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51.
The electric conductivity and the optical absorption of PbTiO3 crystals are explained by the existence of Pb3+ ions and free holes as electronic defects and of O– vacancies and interstitials as ionic anti-Frenkel defects. The absorption band at 475 nm is caused by Pb3+ ions and produces the yellowish or brownish colour of the crystals. Using fluoridic fluxes instead of oxidic fluxes for the crystals growth a small part of O– ions is replaced by F− foreign ions. This substitution diminishes the Pb3+ ion content and causes an increased optical transmission of the PbTiO3 crystals. 相似文献
52.
W. Bollmann 《Crystal Research and Technology》1992,27(5):661-672
The formation volume Vv of vacancies is given by Vv = (hv/L) ΔVf with hv = 8L (formation enthalpy hv of vacancies and heat L of fusion given in same units; ΔVf = change of volume due to melting). If there are phase transitions within the solid, L and ΔVf must be replaced by (L + Δ Ht) and by (ΔVf + ΔVt), respectively (Δ Ht and Δ Vt refer to the heat (s) of transition (s) and to the volume change(s) due to transition(s), resp.). The pressure dependence of the melting point is dTm/dp = (TmVv)/hv. Independent of the sign of Vv any increase of the vacancy concentration above the maximum concentration possible within the solid decreases the melting point thus resulting in the observed surface melting. The melting point is fixed by the characteristics of vacancy formation (hv, Sv, Vv) and by the bulk modulus of the solid (Sv = formation entropy of vacancies). 相似文献