Antistructure Pairs Research Articles

Polymorphism and high-temperature conductivity of Ln 2M 2O 7 (Ln = Sm ─Lu; M = Ti, Zr, Hf) pyrochlores

We present a comparative analysis of the order ─disorder transitions in Ln 2(M 2 ─ x Ln x )O 7 ─ δ (Ln = Sm ─Lu; M = Ti, Zr, Hf; x = 0, 0.096) pyrochlore-like compounds and solid solutions existing in the Ln 2O 3 ─MO 2 systems. In the range ~ 600 ─1200 °C, Ln 2Ti 2O 7 (Ln = Sm ─Lu) and Ln 2Zr 2O 7 (Ln = Sm ─Gd) undergo ordering transitions, F⁎ → PI → P, which culminate in the formation of an ideal pyrochlore structure, P, existing between 1100 and 1300 °C. Above 1300 °C, Ln 2Ti 2O 7 (Ln = Gd ─Lu), Ln 2Zr 2O 7 (Ln = Sm ─Gd) and Ln 2Hf 2O 7 (Ln = Eu ─Tb) exist as oxygen-ion-conducting phases, PII, disordered in both the oxygen and cation sublattices. Ionic conductivity data for Ln 2(M 2 ─ x Ln x )O 7 ─ δ (Ln = Sm ─Lu; M = Ti, Zr, Hf; x = 0, 0.096) synthesized at 1600-1670 °C indicate that the highest conductivity in these systems is typically offered by nominally stoichiometric (Ln:M = 1:1), disordered Ln 2M 2O 7 (Ln = Sm ─Lu; M = Ti, Zr, Hf) pyrochlores containing anti-structure pairs (Ln M ' + M Ln •) and oxygen vacancies ( V O ••) on the 48 f (O2) site. The highest conductivity of Yb 2Ti 2O 7, in which the cations have the smallest radii among the lanthanides and Group IVa metals, seems to be due to the increased role of the geometric factor in the Ln 2Ti 2O 7 (Ln = Sm-Lu) pyrochlores with predominantly covalent metal ─oxygen bonding M-O (Ti-O). The ion transport parameters in these materials are determined primarily by the relationship between the sizes of the mobile oxygen ions and conduction channels.

Effect of non-stoichiometry and synthesis temperature on the structure and conductivity of Ln 2+ xM 2− xO 7− x/2 (Ln = Sm–Gd; M = Zr, Hf; x = 0–0.286)

The effect of non-stoichiometry on the crystal structure and total conductivity of Ln 2+ x Zr 2− x O 7− x/2 (Ln = Sm–Gd; x = 0–0.286) was investigated. The intensity of the pyrochlore superlattice reflections from Ln 2+ x Zr 2− x O 7− x/2 is shown to decrease with increasing Ln concentration. Within the homogeneity range of the pyrochlore phase of Ln 2+ x Zr 2− x O 7− x/2 (Ln = Sm–Gd), the activation energy of high-temperature conduction in samples prepared by 1600 °C sintering of mechanically activated oxide mixtures (Ln 2O 3 and ZrO 2) is ∼ 0.87–1.04 eV. The highest conductivity among the Ln 2+ x Zr 2− x O 7− x/2 (Ln = Sm–Gd) materials is offered by stoichiometric Ln 2Zr 2O 7 samples with a pyrochlore structure, which contain 5–8.1% Ln Zr + Zr Ln anti-structure pairs, except for Gd 2Zr 2O 7 (∼ 22%). The crystal structure of Ln 2+ x Hf 2− x O 7− x/2 (Ln = Sm–Gd) is investigated after sintering at 1000–1670 °C. The compounds Ln 2.096Hf 1.904O 6.9 (Ln = Eu, Gd) prepared by 1200 °C sintering of mechanically activated oxides (Ln 2O 3 and HfO 2) undergo a fluorite-type to pyrochlore phase transition above 1200 °C. The conductivity of Gd 2Hf 2O 7 and Sm 2.096Hf 1.904O 6.952 sintered at 1600 °C seems to be ionic above 780 °C, with an activation energy of 0.77 and 0.82 eV, respectively. In this work, using mechanical activation of starting mixtures, the conductivity of the Ln 2+ x Hf 2− x O 7− x/2 (Ln = Sm–Gd) hafnates was raised close to the level of Ln 2+ x Zr 2− x O 7− x/2 (Ln = Sm–Gd). The hafnates synthesized by the procedure in question are similar in structural disorder to Ln 2+ x Zr 2− x O 7− x/2 (Ln = Sm–Gd), and the disorder ensures high oxygen ion mobility and, accordingly, significant high-temperature conductivity.

