Inorganic Green Pigments Based on LaSr2AlO5

Yamaguchi, Kazuki; Takemura, Akari; Furumoto, Saki; Oka, Ryohei; Masui, Toshiyuki

doi:10.3390/ceramics6040138

Open AccessArticle

Inorganic Green Pigments Based on LaSr₂AlO₅

¹

Department of Chemistry and Biotechnology, Faculty of Engineering, and Center for Research on Green Sustainable Chemistry, Tottori University, 4-101, Koyama-cho Minami, Tottori 680-8552, Japan

²

Field of Advanced Ceramics, Department of Life Science and Applied Chemistry, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan

^*

Author to whom correspondence should be addressed.

Ceramics 2023, 6(4), 2269-2281; https://doi.org/10.3390/ceramics6040138

Submission received: 29 September 2023 / Revised: 25 October 2023 / Accepted: 10 November 2023 / Published: 22 November 2023

(This article belongs to the Special Issue Advances in Ceramics, 2nd Edition)

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Abstract

:

La_1.03Sr_1.97Al_0.97M_0.03O₅ (M = Fe, Co, Ni, and Cu) samples were synthesized using a citrate sol–gel method to develop a novel environmentally friendly inorganic green pigment. Among them, the Co-doped sample exhibited a vivid yellow, but not green. Then, (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ was synthesized and characterized with respect to the crystal structure, optical properties, and color. The sample was obtained in a single-phase form and the lattice volume was smaller than that of the (La_0.94Ca_0.06)Sr₂AlO₅ sample, indicating that Mn ions in the lattice of the sample were pentavalent. The sample exhibited optical absorption at a wavelength below 400 nm and around 650 nm. These absorptions were attributed to the ligand, the metal charge transfer (LMCT), and d-d transitions of Mn⁵⁺. Because the green light corresponding to 500 to 560 nm was reflected strongly, the synthesized sample exhibited a bright green color. (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ showed high brightness (L* = 50.1) and greenness (a* = −20.8), and these values were as high as those of the conventional green pigments such as chromium oxide and cobalt green. Therefore, the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ pigment is a potential candidate for a novel environmentally friendly inorganic green pigment.

Keywords:

green pigment; environmentally friendly; Mn⁵⁺ ion; yellow pigment; Co²⁺ ion; lanthanum strontium aluminate

Graphical Abstract

1. Introduction

Inorganic pigments are widely used as coloring materials for ceramics, glasses, plastics, and glazes because they have good thermal stabilities and resistances to climatic conditions. Among various colors, a greenish color makes us feel reposed, healed and relaxed; therefore, it has been used for a variety of applications. A characteristic of green is that it brings peaceful images of woods and forests. In addition, green signifies safety on traffic signs and public guideboards, and such feelings are widely accepted by people [1]. Thus, green is also used for information boards in evacuation centers and an emergency exits, etc. Moreover, because green is an intermediate color on the hue circle, it is used in various situations for harmonization with other colors.

Currently, chromium oxide (Cr₂O₃), chromium green, cobalt green (CoO·nZnO), and so on are used as commercially available inorganic green pigments. For Cr₂O₃, studies have been conducted to improve its optical reflectance properties by mixing it with TiO₂, Al₂O₃, and V₂O₅ [2], or by substituting other metal ions at the Cr³⁺ site, as in Cr_2−xAl_xO₃ [3]. Sangeetha et al. found that Cr₂O₃ added with rare earth elements improved the greenness of the reflectance without decreasing the greenness [4]. Cr³⁺ is not a harmful ion but Cr₂O₃ has been synthesized by reducing the Na₂Cr₂O₇ or pyrolysis of CrO₃, which contains Cr⁶⁺ [5]. Chromium green is composed of iron blue (Fe₄[Fe(CN)₆]₃) and chrome yellow (PbCrO₄) and is used in paints because of its high hiding power. However, chrome yellow contains Pb and Cr⁶⁺. Cr⁶⁺ is identified as a carcinogen by the International Agency for Research on Cancer (IARC), and so is well known as a highly toxic element. At the same time, Pb is identified as possible carcinogenesis. On the other hand, the safety of CoO in cobalt green has also been questioned due to the concern about adverse effects on the human body. The use of these pigments has been restricted because they contain harmful elements (e.g., Cd, Pb, and Cr) and CoO, which has deleterious effect on the human body and the environment. Therefore, there is a strong desire to develop environmentally friendly inorganic green pigments to replace harmful inorganic pigments.

