Synthesis and properties of GdMCr2O5 (M = Na, K, Cs)

The GdMCr₂O₅ (M=Na, K, Cs) chromites are synthesized from Gd₂O₃, Cr₂O₃, and M₂CO₃ by solid-state reactions and are shown to have a tetragonal structure. The heat capacity of the chromites is measured from 298.15 to 673 K using a heat-pulse calorimeter. The data for GdKCr₂O₅ and GdCsCr₂O₅ show anomalies at 373 and 398 K, respectively, attributable to a second-order phase transition. The best fit equations for the C _p ⁰ (T) data are derived. Electrical measurements in the range 303–383 K attest to semiconducting behavior of the chromites.     

Electrical Conductivity of M2V12O30 · nH2O (M = K, Rb, Cs)

The ac electrical conductivity of the M₂V₁₂O₃₀ · nH₂O (M = K, Rb, Cs) polyvanadates prepared via hydrolytic precipitation is measured at 1 kHz between 298 and 423 K in air of 12% relative humidity. The 298-K conductivity of the samples is found to decrease with increasing cation size (in going from K to Rb and to Cs). The activation energy for conduction in the polyvanadates is 0.31–0.34 eV as determined during heating in the range 323–373 K and 0.20–0.23 eV above 373 K and during cooling. The activation energy is higher in Rb₂V₁₂O₃₀ · 6H₂O. The 298-K ionic transference numbers inferred from dc conductivity measurements are 0.7, 0.2, and 0.7 for K⁺, Rb⁺, and Cs⁺, respectively.     

Magnetic Susceptibilities of MgCu2O3 and Doped MgCu2O3 with Zn and Co

The magnetic susceptibility of MgCu ₂ O ₃ powder was measured between 0.2 K and 800 K. The results above TN can be fitted by a Curie–Weiss law plus a one-dimensional (1 — d) antiferromagnetic susceptibility and a constant susceptibility. Below TN the magnetic susceptibility has the large contribution from the paramagnetic impurity of free Cu ²⁺ ions in the compound. When we add Zn ²⁺ ions in the compound the Néel temperature decreases. On the other hand the Co doped MgCu ₂ O ₃ enhance the Néel temperature. Only a small number of Co ions changes the magnetic property of MgCu ₂ O ₃ drastically.     

Crystal Growth, Thermal Stability, and Electrical Properties of LiCu2O2

LiCu₂O₂ single crystals up to 6 × 10 × 10 mm in dimensions are grown by slow melt cooling and are characterized by thermal analysis, dc and ac (0.1–100 kHz) resistivity measurements from 20 to 300 K, and thermoelectric power measurements in the range 130–300 K. The temperature stability range of LiCu₂O₂ is 890–1050°C, and its cation composition may experience deviations from stoichiometry. LiCu₂O₂ is shown to be a p-type semiconductor. Between 80 and 260 K, its dc resistivity follows Mott's law, = Aexp(T ₀/T)^1/4, and charge transport is dominated by hopping conduction between localized states near the Fermi level.     

Ion-exchange equilibria between (Mn,Co)O solid solution and (Mn,Co) Cr2O4 and (Mn,Co) Al2O4 spinel solid solutions at 1100° C

The compositions of the (Mn,Co)O solid solution with rock salt structure in equilibrium with (Mn,Co)Cr₂O₄ and (Mn,Co)Al₂O₄ spinel solid solutions have been determined by X-ray diffraction measurements at 1100° C and an oxygen partial pressure of 10^–10 atm. The ion exchange equilibria are quantitatively analysed, using values for activities in the (Mn,Co)O solid solution available in the literature, in order to obtain activities in the spinel solid solutions. The MnAl₂O₄-CoAl₂O₄ solid solution exhibits negative deviations from Raoult's law, consistent with the estimated cation disorder in the solid solution, while the MnCr₂O₄-CoCr₂O₄ solid solution shows slightly positive deviations. The difference in the Gibbs free energy of formation of the two pure chromites and aluminates derived from the results of this study are in good agreement with recent results obtained from solid oxide galvanic cells and gas-equilibrium techniques.On leave from the Institute for Metals Research, Polish Academy of Sciences, 25 Reymonta St, 30-059 Krakow, Poland.     

