VALVES EXHAUST GAS
3. Further aspects of crystal growth from metallic fluxes 1. Diagnostics
In o r d e r to determine the suitability of various fluxes for the growth of single crystals of a particular c o m p o u n d , a n u m b e r of small scale and fairly rapid trials are generally required. Crystals p r o d u c e d in this way will most often be small, and it is useful in such cases to examine the melt microscopically. This is especially the case when there are problems associated with chemically separating the crystals from the flux. We have found it worthwhile in these situations to saw with a diamond wheel through the crucible and melt, and then examine the cross-section after polishing it metallurgically. A cut vertically through the crucible gives the best representation of what is actually happening in the melt during cooling. It is perhaps not surprising that crystals often grow much larger in melts placed in special locations in a given furnace due to the local t e m p e r a t u r e gradients, and aspects of this will be visible in such metallurgical examinations. Elemental analysis of these cross-sections in an appropriately equipped SEM is also useful.
3.2. Containers
Containers are a central problem in crystal growth using metal fluxes. Aspects of this have been m e n t i o n e d above. A short list of useful crucible materials is given in appendix 2. It is important to realize that solute reactivities in different melts vary enormously. For example, it was found possible to grow large rare
64 Z. FISK and J.P. REMEIKA
earth rhodium stannide crystals from excess molten tin in sealed quartz tubes.
Some rare earth attack on the quartz was observed, being greatest for the light rare earth compounds, but this attack was sufficiently slow to allow large crystals to grow anyway. In view of the exceptional stability of rare earth oxides, this indicates that the rare e a r t h - t i n interaction is quite strong, reducing the rare earth activity in the melt substantially. By contrast, rare earths and U dissolved in molten Cu are f o u n d to be quite reactive towards container materials, and this might be guessed at by the low melting temperatures of compounds o f rare earths and U formed with Cu. Crystals of rare earth-iridium C-15 Laves phase com- pounds have been grown from molten Cu. Ir dissolved in Cu will attack Ta, but the rate of this attack depends on which rare earth is in the melt. This attack problem was found to be severe for the case of Celr2, but not for T m l r 2. We were not able to find a completely suitable container for the CeIr 2 growth.
Ta, it should be said, is a convenient material to work with because tubing is readily available, and it is easily sealed shut in an arc furnace. W h e n welding a melt load shut in Ta in the arc furnace, it is useful to hold the broad, flattened part of the tube shut in a clamp made from two Cu bars. This large thermal mass keeps the tube cool and prevents expansion of gases inside the T a tube from spreading it apart in the region to be welded. Mo and W are also useful crucible materials, but are not ductile enough to crimp flat for welding. T h e s e crucibles can also be welded shut in an arc furnace if a fitted lid is fabricated. Some kind of copper clamp for holding the crucible while welding it shut is important. This holder should be bolted to the arc furnace hearth for effective heat exchange, since keeping the crucible charge cooled is important for these high t e m p e r a t u r e melting materials. Electron b e a m welding is, of course, a better alternative, but this is not as readily available.
3.3. Flux incorporation
One of the problems attendant to flux growth of single crystals is that macroscopic voids filled with the flux can often be found within crystals, and the presence of these voids is not necessarily evident from a simple visual inspection of the crystals. In the growth the U- and rare earth-Be13 crystals from At, lamellae of A1 are often found. We have often had to separate our crystals from these lammellae by spark machining, followed by leaching in N a O H solution.
A n y residual A1 can be detected by looking for the diamagnetic signal of A1 near 1 . 1 K . The presence of incorporated flux is generally due to unstable crystal growth conditions which usually occur in the initial growth stages, as discussed earlier. It is frequently possible to recognize where this initial growth occurred and mechanically remove this part of the material. Problems of this kind can be ameliorated by providing deliberate nucleation sites, although this complicates the experiment considerably. In the growth of rare earth rhodium stannides from excess Sn, we were never able to completely eliminate Sn inclusions, except in smaller crystals from a melt that had reached equilibrium.
GROWTH OF SINGLE CRYSTALS FROM MOI,TEN METAL FLUXES 65 3.4.
Stoichiometry
Many materials exist over a range of compositions. This variable stoichiometry is a source of problems in flux growth because the melt composition as well as phase boundaries vary during the cooling cycle. This sometimes results in a composition gradient in the grown material. In a n u m b e r of cases, one composi- tion b o u n d a r y is a vertical line in the phase diagram. This allows an excess to be added to the melt corresponding to forcing the growth always to occur at this limit of stoichiometry. It appears, for example, that the c o m p o u n d CePt 2 can form with deficient but not with excess Ce. We have grown crystals of this material from Pb melts, and found that we could approach quite closely the theoretical stoichi- ometry of 1 : 2 using large excess of Ce in the Pb melts. It is important to realize that these excesses may be much larger than one might expect, as we have found that certain melts " h o l d b a c k " certain elements - namely, the material that grows is very different in composition f r o m the charge.
