Dielectric properties and phase transitions in crystals of TlInS x Se2 − x solid solutions

This paper reports on a study of the dielectric permittivity and electrical conductivity of single-crystal TlInS _x Se_{2 − x} solid solutions as functions of temperature and composition. It has been found that the dielectric permittivity ɛ and electrical conductivity σ decrease with increasing x and increase with growing temperature. The temperature dependences ɛ = f(T) and σ = f(T) for TlInS _x Se_{2 − x} crystals have been demonstrated to reveal anomalies in the form of maxima, which suggests that these crystals undergo phase transitions. The temperatures of the phase transitions increase with increasing x.     

Influence of silver on the galvanomagnetic properties and energy spectrum of mixed (Bi1− x Sbx)2Te3 crystals

An investigation is made of the temperature dependences of the resistivity in the range 4.2–300 K, the Hall effect, and the Shubnikov-de Haas effect in magnetic fields up to 40 T in (Bi_{1? x} Sb_x)₂Te₃Ag_y single crystals (0 ≤ x ≤ 0.75). Doping (Bi_{1? x} Sb_x)₂Te₃ crystals with silver showed that in Sb₂Te₃ and (Bi_{1? x} Sb_x)₂Te₃ crystals unlike Bi₂Te₃ silver exhibits acceptor properties. The angular and concentration dependences of the Shubnikov-de Haas effect were studied in (Bi_{1? x} Sb_x)₂Te₃Ag_y. It was established that the anisotropy of the ellipsoids of the upper valence band in Bi_0.5Sb_1.5Te₃ remains unchanged as a result of silver doping.     

Effect of bond polarity in Sb2Te3−x Se x single crystals

Sb₂Te_3?x Se _x (x=0·00?1·25) single crystals were prepared from 5N purity elements using a modified Bridgman method. Measurements of the reflectivity spectra in the plasma resonance frequency range, Hall constantR _H(B∥c) and electrical conductivityσ _⊥C were carried out on these samples at room temperature. With increasing selenium content a shift of the reflectivity minimum towards longer wavelengths was observed as well as an increase of the Hall constant and a decrease of the electrical conductivity — the incorporation of Se atoms into the Sb₂Te₃ crystal lattice results obviously in a decrease in the concentration of free carriers. This effect is accounted for by a change in the polarity of bonds in the Sb₂Te₃ crystal lattice, due to the formation of Se _Te ^x substitutional defects.     

Birefringence of CuGa(S1−x Sex)2 single crystals

The magnitude and dispersion of birefringence of single crystals of CuGa(S_1?x Se_x)₂ solid solutions is studied in the spectral region of 0.5–2.5 μ at T=300 K. The effect of the substitution of selenium for sulfur on special features of birefringence dispersion is analyzed within the framework of the single-oscillator model.     

Optical coefficients of Hg1 − x − y Cd x Eu y Se crystals

Optical properties of Hg_{1 ? x ? y} Cd _x Eu _y Se crystals grown by the Bridgman method have been investigated based on the independent reflectance and transmittance measurements, which were performed on a Nicolet 6700 spectrometer at T = 300 K in the wavelength range 0.9 ≤ λ ≤ 26.6 μm. The values of refractive index n, absorption index k, and absorption coefficient α have been determined for the crystals studied. Based on the dependences α = f(hν), the presence of direct allowed interband optical transitions in the crystals is established and the band-gap values are determined. The influence of temperature on the transmittance and band gap are investigated in the range T = 114–300 K.     

Dynamic shift of the phase-transition temperature in (Sn1−x In(2/3)x)2P2S6 crystals with incommensurate phase

The dielectric properties of ferroelectric (Sn_1?x In_(2/3)x )₂P₂S₆ crystals were studied in the region of incommensurate phase transitions occurring upon fast cooling and heating (0.2–11 K/min). The dynamic phase-transition shift observed in these materials was examined.     

