CsCoPO4 synthesis

[Brian Joy]

I think I can state with some confidence – based on a handful of quantitative analyses – that I’ve managed to synthesize a small amount of CsCoPO₄ using Cs₂CO₃, CoCl₂·6H₂O, and H₃PO₄ as starting materials.  I actually aborted the run after about two hours when I realized that I had used incorrect amounts of the reactants.  At that point, the furnace had reached a temperature of ~750ºC.  The run products comprise a variety of materials -- including more than one Cs-Co phosphate -- that are obviously not representative of equilibrium.  However, it was heartening to see production of apparent CsCoPO₄ after such a short period at relatively low temperature.  I now have a second run in the furnace and intend to keep it at 850ºC for a few days.  I’m relying on the following reaction to produce the CsCoPO₄:

5Cs₂CO₃ + 2CoCl₂ + 6H₃PO₄ = 2CsCoPO₄ + 2Cs₄P₂O₇ + 4HCl + 7H₂O + 5CO₂

The Cs₄P₂O₇ produced by the reaction acts as a flux.  Thankfully it’s easily dissolved with water after the run is complete.  I’ll update on the results of this run as soon as I can.

I haven’t done a thorough investigation of beam sensitivity of the compound yet, but it’s definitely not as easily beam-damaged as the Sr-Cs phosphate that I grew previously.  I’m going to try another run in Sr-bearing system at higher temperature than before in an attempt to produce the hexagonal SrCs₄(PO₃)₆ phase -- I should get to this during the upcoming week.

It would be of great help to me if someone could dig up a phase diagram for the CsPO₃-Co(PO₃)₂ system -- I haven’t been able to find one.

Here is a BSE image showing the likely CsCoPO₄ (brightest) intergrown with another Cs-Co phosphate and a few other compounds:

[Brian Joy]

And here is a quickly-collected ED spectrum (beam energy = 15 keV) of the bright phase:

[Probeman]

No obvious overlaps there!  If it can be produced as a pure phase, and is beam stable, it could be a good candidate standard for Cs.

[Brian Joy]

I’ve done some testing of the stability of CsCoPO₄ under the beam.  The results aren’t exactly fantastic.  I'll let the plots below do most of the talking.  With accelerating potential = 15 kV and Faraday cup current = 10 nA, I used the JEOL chart recorder to record count rates over periods of ten minutes with the beam fully focused and then with the beam defocused to 10 microns.  The sampling interval was 0.5 s.  Below the plot for CsCoPO₄, I show a corresponding plot for natural pollucite.

The CsCoPO₄ could certainly be very useful, but the beam absolutely must be defocused to prevent migration of Cs.

[Probeman]

Too bad!   Do you have another candidate Cs compound in mind?

[Brian Joy]

Too bad!   Do you have another candidate Cs compound in mind?

I'm going to try once more to grow the hexagonal compound in the Sr(PO3)2-CsPO3 system.  If I can't, then I'll try to grow BaCs4(PO3)6, as this compound has definitely been grown before.  The problem with it is that Ba Ln is uncomfortably close to Cs La using PET (but not LiF).  Also, I have a small amount of ZrO2, and so I might even try CsZrOPO4 just for kicks.

[crystalgrower]

To tell the truth Brian, your other synthesis looks a lot closer to the desired product for a first run.

Phase diagram for Cu and Cs metaphosphates is at Materials Research Bulletin Vol 7 p 1525-1534 from 1972.  They also refer to Zn-Cs metaphosphate mixes.

Direct synthesis of CsFePO4 is fully discussed in ACS journals, their search engine works best.  Co will have less of a redox issue.

[crystalgrower]

Hi Brian.

If you are interested in pursuing  CsCoPO4 crystals, then here is an option.  I say crystals as opposed to synthesis because your synthesis is  well done.  Many structure papers mention two different chemical reactions, one to see which phases form, and the other to produce single crystals. 

Use CsCl as the flux and add MnCl2 and Cs3PO4 as reagents. The principle is to use only the ions that you want in the crystals.  Make a little fresh Cs3PO4 and bake it dry on a hotplate.   Hydrated MnCl2 is OK.

Start  with a crucible that is free of nanoseeds.

Calculate equal moles of Mn and PO4 and then use 102% of PO4 (a 2% mole excess of PO4 over Mn).   Use reagents as about 10% of flux weight.  Grind all three compounds together in an agate mortar with a small amount of isopropanol to a homogeneous mix.  Load up your crucible and heat at 150C for a few hours to dry.  Then  slowly raise heat to 750C  and hold it there for 2 days if possible.  Cool crucible slowly and leach with water as per H3PO4 fluxed syntheses.   Leached CsCl can be recycled.

I don’t know if you need am inert   atmosphere to prevent oxidation of Mn+2 to +3.  Some  compounds of Cs-Mn+3-PO4  have been reported as being made from in-situ reduction of Mn+4 as MnO2 plus  other solid state ingredients.  The Russian group has also prepared similar R-Cs metaphosphates.

You will get only one phase because the low Mn limits the possible products.  If Mn > P then you might get some Mn2PO4Cl.  If Mn goes to +3 then you might get some MnPO4.  Both are nice but not the goal here. 

If you want some CsCl to try, I will send what I have. 

References

The venerable paper for CaCl2 and CaF2 as  fluxes for apatites by J. S. Prener, J Electrochem Soc, v114 pp 77-83 (1967). 
Another use of same method Banks et al, Journal of Solid State Chemistry, vol 3 pp 308-313 (1971) 
The Prener paper has been cited many times.  Please note that apatite must be formed above 1040C.  Evaporation of salt solvent below 800C is minimal.

