Shigeyuki Kimura

Comparative study of defect structures in lithium niobate with different compositions

Structure refinements were conducted on LiNbO[sub 3] crystals with four different compositions, ranging from near stoichiometric (Li/(Li + Nb) = 0.498) to highly nonstoichiometric (Li/(Li + Nb) = 0.470), by the X-ray single-crystal diffraction and the TOF neutron powder diffraction methods to clarify the major defect mechanism of LiNbO[sub 3] governing its nonstoichiometry. Two models - Li-site vacancy model and Nb-site vacancy model - were chosen on the basis of density data and examined in the refinements. The former is expressed as [Li[sub 1-5x]Nb[sub x][open square][sub 4x]][Nb]O[sub 3] and the latter [Li[sub 1-5x]Nb[sub 5x]] [Nb[sub 1-4x][open square][sub 4x]]O[sub 3], where [open square] denotes a vacancy. The refinement results revealed that the amount of Nb occupancy was composition-independent and that Li[sup +] ions were replaced by the Nb[sup 5+] ions, creating vacancies at the Li site. Rietveld analysis of the neutron diffraction data was consistent with the X-ray refinement. These results strongly support the Li-site vacancy model.

Increased optical damage resistance in Sc2O3‐doped LiNbO3

The optical damage of 1 mol % Sc2O3‐doped LiNbO3 was approximately two times smaller than an optical grade LiNbO3 measured as a function of Ar+ (λ=488 nm) irradiation time. Severe Ar+ beam distortion observed in the undoped LiNbO3 was not present in the Sc2O3‐doped LiNbO3. There was negligible shift in the OH− absorption band but a 10 nm blue shift was observed in the absorption edge, indicating that Sc3+ and Mg2+ incorporation may proceed by a different mechanism. This is the first of reported results, to the authors’ knowledge, of a trivalent dopant increasing the damage resistance level in LiNbO3.

Crystal chemistry of hexaaluminates: β-alumina and magnetoplumbite structures

Abstract Hexagonal aluminates are known to have a layer structure composed of spinel blocks and conduction layers which are stacked alternately. The structural parameters are influenced by the large cations in the conduction layer. Two typical types of hexagonal aluminates, β-alumina and magnetoplumbite, are studied and reviewed from this point of view. The conclusions are that the structure type of hexaaluminates is determined by the charge and radius of the large cations in the conduction layer, and that the conduction layer thickness decreases as the radii of the large cations in the conduction layer decreases and as the population increases. The spinel block thickness increases according to the increase in the amount of Al 3+ defect within the spinel block.

The crystal structure of barium hexaaluminate phase I (barium β-alumina)

Abstract The crystal structure of barium hexaaluminate phase I (Ba0.79Al10.9O17.14) was determined by single crystal X-ray reflection data. The refinements were carried out by the least-square method to give a final R-value of 0.023. The structure was revealed to be essentially of a β-alumina type and the Ba ion was detected only at the 6h site near the Beevers-Ross site (2d site). The charge compensation for nonstoichiometry was found to be principally effected by the interstitial oxygen due to Frenkel defects of Al ions. From the structural point of view, phase I was referred to as “barium β-alumina.”

The crystal structure of lanthanum hexaaluminate

Abstract The least-square refinement of lanthanum hexaaluminate (La0.827Al11.9O19.09) was accomplished using single crystal X-ray diffraction data. The result of the final anisotropic refinement, corresponding to an R-value of 0.039, revealed the structure of a magnetoplumbite type. In the structure interstitial Al ions were found, which were probably formed by a Frenkel defect mechanism. These interstitial Al ions are proposed to be situated in pairs making a bridge between spinel blocks, and to cause Al and La defects in the intermediate layer ( z ⋍ 0.25 ). The nonstoichiometry of lanthanum hexaaluminate is attributed to these defects.

