Jeannine Thery

Structural and Optical Properties of Calcium Neodymium Hexaaluminates Single Crystals, Potential Laser Materials

Abstract The structural and optical properties of calcium-neodymium hexaaluminates crystals Ca1−xNdxMgx Al12−xO19 (labeled Ca1−xNdx) with a magnetoplumbite (MP) structure are investigated. The floating zone method is used to grow single crystals in the composition range 0.1 ≤ x ≤ 0.7, although for high calcium content, the melting of the compounds is no longer congruent. The X-ray structural determination, optical absorption at 4 K, and ESR investigation agree in the localization of Nd3+ at the regular large cations site of the MP structure with axial (D3h) symmetry. A set of crystal field and free ion parameters which fits the absorption spectrum of Nd3+ in this site is calculated. When x increases, Nd3+ ions tend to occupy also a second site with lower symmetry. Moreover some anomalous behavior observed in the absorption and ESR spectra at high neodymium concentration may be related to Nd3+-Nd3+ ion pairing. Fluorescence intensity and lifetime measurements as a function of the x value are reported. There is evidence of strong cross-relaxation between neighboring neodymium ions for high x values. The results obtained for the Ca1−xNdx compounds can be extended to other series in which Nd3+ is replaced by another lanthanide ion. Preliminary investigations have been performed with Pr3+ and are also reported.

BaMg6Ti6O19, a compound with magnetoplumbite structure: Investigation of its properties and X ray induced defects

Abstract Solid state reaction between oxides in the ternary phase diagram BaOMgOTiO 2 leads to the synthesis of a compound with hexagonal structure and real composition Ba 0.96 Mg 6.12 Ti 5.88 O 18.84 . This compound has a c/a ratio equal to 3.936. X ray irradiation creates V- centers in it, and moreover, attemps to replace Ba 2+ by Ce 3+ or Cu 2+ by ionic exchange are unsuccessful. This indicates that the structure is of the Magnetoplumbite type rather than of the β alumina one. Small single crystals have been grown from a BaF 2 flux. This material could be interesting as a matrix to accomodate di- or tetravalent luminescent ions, or - owing to its large unit cell parameters - as substrate for epitaxial growth of hexagonal ferrite films.

Synthesis characterization and spectroscopic investigations of mixed barium lanthanides (La, Nd) hexaaluminates, with a structure related to magnetoplumbite and β alumina

The mixed compounds barium-lanthanum aluminate (BLA), and barium-neodymium aluminate (BNA), as well as their solid solutions (BLnA) have been synthetized either by solid state reactions at high temperatures or by the melting of mixtures with the following starting compositions: x2BaCO3 - x6Ln2O3 - 6 Al2O3 (1.2 ⩽ x < 1.6). In both cases the powder x-ray diffraction patterns are very similar to those of magnetoplumbite (MP) or β-alumina (β). Furthermore the new compounds present 001 (1 = 2n + 1) reflections which are forbidden in the MP and β compounds space group. By solid state reactions, pure compounds are obtained in a wide range of composition. The powder patterns of some of them exhibit extra reflections which could be attributable to an a√3 x a√3 x c superstructure. Whatever the x value, the small crystals obtained after the melting of the starting materials and cooling them, always present the approximate composition 0.43 BaO - 0.2 Ln2O3 - 6 Al2O3 as determined by electron probe micro-analysis. In this compound there is no evidence of the a√3 x a√3 x c superstructure. Diffuse reflection and electron spin resonance of Nd3+ ions in BNA and BLnA indicate that the neodymium environment is close to that found in MP type lanthanide aluminates. Ba2+ ions appear to be far from Nd3+ ions and this suggests a distribution of Ba2+ and Ln3+ ions in different and alternating 001 planes.

Optical properties of F and F+ type centers in barium β-alumina

X-ray induced F and F + type centers in the conduction planes of Ba 2+ β-alumina are characterized by optical absorption, Electron spin resonance (ESR) and time resolved fluorescence spectroscopies. These two centers exhibit similar optical absorption properties, although F centers are optically bleached by UV irradiation while F + centers are relatively stable under the same conditions. The latter are characterized by a strong fluorescence at 425 nm.

