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An ab initio density functional theory calculations on the K2YF5 crystal containing hydroxyl impurities
Journal of Molecular Structure 1051 (2013) 177–179
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
An ab initio density functional theory calculations on the K2YF5 crystal containing hydroxyl impurities q A.A. Gallegos-Cuellar a,⇑, R. Licona-Ibarra b, J.F. Rivas-Silva c, A. Flores-Riveros c, J. Azorín Nieto d, J.F. Casco-Vásquez a a
Instituto Tecnológico de Apizaco, Av. Instituto Tecnológico S/N, CP 90300, Apizaco, Tlaxcala, Mexico Facultad de Ciencias Químicas, Benemérita Universidad Autónoma de Puebla, 14 Sur y Ave. San Claudio, CP 72570, Puebla, Pue., Mexico Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apartado Postal J-48, 72570, Puebla, Pue., Mexico d Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco 186, 09340 México, DF, Mexico b c
h i g h l i g h t s +3
Theoretical modeling of the IRS of K2YF5:Tb , in comparison to the experimental. IRS lines, show up only for a symmetry of the OH impurity with the plane (002). The DFT calculations confirm the existence of OH aggregates substituting flour. Appearance in the DOS of new orbitals in the valence band.
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 31 July 2013 Accepted 31 July 2013 Available online 6 August 2013 Keywords: Calculations with density of functional theory Theoretical modeling of the infrared spectrum Models where OH impurities are substitutionally introduced
a b s t r a c t High frequency absorption spectral lines not matching any of the chemical constituents were observed while analyzing the infrared experimental spectrum of a K2YF5:Tb+3 sample. We ascribe these lines to the presence of impurities that inadvertently contaminated the crystal compound during synthesis, whose mass and electronegativity apparently indicate OH substitutional ions occupying fluorine sites. In this report we have performed ab initio calculations by means of a solid state computational code, applied to a model consisting of a potassium-yttrium-double fluoride structure where OH ion aggregates are introduced on F sites, which indeed confirm such assignment. Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1. Introduction Gallegos et al. [1] studied the phenomenon of thermo-transferred thermoluminiscence (TTTL) during characterization of the compound K2YF5:Tb+3 [2,3]. Since the thermoluminiscent process involves several metastable states associated with impurities either substitutionally introduced or as a result of contamination
q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. Tel.: +52 241 41 7 20 10; fax: +52 241 41 8 03 21. E-mail addresses: [email protected] (A.A. Gallegos-Cuellar), rliconai@ sirio.ifuap.buap.mx (R. Licona-Ibarra), [email protected] (J.F. Rivas-Silva), fl[email protected] (A. Flores-Riveros), [email protected] (J. Azorín Nieto).
during synthesis or growth, it is essential to fully understand the nature of such defects. The Infrared medium (IRM) spectroscopy allows for determination of aggregates or substitutional ion sites in a nondestructive way. The compound here analyzed was studied by D. Zverev et al. [4–6], who, by means of an EPR technique, predicted the existence of substitutionally introduced OH ions in the crystal. The Density Functional Theory (DFT), based on the solution of the Kohn–Sham [7,8] equations, represents a quantum mechanical description that has often been utilized to analyze the electronic structure of solids [9]. Comparison of theoretical results with available experimental data has proved DFT to be a powerful and accurate theoretical tool. The spectral analysis was performed by means of infrared spectroscopy (IR) with a Perkin–Elmer, Spectrum One, FT-IR Spectrometer with a Universal ATR Sampling Accessory, in the range
0022-2860/$ - see front matter Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.07.059
178
A.A. Gallegos-Cuellar et al. / Journal of Molecular Structure 1051 (2013) 177–179
Fig. 3. Theoretical infrared spectrum of the K2YF5 crystal, where the presence of OH ions is undetected.
Fig. 1. Optimized K2YF5 cell crystal structure.