Defect identification in theAsGafamily in GaAs

Ab initio total-energy calculations are presented for intrinsic defects in GaAs with a particular emphasis on hyperfine interactions in order to clarify the atomic structure of the various ${\mathrm{As}}_{\mathrm{Ga}}$-related defects. For the ${\mathrm{As}}_{\mathrm{Ga}}\text{\ensuremath{-}}{X}_{2}$ defect complex the interpretation as an ${\mathrm{As}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{V}}_{\mathrm{As}}$ antisite-vacancy pair as was considered so far is challenged. An ${\mathrm{As}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{Ga}}_{\mathrm{As}}$ antistructure pair is the most likely identification. It is also unlikely that the ${\mathrm{As}}_{\mathrm{Ga}}\text{\ensuremath{-}}{X}_{1}$ defect can be identified as a distant antistructure pair as was considered from magnetic resonance experiments. The theoretical results obtained for the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ point defect agree with the experimental data reported for the defect identified as isolated ${\mathrm{As}}_{\mathrm{Ga}}$ and, with the exception of a small broadening of the nearest-neighbor lines and of a moderate splitting in the fifth shell, for the EL2 as well. We speculate that at room temperature the EL2 will be an isolated ${\mathrm{As}}_{\mathrm{Ga}}$ defect which lowers its symmetry attracting some other mobile defect at the low temperatures required to perform magnetic resonance experiments. We have calculated the binding energies of antisites bound to a distant shallow acceptor and the influence of the pairing on the hyperfine interactions. We show that this mechanism could explain the broadening of the nearest-neighbor lines but not the splitting in the fifth shell.

Ionic Conductivity of Ln2 + x Zr2 − x O7 − x/2 (Ln = Sm-Gd) Solid Solutions

The electrical conductivity of Ln2 + xZr2 − xO7 − x/2 (Ln = Sm-Gd) solid solutions prepared from mechanically activated Ln2O3 and ZrO2 is shown to correlate with their structural properties. In the three systems, the x-T regions are determined in which electrical transport is dominated by oxygen-ion conduction. In the Sm2O3-ZrO2 system, ionic conductivities from 5 × 10−4 to 6 × 10−3 S/cm at 740°C are found in Sm2 + xZr2 − xO7 − x/2 with 26.6, 33.3, 35.5, 37, and 40 mol % Sm2O3 prepared at 1450, 1530, and 1600°C. Eu2 + xZr2 − xO7 − x/2 and Gd2 + xZr2 − xO7 − x/2 containing 33.3 to 37 mol % Ln2O3 have 740°C ionic conductivities of 10−3 to ∼7.5 × 10−3 and 10−3 to 7 × 10−3 S/cm, respectively. The activation energy of conduction in Ln2 + xZr2 − xO7 − x/2 (Ln = Sm-Gd), Ea = 0.84–1.04 eV, increases with the atomic number of Ln and x. The highest ionic conductivity is offered by the stoichiometric Ln2Zr2O7 (Ln = Sm-Gd) pyrochlores prepared at 1600°C, owing to the optimal concentration of LnZr + ZrLn antistructure pairs (∼5–22%). The grains in the ceramic samples studied range in size from 0.5 to 2 µm.

