Crystal structure,dielectric properties of (K0.5Na0.5)NbO3 single crystal grown by flux method using B2O3 flux

Lead free piezoelectric single crystals of sodium potassium niobate (K_0.5Na_0.5)NbO₃ (KNN) were grown by high‐temperature solution method using two different fluxes; one with a mixture of NaF and KF and other with addition of B₂O₃ along with the mixture. In the present study, the growth of KNN crystals without B₂O₃ flux and the same with B₂O₃ flux were compared. It was found that additions of small amounts of B₂O₃ lowered the melting temperature of the solid solution and enabled better dielectric properties. Phase analysis showed that all samples were crystallized in pure orthorhombic perovskite phase. AFM morphological studies showed that the addition of B₂O₃ flux increased the roughness of the grown crystal. Further, addition of B₂O₃ flux slightly decreased the orthorhombic to tetragonal phase transition temperature T_(O—T) and the Curie temperature T_C. The ferroelectric behaviour of KNN single crystal has been investigated at room temperature. The crystal grown using B₂O₃ flux exhibited a remanent polarization (P_r) ∼ 32 μC/cm² and coercive field (Ec) of ∼11.8 kV/cm whereas the crystal grown without the use of B₂O₃ flux had a remanent polarization (P_r) ∼ 36 μC/cm² and coercive field (Ec) of ∼14.6 kV/cm.     

Effective kinetic coefficients for some binary metallic alloys within the small and finite supercooling region

The normal crystallization kineties concerning the base of In, Rb, Ga some binary metallic melts either within the small supercoolings (Δ T ≦ 1 K) or finite ones (ΔT ≦ 20 ÷ 60 K) has been considered. When crystallizing the binary alloys: In + 2.3 at.% Pb; Rb + 10 at.% K; Ga + 2 at.% Sn; Ga + (2; 4; 8) at.% In; Ga + (0.5; 3.5) at.% Zn; Ga + (5; 8; 10) at.% Zn very dilute binary solid solutions are supposed to be formed. The solid solutions have the B-component distribution coefficients which are either much less or less compared with unit, i.e. k_B ≦ 1, k_B < 1 in all the layers of diphase transition region separating the melt from the crystalline phase. By virtue of fluctuation theory of normal crystal growth within the small and finite supercoolings' region for the above-mentioned binary metallic melt some effective kinetic coefficients have been calculated. A good agreement between theoretical effective kinetic coefficients and available experimental ones in the case of given binary systems has been established.     

Crystallization of a germanium melt in an external electric field

Data are presented on experimental studies of the influence of an external electric field on crystallization of a germanium melt under the layer of a B₂O₃ flux. It has been found out that with the field supercoolings of the melt sharply change. This effect is due to the change of the number of active nucleation centres at the germanium – B₂O₃ flux interface. The maximum supercoolings of the germanium melt ΔT⁻ = 190 K were obtained when a negative potential was connected to germanium. The dependences of supercooling on preliminary melt overheating were measured.     

Transport Coefficient of Gallium-doped Bi2Te3 Single Crystals

Gallium-doped Bi₂Te₃ single crystals were prepared by means of a modified Bridgman method. Temperature dependences of the Hall constant R_H( B ∥ c ), electrical conductivity σ⊥c and Seebeck coefficient S(Δ T ⊥ c) were measured on the samples of these crystals in the temperature interval of 100–400 K. The variations of the investigated transport coefficients with increasing Ga content in the samples showed that Ga atoms in the Bi₂Te₃ crystal lattice behave as donors. This effect is qualitatively explained on the basis of a model of point defects in Bi₂Te₃(Ga) crystals; singly ionised gallium atoms in interstitial sites; Ga_i, are considered to be the most probable defects.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

Pyroelectric Properties of Potassium and Rubidium Titanyl—Arsenate Single Crystals in the Temperature Range of 4.2–300 K

The temperature dependences of the pyroelectric coefficients of KTiOAsO₄ and RbTiOAsO₄ single crystals grown by flux crystallization have been investigated in the temperature range of 4.2–300 K. With an increase in temperature, superionic conductivity first arises in KTiOAsO4 (at T > 200 K) and then (at T > 270 K) in RbTiOAsO₄. This conductivity is much higher in the samples polarized at T = 4.2 K. An exponential change in the crystal resistivity along the polar direction is simultaneously observed. The results of measurements in the range of 4.2–200 K indicate larger values of pyroelectric coefficients when compared with potassium and rubidium titanyl-phosphate crystals. A correlation between the pyroelectric coefficients and a change in the lattice constants at isomorphic substitutions of K atoms for Rb and P atoms for As has been revealed within the symmetry approach.     