Based on this background, in order to develop novel environmentally friendly inorganic green pigments, we took note of pentavalent manganese (Mn⁵⁺) ions as a coloring source. Mn ions are not only harmless but also inexpensive and exhibit various valences to be utilized in multiple applications. Mn⁵⁺ absorbs light with a wavelength of 400 nm or less based on ligand-to-metal charge transfer (LMCT) transition and d-d transition (³A₂ → ³T₁ (³P)) [6,7]. Visible light around 650 nm is also absorbed by d-d transition attributed to ³A₂ → ³T₁ (³F), and therefore, green coloration can be expected by using Mn⁵⁺. Although Mn⁵⁺ ions tend to be generally unstable, they can be stable in the following situation: Mn ions are substituted in tetrahedral sites as well as a host compound, which is comprised of larger ionic radius elements such as alkali earth metal (Sr and Ba) and smaller ionic radius elements such as P and Al [6]. Actually, various Mn⁵⁺-containing compounds have been reported [7,8,9,10,11,12] such as turquoise color pigments based on apatite-type Ba₅Mn_3−xM_xO₁₂Cl (M = V, P) [13] and Ba₃(MO₄)₂:Mn⁵⁺ (M = V, P) phosphors [14].

We focused on LaSr₂AlO₅ as a host material because it contains only low toxicity elements. LaSr₂AlO₅ adopts a tetragonal structure (space group: I4/mcm) [15]. This complex oxide is stacked alternately with an 8-coordinated La³⁺/Sr²⁺ layer, a 10-coordinatd Sr²⁺ layer, and a 4-coordinated Al³⁺ layer through O^2-. A number of papers have also reported the use of LaSr₂AlO₅ as a base material for photocatalysts and phosphors [15,16,17,18,19,20]. There are many compounds in which La³⁺ is replaced by other elements, but few in which the Al³⁺ is replaced.

In this study, therefore, La_1.03Sr_1.97Al_0.97M_0.03O₅ (M = Fe, Co, Ni, and Cu) samples, whose Al³⁺ site was substituted by various transition metal elements, had their color properties characterized. Among them, the Co-doped sample exhibited a vivid yellow, but not green. Thus, La_1−2xCa_2xSr₂Al_1−xMn_xO₅ (x = 0 and 0.03) samples were synthesized to obtain a new environmentally friendly green pigment. The Al³⁺ site in the LaSr₂AlO₅ host was partially substituted by Mn⁵⁺ ions. At the same time, the La³⁺ site was replaced by Ca²⁺ for charge compensation. The color properties of the samples were evaluated and compared with commercially available inorganic green pigments.

2. Materials and Methods

2.1. Synthesis

The LaSr₂(Al_0.97Fe_0.03)O₅, La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Ni and Cu), and La_1+xSr_2−x(Al_1−xCo_x)O₅ (x = 0, 0.03, 0.05 and 0.10) samples were synthesized using a citrate sol–gel method. La(NO₃)₃·6H₂O (FUJIFILM Wako Pure Chemical Industries Ltd., Osaka, Japan, 99.9%), Sr(NO₃)₂ (FUJIFILM Wako Pure Chemical Industries Ltd., 98.0%), Al(NO₃)₃·9H₂O (FUJIFILM Wako Pure Chemical Industries Ltd., 99.9%), Fe(NO₃)₃·9H₂O (FUJIFILM Wako Pure Chemical Industries Ltd., 99.9%), Co(NO₃)₂·6H₂O (FUJIFILM Wako Pure Chemical Industries Ltd., 99.5%), Cu(NO₃)₂·3H₂O (FUJIFILM Wako Pure Chemical Industries Ltd., 99.9%), and Ni(NO₃)₂·6H₂O (Kishida Chemical Co. Ltd., Osaka, Japan, 98.0%) were weighed so as to obtain the objective compositions and were dissolved in deionized water to adjust the La, Sr, and (Al + M) concentrations to 0.2 mol L⁻¹. After the solution was stirred homogeneously, citric acid (FUJIFILM Wako Pure Chemical Industries Ltd., 98.0%) was added as a chelating agent to complex the cations into the solution in the mole ratio 2:1 with regard to the total cations (La, Sr, Al, and M). The mixed solution was stirred at 80 °C until a gel was obtained. Then, the gel was heated in an oven at 120 °C for 24 h and completely dried at 350 °C on a mantle heater. The dried gel was calcined in an alumina crucible at 500 °C for 8 h in air. After the calcination, the sample was heated again in an alumina boat at 1400 °C for 9 h in air. Before characterization, the sample was ground in an agate mortar with agate pestle.