Effects of the synthesis procedure, doping and non-stoichiometry on the order–disorder transformation in Ln 2Ti 2O 7 (Ln = Tm–Lu) oxygen-ion conductors

The ionic conductivity of the Ln 2Ti 2O 7 (Ln = Tm–Lu) pyrochlores is found to correlate with the degree of anti-site cation disordering in these materials. The highest ionic conductivity is exhibited by Ln 2Ti 2O 7 containing 3.53–4.5% Ln Ti + Ti Ln anti-structure pairs. This degree of cation disordering can be achieved by heat treatment just below the melting point of the material, by mechanical activation with subsequent high-temperature firing and by doping with small amounts of cations of different sizes (as, e.g., in the case of (Yb 0.91Sc 0.09) 2Ti 2O 7). The Ln Ti + Ti Ln anti-structure pairs seem to interact with oxygen vacancies. The defect structure of Lu 2+ x Ti 2− x O 7− x/2 (34.5–44 mol% Lu 2O 3) oxygen-ion conductors is investigated. The materials with x = 0.052 (34.5 mol% Lu 2O 3); 0.096 (35.5 mol% Lu 2O 3); 0.286 (40 mol% Lu 2O 3) contain the whole one type of anti-site defect, Lu Ti, and possess an ionic conductivity of about 10 − 3 S/cm at 740 °C. The conductivity of the materials containing Lu Ti defects is lower than that of the stoichiometric materials containing Lu Ti + Ti Lu anti-structure pairs. A strong correlation is found between the variable positional parameter X of O(2) (position 48f) in the structure of Lu 2+ x Ti 2− x O 7− x/2 ( x = 0, 0.052, 0.096, 0.286) nanoceramics and the I 311/ I 222 and I 331/ I 222 intensity ratios in their X-ray diffraction patterns.

Influence of structural defects on the electrical conductivity of (Yb1 − x Scx)2Ti2O7 (x=0, 0.09, 0.3)

The ionic conductivity of Yb2Ti2O7 and (Yb0.91Sc0.09)2Ti2O7 is shown to correlate with the degree of antisite cation disordering in these materials. The highest ionic conductivity is exhibited by materials containing 3.53–4.5% YbTi + TiYb antistructure pairs. This degree of cation disordering can be achieved by heat treatment just below the melting point of the material, by mechanical activation with subsequent high-temperature firing, and by doping on the Yb or Ti site. The YbTi + TiYb antistructure pairs and the associated Frenkel defects in the oxygen sublattice are responsible for oxygen ionic conduction.

Order–Disorder Transformations in Ln2Ti2O7(Ln = Lu, Yb, Tm, Gd)

The ordering processes in the rare-earth titanates Ln2Ti2O7 with Ln = Lu, Yb, Tm, and Gd are studied by x-ray diffraction, thermal analysis, IR spectroscopy, electron microscopy, and electrical conductivity measurements. The compounds are prepared via hydroxide coprecipitation, followed by freeze-drying and heat treatment in the temperature range 350–1700°C. The compounds Ln2Ti2O7 with Ln = Lu, Yb, and Tm are found to have the fluorite structure between 600 and 800°C. Above 800°C, they undergo a fluorite-to-pyrochlore transformation, with cation disordering and the formation of LnTi + TiLn antistructure pairs. Gd2Ti2O7 has the pyrochlore structure over the entire temperature range studied and contains no antistructure defects. In contrast to Gd2Ti2O7 , the compounds Ln2Ti2O7 with Ln = Lu, Yb, and Tm undergo a high-temperature pyrochlore-to-fluorite phase transition around 1700°C. The 750°C conductivity of Ln2Ti2O7 (Ln = Lu, Yb, Tm) samples sintered at 1700°C is 5 × 10–3 to 10–2 S/cm, which is two orders of magnitude higher than that of ceramics of the same composition prepared at lower temperatures (950 or 1400°C). The conductivity of the Gd2Ti2O7 ceramic prepared at 1500°C is two orders of magnitude lower than that of Ln2Ti2O7 with Ln = Lu, Yb, and Tm.