The o t h e r important consideration concerns substitutional incorporation of flux in the growing crystals. For example, it has b e e n found possible to grow CeCu2Si 2 single crystals from In or Sn flux. Analysis shows that small amounts of In and Sn enter substitutionally into the CeCu2Si 2. T h e properties of CeCu2Si 2 are particu- larly sensitive to such substitutions, and for many experiments these crystals, therefore, proved to be unacceptable. T h e r e are m a n y cases of this kind of problem, and this makes it important to examine each possible flux from this standpoint. Chemical or atomic size similarity of the elements in the flux to one of the c o m p o n e n t s of a c o m p o u n d should alert one to this possibility.
3.5.
Separation oJ" crystals' and flux
We have discussed in various places above the problem of flux removal. T h e r e are two general approaches, chemical and mechanical. L e t us discuss chemical techniques first.
The stability of a c o m p o u n d in various acids and bases can be d e t e r m i n e d using polycrystalline material, and it is a good idea to investigate this before embarking on a lengthy attempt to grow crystals from a given flux. Specialized reagents do exist for certain elements, and it is often worthwhile to consult a practicing inorganic chemist for ideas connected with chemical etching. T h e r e are interesting effects, f u r t h e r m o r e , which appear to result from electrochemical differences between fluxes and crystals. In the growth of rare earth rhodium stannides from excess Sn, the crystals can be leached out with dilute HC1. As long as the crystals are in contact with the Sn flux, they remain shiny and metallic. Crystals which have separated from the Sn, however, quickly form a black tarnish from which, microprobe analysis reveals, the rare earth has been extracted; and the black material appears to be an a m o r p h o u s mixture of Rh and Sn. Thus, removal of the crystals from the leaching solution as soon as they are free of Sn flux is desirable.
As a n o t h e r case, we mention the growth of CeCu2Si ~ from molten Cu. It was
66 z. FISK and J.P. REMEIKA
reported that the Cu stoichiometry of CeCu2Si 2 crystals was very relevant to its superconducting properties. We t h e r e f o r e felt that growing CeCu2Si 2 from pure Cu would provide the limiting Cu stoichiometry for this material. T h e p r o b l e m came with the Cu removal, since, although an examination of the crystallization runs showed the presence of well f o r m e d crystals, Cu and the crystals are chemically attacked at nearly the same rate. Ultimately, we were able to slowly extract crystals using acetic acid-hydrogen peroxide mixtures, the process taking approximately one month.
A less often used, but useful, technique is to electrochemically r e m o v e the flux, using it as an anode in an electrochemical cell. By suspending the melt anode in the bath, the crystals can fall free as the etching proceeds, and if this electrolyte does not attack the crystals, they can be easily recovered. T h e advantage of this technique is that the voltage used in the erosion is easily controlled. T h e r e can also be " p r o t e c t i v e " voltages developed between crystals and flux of the kind pointed out earlier in the chemical etching of stannide melts.
A n o t h e r technique of some use is the solution of one metal by a n o t h e r after the growth is complete. As a case in point, we look at the growth of C e l n 3 crystals from excess In. T h e left over In after the growth can be dissolved in Hg. T h e crystals can then be r e m o v e d from the liquid (at r o o m t e m p e r a t u r e ) I n - H g solution, and the H g film remaining on the C e l n 3 crystals can be p u m p e d into a trap in a vacuum system with gentle heating.
The usefulness of centrifuging off low melting fluxes has been discussed above.
Such a process can easily be used in the C e l n g - I n case just mentioned. T h e r e is the additional possibility of flux removal of high vapor pressure metals by simple evaporation after the growth process in a pressure containing vessel. T h e r e is also the less elegant m e t h o d of cutting crystals from melts, exactly as is done from zone refined rods.
3.6. Solution kinetics"
Some flux growth attempts appear to fail because of problems having to do with slow solution kinetics. T h e r e are cases where such slow kinetics can be used to advantage, but more often it constitutes a nuisance. As an example we can look at C u - B solutions. T h e r e are no compounds in the phase diagram, and there is a broad eutectic melting near 1000°C. If we can believe this r e p o r t e d phase diagram, this suggests that Cu should be a very promising solvent for growth of borides. A n u m b e r of ternary superconducting borides have in fact been grown from Cu. A problem, however, is that B seems to dissolve into m o l t e n Cu very slowly. This means that a long soak time at the maximum running t e m p e r a t u r e is required. In addition, if the growth charge consists of the elements of the c o m p o u n d and Cu, it can h a p p e n that the elements other than B will dissolve in Cu first, and then react out on the B, possibly making further solution of B difficult. This can be avoided by pre-reacting the c o m p o u n d and using it in the charge, rather than the elements. It is also sometimes useful to make actual
GROWTH OF SINGLE CRYSTALS FROM MOLTEN METAL FLUXES 67 solubility tests of a c o m p o u n d in a given flux. This can be done in an approximate m a n n e r by soaking arc-melted pellets of the c o m p o u n d in given amounts of flux at fixed temperatures, quenching to r o o m t e m p e r a t u r e , and subsequently measuring the weight loss of the leached-out pill of compound.
4. Extensions of the technique