Ionic Conductivity of (PbOH)2V12O31 · nH2O

The 333-K complex impedance and temperature-dependent 10-kHz electrical conductivity of the cell Ni|(PbOH)₂V₁₂O₃₁ · nH₂O|Ni were measured. The results demonstrate that heat treatment changes the conductivity of (PbOH)₂V₁₂O₃₁ · nH₂O. After the removal of hydrate water, the activation energy for conduction in the range 203–485 K is 0.27 eV. The polarization resistance of the Ni electrodes is 31% of the total resistance of the cell at 296 K and is nearly zero above 303 K. The potential of the system lead polyvanadate/lead varies linearly with the Pb²⁺ concentration in aqueous Pb(NO₃)₂ solutions.     

Seebeck coefficient of V2O5-R2O3-TeO2 (R = Sb or Bi) glasses

The thermoelectric power of glasses in the systems V₂O₅-Sb₂O₃-TeO₂ and V₂O₅-Bi₂O₃-TeO₂ was measured at temperatures in the range 373–473 K. The glasses in both systems were found to be n-type semiconductors. The Seebeck coefficient, Q, at 473 K was determined as –192 to –151 VK^–1 for V₂O₅-Sb₂O₃-TeO₂ glasses, and –391 to –202 VK^–1 for V₂O₅-Bi₂O₃-TeO₂ glasses. For these glasses in both systems, Heikes' formula was satisfied adequately for the relationship between Q and In [C _v/(1-C_v)] (C _v = V⁴⁺/V_total, C _v is the ratio of the concentration of reduced vanadium ions), and discussions confirmed small polaron hopping conduction of the glasses in both systems. Mackenzie's formula relating to Q and V⁵⁺/V⁴⁺ was also applicable to the glasses in both systems, and it was concluded that the dominant factor determining Q was C _v.     

Reactive deposition of Y1Ba2Cu3O7 −x superconductor films by pulsed laser ablation from an unreacted mixture of Y2O3, BaCO3 and CuO

Superconducting thin films of Y₁Ba₂Cu₃ O_{7 −x} have been deposited on (100) Y-ZrO₂ substrates by pulsed excimer laser ablation from anunreacted mixture of Y₂O₃, BaCO₃ and CuO. The films deposited at substrate temperature of 680°C and oxygen partial pressure of 200 mtorr were found to be superconducting with zero resistive transition temperature of 89 K and critical current density of over 3 × 10⁵ A/cm² at 77 K. These results are compared with those obtained by laser ablation from a sintered superconducting pellet.     

Oxygen potentials,Gibbs' energies and phase relations in the Cu-Cr-O system

Thermodynamic properties of ternary compounds, cuprous and cupric chromites (CuCro₂, CuCr₂O₄), and oxygen potentials corresponding to three three-phase regions in the Cu-Cr-O system have been measured in the temperature range 900 to 1350 K using a solid state galvanic cell incorporating calcia-stabilized zirconia. Cuprous chromite was found to be nearly stoichiometric. The compositions of non-stoichiometric cupric chromite saturated with CuO and Cr₂O₃ have been determined using electron microprobe and energy dispersive X-ray analysis. The results of this study resolve discrepancies in Gibbs' energies of cuprous and cupric chromites reported in the literature. A ternary phase diagram for the Cu-Cr-O system at 1150 K and phase relations in air for the Cu₂O-CuO-Cr₂O₃ system as a function of temperature have been derived based on the new thermodynamic data. The phase diagram given in the literature is found to be inaccurate.     

Fracture toughness of CaO-P2O5-B2O3 glasses and glass-ceramics determined by indentation

The fracture toughness, (K _IC) of CaO-P₂O₅-B₂O₃ glasses and glass-ceramics was investigated using both Vickers indentation and the notched beam technique (NBT). Five representative equations were applied and it was found that for the variation of K _IC with B₂O₃ content, the Lawn and Fuller equation showed the best correspondence with the NBT. The values of fracture toughness obtained from the Lawn and Fuller equation showed the same trend with B₂O₃ content as that determined by NBT, although the values from indentation were on average 33% lower. The determination of absolute fracture toughness by indentation requires a correction factor which can be obtained by calibration using NBT. A significant increase in K _IC occurred after a 37CaO-37P₂O₅-20B₂O₃-6Al₂O₃ (mol%) glass was converted to a glass-ceramic. The much higher K _IC for the glass-ceramic measured by NBT (1.32 MN m^–3/2) compared with that from indentation (0.89 MN m^–3/2) is attributed to internal stresses due to thermal expansion differences between the crystalline and residual glass phases leading to additional microcrack toughening.     