(La_1−2xCa_2x)Sr₂(Al_1−xMn_x)O₅ (x = 0 and 0.03) samples were prepared by the same procedure described above. The starting materials, La(NO₃)₃·6H₂O, Ca(NO₃)₂·4H₂O (FUJIFILM Wako Pure Chemical Industries Ltd., 98.5%), Sr(NO₃)₂, Al(NO₃)₃·9H₂O, and (CH₃COO)₂Mn·4H₂O (FUJIFILM Wako Pure Chemical Industries Ltd., 99.9%), were weighed stoichiometrically.

2.2. Characterization

X-ray powder diffraction (XRD) was conducted using an X-ray diffractometer (Rigaku Corporation, Tokyo, Japan Ultima IV) to identify the crystal phase and structure. The XRD patterns were taken with Cu-Kα radiation operating with a tube voltage of 40 kV and a tube current of 40 mA. The data were collected by scanning over the 2θ range of 20–80°. The sampling width was 0.02° and the scan speed was 6° min⁻¹. The lattice volumes were calculated from the XRD peak angles refined by α-Al₂O₃ as a standard and using the CellCalc Ver. 2.20 software. The Rietveld refinement of the resulting XRD patterns was performed by the RIETAN-FP software (version 3.12) package to determine the precise crystal structure [21]. From the Rietveld refinement, the following final R-factors were obtained: Rwp (R-weighed pattern), Rp (R-pattern), Re (R-expected), S (goodness-of-fit indicator), and RF (R-structure factor). The optical reflectance spectra of the as-prepared samples were recorded on an ultraviolet-visible (UV-Vis) spectrometer (JASCO Corporation, Tokyo, Japan, V-770 with an integrating sphere attachment) using a standard white plate as a reference. The step width was 1 nm and the scan rate was 1000 nm min⁻¹. The color properties of the powder samples were evaluated in terms of the Commission Internationale de l’Éclairage (CIE) L*a*b*Ch° system using a colorimeter (Konica-Minolta, INC., Tokyo, Japan, CR-400). A standard C illuminant was used for the colorimetric measurements. The L* parameter indicates the brightness or darkness in a neutral grayscale. The a* values represent the red–green axes, and the b* value yellow–blue axes. The chroma parameter (C) means the color saturation and is calculated with the formula, C = [(a*)² + (b*)²]^1/2. The hue angle (h°) ranges from 0 to 360° and is calculated with the formula h° = tan⁻¹(b*/a*). The standard deviations of all values for the L*a*b*Ch° color coordinate data were less than 0.1.

3. Results and Discussion

3.1. La_1+xSr_2−x(Al_1−xM_x)O₅ (M: Fe, Co, Ni, and Cu)

3.1.1. X-ray Powder Diffraction (XRD)

Figure 1 shows the XRD patterns of the LaSr₂AlO_5, LaSr₂(Al_0.97Fe_0.03)O₅, and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) samples. In the sample containing Fe, the objective LaSr₂AlO₅ phase was obtained in a single-phase form. On the other hand, in the samples containing Co, Ni, and Cu, the target LaSr₂AlO₅ phase was obtained as the main phase, but a few peaks of SrLaAlO₄ and SrLaNiO₄ were observed as impurities, resulting in a mixed phase.