Optical properties of ternary ZnS x Se1−x polycrystalline thin films

Ternary ZnS_xSe_1–x polycrystalline thin films were prepared by evaporation in vacuum of 10^–5 Torr. The molecular fractionx varied in the region ox1. The optical constants (the refractive indexn, the absorption indexk, and the absorption coefficient) were determined in the wavelength range 300–1600nm. A plot representing ₂=f(hv) shows that the ZnS_xSe_1–x polycrystalline thin films of different compositions have two direct transitions corresponding to the energy gapsE andE+. The variation in eitherE orE+ withx indicates that this system belongs to the amalgamation type. Such variation follows a quadratic equation. The bowing parameter was found to be 0.456 eV, roughly equal to the calculated value 0.60 eV using the empirical pseudopotential method based on the virtual-crystal approximation, in which the disorder effect has not been taken into account.     

Mechanism transition of self-pumped phase conjugation in KTa1 −xNbxO3:Fe crystals

Mechanism transitions of Self-Pumped Phase Conjugation (SPPC) with wavelength and doping concentration are observed in KTN:Fe (KTa_{1 –x} Nb _x O₃:Fe with _x = 0.48) crystals. The SPPC mechanism in KTN: Fe (0.4 wt. %) crystal transforms from Stimulated Photorefractive Backscattering and Four-Wave Mixing (SPB-FWM) to cat (or total internal reflection) as the wavelength increases from 514.5 nm to 620 nm. SPPC at 514.5 nm is formed with the cat mechanism in a 0.2 wt. % doped KTN:Fe crystal, while with the SPB-FWM mechanism in a 0.4 wt. % doped one. These mechanism transitions are discussed with respect to the dependence of the backscattering gain coefficient of the crystals on wavelength and doping concentration.     

Dielectric characteristics of the relaxor state of the perovskite ceramics 0.9(Na1 − x K x Bi)1/2TiO3-0.1Bi(ZnTi)1/2O3 near the morphotropic phase boundary

X-ray diffraction investigations have revealed that, in the system of solid solutions 0.9(Na_{1 ? x} K _x Bi)_1/2TiO₃-0.1Bi(ZnTi)_1/2O₃, there is a morphotropic phase boundary in the potassium concentration range of x ≈ 0.25, which separates the regions of compositions with rhombohedral (R3c) and tetragonal (P4mm) structures. It has been shown that, in the vicinity of this boundary, ceramic samples of the studied system exhibit properties of relaxor ferroelectrics. The results of the investigation into the dielectric properties of relaxor ceramics of the composition x = 0.3 with the use of the impedance spectra measured in the frequency range from 25 to 10⁶ Hz at temperatures from 100 to 900 K have been presented. It has been found that, in the temperature region of the existence of the relaxor state lying below the temperature corresponding to the maximum of the real part of the permittivity (T′ _m = 550 K), the dielectric polarization is determined by the sum of the contributions from the matrix and dipole clusters. The temperature dependence of the contribution from the clusters, which is determined by the kinetics of their formation and freezing, is characterized by a curve with the maximum at approximately 400 K. The process of freezing of dipole clusters occurs over an extended temperature range of more than 200 K.     

Concentration dependence of EPR spectra in mixed (BaF2)1 − x (CeF3) x crystals

Physics of the Solid State - In the EPR spectra of the (BaF2)1 − x (CeF3) x system, additional lines appear as the concentration x in the mixture increases from 10−3 to 10−2, some...     

An intermediate monoclinic phase and electromechanical interactions in xPbTiO3-(1−x)Pb(Zn1/3Nb2/3)O3 crystals

The elastic matching of phases in the vicinity of the morphotropic phase boundary in xPbTiO₃-(1?x)Pb(Zn_1/3Nb_2/3)O₃ crystals is investigated in an external electric field with strength E ‖ [001]. The field dependences of the unit cell parameters of the monoclinic phase are determined experimentally in the range 0≤E≤2 MV/m. The results obtained are used in analyzing specific features in the electromechanical properties of xPbTiO₃-(1?x)Pb(Zn_1/3Nb_2/3)O₃ crystals (0.08?x?0.09), in which the monoclinic phase is intermediate between the rhombohedral and tetragonal phases and can coexist with these phases. A correlation between the optimum volume concentrations of domains or twins in different two-phases states is revealed and interpreted for the first time.     

