Electronic calculations on ¯uorides and oxides of Zr, Hf and Th J.F. Rivas-Silva a, * , J.S. Durand-Nicono a , M. Berrondo b a Instituto de F  õsica, Benem erita Universidad Aut onoma de Puebla, Apartado Postal J-48, 72570 Puebla, Puebla, Mexico b Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA Accepted 12 May 1999 Abstract The band gaps of the oxides and ¯uorides of zirconium, hafnium and thorium are calculated by means of two quantum chemical methods. Through the ®rst the gap is estimated as a one-electron energy given by the HUMO± LOMO splitting, while through the second it is obtained as the energy dierence between electronic potentials of crystal clusters computed at their experimental con®guration. Doping eects for these compounds are also analyzed via substitutional impurities on Pr 4 sites. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: Electronic structure; Ab initio methods; Semi-empirical calculations; Optical properties of materials; Detection of radiation 1. Introduction In this work, we present some theoretical cal- culations performed on zirconium, hafnium and thorium ¯uorides and oxides, aimed at exploring optical properties related to their capability as gamma radiation detectors, as speci®cally induced by praseodymium doping. After the works of Lempicki and Wojtowicz [1] and Blasse [2] some basic rules associated with potentially good radi- ation detectors can be outlined: 1. By the nature of the common photomultipli- ers currently in use, the emitted photon should be on the visible frequency range. 2. To allow an ecient outgoing path of the pho- ton across the crystal this ought to be transpar- ent precisely throughout the visible frequency range. 3. The emission of the photon must be a fast enough process so as to make the counting of the arrival events ecient. 4. The material must present a high scattering section to the incoming particles. An essential feature to be considered in these materials is the value of the forbidden band gap, E g . On one hand, it plays a key role in rule (2) above, where quantitatively E g should be 3 eV or larger to satisfy the associated requirement. On the other hand, the band gap enters the expression for the eciency accomplished at the ®rst stage of the detection process, i.e. when the arriving radiation produces all the primary defects and ionization, in addition to energy thermalization. At this stage a number of secondary electrons and holes are www.elsevier.com/locate/commatsci Computational Materials Science 18 (2000) 193±198 * Corresponding author. Tel.: +52-22-45 76 45; fax: +52-22- 44 89 47. E-mail address: rivas@sirio.ifuap.buap.mx (J.F. Rivas-Silva). 0927-0256/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 5 6 ( 0 0 ) 0 0 0 9 5 - 1