How do dyes incorporate into KDP crystals?

Potassium orthophosphate (KDP) crystals can be used as Q-switches and laser radiation converters in laser fusion applications to increase net energy output, as in the NIF (National Ignition Facility) laser array at the Lawrence Livermore National Laboratory (LLNL). The LLNL research scientist, Dr. Natalia Zaitseva, using techniques developed at Moscow State University in Russia, perfected methods of producing rapidly grown crystals. Dr. Zaitseva's research shows the conditions for optimized growth (temperature, level of supersaturation, impurity concentration) as well as how impurities and different dyes incorporate into KDP crystals.

Impurities affect KDP crystal growth in a variety of ways. Some impurities (like magnesium) show no visible affect, even at very high concentration. Other impurities (like aluminum) are very detrimental to KDP crystal growth. In general, impurities of aluminum, chromium, and iron incorporate preferentially into prismatic faces. On the other hand, work done previously, showed that some dyes incorporate into the pyramidal faces.

Dr. Zaitseva's crystal growth experiments with various dyes showed some interesting differences in dye incorporation into KDP crystals. Four dyes were studied: Chicago Sky Blue, Amaranth, Carminic Acid, and Hematoxylin. Of these four, two (Carminic Acid and Hematoxylin) appear to incorporate into both pyramid and prism. The other two (Chicago Sky Blue and Amaranth) incorporated only into the pyramidal faces, and are therefore of much more interest. Chicago Sky Blue and Amaranth did not incorporate uniformly. The crystals took color only after the growth temperature dropped to some fixed point.

It is not surprising that impurities, such as aluminum ions, incorporate into crystals due to their small size. The size of the dyes, however, is much larger and more complex. The structures of the dyes studied are shown below.

^{Chicago Sky Blue}

^Amaranth

^Hematoxylin

^{Carminic Acid}

These crystals all have normal shape, irrespective of which dye was added. The ratio of width to height was approximately 1.2, the same as for those crystals grown without dye in solution. Table 1 shows data for the 19

Table 1

#	Dye	Dye Amount	Temp	H	A/B	Mass	Comments
212	Chicago Sky Blue	40 mg	64.2	87	88	1695	subindividual crystal; crash, dye incorporated at the same time subindividual crystal
215	Chicago Sky Blue	~30 mg	33	50	52	256	same sol after 212; cracks, inclusion on prism final crystal
222	Chicago Sky Blue	4.0 mg	47.6	75	94	1342	crash at end; pyramids have very steep hillocks
233	Amaranth	1.2 g	62.7	96	118	2128	no color
240	Amaranth	1.2 g	65.5	79	103		color into pyramid only
247	Amaranth	1.2 g	65.5	78	103	1239	when pyramid started to take color it spoiled, crystals on bottom
369	Amaranth	1.2 g	62	63	73	748	removed as soon as it took color to preserve crystal
373	Amaranth	1.2 g	62.5	70	92	940	slight inclusion on pyramid before color incorporates
399	Hema-toxlyin	0.8 g	55.9	69	96	890	No visible color, remelted.
405	Amaranth	1.2 g	65.1	47		1522	horizontal crystal
408	Hema-toxylin	0.8 g	55.9	94	116	2011	No visible color; crystal appeared uniformly yellow when removed from solution
670	Amaranth	1.2 g	62.4	90	85	1146	clean, increased growth rate, then stop growth rate before crystal takes color
697	Carminic Acid	0.8 g	56.2	84	93	1122	No color; crystal appeared uniformly pink when removed from solution
747	Chicago Sky Blue	37 mg	42.1	67	91	714	defects on pyramid when crystal takes color, crash
748	Amaranth	1.2 g	48.1	65	67	534	pyramid spoiled, grow by blocks, crystal on bottom
760	Amaranth	1.2 g	51.1	64	82	649	no crystals on bottom
762	Chicago Sky Blue	37 mg	45.7	58	80	332
770	Chicago Sky Blue	37 mg	43	72	89	1026	no crystals on bottom
777	Chicago Sky Blue	37 mg	42.4	55	74	490	no crystals on bottom

crystals grown with the four dyes. Interestingly, Amaranth and Chicago Sky Blue did not incorporate into the crystal at the beginning of growth at high temperature (Table 2 and 3) while Carminic Acid and Hematoxylin seem to have colored the crystal throughout growth. When Chicago Sky Blue and Amaranth incorporation began other changes occurred in the crystals, as indicated in Table 1. Even though the dye only colored the pyramid, defects appeared in the prisms. Subindividual crystals and spontaneous crystals formed at the incorporation temperature. The crystals appeared to take dye at lower temperatures when smaller amounts of dye were added (Tables 2 and 3).

Table 2

KDP Crystal	Dye	Dye Amount	Temperature Dye Incorporates (°C)
212	Chicago Sky Blue	40 mg	26.6
215	Chicago Sky Blue	~30 mg	27.9
222	Chicago Sky Blue	4.0 mg	19.6
747	Chicago Sky Blue	37 mg	28.2
762	Chicago Sky Blue	37 mg	33.3
770	Chicago Sky Blue	37 mg	32.9
777	Chicago Sky Blue	37 mg	31.1

Table 3

KDP Crystal	Dye	Dye Amount	Temperature Dye Incorporates (°C)
233	Amaranth	0.12 g	<20
240	Amaranth	1.2 g	40.8
247	Amaranth	1.2 g	40.0
369	Amaranth	1.2 g	45.6
373	Amaranth	1.2 g	42.6
405	Amaranth	1.2 g	42.8
670	Amaranth	1.2 g	43.3
748	Amaranth	1.2 g	38.3
760	Amaranth	1.2 g	40.6

When the crystals were removed from solution, it became apparent that the dye had not distributed uniformly, as seen in Figure 1 (Amaranth, 1.2 g) and Figure 2 (Chicago Sky Blue, 37 mg) below.

Figure 1

Figure 2

With colorless contaminants (such as aluminum) it was impossible to determine how uniform the incorporation was. The only visual signs of poisoning were shape change or crystal failure. However, by using a colored organic contaminant (Chicago Sky Blue, Amaranth, Hematoxylin, and Carminic Acid) it was possible to visualize the dye incorporation. This reduced the need for specialized analysis of the KDP material for the presence of impurity. Dyes clearly showed not only incorporation, but localized concentration along growth hillocks (1, 2 in figures 1 and 2).

Some questions remained unanswered after this series of experiments. It is still unknown why Chicago Sky Blue and Amaranth incorporated into the pyramids while Hematoxylin and Carminic Acid incorporated into both the prism and pyramid. All four seemed to have similar ring structures, although Chicago Sky Blue and Amaranth contained double bonded nitrogens (which the other two lacked). Also, more research needs to be done to determine why Chicago Sky Blue and Amaranth incorporated only after the temperature had dropped to a fixed point and why this incorporation appeared to be dye concentration dependent. Finally, when impurities of relatively small size (like aluminum ion) caused crystal failure at low concentrations, how did complex organic molecules (Hematoxylin and Carminic Acid) incorporate and not affect crystal structure.

Acknowledgement

Thanks to Dr. Zaitseva for the use of her research notebooks.