Frequency dependence of loss tangent of thermally annealed undoped lead iodide crystals in the dark

The effect of isothermal annealing ( T an ⩽ 120 ° C ) on the frequency dependence of the loss tangent, tan δ, of undoped lead iodide (PbI2) crystals in the dark has been investigated in the range 0.1–15 kHz at different ambient temperatures ( T = 15 – 120 ° C ) without and with DC biasing ( V DC ⩽ 2 V ). Thermal annealing of as-grown PbI2 crystals modified both the magnitude and frequency dependence of their tan δ. The tan δ–ƒ data ( T = 17 ° C ) of unbiased as-grown crystals were found to fit a tan δ∝ƒ−m dependence, with m∼0.94 in the range 100 Hz <ƒ<10 kHz. After annealing these crystals at T an = 70 ° C for 1 h, the tan δ∝ƒ−m relation was obeyed for unbiased samples at any T over most of the frequency range studied, with the exponent m being reduced to around 0.55; a behavior that was also found for isothermally annealed PbI2 crystals when were DC biased, but at high ambient temperatures. The tan δ−T data (ƒ=1 kHz) of the unbiased prolonged annealed ( T an = 120 ° C ) PbI2 crystals exhibited two different thermally activated regimes, with activation energies Ea∼0.23 and 0.087 eV in the ranges 75–120 °C and 15–75 °C, respectively, which were related to trapping cationic Pb+2-vacancies/hole-impurity levels in the crystal band gap. An interpolytypic phase transformation of the 2H-PbI2 upon isothermal annealing may not, however, be entirely excluded.