3.1.2. Ultraviolet-Visible (UV-Vis) Reflectance Spectra

The UV-Vis reflectance spectra of LaSr₂AlO₅, LaSr₂(Al_0.97Fe_0.03)O₅ and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) are depicted in Figure 2. In the case of M = Fe, weak optical absorption due to the d-d transition of Fe³⁺ [22] was observed in the visible region around 420 nm. When M was Ni, the sample absorbed visible light around 650 nm because of the d-d transition of Ni²⁺ [23]. The sample with Cu absorbed visible light less than 400 nm and longer than 700 nm. These absorptions were attributed to the charge transfer between O_2p-Cu_3d orbitals and the d-d transition of Cu²⁺ [24]. In the sample with Co, visible light less than 450 nm was absorbed, which was based on the charge transfer transition between O_2p-Co_3d orbitals and the d-d transition of Co²⁺ [25,26]. In general, the energy gap of the d-d transition of Co²⁺ at the tetrahedral site corresponds to green–red light from 530 to 630 nm. In this study, the Al³⁺ site (ionic radius: 0.039 nm, 4-coordination [27]) was substituted with Co²⁺ (ionic radius: 0.058 nm, 4-coordination [27]), resulting in a stronger crystal field surrounding Co²⁺ and the absorption of higher energy light around 450 nm, corresponding to the blue light region.

3.1.3. Color Property

The L*a*b*Ch° color coordinate data of the LaSr₂AlO₅, LaSr₂(Al_0.97Fe_0.03)O₅, and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) powder samples are summarized in Table 1. The sample photographs are also shown in Figure 3. The LaSr₂AlO₅ sample reflected the entire visible light region and was white. For M = Fe³⁺ and Cu²⁺, the visible light region was almost completely reflected due to the weak d-d transition, and the samples were almost white. The sample with Ni²⁺ was a dingy green. This is due to the impurity phase of SrLaNiO₄, which reduces the reflectance of visible light. In the case of Co²⁺ doping, a bright yellow color was obtained. In general, Co²⁺ ions exhibit a vivid blue color in tetrahedral coordination, as in cobalt blue (CoAl₂O₄) [28,29]. In the Co-doped sample in this study, Co²⁺ is also in a 4-coodinated environment. Despite this, interestingly, the sample exhibited a yellow color. This is a rare case.

To investigate the composition dependence of the yellow-colored Co-doped samples, La_1+xSr_2−x(Al_1−xCo_x)O₅ (x = 0.03, 0.05, and 0.10) samples were synthesized. X-ray powder diffraction measurements showed that the samples with x = 0.05 and 0.10 were mixed phases, as was the sample with x = 0.03, but the target LaSr₂AlO₅ phase was obtained as the main phase. The Co²⁺ content dependence on the lattice volume of the samples is summarized in Table 2. Since the ionic radius of Co²⁺ (0.090 nm, 8-coordination [27]) is smaller than that of Sr²⁺ (0.126 nm, 8-coordination [27])/La³⁺ (0.116 nm, 8-coordination [27]), the lattice volume should shrink when the 8-coordination Sr(1)/La sites in the host material are replaced with Co²⁺. However, the lattice volume of the sample increased with Co²⁺ content up to x = 0.05. As noted above, this indicates that the Al³⁺ sites in the host lattice were partially replaced by larger Co²⁺ ions. Therefore, the chromophore in this sample is considered to be a 4-coordinated Co²⁺ ion.

Table 3 summarizes the L*a*b*Ch° color coordinate data for La_1+xSr_2−x(Al_1−xCo_x)O₅ (x = 0, 0.03, 0.05, and 0.10). The sample photographs are also shown in Figure 4. All samples exhibited a yellow color but become slightly darker with increasing Co²⁺ concentration.

Unfortunately, the LaSr₂(Al_0.97Fe_0.03)O₅ and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) samples did not exhibit a green color, so we synthesized (La_1−2xCa_2x)Sr₂(Al_1−xMn_x)O₅ (x = 0, 0.03) and characterized it in the next section.