Barium hexaaluminoferrites with new structural features

Barium hexaferrite and hexaaluminate are of hexagonal symmetry (P63mmc) with a structure respectively of magnetoplumbite (MP) and s-alumina (s) type. The BaAlxFe12−xO19 phases (BAF) have been synthesized by solid state reactions at high temperatures (1350°C – 1500°C). The X-ray powder diffraction patterns are close to those of a MP or s structure type but for 4 < x < 8 they also exhibit some extralines. These new lines can be interpreted as 001, 1 = 2n+1 which are “forbidden” reflexions in the P63/mmc space group. Their intensities are particularly strong for BaAl6Fe6O19 (x = 6) and decrease apart from this composition. The a unit cell constant shows a linear decrease with increasing x, whereas the c vs. x curve presents a marked inflection point for intermediate x values. The c/a ratios show that for x ≤ 4 the structure should be of MP-type while compounds with x ≥ 9 would adopt the s structure. For the intermediate compositions, corresponding to the appearance of the extra lines, a new structural type appears. Models are proposed to describe these new phases.

Microdurete de monocristaux d'hexaaluminates de lanthanides de type magnetoplombite

Abstract Knoop microhardness is investigated as a function of orientation on basal (0001) and prismatic {1120} and {1100} planes of lanthanides hexaaluminates with magnetoplumbite type structure. Two kinds of single crystals are studied, stoechiometric compounds LnMgAl 11 O 19 (Ln = La  Gd) and LaMAl 11 O 19 (M = Mg, Mn, Fe, Co, Ni), and non stoichiometric ones Ln 1−x M 1−y Al 11±z O 19−u (Ln = La, La 0.9 Nd 0.1 ; M = Mg, Mn). For all the samples, hardness is always isotropic in basal planes. In prismatic planes an anisotropy is observed; the maximum hardness is found not along a simple axis but at a direction of about 35°–40° from c. This particular behavior is mainly related to that of spinel blocks in the magnetoplumbite like structure. Measurements of microhardness performed in corresponding planes of a magnesium aluminate spinel single crystal confirm this observation. A comparison between lanthanide hexaaluminates and β aluminas is given. The structures of these two groups of compounds are very similar but their mechanical behaviors are different.

Substituted lanthanum hexagallates as epitaxial growth substrates

Abstract Looking for larger unit cell constants in magnetoplumbite-like hexagallates LaMgGa 11 O 19 , with the purpose of using them as LPE substrates, Mn 2+ and In 3+ are substituted to Mg 2+ and Ga 3+ respectively. Simultaneous replacement of both ions results in a phase with the largest parameters ( a = 5.866 A ; c = 22.986 A ). An X-ray structural investigation has been achieved on a single crystal with effective composition La 0.96 MnIn 0.5 Ga 10.5 O 18.94 and localization of Mn in tetrahedra and In in octahedra is deduced from the refinement of the structure.

Synthese et etude par microscopie electronique de la tainiolite KLiMg2Si4O10F2

Taeniolite KLiMg2Si4O10F2 has been synthesized by two methods: solid state reaction and from the molten state. The optimal conditions for the formation of taeniolite are determined. Synthesized crystals obtained from the melt have a flaky morphology shown by scanning electron microscopy. Crystallographic studies and TEM observations of taeniolite single crystals confirm that this compound is a single layered mica. The measured values for unit-cell parameters are a = 5.23A, b = 9.13A, c = 10.16A and β = M100.6°.

F+ center as a paramagnetic probe in the EPR investigation of barium hexaaluminate phases I and II and barium-lanthanum hexaaluminate ceramics

An X-ray-induced F{sup +} center is used as a paramagnetic probe to study sintered powders of barium hexaaluminates phases I and II (referred to as Ba({Phi}{sub I}) and Ba({Phi}{sub II})) and mixed barium-lanthanum hexaaluminate (referred to as BLA). This F{sup +} center was previously shown to exist only in {beta}-alumina-type mirror planes containing interstitial oxygen ions as charge-compensating defects, so that its study yields information about the local structure of this type of mirror plane. By comparison of the EPR spectra of Ba({Phi}{sub I}) and Ba({Phi}{sub II}) it is shown that the latter contains only one type of F{sup +} center with a reduced hf interaction. This indicates that there is only one type of mirror plane containing interstitial oxygen in Ba({Phi}{sub II}) and that the separation between spinel blocks is larger in Ba({Phi}{sub II}) than in Ba({Phi}{sub I}). This result agrees with models proposed by other authors for barium-lead hexaaluminate and barium hexagallate. Investigation of the F{sup +} center in mixed barium-lanthanum hexaaluminate ceramics clearly supports the hypothesis of segregation of Ba{sup 2+} ions in {beta}-alumina-type mirror planes. It is also shown that the structure of these planes is very similar to that of Ba({Phi}{sub II}) defect planes.