CsNbOB2O5 ("CBO") as a potential Cs standard

[Brian Joy]

Just in case people aren't totally sick of my posts on this subject yet, I’ve come across another possibility for a Cs standard:  CsNbOB₂O₅, which contains 41.03 wt% Cs₂O, 38.70 wt% Nb₂O₅, and 20.27 wt% B₂O₃.  As described in the attached paper, it can be synthesized using a self-fluxing method with starting materials Cs₂CO₃, Nb₂O₅, and B₂O₃, the last of which can be obtained by dehydration of boric acid, H₃BO₃.  The compound is insoluble in water, but, conveniently, the flux is not.  Using TAP, Cs La(2) overlaps Nb La, but this is not a problem on PET.

[Brian Joy]

I’ve managed to get a hold of some small platinum crucibles and have just recently synthesized CsNbOB₂O₅ by means of a self-fluxing method.  The yield was high, and I obtained tabular and flattened prismatic crystals up to perhaps 3 mm in the longest dimension.  The compound appears to possess two perfect cleavages, and this can make polishing a little difficult depending on grain orientation.  Below is an ED spectrum; the small peak at ~2.7 keV is Nb Lβ_3,4, and the unlabeled peaks below 1 keV are Cs M lines.



I’ve subjected the compound to my usual test of exposing it to a 15 keV/10 nA focused beam for ten minutes, and it gets an "A" grade:



I still need to collect WD spectra to check for minor impurities and also need to analyze my pollucite standard using the CsNbOB₂O₅ as the Cs standard, but so far this looks like a very promising Cs compound possibly of comparable quality to CTA.

[Probeman]

I’ve managed to get a hold of some small platinum crucibles and have just recently synthesized CsNbOB₂O₅ by means of a self-fluxing method...  [snip]

I still need to collect WD spectra to check for minor impurities and also need to analyze my pollucite standard using the CsNbOB₂O₅ as the Cs standard, but so far this looks like a very promising Cs compound possibly of comparable quality to CTA.

This material looks very promising indeed!   I want some!

Nice that there doesn't seem to be any spectral interferences, even for EDS.

[Brian Joy]

I’ve collected a set of wavelength scans on the CsNbOB₂O₅ that I synthesized, and I see no easily detectable impurities.  The compound could be used as a standard for Nb as well, though one must take care to avoid interference from Cs Lα(2) at the upper background offset:

(EDIT: Satellites of Cs Lα(2) interfere at/near the Nb Lα peak position.)



Although the B Kα peak is identifiable on the scan using LDE2, both first- and second-order M lines of Nb interfere at both peak and background positions.

When analyzing my pollucite standard (32 analyses) for Cs using CsNbOB₂O₅ as the standard and keeping other oxide concentrations constant, I get the following results:

PAP/MAC30


Armstrong/FFAST


As has been the case with the other Cs compounds that I’ve synthesized, the atomic number correction constitutes the largest of the matrix correction factors and is the source of the discrepancy between the two models presented above.  The results are comparable to what I've gotten using my other Cs compounds while assuming a stoichiometric formula unit.

I’ve ordered some high-purity B₂O₃ and plan on synthesizing another batch of CsNbOB₂O₅ soon.  I’m working away furiously during evenings to complete my furnace temperature controller assembly, and I hope that this device will help me grow larger crystals that are relatively free of melt inclusions.  However, the largest Pt crucible that I have is ~25 mL, and so I’m limited to small batches.

[Brian Joy]

And here is a plot of Cs Lα count rate on CsNbOB₂O₅ using a 15 keV, 50 nA, focused beam for ten minutes.  The absorbed current falls just slightly from 36.29 to 36.23 nA.  After the test, only a faint, small contamination ring was visible in the secondary electron image, and no change was visible in reflected light.  The only apparent drawback to the compound is the presence of two perfect cleavages, though their effects during polishing can be minimized by orienting grains (but I need to study the compound more closely under the microscope to determine what the best orientation is).

[Probeman]

Although I suspect that any change in Cs La intensity for this awesome material will be well within any normal measurement precision, the other good news is that the time dependent intensity (TDI) correction can be turned on for other standards (as well as unknowns) to correct for any change in intensity over time for improved quantification in Probe for EPMA.

See the Special Options button in the Acquire! window, and also here for more details:

https://probesoftware.com/smf/index.php?topic=11.msg252#msg252

[Brian Joy]

Just this past week I completed a new (automated) synthesis of CsNbOB₂O₅; however, the results were a little perplexing.  I followed the directions of Becker et al. (1995, attached above) fairly closely.  However, it appears that a different phase was stable at high temperature (after soaking at 1000˚C for 20 hours) and that CsNbOB₂O₅ only nucleated relatively late at lower temperature while ramping downward at 2.2˚C per hour (more slowly than Becker et al. did).  The BSE image below illustrates textural relations between the two phases; the CsNbOB₂O₅ is the finer-grained, darker phase.  At first I thought that the high temperature phase might be Cs₂Nb₄O₁₁, which also has potential as a Cs standard.  However, I collected a set of quantitative analyses and found that the formula unit was difficult to establish.  The oxide totals were a little low, and so it’s possible if not likely that the phase is also boron-bearing.



I just started a new CsNbOB₂O₅ synthesis run about 30 minutes ago.  In this run, ironically, I’ll be trying to emulate the conditions that I used for my first run, in which I adjusted the temperature manually and had to allow the crucible to soak during nights.  In my new, automated run, I’ve set the initial soak temperature at 950˚C, followed by a relatively quick ramp downward in temperature and then slower ramps interspersed with 12-hour soaks.  I should have results by next weekend.

Lastly, I’ve now tested exposure of CsNbOB₂O₅ to a focused beam at 15 keV and 100 nA for ten minutes.  It holds up fantastically even under these conditions:



Dare I try 200 nA?

[Brian Joy]

I just had a look yesterday at the results of my most recent CsNbOB₂O₅ (CNB) run.  Slowing down the cooling rate appears to have allowed most of the high-temperature phase of unknown identity to react away, though rare inclusions of it are still present in some of the larger CNB grains:



I regard the run mostly as a success.  However, although some grains measure a couple mm in length, the grain size overall is relatively fine (generally a couple hundred microns).  Some grains also show hopper forms (see below), and so perhaps decreasing the cooling rate even further would produce better results.  In the roughly representative image below, note that grain orientation has a fairly dramatic effect on the quality of the polish.  However, as long as numerous grains are mounted, a significant percentage have good surfaces suitable for probing. 



When I get a chance, I’ll send some of this material to people who have expressed interest in it.  Currently I’m running a new batch of CsTiOAsO₄ in order to try to remedy problems that I encountered in my last CTA run (very few grains, numerous TiO₂ inclusions).