DSC study on the phase transition of BaSm2Mn2O7 and BaEu2Mn2O7

The phase transition of BaSm₂Mn₂0₇ and BaEu₂Mn₂O₇ were studied by differential scanning calorimetry (DSC). Transition temperatures and enthalpy changes accompanying the phase transition for stoichiometric BaSm₂Mn₂O₇ and BaEu₂Mn₂0₇ were measured. The transition temperatures, 525K and 550K for BaSm₂Mn₂O₇ and BaEu₂Mn₂O₇, respectively, determined in this study, are in good agreement with the previous results of high temperature X-ray diffractometry. The enthalpy changes of the transition between orthorhombic and tetragonal structures, ΔH_tr^O, were 0.6 kJ/mol and 1.9 kJ/mol for BaSm₂Mn₂O₇ and BaEu₂Mn₂O₇, respectively.     

Bulk thermal expansion studies of BiCaSrCu2O x and Bi2CaSr2Cu2O x

The compounds BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x were prepared by ceramic techniques and characterized by X-ray powder diffractometry (XRD) and microthermogravimetry (TG) and their bulk thermal expansion measurements were carried out using dilatometry in the temperature range 298T1073 K in air. The results have been analyzed and are compared with those obtained earlier for YBa₂Cu₃O₇. The XRD analysis shows that both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x are single phase in nature, having an orthorhombic symmetry. The TG analysis carried out in oxygen, air, and nitrogen shows negligible weight loss (R~0.1%) on heating to 1073 K, indicating that these two compounds, unlike YBa₂Cu₃O₇, are quite stable. The analysis of bulk thermal expansion data reveals that the average linear thermal expansion coefficient ( ₁) for both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is almost the same ( ₁ 10.5×10^–6 K^–1) and is found to be nearly half of that for YBa₂Cu₃O₇ ( ₁ 18×10^–6 K^–1), suggesting that the interatomic bonding in both BiCaSrCu₂O _x and Bi₂CaSr₂Cu₂O _x is stronger as compared to YBa₂Cu₃O₇.     

Pyroelectricity and related properties in the fresnoite pseudobinary system Ba2TiGe2O8-Ba2TiSi2O8

Complete solid solubility is found in the pseudobinary system Ba₂TiGe₂O₈-Ba₂TiSi₂O₈ by studies on ceramics and single crystals (Czochralski technique). The spontaneous birefringence perpendicular to the polar axis of the orthorhombic low temperature phase has been measured versus temperature and silicon contents: the temperature of the phase transition of species 4mmFmm2 is of second order and decreases from 1103 K at 0 at % Si to about 400 K at 40 at % Si. The pyroelectric coefficient in the orthorhombic mm2 and the tetragonal 4 mm phase is of the order of 10^–6 C m^–2 K^–1 in the entire system and has a positive temperature coefficient for all compositions studied except for Ba₂TiGe₂O₈, in which the pyroelectric coefficient changes sign at about 308 K. The spacegroup of the orthorhombic phase is proposed to be C _2v ¹¹ -Cmm2.     

Laser-induced surface decomposition of Al2O3 excited in the V-center absorption band

The primary products of desorption from Al₂O₃ surface excited by laser pulses (pulse duration τ∼15 ns; wavelength λ=354 nm, radiant power density P/S<10⁸ W/cm²) in the V-center absorption band were studied by the time-of-flight (TOF) spectroscopy. The TOF spectra show evidence of the desorption of one “ cold” (T ₁=300 K) and two “hot” (T ₂=1000 K, T ₃=4300 K) groups of oxygen molecules with the Maxwell velocity distributions, as well as of the hot Al and O atoms with nonequilibrium energy distributions (E ₁=0.37 eV, E ₂=0.38 eV). A model describing the oxygen desorption as initiated by the electron transitions is suggested, in which escape of the cold O₂ molecules from the surface is related to discharge of the O ₂ ⁻ anions adsorbed on the V-centers, desorption of the hot atoms is attributed to discharge of the surface O⁻ anions, and the appearance of the hot O² molecules is related to the associative desorption of two O⁻ anions localized at the same V-center discharged by a pair of excitons.     

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.     

Synthesis, Structure, and Electrical Properties of NdMFe2O5 (M = Li, Na, K, Cs) Ferrites

The NdMFe₂O₅ (M = Li, Na, K, Cs) ferrites are prepared by solid-state reactions using mixtures of Nd₂O₃, Fe₂O₃, and M₂CO₃. The ferrites are shown to have a tetragonally distorted perovskite structure (Z= 16) with the following lattice parameters: a = 10.94 Å, c = 13.83 Å,V = 1655.2 ų for NdLiFe₂O₅; a= 10.98 Å, c = 15.10 Å, V = 1820.5 ų for NdNaFe₂O₅; a= 10.96 Å, c = 16.82 Å, V= 2020.5 ų for NdKFe₂O₅; a= 10.93 Å, c= 17.94 Å, V = 2143.2 ų for NdCsFe₂O₅.     