Spectral and conductivity features of novel ternary Tl1–xIn1–xSnxS2 crystals

Complex studies of novel Tl_1–xIn_1–xSn_xS₂ single crystalline alloys (x = 0.1; 0.2; 0.3; 0.4; 0.5) were performed. Here we present the study of the effect of the partial cation substitution of Sn ions by In ions on their optical absorption and photoconductivity characteristics. The dependences of optical and electric properties of the crystals of the crystalline solid solutions on temperature and composition are discussed within a framework of intrinsic defect states forming then fundamental absorption. The calculations of the charged defects responsible for the experimentally determined values of Δ₀ were done. Some increase of the concentration for charged defects with the sample temperature is probably related to the thermal ionization of some of the defects that were neutral at lower temperature. The results obtained will be analyzed within the framework of the intrinsic defect's model.     

Structural Studies on Ni75(SnSi)25 and Ni75(SnBi)25 Alloys

Ternary alloys on the quasi-binary system Ni₃Sn(r) in equilibrium condition. It has been observed that these phases undergo transformations at high temperatures but upto 500 °C room temperature modifications are stable. The two phases do not have any appreciable solid solubility in either of them. The phase Ni₃Si(r) crystallizes in to Cu₃Au (Li₂) structure where as Ni₃Sn(r) is based on c.p.h. structure with a = 5.305 A°, c = 4.254 A°. No new ternary phase has been detected in Ni₃Sn Ni₃Si section. The investigated alloys of the Ni Sn Bi system contain 75 at.% Ni. All the ternary alloys show simultaneous occurrence of three phases, namely Ni₃Sn(r), NiBi and Ni(Sn) in equilibrium state. The phase NiBi has NiAs(B8) type of structure. Due to non-existence of isostructural phases in the two binary systems (Ni Sn and Ni Bi), single solid solution phases are not formed. Widely differing atomic sizes of nickel and bismuth atoms restrict the formation of solid solution of bismuth in nickel in contrast to Ni(Sn) where atomic size factor is favourable.     

Temperature dependence of the dielectric parameters of nonstoichiometric SrTiO3 single crystals

The results of experimental studies of the temperature dependence of the low-frequency permittivity ε ₀ and the loss tangent taδ of nominally undoped and doped single crystals of strontium titanate at T = 10–300 K are given. The samples were doped with ions of iron-group metals (V, Mn, Fe, Co) and/or ions of rare earth metals (Pr, Nd, Sm, Tm) with concentrations of 10^?3?5 × 10^?1 at %. Anomalous temperature dependences ε₀ T and tanδ(T) were found for a number of samples. The anomalies found were attributed to the violation of stoichiometry of the single crystals under study and the transition of some fraction of Ti⁴⁺ ions to the Ti³⁺ state.     

Crystallization of amorphous Ni35Zr65 and Fe40Ni40P14B6 under proton irradiation

Two amorphous alloys, Ni₃₅Zr₆₅ and Fe₄₀Ni₄₀P₁₄B₆, were irradiated using 400 keV protons at several temperatures below the crystallization temperature, T_x, to peak doses in the neighborhood of 3.5 to 4.5 dpa. Irradiation at 250°C resulted in the crystallization of both alloys, which were examined by transmission electron microscopy of samples electrolytically polished to various distances from the irradiated surface to study the effect of dose. Samples masked from the proton beam remained amorphous during irradiation. In the Ni₃₅Zr₆₅ alloy crystallization of the equilibrium phases propagated throughout the entire sample, while the in the Fe₄₀Ni₄₀P₁₄B₆ alloy crystallization was observed only in those parts of the samples lying within the proton range. Neither alloy crystallized during irradiation at 100°C. In both these alloys the amorphous phase is therefore evidently stable at irradiation temperatures below approximately 0.6 T_x. An examination of the literature on irradiation damage of binary alloys and intermetallic compounds suggests that there is a tendency for initially amorphous alloys to remain amorphous at irradiation temperatures, T_irr < 0.3 T_L, where T_L (≈T_x) is the “melting” temperature (either a eutectic, peritectic or congruent melting temperature). Also, these same alloys, even when they are initially crystalline, transform to the amorphous state during irradiation at T < 0.3 T_L. Some other crystalline alloys have also been shown to transform to the amorphous state at T_irr < 0.3 T_L even though they have never been prepared in this condition by rapid quenching techniques. The temperature 0.3 T_L appears to be a lower limit, however, since the crystalline to amorphous transformation occurs in many of these alloys at temperatures greater than 0.3 T_L. It is suggested, by analogy with results on void formation in irradiated metals, that this low temperature limit is related to the low mobility of vacancies in these materials, although the mechanism of crystallization, or conversely amorphization, is not fully understood.     

