Rare Earth Sulfides and the Origins of their Color

31 августа, 2015

Pokraskin

Rare earth sulfide materials have received considerable attention over past years, mainly because of their interesting optical and magneto-optical properties.

Rare earth elements and sulfur combine to form a wide range of compounds as sulfides or oxysulfides [3, 4]. Among them, sesquisulfides appeared to have the best potential as far as color is concerned (Table 4.1). We gave most of our attention to cerium sulfide, which was the most promising in terms of color intensity and purity.

Table 4.1 Color of some rare earth sesquisulfides in their y-form.

La2S3	Ce2S3	Pr2S3 Nd2S3 Gd2S3	Tb2S3	Dy2S3
White yellow	Dark red	Green Light green Sand	Light yellow	Orange

Rare earth sesquisulfides exist in different allotropic forms (Figure 4.1). Cerium sesquisulfide is known to exist in 3 different allotropic forms: a, b, and y, all stable at different temperatures and exhibiting different colors (Table 4.2).

La Ce Pr Nd Sm Eu Gd Tb Dy (Y) Ho Er Tm Yb Lu (Sc) a-L^ S^ Orthorhombic

Tetragonal

p-Ln2$3

Cubic

y-L^S3

Monoclinic

5-L^S3

Rhombohedric

£-Ln2$3

Cubic

T-Ln2S3

Figure 4.1 Different allotropic forms of the sesquisulfides of rare earth elements[5]. Table 4.2 Color and properties of rare earth sesquisulfide allotropic forms.

	a-Ce2S3	P-Ce2S3 — Cei0S 14OxS 1-х (0 < x < 1)	-Ce2S3 — Ce3-x^x S4 (□ — cation vacancy)
Color	Black/brown	Burgundy	Dark red
Structure	Orthorhombic	Tetragonal	Cubic
Temperature of formation	< 900 °C	> 900 °C	>1200°C

The low-temperature a-Ce2S3 phase is exactly stoichiometric and is stable up to 900 °C in non-oxidizing atmospheres [6, 7]. This phase is readily produced, but has very little interest in terms of color because of its brownish-black shade.

The b-Ce2S3 phase is in fact an oxysulfide containing a minor amount of oxygen substituting sulfide atoms. Its formula can be written as follows: Ce10S14OxS1-x with 0<x<1 [8-10] (Figure 4.2). It is burgundy in color, but it is possible to vary this from dark to light shades by adjusting the amount of oxygen in the molecule.

Подпись: Figure4.2 b-Structure: tetrahed- ric sites occupied either by oxygen atoms (white) or sulfur atoms (gray) in Ce^S^S^ (cerium atoms in black) [12].

The lower the oxygen content the lighter is the tint [11].

Подпись: Figure 4.3 y-Ce2S3 structure: dodecahedra joining at one edge [12].

The y-Ce2S3 phase (Figures 4.3 and 4.4) [12] is dark red, but can only be produced at temperatures above 1100 °C, which makes it very difficult to prepare on the industrial scale. This phase is, in fact, isomorphous with Ce3S4 and has the ability to accommodate other cations in its metal vacancies, for example alkaline earth cations or other non-cerium lanthanides [13-17]. Its formula can be expressed as Ce3-xS4 where x stands for cationic vacancies.

In the light of the above considerations, only the b — and y — forms were chosen to be developed as pigments. This, however, limited the available color range to burgundy and red.

Research was thus carried out to study the possibility of enlarging the color range by considering modifications of y-Ce2S3. Our results showed that the dark red color of y-Ce2S3 was due to electronic transitions from the Ce4f level into the Ce5d conduction band, corresponding to an energy gap of about 1,9 eV (Figure 4.5). It was also found that the value of the 4f-5d energy gap correlates to the ionicity of the Ce-S bond [18]. This conclusion was important as it meant that it should be possible to enlarge the color range by modifying the ionicity of this bond.

——————————————- VB Figure 4.5 Electronic transition at the origin of the

Ssp color for y-Ce2S3.

It was next found that modification of the ionicity of the Ce-S bond could be achieved by incorporating dopants in the cationic lattice vacancies. The color of Ce3-xS4 doped with alkali cations (Li, K, Na) could thus be tuned from red (Eg=1.9 eV) to orange (Eg= 2.1 eV), depending on the nature and the amount of dopant used. This phenomenon was attributed to an increase ofionicity ofthe Ce-S bond due to the lower electronegativity of the alkali cation, compared to that of Ce3+.

Because the amount of alkali cation dopant that could be incorporated within the vacancies is limited for structural reasons to a ratio of [D]/[Ce] = 0.2, only a small modification of the Ce-S bonding ionicity could be induced, and only red/orange-doped y — Ce2S3 materials could be obtained. A brighter orange shade (light orange) could however be obtained by partially substituting cerium atoms with lanthanum cations.

With this understanding of the properties of lanthanide sesquisulfides it was possible (by addition of alkali metal and/or alkaline earth elements) to enlarge the color range of cerium sesquisulfides from burgundy to light orange (Figure 4.6). Serendipitously, this chemical modification of the у-phase Ce2S3 also permitted stabilization at lower temperatures (< 800 °C), thus making industrial production more practical [19].

Figure 4.6 Reflectance curves of orange, red and burgundy cerium sulfide pigments.

Further research in this field is under way to extend the color range to yellow. Because of the limitations of the y-Ce2S3 doping approach, other systems than cerium sesquisulfides are under study. Sm2S3 appears to be a good candidate [20].

4.3