Chemical and structural properties of the system Fe2O3-Cr2O3

Chemical and structural properties of the mixed metal oxides (1–x)Fe₂O₃+xCr₂O₃ were studied by different techniques. X-ray powder diffraction showed the existence of solid solutions, (Fe_1–x Cr _x )₂O₃, over the whole concentration region, 0x1. The gradual replacement of Fe³⁺ with Cr³⁺ ions in samples prepared at 900°C caused changes in unit-cell parameters; most of these changes took place in the region fromx0.3–0.9. The samples having the fraction of Cr₂O₃ in the region from 0.7–0.8, contained two closely related phases, with slightly different compositions. After an additional heat treatment at 1100°C, these samples contained only one phase.⁵⁷Fe Mössbauer spectroscopy showed a gradual decrease of hyperfine magnetic field with increasing Cr₂O₃ content. The sample having the fraction of Cr₂O₃ of 0.7, and prepared at 900°C, exhibited two separated sextets at room temperature, in comparison with other compositions showing one sextet. It was shown that Fourier transform infrared (FT-IR) spectroscopy is a powerful method for the investigation of structural changes in these solid solutions. The increase in the Cr₂O₃ content resulted in shifts of the corresponding infrared bands. In addition, a gradual transition of the spectrum typical for -Fe₂O₃ to the spectrum typical for Cr₂O₃ was shown. The transition effects observed in the FT-IR spectra were correlated with the X-ray powder diffraction and⁵⁷Fe Mössbauer spectroscopic results.     

Superconducting properties of the lithium-substituted YBa2Cu4O8 prepared by the sol-gel method

The effects of the lithium substitution for copper on the properties of the superconducting YBa₂Cu₄O₈ were studied. Single-phase YBa₂Cu_4–x LixO₈ was successfully prepared by the sol-gel method under ambient pressure over a composition range of 0x 0.08, while impurity phases appeared at x=0.10 and 0.20. The lattice constants of YBa₂Cu_4–x Li _x O₈ were almost invariant with increasing lithium content x. The superconducting transition temperature decreased monotonically with increasing x in the range, 0x0.08. The suppression of superconductivity was discussed in terms of the impurity effect in the CuO₂ planes.     

Low-temperature specific heat of YBa2Cu3O7, YBa2Cu3O6, Y2BaCuO5, YBa3Cu2O7, BaCuO2, and CuO

We have measured the low-temperature specific heat (1.3T20 K) and the dc magnetic susceptibility (100T250 K) of eight samples of the high-T _c superconductor Y _x Ba_3–x Cu₃O_7– (x=0.9, 1.0, 1.1) and of two samples of nonsuperconducting YBa₂Cu₃O₆₊. We have also performed specific heat measurements on the possible impurity phases: YBa₃Cu₂O₇, Y₂BaCuO₅, CuO, and BaCuO_2+x . The superconducting samples all have a nonzero, sample-dependent linear term * and an upturn inC/T at very low temperature. We show that this anomalous behavior is at least partly due to the presence of a small amount (1%) of BaCuO_2+x impurity phase in the measured samples. This is evidenced by the correlation between * and the Curie component of the susceptibility, which is proportional to the amount of paramagnetic impurities.     

Dielectric properties of (Ba,Sr)O-(Sm,La)2O3-TiO2 ceramics at microwave frequencies

