L. H. Brixner Cenwal Research and Development ~epartment' E. I. du Pont de Nemours and Company Experimental Station Wilmington, DE 19898
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On the Structural and Luminescent Properties d the S C T ~ ~ . ~ N System ~,O~
Recent research on the structure and properties of ScTal.,Nh,04, the system of solid solutions formed by ScNhOa and ScTa04 over the entire range of x , has produced results t h a t would form the basis for a n interesting experiment for students in a Junior-Senior level undergraduate inorganic ~lahoratorv. The investieation of the fluorescence of this system a i a function of x illustrates standard state preparation [i,rocedures and nndd he used to introduce studenrs to the use of X-ray powder diffraction a s a n identification tool, ~
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introduction According to Keller ( I ) , the structure of hoth ScNh04 and ScTa04 is that of the mineral Wolframite (Fe,Mn)WOa, which has given its name to the complete class of ABOa compounds crystallizing in this particular symmetry..In addition to IIIIV type Wolframites, such as ScNbO4 or InNhOa, there also exist I I N l Wolframites such as CdWOa or IVDV compounds such a s ZrTi04 (2). Broch (3) first descrihed the Wolframite structure for MgW04, hut incorrectly assigned 4 coordination to tungsten. Keeling (4) did a more detailed structure refinement of NiWO4 and came t o the conclusion t h a t hoth A and B ions are 6-coordinated with the oxygens only slightly distorted from ideal octahedral positions. More recently Brixner and Chen (5) did single crystal work on InNbOa confirming the Wolframite structure for this I I I N cumpound. There is only sparse literature on the luminescence of S c N b 0 4 and ScTa04. Brixner (6) descrihed weak luminescence for ScTa04 at -196'C, and Blasse and Bril(7) reported weak blue emission (15%quantum yield) for ScNhOa. Neither comoound has been used as a host for rare earth activators nor has the luminescence of the complete ScTa1.,Nh,04 system been studied. The present paper will particularly focus on the fluorescence of th& system as a function of x
Generally, the component oxides were weighed to the nearest mg and blended via shaking in a jar ins spex mill for 15 mi". This mixture was prefired at 1400-1500°C for 4-8 hr. After the mixture was hamogenized via ball-milling, pellets were pressed and fired in A1203 containers at 1750'C for 2-4 hr. If a flux was employed, it was mixed with the prereaction powder in a ratio of 1:112 solid to flux, and the reaction was carried out in recrystallized AlzOs containers at 1250°C for 10-14 hr. After coaling the fluxed product, it was water-leached, filtered. washed. and dried. Desnite the lower firing temperature, the fluxed reaction produets yielded more uniform single phase material of about 4-8 p average diameter. X-Ray Examination X-ray powder diffractionpatterns for d l compounds prepared \rere g,hrninrd uith 3Cuinier-Hnggfocusingcamers ir = 4 4 narn,.Theradlntien was monurl~rumartc('tlK~~. , A = 1.541151 ,\I. Si in = 5.43054 A) was used as an internal standard. Line positions on the film were determined to i 5 u with a David Mann film reader: intensities were ritimarrd h) oirill~rsvupiccornpartson ui tilm dmw). with the S ~ I U I I ~ line F ~ I uf thr patrern. Refined cell dimensims were ubtained t,) a lwsr rquarrr trrnrment d the Cuinier dnu (;cnerally 1.1 r u CO lines were indexed without extra lines. Fluorescent Studies Emission and excitation spectra were obtained with a Perkin Elmer MPF-2A spectrophotometer. ~
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Discussion Structure ScNbOl and ScTaO, are hoth is~i,.itructuraIuith InNbOt. 'I'he structure of lnNhOa has been descrit~edin detail recently (5). Both Sc and Nh are in slightly distorted octahedral svmmetrv surrounded hv . oxygen - - which forms near perfect dexagonaily close packed layers. An overview of the structure is shown in Figure 1. It is interesting t o note t h a t octahedra ~~~
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Experimental Preparation Because of the highly refractory nature of the constituent component oxides (especiallySc20Band Ta20s). standard solid state reactions had to be carried out at temperatures above 1700°C in air or axygen to obtain single phase material. The use of fluxes, such as LizSO*or LiCI, was also found to be beneficial and permitted reaction temperatures between 1200 and 1300°C. High purity optical grade Ta2OSand NblOs were obtained from Kawecki/Beryleo Ca. and were not purified further. S-08 from Research Chemicals Corp. of a nominal 99.0%purity was converted to the chloride ScCL?,which was further purified by repeated sublimation in a stream of dry HCI gas. Subsequent hydrolysisand firing yielded a pure white, nonlumineseing Sez03 with a cell edge of 9.8406 i (5) A. This is in fair agreement with the 9.845 A reported by Swanson (8). The slightly smaller parameter suggests that Swanson's SenOa may have been contaminated with some of the larger rare earths. Presented at the ACS National Meeting, March 25, 1980, as part of the State-of-the-Art Symposium on Solid State Chemistry in the Undergraduate Curriculum sponsored by the Division of Chemical Education. Contribution No. 2769.
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588 1 Journal of Chemical Education
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Figure 1. The structure of ScNbO..
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Table 1.
Observed and Calculated d-Values for ScNbOa
i
N
K
H
UOBS)
L
UCALC)
Figure 2. Excitation and emission spectra of
with the same central metal ion are joined via edge sharing, whereas octahedra with dissimilar metals are joined via corner sharing. In the indium case ( 5 ) ,we found InTa04 to have a sliehtlv .. . lareer cell volume than InNbOi. The same holds true in the present ScTal.,Nh,On system. \
