The (twin) composition plane as an extended defect and structure-building entity in crystals

Hyde, B. G.; Andersson, Sten; Bakker, Morten P.; Plug, C.M.; O’Keeffe, Michael

doi:10.1016/0079-6786(79)90002-5

Cited by 128 publications

(46 citation statements)

References 92 publications

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“…1) is closely related to the CaIrO 3 -type structure (32). The space group of the U 2 S 3 -type phase is Pmcn (or Pnma in the standard setting), which is a subgroup of Cmcm, CaIrO 3 -type phase's space group.…”

Section: Resultsmentioning

confidence: 99%

Prediction of an U ₂ S ₃ -type polymorph of Al ₂ O ₃ at 3.7 Mbar

Umemoto

Wentzcovitch

2008

Proc. Natl. Acad. Sci. U.S.A.

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We predict by first principles a phase transition in alumina at Ϸ3.7 Mbar and room temperature from the CaIrO 3-type polymorph to another with the U 2S3-type structure. Because alumina is used as window material in shock-wave experiments, this transformation should be important for the analysis of shock data in this pressure range. Comparison of our results on all high-pressure phases of alumina with shock data suggests the presence of two phase transitions in shock experiments: the corundum to Rh 2O3(II)-type structure and the Rh 2O3(II)-type to CaIrO3-type structure. The transformation to the U 2S3-type polymorph is in the pressure range reached in the mantle of recently discovered terrestrial exoplanets and suggests that the multi-megabar crystal chemistry of planetforming minerals might be related to that of the rare-earth sulfides.alumina ͉ first-principles calculation ͉ high-pressure phase transition ͉ postperovskite ͉ rare-earth sulfide structure A lumina, Al 2 O 3 , is an important compound in high-pressure technology and geophysics. It is used as window material in shock-wave experiments (1). Ruby, Al 2 O 3 doped with chromium, serves as a pressure calibrant in diamond-anvil-cell experiments (2). In the Earth's mantle, Al 2 O 3 is a major chemical component in solid solution with MgSiO 3 garnet, perovskite (PV), and postperovskite (PPV). This formation of solid solutions with alumina changes the properties of the pure phases, such as their equations of state, thermoelastic properties, phase boundaries, electrical conductivity, oxidation, and spin states of iron impurities in MgSiO 3 , etc. (e.g., refs. 3-9).The experimental and theoretical literature on phase transitions in Al 2 O 3 is quite extensive (10-19). Two pressure-induced transitions have been predicted theoretically (10)(11)(12)(16)(17)(18) and confirmed experimentally (13)(14)(15)19): from corundum (space group R3 c), the stable ambient form, to the Rh 2 O 3 (II)-type structure (space group Pbcn) at Ϸ80-100 GPa and then to the CaIrO 3 -type PPV structure (space group Cmcm) at Ϸ130 GPa. These pressures are realized in the Earth's mantle. The CaIrO 3 -type polymorph is the highest-pressure form identified experimentally so far for both Al 2 O 3 and MgSiO 3 . A phase transition in the CaIrO 3 -type phase in Al 2 O 3 or MgSiO 3 should change the properties of their solid solution, a key input in the modeling of planetary interiors. A first-principles study predicted the dissociation of CaIrO 3 -type MgSiO 3 into CsCl-type MgO and cotunnite-type SiO 2 at Ϸ1.1 TPa (20), a pressure relevant for the giant planets and exoplanets. The same type of dissociation was also predicted for NaMgF 3 , a low-pressure analog of MgSiO 3 (21). However, CaIrO 3 -type Al 2 O 3 is expected to undergo a nondissociative post-PPV transition because the equivalent dissociation products (AlO and AlO 2 ) are unlikely to form. Here, we report the first-principles prediction of a phase transition in Al 2 O 3 from the CaIrO 3 -type polymorph to another with the U 2 S 3 -type s...

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Section: Resultsmentioning

confidence: 99%

Prediction of an U ₂ S ₃ -type polymorph of Al ₂ O ₃ at 3.7 Mbar

Umemoto

Wentzcovitch

2008

Proc. Natl. Acad. Sci. U.S.A.

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“…Experimental observations revealed that in steels, the o-form phase occurs more frequently than the h phase 30 and that there are a number of polytypes of Fe 7 C 3 . 27 The formation of polytypes originates from twinning or antiphase ordering of the o structure.…”

Section: Formation Of the Fe 7 C 3 Phasesmentioning

confidence: 99%

“…Calculations have been performed for Fe 3 C cementite as well for the sake of comparison. 30 The relative stability of the two Fe 7 C 3 phases is addressed by means of total-energy calculations with high accuracy and the electronic and magnetic properties are characterized. The information obtained here is helpful to understand the formation and stability of the Fe 7 C 3 phases and contributes to a complete understanding of the structural, electronic, and magnetic properties that are of interest to industrial applications.…”

Section: Introductionmentioning

confidence: 99%

Structural, electronic, and magnetic properties of iron carbideFe7C3phases from first-principles theory

2009

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The iron carbide Fe 7 C 3 exhibits two types of basic crystal structures, an orthorhombic ͑o-͒ form and a hexagonal ͑h-͒ one. First-principles calculations have been performed for the basic Fe 7 C 3 forms and for the related -Fe 3 C cementite phase. Accurate total-energy calculations show that the stability of Fe 7 C 3 is comparable to that of -Fe 3 C. The o-Fe 7 C 3 phase is more stable than the hexagonal one, in contrast to recent atomistic simulations. Furthermore, the calculations also show a rather low energy for a carbon vacancy in the o structure, which implies possible C deficiency in the lattice. Both Fe 7 C 3 phases are ferromagnetic metals. Electronic band-structure calculations show that all Fe atoms exhibit high-spin states with the majority of their 3d states being almost fully occupied. From an analysis of the structural and energetic properties, the formation of the o phase in steel treatment processes and of h form in carburization of ferrite is discussed.

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“…1 and significant interatomic distances are presented in Table 2. The structures of rare-earth metal monosilicides and their relation to other AB-type structures have been thoroughly discussed (Hohnke & Parthr, 1966;Raman & Steinfink, 1967;Raman, 1968;Parthr, 1981) as well as their derivation from an h.c.p, structure by periodic unit,cell twinning ('chemical twinning') (Andersson & Hyde, 1974;Hyde, Andersson, Baker, Plug & O'Keefe, 1979). Gd---Gd:…”

Section: Structure Of Gadolinium Monosilicidementioning

confidence: 99%