Interstitial Oxygen In The Lead Tungstate Crystal And The Doping Mechanism

Since PbWO4 (PWO) crystal is of high density, short radiation length, ns fast decay and cheap price, it has been chosen as a preferred material for the detector at the Large Hadron Collider in CERN and as a scintillator it arouse extensive researchers around the world, but till now some of its properties are still debated.Recently, many annealing experimental results indicated that the 350nm absorption band had relation to the interstitial oxygen in PWO crystal, however, the existent mechanism of interstitial oxygen atoms are not clear. In fact, it is difficult to confirm the optimal position of interstitial oxygen atoms in PWO crystal only with experimental measures. It was found that doping some tervalent ions for example La3+, Y3+ could effectly restricte 420nm absorption, however, with different doping concentrations, some new absoption appeared.The thesis mainly contains five parts. The first part is the introduction. The second part introduces the basic calculation theory and related computer programs. The third part discusses the interstitial oxygen atoms and its effect for PWO crystals. The fourth part discusses the origin of the 420nm absorption band and the effect of tervalent ions La3+, Y3+ doping PWO crystal in different concentrations. The fifth part is the computer simulation of Nb5+ doping PWO crystal.The first part of the thesis introduces the research background of PbWO4 crystal; the status of the research and the content will be discussed in the thesis. The second part of the thesis is the introduction of the basic calculation theory and different simulation codes. Firstly the GULP code used in calculation is introduced. Secondly, the density functional theory is introduced and then different calculation codes based on this theory are discussed in detail.The third part of the thesis is the research of the existent mechanism of interstitial oxygen atoms and its effects for the optical properties of the PWO crystals. The calculated results indicate that the interstitial oxygen atom should locate around the W6+ ion and will combine with the formal lattice oxygen forming oxygen molecular ions O22- or O34-, which will arouse the 350nm absorption.In the fourth part of the thesis, firstly the electronic structure of PWO crystal with one isolated lead vacancy is calculated and the origin of the 420nm absorption band is discussed. Then the doping mechanisms of La3+, Y3+ doping PWO crystals are discussed. The electronic structures of La3+, Y3+ doping PWO crystals with different doping mechanism are calculated using DV-Xαcode. The calculated results indicate that doping La3+, Y3+ can effectively restrict the absorption of the hole centers, enhance the radiation hardness. However, in heavily doped La3+, Y3+, because of the existing of interstitial oxygen ions, it will arouse new absorption in the crystal; reducing the radiation hardness of the crystal, so the doping concentration of La3+, Y3+should be controlled.In the fifth part of the thesis, firstly the formation energies of point defects and defect clusters in the Nb5+ doping PWO crystal have been studied by a computer simulation technique. The optimal electronic compensating mechanism is obtained. Then the electronic structures of pure PWO crystal, PWO crystal with oxygen vacancy, Nb5+ doping PWO crystal were calculated. The optical effect of Nb5+ doping for the crystal is discussed.