RPO4 has been prepared in mixed salt melt systems of fission products by adding PO4. The point of mixed salt having lower melting point does not apply here because you do not want any LiMnPO4.

Volkovich et al,  Phys Chem Chem Phys vol 5 pp3053-3060 (2003) DOI 10.1039/b302280n   They added Na3PO4 pellet to molten salt crucible

Hudry et al, Inorganic Chemistry, vol 48 pp 7141-7150 (2009) DOI 10.1021/ic9003142 They added (NH4)H2PO4  at high temperature (!) Bubbling  decomposition would be unreproducible.  Not to mention, unnecessary.

[crystalgrower]

One last reference which I have not accessed after I had read the others

Korchemkin et al, Journal of Chemical Thermodynamics, v 78 pp 114-119 (2014)  This contains XRD and EPMA analysis and presumably crystal growing details for CsMnPO4

doi  http://dx.doi.org/10.1016/j.jct.2014.06.012

[Brian Joy]

Calculate equal moles of Mn and PO4 and then use 102% of PO4 (a 2% mole excess of PO4 over Mn).   Use reagents as about 10% of flux weight.  Grind all three compounds together in an agate mortar with a small amount of isopropanol to a homogeneous mix.  Load up your crucible and heat at 150C for a few hours to dry.  Then  slowly raise heat to 750C  and hold it there for 2 days if possible.  Cool crucible slowly and leach with water as per H3PO4 fluxed syntheses.   Leached CsCl can be recycled.

I've managed to dig up 20 or 30 grams of CsCl; I still need to look for a source of Mn.  But how do I prevent the alumina crucible from participating in the reaction at 750°C?  When I ran a second CsCoPO₄ synthesis at about 850°C for 35 hours, I mostly got Cs₂Co₂Al(PO₄)₃, which is the dominant phase in the BSE image below.  (It turns out to be more beam-sensitive than CsCoPO₄.)  Even the CsCoPO₄, which is the brightest phase in the image, contains about 0.5 wt% Al₂O₃.  Like I noted in another post, I've also had problems with Pt reacting with Cs phosphate.

[Probeman]

Dang.

This is difficult stuff.

No wonder the Calchemist guys want $5K for 50 grams of a CsZr phosphate.

[Brian Joy]

I’ve pressed into service the Cs₂Co₂Al(PO₄)₃ that I (accidentally) grew as a provisional standard for analyzing the natural pollucite that I had previously been using as a Cs standard.  I’ve assumed the synthetic compound to be stoichiometric.  Based on a combination of microprobe analyses, stoichiometric constraints, and solution ICP-MS (to determine Li content, which turns out to be negligible), my previous best guess as to the composition of the pollucite was as follows (in wt%):

SiO₂:  43.0
Al₂O₃:  16.3
FeO:  <0.01
CaO:  <0.01
Li₂O:  <0.01
Na₂O:  1.4
K₂O:  0.01
Rb₂O:  0.1
Cs₂O:  37.6
H₂O:  1.6

At beam energy = 15 keV, I’ve found that the Cs₂Co₂Al(PO₄)₃ shows no evidence of change in Cs Lα count rate over a period of ten minutes using a 10 nA current with the beam defocused to 10 microns, and so I used these conditions to analyze the pollucite (counting for 10 s peak and 10 s bkg on Cs Lα).  (Normally I defocus the beam anyway when analyzing pollucite.)

Using natural sanbornite (BaSi₂O₅, Si), natural celsian (Al), natural albite (Na), natural adularia (K), RbTiOPO₄ (Rb), and Cs₂Co₂Al(PO₄)₃ as standards and assuming 1.4 wt% H₂O, I get the following results:

PAP/MAC30:


The usual constraints on the pollucite-analcime solid solution for the six-oxygen formula unit (excluding H₂O) are as follows (see Beger, Z. Kristallogr. 129:280-302; Černý, Can. Min. 12:334-341):
n_Si + n_Al = 3
n_Cs + n_H2O = 1
sum large cations = n_Al

For this case (average of 52 analyses),
n_Si + n_Al = 3.000
n_Cs + n_H2O = 1.000 (obviously I had to choose a value of H₂O)
sum large cations = 0.911, n_Al = 0.912

Of course I can’t prove that the result is accurate, but it sure looks pretty reasonable.  Also, note that the matrix correction factors for Cs Lα are close to unity, and so the choice of matrix correction model doesn't have much effect on wt% Cs₂O.  The Armstrong model gives slightly lower wt% Cs₂O due to the smaller atomic number correction.

Armstrong/FFAST:

[crystalgrower]

Hi Brian,

The salt flux is not nearly as reactive to alumina as H3PO4 (which forms P2O5 at about 500C).

This lack of reactivity is very well known from nuclear fuel  experiments using LiCl-NaCl solvent for weeks at a time.

I have 100g of CsCl to contribute.

The other issue of borrowing a Pt crucible might be a piece of crowd sourcing. 

I borrowed a PURE Pt crucible for 100 rounds of synthesis with H3Poi4 and there was no measurable change.  I consider that you have developed enough working skills to ask for this loan.  The price of Pt is low enough now (US$900 per troy ounce of 31.6 grams) to ask for a loan from other forum users.

Single crystals of NaCl are routinely grown from Pt.  Cooking CsCl flux in Pt isn't any different, except it goes at 100C lower.

And for anybody considering purchase, I bought a crucible from Birmingham Metals in the UK.  They cast them,  therefore  far lower labour cost on top of bullion weight.  I was extremely satisfied with the performance.  The business retained the crucible of course.  Otherwise we would probably have decent Cs crystals right now.