3.2. (La_1−2xCa_2x)Sr₂(Al_1−xMn_x)O₅ (x = 0 and 0.03)

3.2.1. X-ray Powder Diffraction (XRD)

Figure 5 shows the XRD patterns of the (La_1−2xCa_2x)Sr₂(Al_1−xMn_x)O₅ (x = 0 and 0.03) and (La_0.94Ca_0.06)Sr₂AlO_4.97 samples. All samples were obtained in a single-phase form. The lattice volume of each sample was calculated from the XRD peak angles and is summarized in Table 4. The lattice volume decreased in the order of undoped > Ca²⁺-doped > Ca²⁺-Mn⁵⁺ co-doped. In (La_0.94Ca_0.06)Sr₂AlO_4.97, some of the La³⁺ (ionic radius: 0.116 nm, 8-coordination [27]) ions in LaSr₂AlO₅ were replaced by smaller Ca²⁺ ions (ionic radius: 0.112 nm, 8-coordination [27]), which resulted in the lattice volume contraction. In the sample co-doped with Ca²⁺ and Mn⁵⁺, the lattice volume was further decreased. The ionic radii of the 4-coordinated Mn²⁺ and Mn⁴⁺ are 0.039 and 0.066 nm, respectively [27], and those are equal to or larger than Al³⁺ (ionic radius: 0.039 nm, 4-coordination [27]), so the lattice volume in Table 4 should be the same or increase when Mn²⁺ or Mn⁴⁺ is dissolved in the Al³⁺ site, but it actually decreased. Accordingly, the decrease in lattice volume was not only because La³⁺ was substituted with Ca²⁺, but also because some of the Al³⁺ ions in the host material were replaced by smaller Mn⁵⁺ ions (ionic radius: 0.033 nm, 4-coordination [27]), although the substitution by Mn⁶⁺ is also in accordance with this result.

The Rietveld analysis of the XRD patterns of LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ was performed to determine the average La–O, Sr–O, and Al–O bond length. The Rietveld refinement profiles of the samples are shown in Figure 6. The detailed crystallographic data are summarized in Table 5. The refined structural parameters of the samples are tabulated in Table 6. The crystallographic parameters of LaSr₂AlO₅ reported by W. B. Im et al. [15] were employed as the initial structure for fitting. The refined crystal structure of (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ illustrated by the VESTA program [30] is shown in Figure 7, where the Ca²⁺ and Mn⁵⁺ cations in (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ were located at octahedral and tetrahedral sites, respectively. The average bond lengths of La−O, Sr(2)−O, and Al−O for the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples are summarized in Table 7. In the case of (La_0.94Ca_0.06)Sr₂AlO_4.97, the average Al−O bond length was longer than in LaSr₂AlO₅. This is because some of the La³⁺ (ionic radius: 0.116 nm, 8-coordination [27]) sites in LaSr₂AlO₅ were replaced by smaller Ca²⁺ ions (ionic radius: 0.112 nm, 8-coordination [27]), resulting in a shorter average La−O bond length. On the other hand, the average Al−O bond length of the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample was shorter than that of the (La_0.94Ca_0.06)Sr₂AlO_4.97 sample. This is considered to be because some of the Al³⁺ ions (ionic radius: 0.039 nm, 4-coordination [27]) in (La_0.94Ca_0.06)Sr₂AlO_4.97 were replaced by smaller Mn⁵⁺ (ionic radius: 0.033 nm, 4-coordination [27]) ions, resulting in shorter average Al−O bond length. Consequently, Mn ions are present as Mn⁵⁺ in the lattice of the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample.

3.2.2. Ultraviolet-Visible (UV-Vis) Reflectance Spectra

Figure 8 shows the UV-Vis reflectance spectra of the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples. The results confirm the presence of Mn⁵⁺ ions in the co-doped sample. The samples without Mn⁵⁺ reflected totally visible light to exhibit a white color. On the other hand, the Mn⁵⁺-doped sample strongly absorbed the light at wavelengths below 400 nm and around 650 nm. The former absorption was assigned to the combination of LMCT transition and the d-d transition attributed to the ³A₂ to ³T₁ (³P) transition of Mn⁵⁺ [6,7]. The latter was attributed to the d-d transition corresponding to the ³A₂ to ³T₁ (³F) transition of Mn⁵⁺ [6,7]. As a result, the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample only reflected light around 520 nm corresponding to green light, and showed a green color. If Mn ions are hexavalent, optical absorption based on Mn⁶⁺ ions appears at wavelengths below 450 nm and around 550 nm, due to the LMCT transition as well as the ²E₁ to ²T₁ transition of Mn⁶⁺ [6]. In that case, the sample should show an indigo color, not green. These crystallographic and optical reflectance properties support the fact that Mn ions are present not as Mn⁶⁺ but Mn⁵⁺ in the sample.

