerably (by approximately 1.1 eV) towards lower energies. It is found that by changing the structural state of GMO:Eu by solid-state amorphization followed by annealing, the spectral characteristics of the sample can be purposefully changed. This is extremely important for solving the urgent problem of designing high-efficiency light-emitting diodes producing “white” light.

Dynamic magnetoelectric effect in terbium molybdate

The electric polarization induced in ferroelectric terbium molybdate by a magnetic field linearly varying with time is measured. The measurements are performed in fields up to 19 T at different specified rates of change in the magnetic field at temperatures of 273 and 219 K. The results obtained indicate that there are magnetoelectric effects of two types. One of them is a conventional magnetoelectric effect, which is appropriately referred to as the static magnetoelectric effect. The other effect is characterized by the fact that the electric polarization increases with an increase in the rate of change in the magnetic field and relaxes with time to zero at a fixed nonzero field. This phenomenon is termed the dynamic magnetoelectric effect.

Magnetoelectric effect in samarium molybdate

This paper reports on the nonlinear magnetoelectric effect (MEE) in the orthorhombic ferroelectric ferroelastic β′ phase of samarium molybdate Sm2(MoO4)3 observed in magnetic fields up to 20 T and temperatures from 4.4 to 0.43 K. The magnetic-field-induced electric polarization in Sm2(MoO4)3 is an order of magnitude larger than that in isomorphic Gd2(MoO4)3. This provides support for the magnetostriction mechanism proposed by us for the MEE in rare-earth molybdates. The polarization in Sm2(MoO4)3 was found to fall off with time. The relaxation time constant τ increases with decreasing temperature from τ=102 s at T=4.4 K to τ≈103 s at T=0.43 K.

Low-temperature specific heat of crystalline and amorphous Eu2(MoO4)3

The specific heat Ctotal of crystalline and amorphous Eu2(MoO4)3 is measured in the temperature interval 4.5–30 K. The amorphous state is obtained by applying pressure ∼7 GPa at room temperature. It is found that the specific heat of the crystal at T⩽7.5 K is described by a cubic function of temperature, while the specific heat of the amorphous sample has a strongly non-Debye character in the entire experimental temperature interval. The curve of Ctotal for amorphous europium molybdate is analyzed in a model of soft atomic potentials, and it is shown that it agrees well with universal low-temperature anomalies of the specific heat of classical glasses obtained by quenching from the liquid.

Features of the pressure-induced phase transitions in Eu2(MoO4)3 single crystals

The structural changes induced by a 9-GPa pressure in Eu2(MoO4)3 single crystals at room temperature have been studied using x-ray diffraction. It is established that a structural phase transition from the initial tetragonal phase to the new high-pressure tetragonal phase occurs rather than solid-phase amorphization that was observed previously in polycrystalline samples. The samples in the observed transition remain single-crystalline despite a significant difference (ΔV ∼ 18%) between the specific volumes of the initial and final phases. It is shown that the transition from the initial state to the high-pressure phase occurs via the formation of broad transition zones featuring a continuous and smooth change of the crystal lattice parameters.

Peculiarity of O2− and Li+ electrodiffusion into lithium niobate single crystals

Abstract Congruently grown LiNbO3 single crystals show both a high oxygen and lithium ionic conductivity at temperatures above 500°C. The high oxygen ionic conductivity can be understood in terms of a certain amount of oxygen vacancies already present in congruently as-grown LiNbO3 single crystals. The electrocoloration phenomenon in LiNbO3 single crystals is due to a process where lithium ions and electrons are injected by a double charge mechanism into LiNbO3. Optical and electrical properties of electrocolored LiNbO3 crystals were investigated. The motion of stoichiometric domains (regions with different stoichiometry) through a LiNbO3 crystal from one electrode to the other is studied and described in terms of electrodiffusion of the ions and electrons.

Anisotropy of the magnetoelectric effect in terbium molybdate

Anisotropy of the nonlinear magnetoelectric effect in a single-crystal, single-domain sample of the β′ metastable ferroelectric paramagnetic phase of terbium molybdate Tb2(MoO4)3 was studied experimentally in dc magnetic fields of up to 6 T at temperatures of 4.2 and 1.8 K. It was shown that the existing models of the magnetoelectric effect cannot explain the experimental dependences of magnetic field-induced electric polarization on the direction of the applied magnetic field. A model of the magnetoelectric effect is proposed that qualitatively describes the observed angular dependence of the magnetic field-induced electric polarization.

Anisotropy of the magnetoelectric effect in β′-Gd2(MoO4)3

An experimental study is reported of the nonlinear magnetoelectric effect in the metastable orthor-hombic ferroelectric ferroelastic paramagnetic β′ phase of Gd2(MoO4)3 in magnetic fields of up to 20 T and at temperatures of 4.2 and 0.4 K. It is shown that the present models of the paramagnetoelectric effect can be reconciled with experiments only for low magnetic fields. A new approach to the description of the magnetoelectric effect in rare-earth molybdates is proposed, which is based on a model of single-ion magnetostriction.

Sodium metavanadate NaVO3 crystal: Frustrated ferroelectric and electret

We measured the temperature behavior of the spontaneous electric polarization of a monocrystalline sodium metavanadate NaVO3 sample along the polar axis [010] at temperatures from 20 to 610°C in zero external field. It was found that the spontaneous electric polarization has a wide maximum in the range from 200 to 300°C and remains nonzero up to 610°C. Measurements of electric polarization of the sample, subjected to the thermoelectric treatment, proved that NaVO3 is an electret. The electrical hysteresis loop in an alternating electric field with a frequency of 50 Hz and amplitude of 450 V/cm was measured in the vicinity of the temperature maximum.