Optical absorption of Fe3+ Ion in LiF crystals

The absorption spectrum of LiF—Fe³⁺ single crystals was investigated at liquid nitrogen temperature. Six new absorption bands, unobserved earlier in other works, were found, which were located at 11 200, 16 000, 22 700, 27300, 30 500, and 41 900 cm⁻¹. The spectrum was interpreted in the cubic cristalline field approximation with D_q = 1397 cm⁻¹, B = 657 cm⁻¹, C = 3226 cm⁻¹. Above mentioned bands were assigned to the transitions from a ground state ⁶A_1g to the levels ⁴T_1g(G), ⁴T_2g(G), ⁴A_1g(G) (⁴E_g), ⁴E_g(D), ²T_1g(F), and ⁴T_1g(F), respectively.     

Absorption spectrum of octahedral CoCl4(H2O)2 units in [(CH3)3NH]CoCl3·2H2O single crystal

The optical absorption spectrum of the octahedral moiety CoCl₄(H₂O)₂ in single crystal [(CH₃)₃NH]CoCl₃·2H₂O has been studied at room and liquidair temperatures. The observed ambient bands at 6800, 16,300, and 19,000 cm^–1 have been assigned, respectively, to the transitions⁴ T ₁(F) ⁴ T ₂(F),⁴ A ₂(F), and⁴ T ₁ (P). The crystal field parameters evaluated areD _q =B=850 cm^–1, andC=4.63B. At liquid-air temperatures the⁴ T ₁(P) band is seen to split into the expected four components ₇, ₈, ₈, and ₆, due to spin-orbit coupling, and the coupling coefficient was computed to be 525 cm^–1.     

On the position of the upper limit temperature of homogeneous nucleation of precipitates in low concentrated Al?Zn alloys

The upper limit temperature T_hn of the onset of homogeneous nucleation of Guinier-Preston (GP) zones was determined for three Al Zn alloys with xZn = 4.5, 6.0 and 8.0 at.% by SAXS investigations (cooling from the range of homogeneity to various temperatures T_a and ageing at T_a). The results are T_hn = (95 ± 3) °C for xZn = 4.5 at.%, (118 ± 3) °C (6.0 at.%) and (154 ± 3) °C (8.0 at.%), respectively. The obtained results fit well the data known for the alloys with higher contents of Zn. It is stressed that one has to distinguish between T_hn, determined by isothermal measurements after a direct quench or cooling to the respective T_a, and the upper limit temperature T_rhn of the onset of the rapid homogeneous nucleation of GP zones (continuous cooling).     

The Description of the Isothermal Decomposition Processes in Al?Sc Alloys by means of Electric Resistivity Measurements and TEM Investigations

The decomposition behaviour of two AlSc alloys (c_Sc = 0.18; 0.36 at.%) after direct quench to room temperature and subsequent ageing at temperatures in the range 200 °C ≦ T_a ≦ 550 °C was investigated by means of isothermal resistivity measurements and TEM. In the temperature range 325 °C ≦ T_a ≦ 400 °C both alloys show independent of the Sc content an “inversion” of the decomposition kinetics, obviously caused by the transition of the coherent into the incoherent Al₃Sc phase.     

Optical Absorption Spectrum of Co2+ Ion Impurities in Lithium Sodium Potassium Sulphate Single Crystal

Optical absorption spectrum of Co²⁺ ions doped in lithium sodium potassium sulphate single crystal has been studied at room and liquid nitrogen temperatures. The observed bands are assigned as transitions from the ground ⁴T_1g(F) to various excited quartet and doublet levels of Co²⁺ ion in octahedral symmetry. The splitting in one of the bands at liquid nitrogen temperature has been explained as due to spin-orbit interaction. All the observed band positions have been fitted with the parameters B, C, D_q and ζ.     

