Tri-Color-Change Holmium-Doped Synthetic CZ
Colored yttria-stabilized synthetic cubic zirconia (CZ) has been an attractive diamond substitute since the late 1970s (R.T. Liddicoat and J.I. Koivula, “Synthetic cubic stabilized zirconia,” Summer 1978 G&G, pp. 58–60). The different colors seen in synthetic CZ are caused by the introduction of specific transition metal elements and rare earth elements (K. Nassau, “Cubic zirconia: an update,” Spring 1981 G&G, pp. 9–19). At this year’s Tucson Gem and Mineral Show, the authors obtained an interesting synthetic CZ rough that exhibited an unusual color-change behavior (figure 1). Unlike traditional color-change stones such as alexandrite, color-change corundum, and color-change garnet, this material did not exhibit different colors when illumination alternated between incandescent light and daylight conditions (approximated by CIE standard illuminants A and D65, respectively). Surprisingly, it showed three distinct hues in daylight/incandescent light (yellowish green), in fluorescent lighting corresponding to CIE standard illuminant F9 (green-blue), and in CIE standard illuminant F10 (purplish violet). One wafer with 3.32 mm thickness was polished and analyzed to understand this color-change phenomenon.
The wafer’s chemical composition was obtained using a Thermo Fisher iCAP Q ICP-MS coupled with a New Wave Research UP-213 laser ablation unit. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses were performed in the same spot where the spectroscopic data was collected. Based on its calculated chemical formula of Zr0.52Y0.45Ho0.02Hf0.01O1.79 (the formula is nonstoichiometric and calculated by forcing cations to 1 atom per formula), the wafer was a holmium-doped yttria-stabilized CZ (S. Gutzov et al., “High temperature optical spectroscopy of cubicholmium doped zirconia, Zr0.78Y0.21Ho0.01O1.90,” Physical Chemistry Chemical Physics, Vol. 9, No, 4, 2007, pp. 491–496). Holmium was the only detected chromophore, at a concentration of about 7780 ppma.
Visible spectra were collected with a Hitachi U-2910 spectrometer with a 1 nm spectral resolution at a scan speed of 400 nm/min. The wafer’s spectrum matched the holmium-doped CZ spectrum reported by Gutzov et al. (2007). The unusual color change under different illuminants can be qualitatively understood by examining figure 2. The violet and orange emission peaks of the F10 illuminant are not absorbed by Ho-doped CZ; however, there is significant absorption by holmium of the green emission peak for the F10 illuminant. This selective absorption of the F10 emissions creates a purplish violet color (figure 2, top). The F9 illuminant has more of a broadband emission, which is unaffected by the strong but narrow absorption by holmium of green light at 540 nm. This results in a blue-green color under F9 illumination (figure 2, middle). The D65 illuminant is essentially a broadband emission roughly corresponding to a black body irradiator at 6500 K. Under D65 illumination, a yellowish green color is produced because of the strong absorption of blue light by holmium (figure 2, bottom).
The wafer’s visible absorption spectrum was reflection corrected by subtracting the absorbance value at 850 nm, where no chromophoric absorption is expected, from values for every other data point along the rest of the spectrum. The reflection-loss-corrected visible spectrum can then be used to quantitatively calculate the color of this material at a wide range of path lengths and under different lighting conditions (Z. Sun et al., “Vanadium- and chromium- bearing pink pyrope garnet: Characterization and quantitative colorimetric analysis,” Winter 2015 G&G, pp. 348–369). There are large differences in the CIE L*, a*, b* color coordinates between D65, F10, and F9 (figure 3). One way to judge the quality of a color-change stone is to plot the color pair in the CIE 1976 color circle. Well-defined color-change pairings show a large hue angle difference, a small chroma difference, and high chroma values. The color coordinates of the material with a 10 mm light path length in D65, F9, and F10 were plotted in the CIE 1976 color circle shown in figure 3; calculated color panels for the illuminants were also placed alongside the faceted material (see below). The fact that the material shows three distinct hues in three different white lights makes it unique.
