CsTiOAsO4 ("CTA") as a potential Cs standard

[Brian Joy]

While following a trail of references related to potential Cs standards, I came across a handful of papers detailing synthesis of CsTiOAsO₄ (“CTA”), which is isostructural with RbTiOPO₄; I’ve attached those papers.  CTA contains 41.97 wt% Cs₂O, 34.23 wt% As₂O₅, and 23.79 wt% TiO₂.  Of course in EDS the Ti K lines interfere with the Cs L lines, but in WDS no interferences are present as long as one takes care in placing the low background offset for Cs La when using PET (likewise for Ti Ka).  I’m continuing to pursue synthesis of SrCs₄(PO₃)₆, but perhaps CTA should be investigated as well.  I might try to synthesize small crystals of it, but I’m definitely not well enough equipped to grow large crystals in quantity.

[Probeman]

I believe Marc Schrier mentioned this compound to me during our initial discussions on Cs synthesis as a possibility. With such arsenates we were wondering about its potential beam stability...   also there's the toxicity issue.

[Brian Joy]

I believe Marc Schrier mentioned this compound to me during our initial discussions on Cs synthesis as a possibility. With such arsenates we were wondering about its potential beam stability...   also there's the toxicity issue.

Note that Kunz et al. (1995) also grew isometric CsTiPO₅.  They were only able to grow ~50-micron crystals by spontaneous nucleation from a melt cooled slowly from 1050°C to 800°C.  Presumably larger crystals could be grown using these as seeds.

[Probeman]

I believe Marc Schrier mentioned this compound to me during our initial discussions on Cs synthesis as a possibility. With such arsenates we were wondering about its potential beam stability...   also there's the toxicity issue.

Note that Kunz et al. (1995) also grew isometric CsTiPO₅.  They were only able to grow ~50-micron crystals by spontaneous nucleation from a melt cooled slowly from 1050°C to 800°C.  Presumably larger crystals could be grown using these as seeds.

Hi Brian,
I know nothing about crystal growing but if CsTiPO5 is the same as CsTiOPO4 then see this response here:

http://probesoftware.com/smf/index.php?topic=560.msg6674#msg6674

[Brian Joy]

I believe Marc Schrier mentioned this compound to me during our initial discussions on Cs synthesis as a possibility. With such arsenates we were wondering about its potential beam stability...   also there's the toxicity issue.

Note that Kunz et al. (1995) also grew isometric CsTiPO₅.  They were only able to grow ~50-micron crystals by spontaneous nucleation from a melt cooled slowly from 1050°C to 800°C.  Presumably larger crystals could be grown using these as seeds.

Hi Brian,
I know nothing about crystal growing but if CsTiPO5 is the same as CsTiOPO4 then see this response here:

http://probesoftware.com/smf/index.php?topic=560.msg6674#msg6674

Hi John,

The family of compounds with formulas ATiOBO₄, in which “A” represents an alkali metal or thallium and “B” represents phosphorus or arsenic, has orthorhombic symmetry.  To my knowledge, all attempts to synthesize CsTiOPO₄ have failed.  In contrast, CsTiPO₅ has isometric symmetry and can be synthesized by spontaneous nucleation from a melt, as was done by Kunz et al. (1995).

I should note that CsTiOAsO₄ itself is not likely toxic, as it is insoluble in water.  Of course the As₂O₅ starting material is quite dangerous.  Owen Neill should be able to speak to the stability of RbTiOAsO₄ under the beam, as he accidentally received some of this material when ordering RbTiOPO₄ from Marc.

[Brian Joy]

I’m making an attempt at growing CsTiOAsO₄, roughly following the instructions in the attached patent document under the heading, “Comparative Example A.”  Instead of using a Pt crucible, I’m using a 15 mL alumina crucible lined with Pt foil and have scaled the amounts of starting materials down by a factor of about 75.  I increased the furnace temperature slowly in order to allow any water and carbon dioxide to escape non-destructively.  When I last checked the crucible early yesterday PM after it had been sitting at 500°C for nearly an hour (to calcine the Cs₂CO₃), everything looked stable.  I allowed the crucible to sit in the furnace at temperature just below 1000°C for 16 hours and am now in the process of slow cooling.  Unfortunately, I have no digital temperature control, and so I simply have to lower the set temperature manually in order to induce crystal nucleation (and it’ll have to sit at constant temperature during nights).  I should have some sort of result by the weekend.  I'll be ecstatic if I can grow even just 0.5 mm crystals, though the patent is aimed at production of large crystals for laser frequency doubling.

