Defect Energetics and Nonstoichiometry in Lanthanum Magnesium Hexaaluminate

Park, Jae–Gwan; Cormack, Alastair N.

doi:10.1006/jssc.1996.7141

Cited by 30 publications

(23 citation statements)

References 34 publications

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“…Based on modeling 27 and qualitative arguments, 31 the magnesium ions are believed to occupy the Al͑2͒ sites. But there are four Al͑2͒ sites: Site 1 1/3 1/3 z Site 2 2/3 1/3 z + 1/2 Site 3 2/3 1/3 −z Site 4 1/3 2/3 −z + 1/2 with z = .024.…”

Section: Resultsmentioning

confidence: 99%

Theoretical Investigation of Structure and Stability of Reidinger Defects in Barium Magnesium Aluminate

Mishra

2005

J. Electrochem. Soc.

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An atomistic simulation method has been used to study the equilibrated geometry, defect structure, and phase mixing of barium magnesium aluminate ͑BAM͒ and related compounds. The calculated lattice energies are used to study the stability of defect structures in the ␤-alumina lattice, particularly in the presence of excess alumina in the starting materials. It is shown that excess alumina in the starting compounds could lead to the formation of Reidinger defects in BAM. The calculated structural modification of the lattice due to a Reidinger defect is in excellent agreement with the experimental results. These defects are responsible for generation of F + and F-type color centers by ͑vacuum͒ ultraviolet radiation and bombardment of energetic particles from the discharge. BAM could be made resistant to the formation of color centers by adhering to a strictly stoichiometric formulation of this phosphor.Barium magnesium aluminate ͑BaMgAl 10 O 17 ͒ activated by divalent europium ions is a blue-emitting phosphor. This phosphor is popularly known as BAM in lighting applications. It is used as a blue-emitting phosphor in triband phosphor coatings in low-pressure Hg-discharge lamps. Unlike the other components of triband phosphors, namely, the red-emitting yttrium oxide phosphor activated by trivalent europium, and the green-emitting lanthanum phosphate phosphor activated by terbium, BAM is relatively unstable during high-temperature processing ͑ϳ400°C͒ of certain fluorescent lamps in an oxidizing environment. 1,2 Similar processing conditions are also encountered in plasma display panel applications with similar results. BAM also tends to degrade faster during the lamp life under high loading conditions, a leading to "unacceptable" color shifts during the lamp life. The last few years have seen intense activities in the luminescent community toward developing an understanding of the nature of the thermal degradation process. It is believed that the thermal degradation of BAM results from the oxidation of europium ions. 3 However, similar efforts to understand the nature of the degradation process during the lamp life are still lacking. This paper presents the results of a theoretical investigation of the structure and origin of defects that could contribute to the generation of color centers during the lamp life.One of the reasons for phosphor degradation in the lamp environment is the generation and growth of color centers in phosphors due to lamp operation. Inside the fluorescent lamps, the phosphors exist in a very harsh environment of an arc discharge. During the lamp operation, the surfaces of microcrystalline phosphor particles are continuously subjected to bombardment of energetic electrons, ions, and neutral atoms and irradiation by vacuum ultraviolet ͑VUV͒ photons. The color centers are induced by these discharge-related processes.The lamp phosphors are usually made from complex inorganic hosts. The color center literature is sparse on the investigation of such systems. The haloapatite phosphors are the only cl...

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Section: Resultsmentioning

confidence: 99%

Theoretical Investigation of Structure and Stability of Reidinger Defects in Barium Magnesium Aluminate

Mishra

2005

J. Electrochem. Soc.

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“…LHA is a defective and nonstoichiometric compound, [9] therefore with increase in the LHA content the defect concentration is significantly increasing in the composite ceramics. The net result is that in microwave electric fields charged defects can be driven away from, or pulled toward a surface, producing a near-surface depletion or accumulation.…”

Section: Discussionmentioning

confidence: 99%

“…This arises because its molecular stoichiometry, LaAl 11 O 18 , is incompatible with the stoichiometries of its putative parent structures, MAl 11 AlO 3 ] À , respectively, in which the stabilizing cations (M þ or M 2þ ) reside. [8] Park and Cormack [9] by using computerbased atomistic simulation techniques, elucidated that for the stoichiometric LHA, the mixed b/MP-type phase would be most likely. The energetically most favored structure of this phase has ideal b-type and MP-type mirror planes alternating in the c direction, with aluminum vacancies (one per the basic cell) at the central spinel block to provide the electroneutrality.…”