650–4000 cm 1, applied on a K2YF5:Tb+3 sample, experimentally prepared by Prof. J. Azorín-Nieto at Universidad Autónoma Metropolitana-Iztapalapa (Mexico) and Prof. N. Khaidukov at Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow (Russia). The theoretical calculations were performed on a crystal model for the compound consisting of a symmetry group Pna21 (33) cubic cell of 10.791, 6.607 and 7.263 Å, containing, for the K2YF5 [10] pure system, 4 yttrium, 8 potassium and 19 fluorine atoms, whereas for the system with aggregates, one or two OH ions substitutionally occupying F atomic sites, with no regard of Tb doping since this has no influence on the IR spectrum range we are interested in. The Kohn–Sham equations were solved in the cell reciprocal space by means of a Perdew–Zunger GGA exchange–correlation potential. The geometry of the compound cell was optimized by minimizing the total energy, yielding a structure on which the theoretical IR spectrum was obtained by making use of the hessian through the IR/Raman module of Cerius2 software package [11].
Fig. 4. K2YF5(OH) crystal with the symmetry plane (0 0 2).
2. Results The optimized cell crystal structure is shown in Fig. 1, where the location of each of the constituent elements is clearly indicated. In Fig. 2 is illustrated the sample IR experimental spectrum, where the main peaks are found at two separate ranges: 681, 790, 810 and 932 cm 1 and 1076, 1534, 1700, 2976 and 3750 cm 1.
Fig. 2. K2YF5:Tb+3 sample IR experimental spectrum.
A.A. Gallegos-Cuellar et al. / Journal of Molecular Structure 1051 (2013) 177–179
179
Fig. 5. Infrared theoretical spectrum of K2YF5(OH) crystal, where activation of the OH group lines can be observed. Table 1 Unit cell parameters for some alternative models of the K2YF5 crystal with OH impurities. Substituted atomic site F
a (Å)
b (Å)
c (Å)
a,b,c (°)
EXP No symm. OH (OH)2
10.791 10.891 10.920 11.308
6.607 6.680 6.666 6.662
7.263 7.281 7.298 7.275
90 90 90 90
The values at the second range are precisely those which, given their magnitude, cannot be associated with vibrations of either K, Y or F atoms for the pure crystal, and must therefore correspond to substitutionally introduced impurities. On the other hand, in the obtained theoretical spectrum, shown in Fig. 3, only three peaks can be discerned in the range 100–800 cm 1, whereas those falling within the second range are apparently missing. We thus conclude that, along this theoretical range of wavenumbers, there appear only vibrations that correspond to the constituent elements of the pure crystal and no trace of the OH impurities. In order to theoretically obtain the missing peaks, in view of the above findings, we here propose several models where OH impurities are substitutionally introduced. A band at 3775 and 1650 cm 1 is obtained when an F atomic site is substituted by an OH ion placed in a position where the O atoms are on (and the H atom out of) the symmetry plane (0 0 2), as shown in Fig. 4. The vibrational modes of the OH group give rise to a strong IR spectral line at 3775 and 1650 cm 1, as illustrated in Fig. 5. Although other models can be proposed for the same impurity, as indicated in Table 1, the essential idea is to place the OH ion in a position and associated symmetry leading to the activation of the expected spectral bands. In order to complement the present report we have also obtained the density of states (DOS) of the K2YF5(OH) compound, shown in Fig. 6, where a line is shifted in the forbidden band when comparing the pure crystal K2YF5 (at 6.55 eV) and the one with the impurity (at 5.16 eV), giving rise to the presence of new states above the valence band, beyond the Fermi level (at 0 K). 3. Conclusions The DFT calculations here performed and proposed models theoretically confirm the existence of OH aggregates substituting F atomic sites in the crystal structure of K2YF5, through the appearance of spectral lines in the range 800–4000 cm 1, otherwise missing when obtaining the IR spectrum for the pure crystal compound. However, such lines show up only for a particular symmetry adopted by the OH impurity with respect to the plane (0 0 2) in the crystal.
Fig. 6. Density of states of the K2YF5(OH) crystal compound, where metastable states show up above the valence band.
