Formation of titanium-solute clusters in alumina: A first-principles study

Matsunaga, Katsuyuki; Mizoguchi, Teruyasu; Nakamura, Atsutomo; Yamamoto, Takahisa; Ikuhara, Yuichi

doi:10.1063/1.1760598

Cited by 20 publications

(14 citation statements)

References 14 publications

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“…We connect this discrepancy with the following. In [15] the band structure was calculated for the Ti 3+ Ti 3+ pair oriented along the [0001] axis while another orientation of that pair was implied in our calculations. The presence of Ti 3+ − Ti 4+ pairs ( Fig.…”

Section: Ti Impurity Levels In the Band Structurementioning

confidence: 99%

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Defect complexes in Ti-doped sapphire: A first principles study

Kravchenko

Fil

2018

Journal of Applied Physics

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First-principles calculations have been performed to study the formation of defect complexes in Ti doped α-Al2O3 crystals. The formation energies of isolated Ti 3+ and Ti 4+ defects, pairs, triples and quadruples of Ti ions and Al vacancies are computed under different equilibrium conditions of Al-Ti-O related phases. Taking into account charge neutrality of the whole system we determine the equilibrium concentrations of simple and complex defects as well as the total equilibrium concentration of Ti in an α-Al2O3 crystal. It is shown that the equilibrium concentration of complex defects can be on the same order of or even larger than the concentrations of isolated substitutional Ti 3+ and Ti 4+ defects. It is found that in Ti-deficient conditions the relative fraction of isolated defects increases and the balance is shifted towards Ti 4+ defects. A universal relation between equilibrium concentrations of isolated and complex defects is obtained. The band structure of the system with complex defects is calculated and extra levels inside the band gap caused by such defects are found.

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Section: Ti Impurity Levels In the Band Structurementioning

confidence: 99%

“…In [15] the formation of Ti clusters in Ti-doped Al 2 O 3 was investigated. It was shown that Ti 3+ clusters have a positive binding energy.…”

Section: Introductionmentioning

confidence: 99%

Defect complexes in Ti-doped sapphire: A first principles study

Kravchenko

Fil

2018

Journal of Applied Physics

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“…Defect energetics in Ti:sapphire were investigated within the DFT approach in [14,15]. It was shown that in the oxidized conditions Ti 4+ ions substituted for Al 3+ ions together with charge compensating vacancies of Al (V 3− Al ) are the most stable defects.…”

Section: Introductionmentioning

confidence: 99%

Control of charge state of dopants in insulating crystals: Case study of Ti-doped sapphire

Kravchenko

Fil

2020

Phys. Rev. Research

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We study mechanisms of control of charge state and concentration of different point defects in doped insulating crystals. The approach is based on the density functional theory calculations. We apply it to the problem of obtaining of Ti-doped sapphire crystals with high figure-of-merit (FOM). The FOM of a given sample is defined as the ratio of the coefficient of absorption at the pump frequency to the coefficient of absorption at the working frequency of Ti:sapphire laser. It is one of standard specifications of commercial Ti:sapphire laser crystals. It is believed that FOM is proportional to the ratio of the concentration of isolated Ti 3+ ions to the concentration of Ti 3+-Ti 4+ pairs. We find that generally this ratio is in inverse proportion to the concentration of Ti 4+ isolated substitutional defects with the coefficient of proportionality that depends on the temperature at which the thermodynamically equilibrium concentration of defects is reached. We argue that in certain cases the inverse proportion between concentrations of Ti 3+-Ti 4+ and Ti 4+ may be violated. The role of codoping in the control of the charge state of dopants is analyzed. We show that codopants that form positively (negatively) charged defects may decrease (increase) the concentration of positively charged defects formed by the main dopants. To evaluate the effect of codoping it is important to take into account not only isolated defects but defect complexes formed by codopants, as well. In particular, we show that codoping of Ti:sapphire with nitrogen results in an essential increase of the concentration of Ti 4+ and in a decrease of the FOM, and, consequently, growth or annealing in the presence of nitrogen or its compounds is unfavorable for producing Ti:sapphire laser crystals. The approach developed can be used for determining appropriate growth and annealing conditions for obtaining doped crystals with the required characteristics.

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“…24) Supercells containing a Zr impurity and an Al vacancy at different positions were calculated. The binding energy between the Zr impurity and the Al vacancy was estimated by the same method as a previous study 25) and the resulting binding energies were compared. Figure 3 shows the results of the calculated binding energies of the Al vacancy as a function of distance from the Zr site and the positional relationship between Zr and Al vacancy.…”

Section: Methodsmentioning

confidence: 99%

Zr segregation and associated Al vacancies in alumina grain boundaries

Buban

Mizoguchi

Shibata

et al. 2011

J. Ceram. Soc. Japan

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Segregated dopants at grain boundary dramatically change the material's mechanical and electrical properties. The knowledge of the behavior of these dopants are therefore key for further material developments. Here, a Zr doped alumina (Al 2 O 3 ) bicrystal was fabricated and the atomic structure of the grain boundary was characterized using scanning transmission electron microscopy (STEM). Zr was found to segregate to specific sites along the grain boundary, which was neighbored by Al columns of weak image intensity. First principles calculations and static lattice calculations revealed that the charge compensating Al vacancies are most stable in Al columns neighboring the location of the Zr sites. Multi-slice simulations of STEM images confirmed that Al columns containing vacancies exhibit a reduction in image intensity. These results indicate that associated Al vacancies strongly influence its atomic site segregation behavior of Zr.

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Formation of titanium-solute clusters in alumina: A first-principles study

Cited by 20 publications

References 14 publications

Defect complexes in Ti-doped sapphire: A first principles study

Defect complexes in Ti-doped sapphire: A first principles study

Control of charge state of dopants in insulating crystals: Case study of Ti-doped sapphire

Zr segregation and associated Al vacancies in alumina grain boundaries

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