I may try to modify another, similar old furnace for use with a digital “PID” ramp/soak controller in conjunction with a solid-state relay.  If anybody has experience with this sort of modification, please let me know how you went about it.

[Brian Joy]

I just removed my CTA run from the furnace.  Although the Pt foil appeared undisturbed, unfortunately most of the reactants bubbled out of the foil enclosure and into the enclosing alumina crucible.  This probably occurred relatively early in the run during dehydration and/or calcination.  I’m currently dissolving the Cs arsenate flux in hot water to see if I produced any CTA at all, but I fear that I may just get ~100 mL of aqueous toxic waste.  Thankfully I have a means of disposing of this safely.

As Crystalgrower has noted in another post, I could likely get these runs to go more smoothly if I were working with a Pt crucible.  However, I simply don’t have the funds available to buy one.  Would anyone out there be willing to lend one to me?

[Brian Joy]

It appears that I did in fact produce some CsTiOAsO₄ during that last, otherwise disastrous run.  It formed blocky crystals that appear to have fractured either during cooling or during removal of the flux.  In some of the crystals, P substitutes for some of the As, as some Cs phosphate flux was probably left over in the crucible from the previous run (and was supposed to have been isolated from the reactants by the Pt foil).  The BSE image below shows one of the CTA crystals surrounded by fine grains of a variety of other phases, some of them Al-rich; the polish isn't so great, as I was in a hurry.  I’ve evaluated the beam sensitivity of the CTA by subjecting it to my usual test of monitoring the Cs Lα count rate at 15 kV and 10 nA for ten minutes with the beam fully focused.  It appears to hold up under the beam remarkably well.  I probably won’t get a chance to do any quantitative analyses until next week.

I think this one shows some real potential as a quality Cs standard, especially if I can manage to grow larger crystals.  However this will require a Pt crucible and better furnace temperature control.

[Brian Joy]

And here are WD and ED spectra.  The tiny peak at ~67 mm on TAP is P Kα.

[Brian Joy]

I made a second mount of some of the products from my recent CsTiOAsO₄ (“CTA”) run after realizing that the CTA was actually quite coarse and not mixed in with the fine-grained fraction.  In fact it was coarse enough (up to ~2 mm) that I was able to pick out individual grains for mounting.  The BSE image below shows two intergrown grains that contain regions rich in inclusions and also regions that are relatively inclusion-free.  The very dark inclusions are TiO₂, and the very bright, tiny ones are Pt metal.  The larger, roughly equant inclusions that are just a little darker than the main phase are a Cs-deficient, nonstoichiometric, likely isometric polymorph that probably nucleated during the early 16-hour soak at ~980°C.  These inclusions probably would have been fully resorbed had I been able to control the cooling rate of the furnace a little better.

After performing quantitative analyses, I find that most CTA grains in the mount contain ~0.25 wt% P₂O₅ contributed by Cs phosphate residue that was present in the enclosing alumina crucible; some contain higher concentrations of P₂O₅, and I’ve omitted these from the results that I show below.  In order to avoid relatively large matrix corrections for Ti and As using the standards that I have available, I’ve recalculated the analysis results assuming stoichiometric amounts of TiO₂ and As₂O₅ and assuming substitution of 0.25 wt% P₂O₅ for As₂O₅, as is consistent with the original measurements. With this concentration of P₂O₅, wt% Cs₂O should be 42.04 for the stoichiometric compound.  The results are as follows, using my earlier-produced Cs₂Co₂Al(PO₄)₃ as a Cs standard:

PAP/MAC30:


Armstrong/FFAST:


Note that the matrix corrections for Cs Lα are small.  In summary, it appears that the material that grew with decreasing temperature does in fact contain a stoichiometric concentration of Cs₂O.  In further support of this, it is quite homogeneous in Cs:  the last column ("ratio") in the results above is the Boyd homogeneity index for the 95 measurements that I made.