Defect identification for the AsGa family

The AsGa family consists of at least four distinctly different point defects including the technologically important EL2 defect. While the different members are easily distinguished from their MCDA spectra, the differences of the hf and shf interactions as derived from ODEPR and ODENDOR are rather small.We present ab initio calculations using the LMTO–ASA Green's function method for a variety of defect models that might be relevant for the identification of AsGa-related defects. We confirm the identification of the isolated AsGa and show that the {AsGa–X2} defect must be identified with the nearest-neighbor antistructure pair rather than with the {AsGa–VAs} pair. For the {AsGa–X1} defect a distant antistructure pair is a likely candidate. For the EL2, the most important member of the AsGa family, we have not found a conclusive defect model. The recent ODENDOR data are similar to those of the distant orthorhombic {AsGa–VGa} pair, which, however is a triple acceptor and not a donor.

Electronic structure of the anti-structure pair in 3C–SiC

We have studied the anti-structure pair (adjacent carbon C Si and silicon Si C antisites) in 3C–SiC using the plane–wave pseudopotential method. We report results for the formation energies, the ionization levels and the geometry of the relaxed structures of the defect in all its possible charge states. The calculated properties are compared with the pseudopotential results for the isolated antisites C Si and Si C. It is found that the defect complex is bound but exhibits otherwise similar behavior as isolated antisites. The electronic structure of the anti-structure pair is discussed in view of experimental data.

Concerted-exchange mechanism for antistructure pair defects in GaAs

We show via first principle calculations why the antistructure-pair (Ga As–AS Ga) in the nearest neighbor (nn) configuration is not experimentally detected. A low energy barrier for the concerted exchange mechanism is seen to be responsible for the recovery of the ideal crystal structure. This explains the fact that, although energetically more favorable when compared with the next nearest neighbor (nnn) case, the nn antistructure-pair is not observed at room temperature.

Theoretical study of defect complexes related with antisites in GaAs

An ab initio calculation based on pseudopotential density-functional theory is used. A supercell with 128 atoms is adopted in Car-Parrinello scheme. We study the antistructure pairs (GaAS-AsGa, both nearest neighbor and next-nearest-neighbor cases, and the vacancy antisite interstitials (VGa + GaAs + Asi), both nearest-interstitial and next-nearest-interstitial cases. For the nearest-neighbor antistructure pair a metastable configuration is found. For the vacancy antisite interstitial defects it is observed that the nearest-interstitial case is spontaneously recombined whereas the next nearest-interstitial case is stable with the final electronic structure similar to the combination of isolated Asi plus VGa + GaAs defects.

Metastability of the antistructure pair in GaAs

We have studied the metastability of the antistructure (arsenic-antisite gallium-antisite) pair in GaAs using self-consistent, parameter-free total energy methods. Our calculations predict that this defect complex exhibits metastability similar to that of the isolated arsenic antisite. However, the antistructure pair has ionization levels in the band gap in the metastable configuration, unlike the isolated arsenic antisite. The ionization levels enable absorption of infrared light in the metastable state. The results are used to discuss and interpret the arsenic-antisite-type defects observed experimentally in electron-irradiated GaAs.

Optically detected magnetic resonance investigation of a Gallium vacancy-related defect in electron-irradiated Gallium arsenide

In different types of GaAs (p-type and semi-insulating) electron-irradiated at 4.2 K and kept below 80 K, a dominant defect with a quartet hyperfine split spectrum was observed with optically detected electron paramagnetic resonance. The defect is attributed to a Gallium vacancy-related defect. It decays at about room temperature while an As antisite-related defect, identified previously as an anti-structure pair, As Ga with Ga As as a second nearest neighbour (nnn), is formed. The spectrum of the magnetic circular dichroism of the Ga vacancy-related defect is interpreted to originate from photo-ionisation transitions to the valence band.