3.2.3. Color Properties

The L*a*b*Ch° color coordinate data for the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ pigments are summarized in Table 8. The photographs are also displayed in Figure 9. As discussed for the UV-Vis reflectance spectra in Figure 8, the samples without Mn⁵⁺ exhibited a white color due to the total reflection of visible light. For the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ pigment, a bright green color with a high L* value (50.1) and a negatively high a* value (−20.8) was achieved because the sample intensely reflected green light around 500~560 nm.

3.2.4. Comparison with Commercially Available Pigments

The color parameters of the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample were compared with those of the commercial green pigments such as chromium green (Cr₂O₃) and cobalt green deep and pale (CoO·ZnO), as listed in Table 9. Their photographs are also shown in Figure 10. The absolute value of a* of the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ pigment was as high as those of the commercial pigments. The b* of the synthesized pigment was positive and low, so its hue angle (h°) was 160°, close to 180°, which means the purest green color. Chromium oxide had negative high a* and positive high b* values, giving it a yellowish green color. Although the h° of cobalt green deep was the most ideal, its brightness (L*) was lower than those of the other pigments, giving it a dark green color. Cobalt green pale had a higher absolute value of a* but a negative b* value, giving it a bluish green color. These results indicate that the color property of the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ pigment is well balanced for an environmentally friendly inorganic green pigment.

4. Conclusions

The (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample was synthesized as an environmentally friendly inorganic green pigment. The sample was obtained in a single-phase form and the lattice volume was reduced compared with those of LaSr₂AlO₅ and (La_0.94Ca_0.06)Sr₂AlO_4.97. These results indicate that a solid solution was successfully obtained, and Mn ions are present in the lattice as Mn⁵⁺. The (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample exhibited optical absorption in visible light at wavelengths below 400 nm and around 650 nm due to the LMCT and d-d transitions of Mn⁵⁺. Therefore, this sample strongly reflected green light between 500 and 560 nm, and showed a bright green color. (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ exhibited negatively high a* (−20.8) and h° (160), which were comparable to those of the conventional green pigments containing toxic elements. In conclusion, (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ could be a novel environmentally friendly inorganic green pigment.

Author Contributions

The following are the author contributions to this study: conceptualization, R.O. and T.M.; methodology, K.Y., R.O. and T.M.; validation, K.Y., R.O. and T.M.; investigation, K.Y., A.T., S.F. and R.O.; data curation, K.Y, A.T., S.F., R.O. and T.M.; writing—original draft preparation, K.Y.; writing—review and editing, T.M.; supervision, T.M.; funding acquisition, T.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by JSPS KAKENHI, grant numbers JP22K04698 and JP20H02439.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of the LaSr₂AlO₅, LaSr₂(Al_0.97Fe_0.03)O₅, and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) samples. The standard data of LaSr₂AlO₅ is based on the literature reported by Im, W.B et al. (2009) [15].

Figure 2. UV-Vis reflectance spectra for the LaSr₂AlO₅, LaSr₂(Al_0.97Fe_0.03)O₅, and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) samples.

Figure 3. Photographs of the LaSr₂AlO₅, LaSr₂(Al_0.97Fe_0.03)O₅, and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) samples.

Figure 4. Photographs of the La_1+xSr_2−x(Al_1−xCo_x)O₅ (x = 0, 0.03, 0.05, and 0.10) samples.

Figure 5. XRD patterns of the (La_1−2xCa_2x)Sr₂(Al_1−xMn_x)O₅ (x = 0 and 0.03) and (La_0.94Ca_0.06)Sr₂AlO_4.97 samples. The standard data of LaSr₂AlO₅ is based on the literature reported by Im, W.B et al. (2009) [15].