The crystal structure and cation distribution of highly nonstoichiometric magnesium-doped potassium β-alumina

Abstract The crystal structure of Mg-doped potassium β-alumina K1.875Ba0.022Mg0.919Al10.081O17.0 (K1.875 · Mg β-alumina), obtained by ion exchange from Ba0.956Mg0.912Al10.088O17.0 (Ba0.956 · Mg β-alumina), has been refined from single-crystal X-ray data with a final R-value of 0.034. K1.875 · Mg β-Alumina contains an extremely large number of K ions while retaining the fundamental β-alumina structure and space group symmetry P6 3 mmc . The structure was found to be consistent with a model composed of singly occupied cells containing a K ion at the Beevers-Ross (BR) site and triplet cells containing three K ions at the mid-oxygen (mO) sites, as in the case of K1+yMgyAl11−yO17 with y = 0.62. The extra cations were assumed to be accommodated in the anti-BR (aBR) sites created by clustering of three triplet cells. The occupancies of K ion sites calculated by this model agree well with those obtained by the refinement. The structure of Ba0.956 · Mg β-alumina was also refined, yielding R = 0.031.

Electron microscopic study of barium hexaaluminates

Abstract The crystallographic relation between phase I and phase II of barium hexaaluminates, which were conventionally considered as the single compound “barium hexaaluminate (BaAl 12 O 19 ),” was investigated using principally the electron diffraction method. Phase I (Ba 0.79 Al 10.9 O 17.14 ) was found to have β-alumina type structure with space group P6 3 mmc . On the other hand, phase II (Ba 2.34 Al 21.0 O 33.84 ) exhibited an a √3 × a √3 superstructure, which is probably due to the ordering of excess Ba ions within BaO layers. Possible structure models of both phases are presented.

Control of interface shape by using heat reservoir in FZ growth with infrared radiation convergence type heater

Abstract A concave interface is unfavorable for the growth of single crystals from the melt, because it enhances the concentration of inclusions and dislocations along the core of growing crystals. The growth of single crystals is often interrupted by these inclusions and dislocations. In FZ growth using an infrared radiation convergence type heater, the growing interface of oxide materials which minutely absorb the near infrared radiation tends to become concave towards the melt. When a heat reservoir in the form of an alumina hollow cylinder is used, heated by radiation from a heating lamp, the temperature distribution in the vicinity of the growing interface is influenced. In the case of YAG, where the interface is concave without a heat reservoir, the interface can be changed to convex towards the melt by using a heat reservoir, thus single crystals of YAG with a low density of inclusions can be grown successfully.

Interface shape and horizontal variations of Al and Ga contents in substituted YIG single crystals grown by the floating zone method

Abstract The shape of the solid-liquid interface in the TSFZ growth of a YIG single crystal was revealed by an etching technique, and the horizontal variations of the Al and Ga contents in the Al- and Ga-substituted crystals were determined by means of EPMA. The interface can be divided into three regions by their characteristics in cross-sectional profile. They are (1) core region, (2) intermediate region, and (3) peripheral region. The characteristics of growth striations in each region and the influence of the rotation rate on the interface shape suggest that the forced convection by crystal rotation is dominant in the melt above the core region, and the thermocapillary convection is dominant in the melt above the peripheral region. EPMA analysis indicated that the concentration of Al2O3 was higher in the peripheral region than in the core, and it was highest at the part 300–500 μm inside from the crystal edge. Such horizontal variation can be interpreted in terms of the local variation of the boundary layer thickness caused by the coexistence of convections of the different types in the molten zone.

The crystal structure of barium lead hexaaluminate phase II

Abstract A refinement was performed on the average crystal structure of barium lead hexaaluminate phase II ((Ba0.8Pb0.2)2.34Al21.0O33.84) using single crystal X-ray diffraction data, giving a final R value of 0.030 with space group symmetry P 6 m2 . The structure is essentially of a β-alumina type, but contains a lot of defects and interstitials. Inside the spinel block were found Ba(Pb) ions at 12-coordinated polyhedral sites, formed by complex defects, including triple Reidinger defects, in the same unit cell. Barium lead hexaaluminate phase II was found to consist of two kinds of unit cells with formulae “(BaPb)3.0Al20.0O35.0” and “Ba2.0Al22.0O34.0” in a 1 : 2 ratio; these three cells combine to form an a√3 × a√3 superstructure.