Dielectric and piezoelectric properties of (Bi1−x−yNdxNa1−y)0.5BayTiO3 lead-free ceramics

New lead-free (Bi_1−x−yNd_xNa_1−y)_0.5Ba_yTiO₃ ceramics were prepared by a conventional ceramic technique and their dielectric and piezoelectric properties were studied. X-ray diffraction studies reveal that Nd³⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and tetragonal phases is formed at 0.04 < y < 0.10. The partial substitutions of Nd³⁺ and Ba²⁺ decrease effectively the coercive field E_c and increase significantly the remanent polarization P_r. Because of lower E_c, larger P_r and the formation of the MPB, the piezoelectric properties of the ceramics are significantly enhanced at x/y = 0.02/0.06: d₃₃ = 150 pC/N and k_p = 30.5%. The ceramics exhibit relaxor characteristic, which is probably resulted from the cation disordering in the 12-fold coordination sites. The depolarization temperature T_d shows a strong compositional dependence and reaches a minimum value at the MPB. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both the polar and non-polar regions near the depolarization temperature T_d, which cause the polarization hysteresis loop become deformed near/above T_d.     

ВЛИЯНИЕ СТАРЕНИЯ НА Э ЛЕКТРИЧЕСКИЕ СВОЙСТ ВА ПОЛУПРОВОДНИКОВОЙ С ИСТЕМЫ Bi2Te3-Bi2Se3

, Bi₂Te₃-Bi₂Se₃. , , , . .

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Influence of ageing on change in electrical properties of semiconducting systems of Bi₂Te₃-Bi₂Se₃

The paper describes the effect of ageing observed on a semiconducting system Bi₂Te₃ — Bi₂Se₃. It is shown that the change in electrical conductivity and thermoelectric force, which takes place during ageing, is caused by the change in concentration of the free electrons. The influence of this process on the efficiency of equipment employing the Peltier effect is analyzed.

     

Special points on the phase diagrams of langbeinites. The case of (x,T)-phase diagram of mixed K2xRb2(1−x)Cd2(SO4)3 crystals

We have studied special points that appear on the (x, T)-phase diagram of mixed K_2xRb_2(1?x)Cd₂(SO₄)₃ crystals. This phase diagram has been constructed issuing from the changes in domain structure occurring in the course of phase transitions. We have found that two triple points on the (x, T)-phase diagram of the above solid solutions are very close to each other in the phase space and can change their places or transform into a quadruple point. These special points are associated with disappearance of ferroelectric phases with the symmetries P1 and P2₁ taking place when Rb₂⁺ are substituted with K₂⁺ ions.     

Effect of pressure on electrical properties of SnS x Se1−x single crystals

The single crystals of tin monosulphoselenides in the form of a series SnS _x Se_1?x (where x?=?0, 0.25, 0.50,0.75 and 1) have been grown using the direct vapor transport technique (DVT). The analysis of the X-ray diffraction patterns reveals that all crystals belong to the orthorhombic crystal structure. Hall effect measurements were carried out on grown crystals at room temperature. The optical absorption measurements at room temperature have been carried out for all crystals. The values of the band gap were determined at atmospheric pressure and also calculated at high-pressure. Simultaneous thermoelectric power (TEP) and a.c. resistance measurements up to 8?GPa were carried out. The results of the effect of high-pressure on the electrical resistance of the grown crystals are reported in this paper.     

Mössbauer study of the ternary system (Fe1−xVx)3 Ge

Three crystallographically different structures of (Fe_1–xV_x)₃ Ge have been studied by⁵⁷Fe Mössbauer spectroscopy. The hexagonal phase, stable for low values ofx, is ferromagnetic with the spins parallel to thec-axis above a critical temperature, where a spin flip to thec-plane takes place. A V/Fe substitution in the near surrounding of an iron atom leads to a decrease in the isomer shift of –0.02 mm/s and an estimated reduction in the magnetic moment of 0.31 _B from 2.07 _B. A result for the intermediate cubic closed packed structure is that V populates only one type of metal sites. Furthermore, from similarities with-Fe the average value of the change in isomer shift is found to be +0.075 mm/s and +0.02 mm/s per Ge/Fe substitution in the 1nn and 2nn shells, respectively. The spin polarization effect on the magnetic hyperfine field for iron is –8.6%, –0.4%, and –0.6% per Ge/Fe (1nn), Ge/Fe (2nn) and V/Fe (3nn) substitutions, respectively. The simple cubic compound (Fe_0.7V_0.3)₃ Ge is non-magnetic down to at least 5 K. Here a decrease in the isomer shift of –0.05 mm/s is found for a V/Fe (1nn) substitution.On leave from the Department of Nuclear Physics, University of Madras, Madras, India.On leave from the Physics Department, Punjabi University, Patiala, India.     