The dielectric properties of (Ba, Sr)O-(Sm, La)₂O₃-TiO₂ material at microwave frequencies were investigated. By varying the amount of strontium from 0–25 mol% in the 0.15(Ba_1–x Sr _x )O-0.15Sm₂O₃-0.7TiO₂ composition, it was possible to adjust the frequency temperature coefficient, _f, from –13 p.p.m. °C^–1 to + 30 p.p.m. °C^–1. When 7 mol% Sr was substituted for barium, _f=0 p.p.m. °C^–1 was obtained. TiO₂ with rutile phase (_f400 p.p.m. °C^–1) acted as a dominant element in _f variation of the 0.15(Ba_1–x Sr _x )O-0.15(Sm_1–y La _y )₂O₃-0.7TiO₂ (0x0.25, 0y0.6) system. Additionally, increasing the quantity of lanthanum substitution for samarium had a greater positive effect on _f than strontium substitution for barium. When 60 mol% La was substituted for samarium with 7mol% Sr substitution barium, _f of the system reached 95 p.p.m. °C^–1. The effect on microwave dielectric characteristics of the 0.15(Ba_0.93Sr_0.07)O-0.15Sm₂O₃-0.7TiO₂ (BSST) ceramics by varying the calcination and/or sintering conditions or doping additives, were studied. The added SnO₂ acted as a firing agent to lower the sintering temperature, and the dielectricQ(Q _d) value was improved by properly adding CdO. With 1 wt% CdO addition, the highestQ _d value of the BSST resonator, after calcination at 1100 °C/2 h and sintering at 1370 °C/4 h, reached 4180 at 4 GHz with a small _f of –4 p.p.m. °C^–1 and an _r of 80.7 was obtained.     

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (Cr_xFe_1–x)₂O₃, 0 x 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, -(Cr_xFe_1–x)₂O₃, which is isostructural with -Fe₂O₃ and phase C, which is closely related to Cr₂O₃. Phase F exists in samples heated up to 900°C, for 0 x 0.95. Phase C exists from x0.27 to x=1 for samples heated up to 900°C and from x0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr₂O₃ in -Fe₂O₃ and 4.0 ± 0.5 mol % of -Fe₂O₃ in Cr₂O₃. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. ⁵⁷Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at 460 and 370 nm (22 000 and 27 000cm^–1) for x 0.5, 500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the ⁶A₁ 4T₁ Fe³⁺ ions in all spectra and the development of the absorption at 13000 cm^–1 due to a metal-metal charge transfer (Cr³⁺ Fe³⁺) transition, might be explained by exchange coupling which has been observed in some spinel compounds.     

Some properties of Sb2Te3−xSex for nonvolatile memory based on phase transition

Composition dependence of properties of Sb₂Te_3–x Se _x in the range 0x<3 were studied using differential thermal analysis and X-ray diffraction. Sb₂Te_3–x Se _x form solid solution for 0<x1.25 and 2.75x<3. A systematic study of crystallization temperature in Sb₂Te_3–x Se _x (0x2.75) thin films prepared by flash evaporation was carried out. In preliminary experiments for some compositions, more than 10³ repetitions between amorphous and crystalline states were attained by the application of electric pulses.     

Y xBayCuzO7− δ solid solutions

Y_xBa_yCu_zO_7– ceramics forming at isobaric conditions were studied by x-ray diffraction analysis, dynamic magnetic measurements and potentiometric titration. It was established that compositions: 0.8 x 1.2 y=2, z=3; x-1, 1.8 y 2.2, z=3; x=1, y=2, 2.7 z 3.4: are in the homogeneity range of 123. It was found that compositions with nonstoichiometric cations rations have minimum T_c.     

Fermi liquid behaviour of the viscosity of3He-4He mixtures

We have investigated³He-⁴He mixtures at³He-concentrations 0.98%x9.5% by the vibrating wire technique in the temperature range 1 mKT 100 mK and at pressures 0 bar p 20 bar. In the degenerate regime of the mixtures the Landau theory of Fermi liquids predicts a temperature dependence of the viscosity proportionalT ^–2. We report on the first observation of this behaviour at 3 mKT 10 mK for all investigated concentrations and pressures. At temperatures below about 20 mK slip corrections had to be taken into account due to the increase of the quasiparticle mean free path at very low temperatures. The low-temperature cut-off in T ² = constant indicates the transition into the ballistic regime of the mixtures, where the mean free path of the quasiparticles exceeds the radius of the vibrating wire. Our results for the pressure dependence of the viscosity as well as for its magnitude show substantial differences from predictions based on pseudopotential theory. However, a calculation of with the quasiparticle interaction potential of recent solubility measurements in mixtures agrees well with our experimental data, in particular the pressure independence of .     

