Nb analysis in pyrochlore

[Julien]

Hi,

Recently I run into some trouble analyzing pyrochlore (ideal formula (Ca,Na)2(Nb,Ta)2O6(OH,F)). I have been using Nb L-alpha, Ca K-alpha, Na K-alpha, Ta L-alpha at 25 kV, 20 nA. The setup includes many other elements (Y, REE, K, Si, Al, Fe, Mg, Mn, Ti, Zr, Ba, Sr, Pb, U, Th) but Ca, Na, Nb, Ti and F makes around 90-95% of the total oxides. My problem concern, I think, the matrix correction. Using Armstrong ZAF correction, all my totals are around 96-98% (including F and recalculated OH based on 1 hydroxyl group site) and the Nb wt-% is apparently too low (around 65-66% element, I would expect more like 68-69%). To check the quality of the results I did perform a mineral formula recalculation assuming a total of 6 oxygens + 1 OH; these yield too low value for the Nb-site (that include all 5+ and 4+ cations plus probably some 3+ cations), but close to 2 for the totals of 1+ and 2+ cations (mainly Ca and Na). However, recalculating the data with either the Heinrich or the P&P matrix correction yield better results, both in term of totals and mineral formula recalculation.

Problems we can rule out...
a) My standards have been recently strongly repolished to remove any oxide layer (~6 months ago), so I doubt this is the issue. Moreover, if this was a problem with the standard (e.g. getting oxidized), I would expect my totals to get too high, not too low.
b) The analyzed samples are well polished, and the standardization is only 2 days old.
c) I've checked the peak position, and there is no significant change between the standard (Nb metal) and the unknown (Nb oxide).
d) An erroneous background position is unlikely. All backgrounds yield coherent values and have been double-checked by numerous scans within the standards. Moreover it would take a huge error on a background value to yield a difference of 3% or so on Nb...
d) I have performed several analyses on the same (unzoned) grain at different beam size (10, 5 and focused) to check for possible beam damage, and all three results are within the error identical.

Unfortunately I have no way to test my analysis using a secondary Nb standard (preferably an oxide), and the only standard I do have for Nb is simply a pure metal.

Does anyone has experience with Nb analysis? Which of the numerous matrix correction available in Probe for EPMA are you using? My tests suggest that Heinrich or Pichou & Pouchoir would be the most appropriate, but...

You will find attached to this email a summary of the various oxide wt-% results (average of 4 points) using the different matrix correction, and (on the second page) a summary of the CalcZAF calculation for one of these points. The most obvious difference is a lower backscatter coefficient (and Z) in Armstrong for Nb, along with a slightly lower MACs calculation and slight difference in absorption coefficient. Any thoughts on this problem would be greatly appreciated!

Thanks in advance for your help!

Julien

[John Donovan]

Hi Julien,
Wow, what a nasty problem. It's so strange that the Nb values are so different.  I can only note that the absorption correction for Nb La at 25 keV is huge:

25 keV:
ELEMENT  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR STP-POW BKS-COR   F(x)u      Ec   Eo/Ec    MACs
   Ta la   .9668  1.0000  1.1593  1.1207  1.3365   .8674   .9302  9.8810  2.5301 123.686
   Nb la  1.7931   .9981  1.0122  1.8116  1.0474   .9664   .3438  2.3710 10.5441 1893.61
   Ca ka  1.5451   .9955   .8805  1.3544   .7804  1.1282   .5888  4.0390  6.1897 847.373
   Na ka  3.9143   .9984   .8500  3.3219   .7210  1.1789   .1722  1.0730 23.2992 3720.04
   F  ka  8.4149   .9993   .8284  6.9660   .7040  1.1767   .0631   .6870 36.3901 8849.39
   O  ka  9.4954   .9996   .7748  7.3542   .6551  1.1828   .0492   .5317 47.0190 10825.9

 ELEMENT   K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
   Ta la  .00000  .40071  44.908  54.836  11.765    .222   25.00
   Nb la  .00000  .12728  23.058  32.985  11.765    .222   25.00
   Ca ka  .00000  .07344   9.947  13.918  11.765    .222   25.00
   Na ka  .00000  .01718   5.706   7.691  11.765    .222   25.00
   F  ka  .00000  .00338   2.357   2.357   5.882    .111   25.00
   O                      13.898    .000  41.176    .778
   H                        .125   1.118   5.882    .111
   TOTAL:                100.000 100.000 100.000   1.889

But at 15 keV it is much better:
ELEMENT  ABSCOR  FLUCOR  ZEDCOR  ZAFCOR STP-POW BKS-COR   F(x)u      Ec   Eo/Ec    MACs
   Ta la   .9879  1.0000  1.2671  1.2517  1.3545   .9354   .9767  9.8810  1.5181 123.686
   Nb la  1.3459   .9986  1.0119  1.3600  1.0513   .9625   .5994  2.3710  6.3264 1893.61
   Ca ka  1.2084   .9982   .8511  1.0267   .7719  1.1026   .7969  4.0390  3.7138 847.373
   Na ka  2.1899   .9988   .8417  1.8409   .7030  1.1973   .3844  1.0730 13.9795 3720.04
   F  ka  4.4182   .9995   .8152  3.5999   .6831  1.1934   .1696   .6870 21.8341 8849.39
   O  ka  5.2521   .9996   .7538  3.9573   .6340  1.1889   .1334   .5317 28.2114 10825.9

 ELEMENT   K-RAW K-VALUE ELEMWT% OXIDWT% ATOMIC% FORMULA KILOVOL
   Ta la  .00000  .35878  44.908   -----  11.765    .222   15.00
   Nb la  .00000  .16954  23.058   -----  11.765    .222   15.00
   Ca ka  .00000  .09689   9.947   -----  11.765    .222   15.00
   Na ka  .00000  .03099   5.706   -----  11.765    .222   15.00
   F  ka  .00000  .00655   2.357   -----   5.882    .111   15.00
   O  ka  .00000  .03512  13.898   -----  41.176    .778   15.00
   H                        .125   -----   5.882    .111
   TOTAL:                100.000   ----- 100.000   1.889

The Ta La is a little low on over voltage but usable.  Is there a reason for running it at 25 keV?
john

[Julien]

The main reason to analyses these at 25 kV is that indeed I can then use Ta L-alpha instead of M-alpha (which might yield to other troubles). The analyses there are low in Ta, but some pyrochlore do have a large amount up to 6% as measured in other grains. Moreover, I have only small diffraction crystals, and jumping to 25 kV give me also a much better count rates on REE and other elements.

J.