By measuring electron spin resonance (ESR) spectra at the X-band frequency and absorption spectra from the visible to ultraviolet region at room temperature, it was confirmed that perovskite single crystal LaAlO3 contains Cr and Fe as impurities. When LaAlO3 is exposed to photons with energies higher than 4.5 eV, the intensities of ESR signals due to Cr3+ and Fe3+ decrease, which indicates that electrons released by the photon irradiation are captured by Cr3+ and Fe3+. Concurrently with this, a broad optical absorption band at about 3.0 eV and two new broad and weak ESR signals appear. The former is attributable to a combination of a hole and a La3+ (or Al3+) vacancy, and the two weak ESR signals are assigned to the O− center and F+ center.

Thermal annealing was given to single crystal LaAlO3 and its effects were examined by measuring electron spin resonance (ESR) and optical absorption. When LaAlO3 was annealed at temperatures above 500 °C in an oxidizing atmosphere, the intensities of ESR signals due to transition metal, likely ascribable to Fe3+, decreased. Concurrently with this, two optical absorption bands at 2.7 and 3.5 eV, attributable to a combination of a hole and a La3+ (or Al3+) vacancy, increased. These results indicate that thermal electron–hole generation is induced by oxidizing annealing and that the generated electrons and holes are then captured by Fe3+ ions and La3+ or Al3+ vacancies, respectively. It is also assumed that captured electrons and holes are released and recombine with each other by reducing annealing.

Impurities present in LaAlO 3 , were identified by analyzing electron spin resonance spectra and optical absorption spectra. Especially, the effects of ultraviolet photon irradiation and thermal annealing on these spectra were studied in detail. The experimental results confirm that LaAlO 3 contains Cr and Fe as impurities. They also indicate that electrons and holes are released by the irradiation of UV photons with energies higher than 4.5 eV or by the annealing in an oxidizing atmosphere at temperatures above 500 o C. The released electrons are captured by trivalent impurity ions (Fe 3+ or Cr 3+ ), while the holes are captured by trivalent vacancies of La 3+ or Al 3+ . It is assumed that the opposite reaction is induced by the irradiation of visible light or by the annealing in a reducing atmosphere at temperatures above 500 o C.