Synthesis of Cr x Ti1−2x Nb x O2O20≤x≤0.5 rutile solid solutions from alkoxides

In this study, Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions have been synthesized from gels built from hydrolysis-condensation of Cr (III) acetylacetonate, NbCl₅ and Ti (IV) isopropoxide mixture (polymeric gel). Characterization of these solid solutions was carried out by X-ray diffraction, ultraviolet-visible and infrared spectroscopy, differential thermal and thermogravimetric analysis and CIELAB (Commission Internationale del'Eclairage L^*a^*b^*) parameter measurements. The results obtained by the polymeric gel method were compared with those obtained by traditional ceramic synthesis. This comparison reveals some differences with regard to synthesis temperatures and reaction mechanisms. The formation of Cr _x Ti_1–2x Nb _x O₂ (0 x 0.5) rutile solid solutions by the ceramic method requires temperatures of about 1200°C and soaking times of several days. These solid solutions are synthesized at 1000°C in 24 h by the polymeric gel method. In ceramic synthesis, the CrNbO₄ compound with rutile structure appears as an intermediate compound in the formation of rutile solid solutions. In polymeric gel synthesis, however, the CrNbO₄ rutile compound was not detected in the samples.     

Investigation of the dominant point defects in tetragonal YBa2Cu3Ox at elevated temperatures

The manner in which oxygen is incorporated into YBa₂Cu₃O _x (YBCO) at 800°C for values ofx close to 6 is shown to be in the form of neutral oxygen interstitials, O _i ^x . The experimental data on which this conclusion is based are obtained from measurements of oxygen partial pressure,P(O₂), as a function of compositionx and temperatureT (5.99x 6.35, 825T1120 K). The data are obtained by a solid-state electrochemical method. Other conclusions of this study include: (a) O _i ^x are noninteracting forx 6. (b) The stoichiometric composition of YBCO isx 6.0. (c) The reaction enthalpy of oxidation is 179 kJ/mol O₂. (d) The Fermi level changes by –0.2 eV asx increases from 6.05 to 6.35.     

Standard Gibbs' energies of formation of BaCuO2, Y2Cu2O5 and Y2BaCuO5

The Gibbs' energies of formation of BaCuO₂, Y₂Cu₂O₅ and Y₂BaCuO₅ from component oxides have been measured using solid state galvanic cells incorporating CaF₂ as the solid electrolyte under pure oxygen at a pressure of 1.01×10⁵ Pa BaO + CuO BaCuO₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=–63.4–0.0525T(K) Y₂O₃ + 2CuO Y₂Cu₂O₂ G _f,ox ^o (± 0.3) (kJ mol^–1)=18.47–0.0219T(K) Y₂O₃ + BaO + CuO Y₂BaCuO₅ G _f,ox ^o (± 0.7) (kJ mol^–1)=–72.5–0.0793T(K) Because the superconducting compound YBa₂Cu₃O_7– coexists with any two of the phases CuO, BaCuO₂ and Y₂BaCuO₅, the data on BaCuO₂ and Y₂BaCuO₅ obtained in this study provide the basis for the evaluation of the Gibbs' energy of formation of the 1-2-3 compound at high temperatures.     