Figure 6. Rietveld refinement profiles of the XRD patterns for the (a) LaSr₂AlO₅, (b) (La_0.94Ca_0.06)Sr₂AlO_4.97, and (c) (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples. The black cross symbols and the solid red lines represent the observed and calculated intensities, respectively. The difference profiles between the observed and calculated patterns are depicted as a blue line at the bottom. The green vertical bars show the Bragg reflection peaks.

Figure 7. (a) Crystal structure obtained via the Rietveld analysis for (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅. (b) The octahedral coordination environment of La/Sr(1)O₈, the decahedral coordination environment of Sr(2)O₁₀, and the tetrahedral coordination environment of AlO₄.

Figure 8. UV-Vis reflectance spectra for the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples.

Figure 9. Photographs of the pellets of LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ powders.

Figure 10. Photographs of the pellets of (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅, chromium oxide, cobalt green deep, and cobalt green pale powders.

Table 1. Color coordinates for the LaSr₂AlO₅, LaSr₂(Al_0.97Fe_0.03)O₅, and La_1.03Sr_1.97(Al_0.97M_0.03)O₅ (M: Co, Ni, and Cu) samples.

Samples	L*	a*	b*	C	h°
LaSr₂AlO₅	96.7	−0.69	+2.84	2.92	−
LaSr₂Al_0.97Fe_0.03O₅	93.2	−0.24	+5.75	5.76	92.4
La_1.03Sr_1.97Al_0.97Co_0.03O₅	76.9	−4.36	+62.2	62.4	94.0
La_1.03Sr_1.97Al_0.97Ni_0.03O₅	55.6	−1.83	+25.7	25.8	94.1
La_1.03Sr_1.97Al_0.97Cu_0.03O₅	82.1	−2.45	+14.5	14.7	99.6

Table 2. Lattice volumes of the La_1+xSr_2−x(Al_1−xCo_x)O₅ (0 ≤ x ≤ 0.10) samples. The numbers in parentheses are the estimated standard deviations.

x	Lattice Volume/nm³
0	0.52431(6)
0.03	0.52486(4)
0.05	0.52538(4)
0.10	0.52541(6)

Table 3. Color coordinate data for the La_1+xSr_2−x(Al_1−xCo_x)O₅ (x = 0, 0.03, 0.05, and 0.10) samples.

x	L*	a*	b*	C	h°
0	96.7	−0.69	+2.84	2.92	−
0.03	76.9	−4.36	+62.2	62.4	94.0
0.05	75.8	−4.52	+67.2	67.4	93.8
0.10	62.2	−1.78	+58.7	58.7	91.7

Table 4. Lattice volume of the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples. The numbers in parentheses are the estimated standard deviations.

Samples	Lattice Volume/nm³
LaSr₂AlO₅	0.52475(2)
(La_0.94Ca_0.06)Sr₂AlO_4.97	0.52394(4)
(La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅	0.52380(4)

Table 5. Crystallographic data for the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples refined via the Rietveld analysis. The numbers in parentheses are the estimated standard deviations.

Samples	LaSr₂AlO₅	(La_0.94Ca_0.06)Sr₂AlO_4.97	(La_0.94Ca_0.06)Sr₂ (Al_0.97Mn_0.03)O₅
Crystal system	Tetragonal	Tetragonal	Tetragonal
a/nm	0.688766(5)	0.688455(5)	0.688338(7)
b/nm	−	−	−
c/nm	1.105570(10)	1.105399(11)	1.105249(14)
V/nm³	0.524481(7)	0.523926(8)	0.523677(10)
R_wp	4.391	4.684	3.647
R_p	2.943	3.122	2.372
R_e	2.663	2.637	2.537
S	1.649	1.776	1.437
R_F	2.403	2.556	2.465

Table 6. Atomic coordinates and isotropic displacement factors (B_iso) for the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples refined via the Rietveld analysis. The numbers in parentheses are the estimated standard deviations.