A more realistic theoretical modeling of the IR spectrum of K2YF5:Tb+3, in comparison to the experimentally known active lines, would probably require a combination of models such as those proposed in Table 1 plus others, taking into account the particular symmetry that the impurities present in the crystal compound. The density of states (DOS) analysis indicates the appearance of new orbitals within the valence band, associated with metastable states resulting from a recombination of electrons released during thermoluminescent emission, as reported in the literature. Acknowledgments This study was made possible due to funding from the Consejo Nacional de Ciencia y Tecnología (CONACyT), as well as resources provided by the Instituto de Física ‘‘Ing. Luis Rivera Terrazas’’, BUAP, its academic group FCMC-SEP-PIFI, and Instituto Tecnológico de Apizaco for support. We thank to the Centro de Cómputo of IFUAP for the provided facilities, and to the Laboratorio de Análisis Instrumental of the Facultad de Ingeniería Química, BUAP, where the IR spectrums were made. We also thank to Prof. N. Khaidukov for give us the materials. References [1] A. Gallegos et al., Rev. Mex. de Fís. S 57 (1) (2011) 65–68. [2] J. Azorín Nieto et al., Radiat. Eff. Defects Solids 161 (8) (2006) 443–449. [3] J. Azorín, A. Gallegos, T. Rivera, J.C. Azorín, N. Khaidukov, Nucl. Instr. Methods Phys. Res. A 580 (2007) 177–179. [4] F. Loncke, D. Zverev, H. Vrielinck, N.M. Khaidukov, P. Matthys, F. Callens, Phys. Rev. B 75 (2007) 144427. [5] D. Zverev, H. Vrielinck, F. Callens, P. Matthys, S. Van Doorslaer, N.M. Khaidukov, Phys. Chem. Chem. Phys. 10 (2008) 1789–1798. [6] D. Zverev, H. Vrielinck, P.F. Smet, D. Poelman, F. Callens, Phys. Rev. B 79 (2009) 224110. [7] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [8] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [9] M.C. Payne, M.P. Teter, D.C. Allan, Rev. Mod. Phys. 64 (1992) 4. [10] E.M. Maddock, R.A. Jackson, M.E.G. Valerio, J. Phys.: Conf. Ser. 249 (2010) 1. [11] Cerius 2 User Guide; Accelrys, Inc., Cerius2 Modeling Environment, Release 4.8, San Diego: Accelrys Software Inc., 2005.
Contents lists available at ScienceDirect
Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
An ab initio density functional theory calculations on the K2YF5 crystal containing hydroxyl impurities q A.A. Gallegos-Cuellar a,⇑, R. Licona-Ibarra b, J.F. Rivas-Silva c, A. Flores-Riveros c, J. Azorín Nieto d, J.F. Casco-Vásquez a a
Instituto Tecnológico de Apizaco, Av. Instituto Tecnológico S/N, CP 90300, Apizaco, Tlaxcala, Mexico Facultad de Ciencias Químicas, Benemérita Universidad Autónoma de Puebla, 14 Sur y Ave. San Claudio, CP 72570, Puebla, Pue., Mexico Instituto de Física, Benemérita Universidad Autónoma de Puebla, Apartado Postal J-48, 72570, Puebla, Pue., Mexico d Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco 186, 09340 México, DF, Mexico b c
h i g h l i g h t s +3
Theoretical modeling of the IRS of K2YF5:Tb , in comparison to the experimental. IRS lines, show up only for a symmetry of the OH impurity with the plane (002). The DFT calculations confirm the existence of OH aggregates substituting flour. Appearance in the DOS of new orbitals in the valence band.
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 31 July 2013 Accepted 31 July 2013 Available online 6 August 2013 Keywords: Calculations with density of functional theory Theoretical modeling of the infrared spectrum Models where OH impurities are substitutionally introduced
a b s t r a c t High frequency absorption spectral lines not matching any of the chemical constituents were observed while analyzing the infrared experimental spectrum of a K2YF5:Tb+3 sample. We ascribe these lines to the presence of impurities that inadvertently contaminated the crystal compound during synthesis, whose mass and electronegativity apparently indicate OH substitutional ions occupying fluorine sites. In this report we have performed ab initio calculations by means of a solid state computational code, applied to a model consisting of a potassium-yttrium-double fluoride structure where OH ion aggregates are introduced on F sites, which indeed confirm such assignment. Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved.