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confidence: 99%

Microwave Hybrid Post‐Heat Treatment of Reaction Sintered Alumina/Lanthanum Hexaaluminate Composite Ceramics

Negahdari

Willert‐Porada

2010

Adv Eng Mater

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Lanthanum hexaaluminate (LHA), with an ideal stoichiometry LaAl 11 O 18 is a candidate coating material for fibermatrix interface adjustment in fiber reinforced ceramic composites as well as an in situ platelet reinforcement for dispersion reinforced composites. [1,2] Fields of application are, e.g., thermal barrier coatings and combustion catalysts. [3,4] LHA is one of the main compounds which is normally formed in addition to the lanthanum aluminate, LaAlO 3 , in the La 2 O 3 -Al 2 O 3 system studied by Bondar and Vinogradova. [5] The reaction sequence for formation of the LHA phase requires formation of LaAlO 3 at lower temperature, been followed by a solid-state reaction between LaAlO 3 and Al 2 O 3 at higher temperatures. [5] According to thermodynamic calculations for the stoichiometric composition, initial temperature of the solid-state reaction between LaAlO 3 and Al 2 O 3 is 1170 8C. [6] Despite the low thermodynamically calculated temperature, the formation of LaAl 11 O 18 in comparison with many solid-state reactions is extremely slow and requires higher temperatures and hours of dwelling. Ropp and Carroll have proposed that the rate-limiting step in formation kinetics is conversion of O 2 to 2O 2À at the phase boundary of LHA that accompanies La 3þ diffusion into A1 2 O 3 building blocks. [7] In addition to its importance as an engineering material, LHA is of intrinsic scientific interest because of its inherent defective structure. This arises because its molecular stoichiometry, LaAl 11 O 18 , is incompatible with the stoichiometries of its putative parent structures, MAl 11 O 17 (b-alumina) and MAl 12 O 19 (magnetoplumbite). The structures of both of these compounds consist of spinel-like blocks [Al 11 O 16 ] þ separated by mirror planes containing the large cations [M þ O] À and [M 2þ AlO 3 ] À , respectively, in which the stabilizing cations (M þ or M 2þ ) reside. [8] Park and Cormack [9] by using computerbased atomistic simulation techniques, elucidated that for the stoichiometric LHA, the mixed b/MP-type phase would be most likely. The energetically most favored structure of this phase has ideal b-type and MP-type mirror planes alternating in the c direction, with aluminum vacancies (one per the basic cell) at the central spinel block to provide the electroneutrality. However, they discussed that these structures are less stable than the nonstoichiometric ones and LHA will not form an ideal stoichiometric phase, but most likely an MP-type nonstoichiometric phase La 0.83 Al 11.83 O 19 (Al/La ¼ 14.2). Their calculations also revealed that the nonstoichiometric LHA contains defect complex composed of a vacancy pair (V La and V Al ) on the mirror plane, and a pair of Al Frenkel defects (V Al and Al i ) formed doubly above and below the center of the vacancy pair.Two aspects in LHA ceramics motivated utilization of microwave heating in reaction sintering of alumina/LHA ceramics: (i) the solid-state reaction of LHA formation can be enhanced by an additional driving force for mass transport while heat...

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“…Substitution of divalent Mn and Mg for trivalent Al could effectively reduce the defectivities of MP structure via a charge compensation mechanism [5], so that the stability of MP structure could be enhanced greatly. According to literatures [21][22][23][24], Mg 2+ enters the structure in tetrahedral sites; and Mn preferentially enters the structure as Mn 2+ in tetrahedral holes and as Mn 3+ in octahedral sites for hexaaluminates. In the presence of Mg and Mn, it is not necessary for bivalence Mn to stabilize MP structure.…”

Section: Influence Of Mn and Mg Introduction On The Structure Stabilimentioning

confidence: 99%

Crystal structure stability and catalytic activity of magnetoplumbite (MP) catalyst doped with Mn and Mg

Teng

Man

Liang

et al. 2007

Journal of Non-Crystalline Solids

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Defect Energetics and Nonstoichiometry in Lanthanum Magnesium Hexaaluminate

Cited by 30 publications

References 34 publications

Theoretical Investigation of Structure and Stability of Reidinger Defects in Barium Magnesium Aluminate

Theoretical Investigation of Structure and Stability of Reidinger Defects in Barium Magnesium Aluminate

Microwave Hybrid Post‐Heat Treatment of Reaction Sintered Alumina/Lanthanum Hexaaluminate Composite Ceramics

Crystal structure stability and catalytic activity of magnetoplumbite (MP) catalyst doped with Mn and Mg

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