  The electronic transport properties and optical properties of lead(II)
chromate (PbCrO4) have been studied at high pressure by means of resistivity,
Hall-effect, and optical-absorption measurements. Band-structure
first-principle calculations have been also performed. We found that the
low-pressure phase is a direct band-gap semiconductor (Eg = 2.3 eV) that shows
a high resistivity. At 3.5 GPa, associated to a structural phase transition, a
band-gap collapse takes place, becoming Eg = 1.8 eV. At the same pressure the
resistivity suddenly decreases due to an increase of the carrier concentration.
In the HP phase, PbCrO4 behaves as an n-type semiconductor, with a donor level
probably associated to the formation of oxygen vacancies. At 15 GPa a second
phase transition occurs to a phase with Eg = 1.2 eV. In this phase, the
resistivity increases as pressure does probably due to the self-compensation of
donor levels and the augmentation of the scattering of electrons with ionized
impurities. In the three phases the band gap red shifts under compression. At
20 GPa, Eg reaches a value of 0.8 eV, behaving PbCrO4 as a narrow-gap
semiconductor.