Atom	Site	Occupancy	x	y	z	100 × B_iso/nm²
LaSr₂AlO₅
La	8h	0.5	0.31923(9)	=x(La) + 1/2	0	0.48(2)
Sr1	8h	0.5	=x(La)	=y(La)	0	=B_iso(La)
Sr2	4a	1	0	0	1/4	0.37(3)
Al	4b	1	1/2	0	1/4	0.39(9)
O1	4c	1	0	0	0	3.7(3)
O2	16l	1	0.1352(5)	=x(O2) + 1/2	0.1510(3)	0.97(10)
(La_0.94Ca_0.06)Sr₂AlO_4.97 ^a
La	8h	0.438(3)	0.3199(1)	=x(La) + 1/2	0	–
Ca	8h	0.062 ^b	=x(La)	=y(La)	0	–
Sr1	8h	0.5	=x(La)	=y(La)	0	–
Sr2	4a	1	0	0	1/4	–
Al	4b	1	1/2	0	1/4	–
O1	4c	0.893(14)	0	0	0	–
O2	16l	1	0.1343(5)	=x(O2) + 1/2	0.1488(3)	–
(La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ ^a
La	8h	0.414(3)	0.3195(1)	0.8195	0	–
Ca	8h	0.086 ^b	=x(La)	=y(La)	0	–
Sr1	8h	0.5	=x(La)	=y(La)	0	–
Sr2	4a	1	0	0	1/4	–
Al	4b	0.973(10)	1/2	0	1/4	–
Mn	4b	0.027 ^c	1/2	0	1/4	–
O1	4c	1	0	0	0	–
O2	16l	1	0.1344(5)	=x(O2) + 1/2	0.1495(3)	–

^a The isotropic displacement parameters (B_iso) for the (La_0.94Ca_0.06)Sr₂AlO_4.97 and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ samples were fixed with LaSr₂AlO₅. ^b (Ca, g) = 0.5 − (La, g). ^c (Mn, g) = 1.0 − (Al, g).

Table 7. Average bond length of La−O, Sr(2)−O, and Al−O for the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample refined via the Rietveld analysis.

Samples	LaSr₂AlO₅	(La_0.94Ca_0.06)Sr₂AlO_4.97	(La_0.94Ca_0.06)Sr₂ (Al_0.97Mn_0.03)O₅
La−O/nm	0.2625	0.2611	0.2615
Sr(2)−O/nm	0.3585	0.3597	0.3593
Al−O/nm	0.1712	0.1721	0.1716

Table 8. Color coordinate data for the LaSr₂AlO₅, (La_0.94Ca_0.06)Sr₂AlO_4.97, and (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ pigments.

Sample	L*	a*	b*	C	h°
LaSr₂AlO₅	96.2	−0.732	+1.89	2.03	111
(La_0.94Ca_0.06)Sr₂AlO_4.97	95.5	−0.854	+3.27	3.38	104
(La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅	50.1	−20.8	+7.65	22.2	160

Table 9. Color coordinate data for the (La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅ sample and the commercially available green pigments.

Sample	L*	a*	b*	C	h°
(La_0.94Ca_0.06)Sr₂(Al_0.97Mn_0.03)O₅	50.1	−20.8	+7.65	22.2	160
Chromium oxide	50.2	−20.5	+18.2	27.4	138
Cobalt green deep	33.3	−19.9	+3.88	20.3	169
Cobalt green pale	52.3	−24.4	−5.53	25.0	193

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Yamaguchi, K.; Takemura, A.; Furumoto, S.; Oka, R.; Masui, T. Inorganic Green Pigments Based on LaSr₂AlO₅. Ceramics 2023, 6, 2269-2281. https://doi.org/10.3390/ceramics6040138

AMA Style

Yamaguchi K, Takemura A, Furumoto S, Oka R, Masui T. Inorganic Green Pigments Based on LaSr₂AlO₅. Ceramics. 2023; 6(4):2269-2281. https://doi.org/10.3390/ceramics6040138

Chicago/Turabian Style

Yamaguchi, Kazuki, Akari Takemura, Saki Furumoto, Ryohei Oka, and Toshiyuki Masui. 2023. "Inorganic Green Pigments Based on LaSr₂AlO₅" Ceramics 6, no. 4: 2269-2281. https://doi.org/10.3390/ceramics6040138

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Inorganic Green Pigments Based on LaSr₂AlO₅

Abstract

1. Introduction