Metastability of arsenic antisite-related defects created by electron irradiation in gallium arsenide

By irradiation of semi-insulating GaAs with high energy electrons at low temperatures three As antisite-related defects are produced, one of which is the well-known EL2 defect. We performed magneto-optical measurements to investigate the metastability properties of the other two antisite-related defects, in particular those of an anti-structure pair which consists of an As antisite defect with a Ga antisite defect in the next nearest neighbour position. All three As antisite-related defects show metastability. Because of striking similarities of the metastability between all As antisite-related defects, we conclude that the metastability must be caused by the same microscopic process, presumably determined by the antisite atom.

Enthalpy of formation of antisite defects and antistructure pairs in III-V compound semiconductors

A formulation for estimating the enthalpy of formation of neutral, isolated antisite defects in III-V compound semiconductors is presented, in which it is assumed that the formation enthalpy is comprised of disorder and electronic contributions. The disorder contribution was previously calculated by Van Vechten [J. Electrochem. Soc. 122, 423 (1975); Handbook on Semiconductors, edited by S. P. Keller (North-Holland, Amsterdam, 1989), Vol. 3, Chap. 1]. The electronic contribution is equal to the difference of the donor (acceptor) levels with the valence-band maximum (conduction-band minimum). The enthalpy of formation of neutral, singly, and doubly ionized, isolated antisites is estimated for GaAs and InP. The enthalpy of formation of neutral antistructure pairs in wide band-gap III-V semiconductors is calculated assuming that the binding energy is given by the electrostatic energy of a doubly ionized nearest-neighbor pair of antisites separated by a normal lattice site. The enthalpy of formation of neutral antistructure pairs in the narrow band-gap III-V semiconductors InAs and InSb is estimated assuming the binding energy is twice the band-gap energy.

Optical and ODMR investigation of anti-site defects in GaP

In p-type GaP:Zn the two luminescence bands at 0.95 and 1.20 eV can be excited with light below the gap energy. They are due to two independent donor-acceptor recombinations. In the 0.95 eV emission the donor is the doublet PP4 anti-site as seen from ODMR. The excitation spectrum of this emission follows the density of valence band states of T2 symmetry with a threshold energy of 1.25 eV. In the 1.2 eV emission the donor is a triplet PP3YP anti-site, the acceptor an impurity with S>1/2 bound to the PP3YP, probably at an interstitial site. A likely acceptor could be Fe3+, which is present as an impurity in the crystal. In the ground state PP3YP is a spin doublet and the proposed acceptor would then be Fe12+. The authors suggest that YP is Ga on a nearest P site (GaP) forming a (PGaGaP) anti-structure pair.

Deviations from the one-defect approximation: antistructure in III-V compounds

In III-V semiconductors a common mode of vacancy migration is by nearest-neighbour hopping. In the presence of Zn diffusion this mode is greatly enhanced. A consequence is the production of antisite defects, which usually become antistructure pairs as the vacancy returns to its original sublattice. Many important deep-level defects involve vacancies during their creation. One should expect to find neutral antistructure pairs in the vicinity of Zn-O luminescence centres, EL2 defects, etc. Much of the mystery of these centres can be attributed to the dipolar interaction of these antistructure pairs with the nominal defects.

Simple Theoretical Estimates of the Enthalpy of Antistructure Pair Formation and Virtual‐Enthalpies of Isolated Antisite Defects in Zinc‐Blende and Wurtzite Type Semiconductors

The few theoretical treatments which have been successful in predicting the thermodynamic parameters of substitutional impurities are examined for their implications regarding antisite defects, or , and antistructure pairs, in AB semiconductors. All such theories predict a large concentration of , especially in III–V 's. All predict that or and or should be important to the properties of III–V 's. Results of the Phillips‐Van Vechten dielectric two‐band model are presented for 23 common semiconductors. These are combined with results for vacancies in the accompanying paper to give a detailed account of native defects in both during crystal growth and as the sample cools. The existence curve is calculated and compared with experiment. The interaction between vacancies and antisite defects is invoked to explain why deviations from stoichiometry have slight effects on carrier concentration but large effects on luminescent efficiency.