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Single-crystal platelets (3×3×0.1 mm) of both tetragonal and orthorhombic lead oxide have been grown from dilute NaOH solutions at high pressures using conventional hydrothermal techniques. The predominant yield of the two phases was varied in accord with the P-T phase diagram of PbO. Electrical measurements include the Hall coefficient and electrical conductivity as a function of temperature, while the corresponding photoeffects and optical absorption with monochromatic radiation were also studied. ``As-grown'' tetragonal crystals were highly conducting n-type with σ(300°K) ∼10−3 to 1.0Ω−1 cm−1, and had Hall mobilities of about 100 cm2V−1 sec−1. In constrast the ``as-grown'' orthorhombic crystals were rather insulating with σ(300°K) ∼10−5Ω−1 cm−1 and a Hall mobility μe∼50 cm2V−1 sec−1. Heat treatment in air altered both the electrical and optical properties. Photoconductivity was observed in both crystalline forms. The optical measurements indicate band transitions at 1.90 eV and 2.75 eV for the tetragonal form and 2.67 eV for the orthorhombic modification.

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