Effect of B2O3 addition on microhardness and structural features of 40Na2O-10BaO-xB2O3-(50-x)P2O5 glass system

Phosphate glasses having composition, 40Na₂O-10BaO-xB₂O₃-(50-x)P₂O₅, wherex = 0–20 mol% were prepared using conventional melt quench technique. Density of these glasses was measured using Archimedes principle. Microhardness (MH) was measured by Vicker’s indentation technique. Structural studies were carried out using IR spectroscopy and³¹P and¹¹B MAS NMR. Density was found to vary between 2.62 and 2.77 g/cc. MH was found to increase with the increase in boron content.³¹P MAS NMR spectra showed the presence of middle Q² groups and end Q¹ and Q⁰ groups with P-O-B linkages. FTIR studies showed the presence of BO₃ and BO₄ structural units along with the depolymerization of phosphate chains in conformity with³¹P MAS NMR.¹¹B NMR spectra showed increase in BO₄ structural units with increasing boron content. The increase in MH with B₂O₃ content is due to the increase of P-O-B linkages and BO₄ structural units as observed from MAS NMR studies resulting in a more rigid borophosphate glass networks.     

Magnetic properties of the phases Fe2V4O13, FeVMoO7 and Fe4V2Mo3O20

The Curie constant,C, Weiss constant, , effective magnetic moment, _eff, and spectroscopic splitting factor,g, were determined for the Fe³⁺ ions in Fe₂V₄O₁₃, FeVMoO₇ and Fe₄V₂Mo₃O₂₀ at 76–300 K based on measurements of magnetic susceptibility of the phases. The Neel temperature,T _N, of interest was based on the temperature dependence of magnetization of the phases. It was shown that a local antiferromagnetic arrangement of the Fe³⁺ ions in Fe₂V₄O₁₃, FeVMoO₇ and Fe₄V₂Mo₃O₂₀ is already involved at a temperature much higher than the Neel temperature, resulting from the cation-anion-cation superexchange between the Fe³⁺ ions with ad ⁵ configuration.     

Electrical properties of Sb2O3–CaO–V2O5 glasses and glass-ceramics

The d.c. conductivity (σ) of (a) glasses prepared by the press-quenching method and (b) glass-ceramics (crystallized glass) produced by post-heat treatment was investigated in the system Sb₂O₃–CaO–V₂O₅ and their conduction mechanism was studied. The glasses were n-type semiconductors with σ = 2.6 × 10^-6 ∼ 2.8 × 10^-5 S cm^-1 at 333 K for varying glass compositions. The conduction was attributed to small polaron hopping in the adiabatic regime. The estimated carrier density was 1.7 ∼ 3.8 × 10²¹ cm^-3 for V₂O₅ = 70 ∼ 80 mol% and the mobility was 3.5 × 10^-9 to 6.9 × 10^-8 cm² V^-1 s^-1. Crystallization raised the conductivity by a factor of 10³. The crystalline product in the glass-ceramics was Ca_0.17V₂O₅. The glass-ceramics were n-type semiconductors, and the conduction was interpreted by a superposition of the small polaron hopping in the crystalline and glassy phases. This revised version was published online in November 2006 with corrections to the Cover Date.     

Superconductivity and Magnetism in R 2CaBa2Cu5O z (R=La, Pr, Nd and Eu)

A series of compounds R ₂CaBa₂Cu₅O _z (R=La, Pr, Nd and Eu) have been synthesized by conventional solid state reaction and characterized for their structural, superconducting and magnetic properties. All compounds crystallize with the tetragonal LaBa₂Cu₃O _z type structure, space group P4/mmm. Among the four compounds studied here, R=La, Nd and Eu are superconductors with superconducting transition temperatures (T _c ^R=0) of 72, 40 and 55 K, respectively. On the other hand, neither superconductivity nor magnetic ordering is seen for R=Pr down to 3 K. The effect of the magnetic field on the susceptibility and (magneto) resistance for the superconducting samples has been investigated. The superconductivity of compounds with magnetic rare-earth ions Nd and Eu exhibit a profound influence of the magnetic field, whereas the application of the magnetic field has a limited effect on the La compound. The Pr compound is paramagnetic and does not exhibit magnetic ordering either.