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

The effect of isothermal annealing (Tan⩽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 (VDC⩽2V). 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 Tan=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 (Tan=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.