1. Introduction Gallegos et al. [1] studied the phenomenon of thermo-transferred thermoluminiscence (TTTL) during characterization of the compound K2YF5:Tb+3 [2,3]. Since the thermoluminiscent process involves several metastable states associated with impurities either substitutionally introduced or as a result of contamination
q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. ⇑ Corresponding author. Tel.: +52 241 41 7 20 10; fax: +52 241 41 8 03 21. E-mail addresses: [email protected] (A.A. Gallegos-Cuellar), rliconai@ sirio.ifuap.buap.mx (R. Licona-Ibarra), [email protected] (J.F. Rivas-Silva), fl[email protected] (A. Flores-Riveros), [email protected] (J. Azorín Nieto).
during synthesis or growth, it is essential to fully understand the nature of such defects. The Infrared medium (IRM) spectroscopy allows for determination of aggregates or substitutional ion sites in a nondestructive way. The compound here analyzed was studied by D. Zverev et al. [4–6], who, by means of an EPR technique, predicted the existence of substitutionally introduced OH ions in the crystal. The Density Functional Theory (DFT), based on the solution of the Kohn–Sham [7,8] equations, represents a quantum mechanical description that has often been utilized to analyze the electronic structure of solids [9]. Comparison of theoretical results with available experimental data has proved DFT to be a powerful and accurate theoretical tool. The spectral analysis was performed by means of infrared spectroscopy (IR) with a Perkin–Elmer, Spectrum One, FT-IR Spectrometer with a Universal ATR Sampling Accessory, in the range
0022-2860/$ - see front matter Ó 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.07.059
178
A.A. Gallegos-Cuellar et al. / Journal of Molecular Structure 1051 (2013) 177–179
Fig. 3. Theoretical infrared spectrum of the K2YF5 crystal, where the presence of OH ions is undetected.
Fig. 1. Optimized K2YF5 cell crystal structure.
650–4000 cm 1, applied on a K2YF5:Tb+3 sample, experimentally prepared by Prof. J. Azorín-Nieto at Universidad Autónoma Metropolitana-Iztapalapa (Mexico) and Prof. N. Khaidukov at Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow (Russia). The theoretical calculations were performed on a crystal model for the compound consisting of a symmetry group Pna21 (33) cubic cell of 10.791, 6.607 and 7.263 Å, containing, for the K2YF5 [10] pure system, 4 yttrium, 8 potassium and 19 fluorine atoms, whereas for the system with aggregates, one or two OH ions substitutionally occupying F atomic sites, with no regard of Tb doping since this has no influence on the IR spectrum range we are interested in. The Kohn–Sham equations were solved in the cell reciprocal space by means of a Perdew–Zunger GGA exchange–correlation potential. The geometry of the compound cell was optimized by minimizing the total energy, yielding a structure on which the theoretical IR spectrum was obtained by making use of the hessian through the IR/Raman module of Cerius2 software package [11].
Fig. 4. K2YF5(OH) crystal with the symmetry plane (0 0 2).
2. Results The optimized cell crystal structure is shown in Fig. 1, where the location of each of the constituent elements is clearly indicated. In Fig. 2 is illustrated the sample IR experimental spectrum, where the main peaks are found at two separate ranges: 681, 790, 810 and 932 cm 1 and 1076, 1534, 1700, 2976 and 3750 cm 1.
Fig. 2. K2YF5:Tb+3 sample IR experimental spectrum.