Absorption spectrum of Ni2+ doped in NH4Br single crystal

The optical absorption spectrum of Ni²⁺ doped in NH₄Br single crystals has been studied at room and liquid-air temperatures. The observed ambient bands at 6300, 11,600, 13,400, 18,500, and 20,000 cm^–1 have been assigned, respectively, to the transitions³ A ₂(F) ³ T ₂(F),³ T ₁(F),¹ E(D),¹ T ₂(D and³ T ₁(P). The crystal field parameters evaluated areD _q=630 cm^–1,B=850 cm^–1, andC=3600 cm^–1. In assigning the location of Ni²⁺ ions within the lattice, evaluations are made of the (limited) extant comparable data. It is then concluded that Ni²⁺ enters an octahedral interstitial, rather than a substitutional, site. It is surrounded by four bromide ions, in a plane, with two polarized water molecules at the opposing fifth and sixth positions.     

About the effective temperature of crystal surfaces

Temperature dependences of self-diffusion coefficients, scattered electron intensities, lattice parameters and atomic bond energies are considered. All curves representing the dependences are “splitted” into two linear ones: for a surface and for a volume of the crystal. This splitting can be eliminated and the lines representing the two linear dependences be matched, if a new variable T_c = nT (where n is a scale factor and T_e is an effective temperature) is introduced. The value of n is shown to vary over not a wide range (from 1.6 to 8) for all the processes considered and in all cases n > 1. A possible physical sense of the effective temperature has been considered.     

Effect of structure on low temperature plasticity and magnetic properties of ageing Pb?Sb alloys below Tc

It was carried out comparative structural, magnetic and mechanical studies of single crystals of Pb—Sb alloys with 0.5–5 at.% Sb, which are II type superconductors. The structure was studied by X-ray diffractometry, small-angle X-ray scattering, microhardness and resistometry methods. It was determined the total volume fraction and the precipitate size. The second critical field H_c2 and the magnetic flux trapping measured at 4.2 K are determined by the solid matrix concentration and the precipitate volume fraction respectively. Plasticity of Pb—Sb crystals was investigated by tension tests at 4.2 K. The dependence of the change of the flow stress, Δτ_sn, at superconducting transition on ageing time and antimony concentration was obtained.     

The effect of Sn addition on the glass-forming ability of Cu-Ti-Zr-Ni-Si metallic glass alloys

The effect of Sn substitution for Ni on the glass-forming ability was studied in Cu₄₇Ti₃₃Zr₁₁Ni_8−xSn_xSi₁ (x=0,2,4,6,8) alloys by using thermal analysis and X-ray diffractometry. With increasing x from 0 to 8, the glass transition temperature, T_g, of melt-spun Cu₄₇Ti₃₃Zr₁₁Ni_8−xSn_xSi₁ alloys increased gradually from 720 to 737 K. On the other hand, the crystallization temperature, T_x, increased from 757 K at x=0 to 765 K at x=2, being nearly same with further increase of x. Partial substitution of Ni by Sn in Cu₄₇Ti₃₃Zr₁₁Ni₈Si₁ promotes the glass formation. Both amorphous Cu₄₇Ti₃₃Zr₁₁Ni_8−xSn_xSi₁ alloys prepared by melt spinning and injection casting showed similar crystallization process during continuous heating in DSC. Temperature range of undercooled liquid region exhibits good correlation with the critical diameter for the formation of an amorphous phase in injection casting.     

Characteristic temperature fields of crystal growth calculated with the aid of heat flow potentials

In order to grow monocrystals of larger and larger diameters, more detailed knowledge about the steady state temperature field and its effects on the growing crystal is needed. It can be acquired from experimental data only by means of a subtilized numerical analysis. A mathematical model based on closed-form solutions for the steady state cylindrical case is given. Using the so called “heat flow potential” defined in KIRCHHOFFS sense, any temperature dependence of the heat conductivity k can exactly be taken into account. It can be shown that the problem treated is linear and is generally of the Laplace type. So, the temperature field T(r, z) in cylindrical coordinates r, z can be calculated if the surface temperature function T(r₀, z), the shape of the growth interface and the temperature-dependent heat conductivity k(T) are known. Numerical examples of temperature fields of silicon crystal growth processes are presented.