1 : 2 Long-range ordering and defect mechanism of WO3-doped perovskite Ba(Mg1/3Ta2/3)O3

The nature of the B-site cation ordering and the associated defect process necessary to stabilize the ordered domains were investigated using the WO₃-doped BaMg_1/3Ta_2/3O₃ BMT system as a typical example of BaB_1/3B_2/3O₃-type complex perovskites. It was shown that only the 1 : 2 long-range ordering of the B-site cation existed in both undoped and WO₃-doped BMT perovskites. The atomic defect mechanism associated with the stoichiometric 1 : 2 long-range ordering was systematically investigated. It is concluded that the substitution of W⁶⁺ for Ta⁵⁺ in the WO₃-doped BMT enhances the degree of the 1 : 2 long-range ordering and produces the positively charged W _Ta sites with a concomitant generation of tantalum vacancies V_Ta and mobile oxygen vacancies V _O for the ionic charge compensation.     

X-ray Diffraction and Thermodynamic Studies of GdLiCr2O5

GdLiCr₂O₅ is synthesized from Gd₂O₃, Cr₂O₃, and Li₂CO₃ by solid-state reaction. This compound has an orthorhombically distorted perovskite structure with lattice parametersa= 10.78 Å, b= 10.63 Å, and c= 11.04 Å (V = 1263.9 ų, V _subcell = 158.0 ų, _x = 3.66 g/cm³, _meas = 3.57 ± 0.09 g/cm³). The heat capacity of GdLiCr₂O₅ is measured between 298.15 and 673 K by dynamic calorimetry. At 423 K, C _p ⁰ exhibits a lambda-type anomaly due to a second-order phase transition. The best fit equations for C _p ⁰(T) are derived, and the thermodynamic functions S ⁰(T), H ⁰() – H ⁰(298.15 K), and "(T) are calculated.     

Nuclear ferromagnetic ordering of141Pr in the diluted Van Vleck paramagnets Pr1−x Y x Ni5

We report on the first investigation of the effect of magnetic dilution on nuclear magnetic spin interactions in metals; we studied this effect in the diluted Van Vleck paramagnets Pr_1–xY_xNi₅. In addition, we investigated the electronic magnetic properties of these intermetallic compounds. For this purpose, we have measured the nuclear and electronic susceptibilities of Pr_1–xY_xNi5 with x = 0 to 0.2 at 50K T 8 mK and at 2.2 K T 300 K. We observe a linear decrease of the electronic Van Vleck susceptibility and of the hyperfine enhancement factor by 13% (per mole Pr³⁺). The hyperfine-enhanced nuclear susceptibility decreases by about a factor of three when going fromx = 0 tox = 0.2. The samples show nuclear ferromagnetic transitions of¹⁴¹Pr with nuclear Curie temperatures T_c decreasing from 370 K to 100 K and Weiss temperatures decreasing from 218 K to 13 K in this concentration range. These data are compared with the results of mean-field calculations. In addition, we report on measurements of the nuclear spin relaxation time of these compounds, for which we find values of a few msec at millikelvin temperatures and a critical speeding-up at T_c. Our data give hints that for x > 0.2 the character of the nuclear magnetic transition may change, possibly to a nuclear spin glass freezing.     

Decomposition of Al2TiO5-MgTi2O5 solid solutions: a thermodynamic approach

The decomposition of Al_1–x Mg _x Ti_1+x O₅ solid solutions with x=0.0, 0.1, 0.2, 0.4, 0.5 and 0.6 was studied in the temperature range 900–1175 °C using a 250 h annealing test. As x increases from 0–0.2 there is a strong stabilizing effect and the decomposition temperature decreases from 1280 °C (Al₂TiO₅) down to 1125 °C. For 0.2x0.5 the decomposition temperature does not decrease further. For x=0.6 no decomposition was observed. For x0.5 decomposition is complete or almost complete at 1000 °C; at 900 °C transformation is kinetically hindered and solid solutions with x=0.2 and 0.4 are unaffected by the thermal treatment. A relationship between the decomposition temperature and the parameter x has been derived using the regular solution model to describe the Al_2(1–x)Mg _x Ti_(1+x)O₅ solid solution.     