A.A. Gallegos-Cuellar et al. / Journal of Molecular Structure 1051 (2013) 177–179
179
Fig. 5. Infrared theoretical spectrum of K2YF5(OH) crystal, where activation of the OH group lines can be observed. Table 1 Unit cell parameters for some alternative models of the K2YF5 crystal with OH impurities. Substituted atomic site F
a (Å)
b (Å)
c (Å)
a,b,c (°)
EXP No symm. OH (OH)2
10.791 10.891 10.920 11.308
6.607 6.680 6.666 6.662
7.263 7.281 7.298 7.275
90 90 90 90
The values at the second range are precisely those which, given their magnitude, cannot be associated with vibrations of either K, Y or F atoms for the pure crystal, and must therefore correspond to substitutionally introduced impurities. On the other hand, in the obtained theoretical spectrum, shown in Fig. 3, only three peaks can be discerned in the range 100–800 cm 1, whereas those falling within the second range are apparently missing. We thus conclude that, along this theoretical range of wavenumbers, there appear only vibrations that correspond to the constituent elements of the pure crystal and no trace of the OH impurities. In order to theoretically obtain the missing peaks, in view of the above findings, we here propose several models where OH impurities are substitutionally introduced. A band at 3775 and 1650 cm 1 is obtained when an F atomic site is substituted by an OH ion placed in a position where the O atoms are on (and the H atom out of) the symmetry plane (0 0 2), as shown in Fig. 4. The vibrational modes of the OH group give rise to a strong IR spectral line at 3775 and 1650 cm 1, as illustrated in Fig. 5. Although other models can be proposed for the same impurity, as indicated in Table 1, the essential idea is to place the OH ion in a position and associated symmetry leading to the activation of the expected spectral bands. In order to complement the present report we have also obtained the density of states (DOS) of the K2YF5(OH) compound, shown in Fig. 6, where a line is shifted in the forbidden band when comparing the pure crystal K2YF5 (at 6.55 eV) and the one with the impurity (at 5.16 eV), giving rise to the presence of new states above the valence band, beyond the Fermi level (at 0 K). 3. Conclusions The DFT calculations here performed and proposed models theoretically confirm the existence of OH aggregates substituting F atomic sites in the crystal structure of K2YF5, through the appearance of spectral lines in the range 800–4000 cm 1, otherwise missing when obtaining the IR spectrum for the pure crystal compound. However, such lines show up only for a particular symmetry adopted by the OH impurity with respect to the plane (0 0 2) in the crystal.
Fig. 6. Density of states of the K2YF5(OH) crystal compound, where metastable states show up above the valence band.
A more realistic theoretical modeling of the IR spectrum of K2YF5:Tb+3, in comparison to the experimentally known active lines, would probably require a combination of models such as those proposed in Table 1 plus others, taking into account the particular symmetry that the impurities present in the crystal compound. The density of states (DOS) analysis indicates the appearance of new orbitals within the valence band, associated with metastable states resulting from a recombination of electrons released during thermoluminescent emission, as reported in the literature. Acknowledgments This study was made possible due to funding from the Consejo Nacional de Ciencia y Tecnología (CONACyT), as well as resources provided by the Instituto de Física ‘‘Ing. Luis Rivera Terrazas’’, BUAP, its academic group FCMC-SEP-PIFI, and Instituto Tecnológico de Apizaco for support. We thank to the Centro de Cómputo of IFUAP for the provided facilities, and to the Laboratorio de Análisis Instrumental of the Facultad de Ingeniería Química, BUAP, where the IR spectrums were made. We also thank to Prof. N. Khaidukov for give us the materials. References [1] A. Gallegos et al., Rev. Mex. de Fís. S 57 (1) (2011) 65–68. [2] J. Azorín Nieto et al., Radiat. Eff. Defects Solids 161 (8) (2006) 443–449. [3] J. Azorín, A. Gallegos, T. Rivera, J.C. Azorín, N. Khaidukov, Nucl. Instr. Methods Phys. Res. A 580 (2007) 177–179. [4] F. Loncke, D. Zverev, H. Vrielinck, N.M. Khaidukov, P. Matthys, F. Callens, Phys. Rev. B 75 (2007) 144427. [5] D. Zverev, H. Vrielinck, F. Callens, P. Matthys, S. Van Doorslaer, N.M. Khaidukov, Phys. Chem. Chem. Phys. 10 (2008) 1789–1798. [6] D. Zverev, H. Vrielinck, P.F. Smet, D. Poelman, F. Callens, Phys. Rev. B 79 (2009) 224110. [7] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [8] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [9] M.C. Payne, M.P. Teter, D.C. Allan, Rev. Mod. Phys. 64 (1992) 4. [10] E.M. Maddock, R.A. Jackson, M.E.G. Valerio, J. Phys.: Conf. Ser. 249 (2010) 1. [11] Cerius 2 User Guide; Accelrys, Inc., Cerius2 Modeling Environment, Release 4.8, San Diego: Accelrys Software Inc., 2005.