An important inorganic phase of synthetic bone applications, calcium hydroxyapatite (HA, Ca 10 (PO 4 ) 6 (OH) 2 ), was prepared as a single-phase ceramic powder. Carbonated HA powders were formed from calcium nitrate tetrahydrate and diammonium hydrogen phosphate salts dissolved in aqueous`synthetic body¯uid' (SBF) solutions, containing urea (H 2 NCONH 2 ), at 37 C and pH of 7 . 4, by using a novel chemical precipitation technique. These powders were also found to contain trace amounts of Na and Mg impurities in them, originated from the use of SBF solutions, instead of pure water, during their synthesis. The characterization and chemical analysis of the synthesized powders were performed by powder X-ray diraction (XRD), Fourier-transformed infra-red spectroscopy (FT±IR), and inductively-coupled plasma atomic emission spectroscopy (ICP±AES). #

The e g ↑ / ͑t 2g ↓ + e g ↓͒ band ratio in cation-substituted La-Fe oxides is identified in O ͑1s͒ x-ray absorption spectra as a linear spectral indicator for conducting electron holes. The t 2g ↓ and e g ↓ bands act as a conductivity inhibitor by ferromagnetic double exchange coupling on the e g ↑ electron. Disorder induced by substitution appears to modulate the hole conduction such that an exponential relation is found between the conductivity and the e g ↑ / ͑t 2g ↓ + e g ↓͒ ratio and hole concentration. The quantitative correlation of conductivity and x-ray absorption spectra from heterovalent-substituted LaFeO 3 lets substitution-driven metal insulator transitions appear in a new light.

Mixed ionic-electronic conducting La 0:5 Sr 0:5 Fe 1Àx B x O 3Àδ (B: Al, Cr, Zr, Ga, Ti, Sn, Ta, V, Mg, and In with x=0, 0.1, 0.2) perovskite materials were produced via solid-state synthesis. In order to study the effect of Bsite substitution on the expansion behavior of these materials, their thermal expansion in air up to 900°C and isothermal expansion at the same temperature from air to Ar were measured by dilatometry. Ti and Ta were found to be the most effective substitutions in suppressing the isothermal expansion. The isothermal expansion at 900°C from air to Ar was reduced by 50% by substitution of 20% Ti or 10% Ta. Therefore, these compositions were further characterized by 4-probe total DC conductivity and permeation measurements under air/Ar gradient. The total conductivity of La 0:5 Sr 0:5 FeO 3Àδ was decreased by more than one order of magnitude at low temperatures and from 430 S/cm, which is the maximum, to around 100 S/cm at 500°C with the addition of Ti and Ta. The normalized oxygen permeation of LSF at 900°C decreased from 0.18 to 0.05 μmol/cm 2 s and 0.07 μmol/cm 2 s with the substitution of 20% Ti and 10% Ta, respectively.

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