LaAlO3-Sr(Al,Ta)O3-Based Transparent Ceramics
Image pickup-type lenses in digital cameras, video cameras, and other devices use multiple lens materials with different refractive indices and dispersion for correcting for color bleeding and image blurring. Thus, while materials with a low Abbe number such as (Ba,Ca)(Ti,Mg,Ta)O3 transparent ceramics are needed, materials with a high Abbe number are also needed. For this reason, we decided to develop materials with a high refractive index and high Abbe number, and we focused on LaAlO3 as a material base. The band gap of LaAlO3 is 5.6eV, and because this is a value larger than the band gap (4.4eV) calculated from the absorption edge of Ba(Sn,Zr,Mg,Ta)O3, we expect a high Abbe number for this material. Also, the refractive index of LaAlO3 based on the Gladstone-Dale relation is 2.06, and so a high refractive index can also be expected. However, because the crystalline structure of the LaAlO3 itself is a rhombohedron, a high transmittance cannot be expected in its unmodified form as a polycrystalline body. LaAlO3 and Sr(Al,Ta)O3-based solid solutions are well-known as LaAlO3-based cubic crystal materials. The composition is 0.25LaAlO3-0.75Sr(Al,Ta)O3, and it is used as a single-crystal substrate material for forming thin films, but no reports on polycrystalline ceramics based on these solid solutions were found. This led us to fabricate ceramics in a wide range of composition ratios based on these solid solutions for examining their crystalline structures and optical characteristics. The measurement results for the transmittance of LaAlO3-Sr(Al,Ta)O3-based solid solution ceramics with various composition ratios are shown in Fig. 4. X-ray diffraction was used to find that the crystals have a cubic structure for a Sr(Al,Ta)O3-based solid solution of 40mol% or more, and it is also clear from the transmittance results that these crystals show a high transmittance in a cubic crystal composition (x=0.4 or higher in Fig. 4). The measurement values for the refractive index and Abbe number are nd=2.06 and vd=42.8, respectively, for LaAlO3(x=0.0) and are nd=2.01 and vd=34.3 for Sr(Al,Ta)O3 (x=1.0), which shows that as the solid solution quantity (x value) of Sr(Al,Ta)O3 increases, nd and vd both decrease monotonically. The refractive index and Abbe number of compositions with a high transmittance in a (1-x)LaAlO3-xSr(Al,Ta)O3 solid solution were, for instance, nd=2.04 and vd=37.8 at x=0.5.
In this way, we were able to obtain transparent ceramics with a high refractive index and high Abbe number in LaAlO3-Sr(Al,Ta)O3-based solid solutions. Figure 5 shows a graph plotted with the refractive indices and Abbe numbers of various types of transparent ceramic materials developed by Murata Manufacturing compared to the values of optical glass materials. Optical glass materials include various types of compositions, but their refractive indices and Abbe numbers are clustered in a band shape as shown in Fig. 5. This shows that the transparent ceramics of Murata Manufacturing have characteristic values that are distinct from this group of optical glasses.
Fig. 4 Relationship Between Composition Ratio and Transmittance of (1-x)LaAlO3
-Based Solid Solution Ceramics This shows the data when the thickness of the measurement sample was changed.
Fig. 5 Refractive Indices and Abbe Numbers of Various Materials
●: Optical glass material, ○: 0.5LaAlO3
, □: (Ba,Ca)(Ti,Mg,Ta)O3