Note that the CTA crystals should be orthorhombic, but their habit is more suggestive of the isometric system.  The crystals are too thick and contain too many inclusions to get a good sense of whether they are isotropic, and I don’t have enough material for powder XRD.

This sure looks like a promising material, and I should be able to grow larger and more inclusion-free crystals once I improve my synthesis setup.

[Probeman]

This looks like very promising material for utilizing as seed crystals... too bad the Ti K and Cs L lines overlap, but it shouldn't be a problem with WDS.

[Brian Joy]

I’ve analyzed my pollucite standard using my recently synthesized CsTiOAsO₄ as a Cs standard and assuming a fixed matrix composition aside from Cs₂O.  These are the results of 30 analyses:

PAP/MAC30:


Armstrong/FFAST:


Note that the source of the difference in results between the two models is the atomic number correction.  Use of Cs₂Co₂Al(PO₄)₃ instead as the Cs standard results in a smaller atomic number correction.  I found that Cs Lα count rate in this latter material was stable over a period of at least 10 minutes when using a 10 nA beam current and with the beam defocused to 10 microns (but was not stable with a focused beam at 10 nA).  I've just started another run to produce more Cs₂Co₂Al(PO₄)₃ (intentionally this time) and hope to report on it within the next 10 days or so.

[Brian Joy]

Now this is worth checking out.  Below are the results of a test of exposure of CsTiOAsO₄ to a focused 50 nA beam for a period of ten minutes.  I focused the beam as tightly as possible (W filament) and made sure that it was as free of astigmatism as possible.  The absorbed current dropped a little during the test, but Cs Lα count rates remained steady within counting error.  In a secondary electron image collected following the test, I could see only a very faint mark at the point of impact of the beam, and I saw no discoloration or relief at all in reflected light.  This is a pretty robust material!

[Brian Joy]

Last month I synthesized a new batch of CsTiOAsO₄ in a Pt crucible.  However I obtained only a few small crystals using my excruciating manual temperature ramping technique.  As before, the crystals contain numerous inclusions of TiO₂.  Since As₂O₅ is hygroscopic and since I didn’t really want to spend more time working with it than I absolutely had to (so I weighed it straight out of the bottle), I suspect that my two runs have ended up containing less As₂O₅ relative to the other starting materials than I had planned.  In the BSE image below, the dark masses near the CTA crystal are Ti arsenate, which is difficult to separate from the CTA.  Also, I suspect that I got larger crystals in the last run because the inadvertent addition of P₂O₅ expanded the range of stability of the compound to lower temperature.  On the whole, CsNbOB₂O₅ ("CNB," not "CBO") appears to be much easier to grow.



My ramp-and-soak temperature controller assembly using the Omega CN8DPT controller appears to be functioning well in tests using a wirewound power resistor as the “furnace,” and so I hope to put it into operation for real at the end of next week.  I’m just waiting for delivery of some wire with teflon and fiberglass (“TGGT”) insulation so that I can connect the output directly to the furnace heating elements.  In the photo below, I’m cooling the resistor (attached to the stand on the left) at a controlled rate from 60º to 45ºC.

[Brian Joy]

I’ve finally managed to grow prismatic crystals of CsTiOAsO₄ that are free of inclusions.  The key to it simply was to decrease the cooling rate and intersperse 12-hour soak periods with the cooling ramps.  The crystals measure as much as ~2 mm in length -- some are acicular, while others are stubby; a minority show radiating habit.  The compound possesses no cleavage and so is easier to mount and polish than CsNbOB₂O₅.  In the BSE image below, the darker fragments are Ti arsenate, which turns into a gelatinous material when exposed to water and is difficult to remove completely (other than manually picking out crystals when dry).  I just started a new run using an even slower cooling rate; it should be done in about two weeks.