The dielectic response of K x Al x Ti8−x O16 and K x Mg x/2 Ti8−x/2 O16

Materials of the hollandite structure with the general formulae K_x Al_x Ti_8–x O₁₆ and K_x Mg_x/2 Ti_8–x/2 O₁₆ have been synthesized in the composition range 1.6x2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10^–3 to 10⁶ Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the universal dielectric relation in which the loss follows the law x() ^n–1, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.     

Dielectric properties and microstructural behaviour of B-site calcium-doped barium titanate ceramics

Dielectric properties and microstructural behaviour of Ba_1–x Sr _x Ti_1–y Ca _y O_3–y ceramics, where strontium and calcium were doped on the barium and titanium sites, respectively, within the range 0x0.24 and 0y0.05, were investigated. Calcium addition decreased the tetragonality,c/a, increased the unit cell volume, and lowered the Curie temperature, which were all attributed to the occupancy of Ca²⁺ ions on titanium sites. When sintered at a low oxygen partial pressure of 10^–9 MPa, a resistivity higher than 10¹¹ cm was maintained for the formulations containing B-site calcium substitution more than 0.5 mol %. With increasing the amount of calcium addition, the Curie peak was depressed and completely broadened for the compositions with calcium addition more than 3 mol %, where the average grain size was smaller than 1 m. Co-firing with nickel electrodes in a reducing atmosphere also depressed the Curie peak and inhibited the grain growth due to the diffusion of nickel into the dielectrics.     

Kinetics of dissolution of silicon in CrO3-HF-H2O solutions

Silicon wafers [p-type, B-doped, 10 cm, dislocation density <500cm^–2, orientation (111)] have been dissolved in CrO₃-HF-H₂O solutions (Sirtl etch) with molar concentration ratios [Cr⁶⁺]/[HF] between 0.03 and 0.72 at temperatures between 10 and 60° C. The dissolution kinetics suggests that silicon suboxides SiO _x (0.67<×<1) are formed, and/or that the rate of diffusion of chromium is three times that of hydrofluoric acid.     

Mean polarizabilities and second optical and density virial coefficients of CH3OH and CCl2F2 between 300 and 355 K

Mean dipole polarizabilities ₀(, T) as well as second optical (or refractive index) virial coefficients b _R(, T) and second density virial coefficients B(T) of gaseous CH₃OH and CCl₂F₂ have been determined by precise measurements of the refractive index n(, T, p) [543 nm 633 nm, 300 K T 355 K, p<0.25 bar (CH₃OH) and p<3 bar (CCl₂F₂)]. ₀ critically compared with the few data in literature. The b _R of these gases was measured for the first time with the cyclic-expansion method. The values of ¦B¦ and b _R=3160(25) cm³ · mol^–1 measured for CH₃OH are considerably greater than the values calculated by Buckingham's statistical-mechanical expressions for a Stockmayer interaction potential. This difference is discussed by assuming dimerization via H bonds, with result H ₂ ⁰ –(28 ... 33) kJ · mol^–1 and S ₂ ⁰ –(116 133) J · mol^–1 · K^–1 for the dimerization enthalpy and entropy for standard conditions, respectively. On the other hand, Buckingham's formulae can be used with success to estimate b _R and B of CCl₂F₂.Dedicated to Prof. Dr. F. Kohler on the occasion of his 65th birthday     

Energy Scales in the Local Magnetic Excitation Spectrum of YBa2Cu3O6+y

The wave-vector integrated dynamical spin susceptibility _2D() of YBa ₂ Cu ₃ O _6+y cuprates is considered. _2D is calculated in the superconducting state from a renormalized mean-field theory of the t–t–J-model, based on the slave-boson formulation. Besides the well-known 41 meV resonance a second, much broader peak ('hump') appears in Im _2D. It is caused by particle-hole excitations across the maximum gap ⁰ . In contrast to the resonance, which moves to lower energies when the hole filling is reduced from optimal doping, the position of this 'hump' at 2 ⁰ stays almost unchanged. The results are in reasonable agreement with inelastic neutronscattering experiments.