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The photoconductive behavior of evaporated layers of lead monoxide in both the tetragonal and orthorhombic crystalline modifications has been investigated. Stoichiometry was controlled on a relative scale by controlling the oxygen pressure during sample preparation, yielding both n- and p-type material. Current—voltage characteristics, optical absorption, and variation of photocurrent with (1) wavelength, (2) applied field, and (3) light intensity are presented and discussed for the two forms. Energy band gaps of 2.0 eV for the tetragonal form and 2.6 eV for the orthorhombic form are indicated. A maximum resistivity of 3×1011Ω·cm with a μτ product of 4×10−7 cm2/V was measured for the tetragonal phase. The corresponding values for the orthorhombic phase were 1×1013Ω·cm and 1×10−9 cm2/V. Whereas in earlier pictures photosensitivity was explained as a contact phenomenon, the present data demand a model such as impurity sensitization or interparticle barrier modulation to explain the observed bulk photosensitivity.

The calcium-strontium phase system has been investigated in the liquid and solid state over the entire range of compositions. The liquidus and_ solidus curves were determined by differential thermal analysis whereas the solid state equilibria were studied by X-ray diffraction methods. Complete miscibility in both the liquid and solid state were observed with a minimum in the liquidus-solidus at 78 ± 2 weight % strontium and at 738°. Calcium, strontium and all calcium-strontium alloys exist in three crystalline modifications, face-centered cubic, hexagonal close-packed and body-centered cubic. There is a minimum in the b.c.c. h.c.p. transition, but the h.c.p. <=* f.c.c. transition in all alloys occurs at temperatures intermediate to the transition temperatures of the pure components. The variation of oo with atomic % at room temperature for f.c.c. alloys is linear in conformity with Vegard's law. At 415°the variation of both Oo and Co of the h.c.p. modification is linear with atomic per cent, for the calcium rich alloys; however, positive deviation from Vegard's law occurs with strontium-rich alloys. Positive deviation from Vegard's law also occurs in the strontium-rich b.c.c. alloys (615°).

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