Low-temperature phase transitions and dynamics of ammonium in (NH4)3H(SO4)2 and [(NH4)1−x Rbx]3H(SO4)2 crystals

The (NH₄)₃H(SO₄)₂ and [(NH₄)_0.82Rb_0.18]₃H(SO₄)₂ crystals are investigated by dielectric spectroscopy, inelastic incoherent neutron scattering (IINS), and neutron powder diffraction. A comparative analysis of the data obtained is given. It is shown that the phase transitions II ? III, III ? IV, IV ? V, and V ? VII in the (NH₄)₃H(SO₄)₂ crystal are accompanied by changes in the orientation ordering of the NH ₄ ⁺ ions. In the [(NH₄)_0.82Rb_0.18]₃H(SO₄)₂ crystal, these phase transitions are completely suppressed and the long-range order inherent in the II phase is retained over the entire temperature range covered (6–300 K). It is revealed that this crystal at the temperature T _g≈70 K undergoes a transition to the dipole glass phase, which is attended by “freezing” the orientation disordering of the ammonium ions.     

Neutron diffraction study of the oxide conducting δ1-phase of (Bi2O3)1−x(Y2O3)x (x=0.25)

The atomic parameters of the average cell of δ¹-Bi(Y)O_1.5 were determined by powder neutron diffraction. The evidence indicates that the disorder in the structure has many features in common with the structure of the oxygen deficient zirconia. Most oxygens (78%) are in sites displaced 0.335 Å along 〈100〉 directions from the normal fluorite positions, while a smaller proportion (22%) are displaced 0.80 Å along 〈111〉 directions and no oxygen remains in normal positions. In addition to the anion displacements a smaller displacement (0.25 Å) along the 〈111〉 direction was found for the cations. Comparison with the structure of β-Bi₂O₃ suggests that the displacements may be precursors to a δ→β phase transition.     

Über Kristallstruktur und elektrische Eigenschaften der Wismutselenide Bi2Se2 und Bi2Se3

The properties of bismuth triselenide (Bi₂Se₃) are already known to a certain extent through the work of several authors, while it was still an open question whether there exists an individual solid phase of BiSe. Further information on this subject could be obtained by the successful growth and investigation of single crystals of both Bi₂Se₃ and Bi₂Se₂. X-ray analysis by means of goniometry, Weißenberg, Laue, and Debye-Scherrer diagrams confirmed the known crystal structure of Bi₂Se₃ (ditrigonal scalenohedral;D _3d ⁵ ?R—m; with the hexagonal axes:a=4·15 Å andc=28·55 Å, and 3 molecules per unit cell). As to Bi₂Se₂ it can be shown that it belongs to the same class but to a different space group (D _3d ¹ ?P— 1m orD _3d ³ ?P—m 1; hexagonal axes:a=4·15 Å,c=22·84 Å, unit cell: 3 molecules, if the formula Bi₂Se₂ is adopted). Common to both is a subcell with the dimensions:a′=a=4·15 Å andc′=5·71 Å. The temperature dependence of electrical conductivity and Hall coefficient was measured on several specimens having different crystal orientations. The most striking difference is the high anisotropy of Bi₂Se₃ (σ _a σ _c =10) as compared with Bi₂Se₂ (σ _a /σ _c <2). All specimens turned out to ben-type. The room temperature carrier concentration observed was:n (Bi₂Se3)=8·10¹⁸ cm^?3 andn (Bi₂Se₂)=4·10²⁰ cm^?3, the carrier mobility:μ(Bi₂Se3)=2·10³ cm²/V·s andμ(Bi₂Se₃)=20 cm²/V·s.