Color tuning by Si/Ge substitution in Cr⁴⁺ doped melilite inorganic pigments

Abstract

Rockery constitutes one of the significant elements of Chinese royal garden design, and it becomes an emerging interdisciplinary topic covering both science and art. In this work, we report an attempt to restore the color of the ancient rockeries by finding the appropriate inorganic pigments. Chromium is doped as activators into strontium melilites with varying Si/Ge ratio, and the synthesized powders give a series of color changing from ice blue to dark-sea green. With the structure and spectroscopic characterizations, we discuss the substitutions of Cr in Mg/Si/Ge sites of the melilite host. The Tanabe-Sugano diagram of 3d² configuration is employed to explain the change of diffuse reflectance spectra caused by Si/Ge substitution. With the work, we hope to inspire the interdisciplinary research and promote the rockery heritage restoration and protection.

Highlights

• •Cr doped strontium melilite for rockery color restoration is studied.
• •Site occupancy and valence state of Cr are explored.
• •Color change is analyzed by Tanabe-Sugano diagram.

Graphical abstract

1. Introduction

Rockery is one of the most unique elements in ancient Chinese garden design, and also an important means of creating naturalistic landscapes [1,2]. Among them, the artificial rock formations in the royal gardens of the Ming and Qing dynasties of China, such as the Three Seas, Three Mountains and Five Gardens, are particularly magnificent, with distinctive shapes, novel colors, and rich variations, making them the top examples of ancient Chinese rockeries [3]. In fact, the selection and construction of rockwork in royal gardens pay attention to the choice of stone colors and mountain colors, reflecting the elegance and luxury of the imperial gardens. However, due to the destruction by war and natural weathering, the existing rockery material has deteriorated, causing the colors to become darker and rustier, often appearing in dark or gray tones, losing the original color information, as depicted in Fig. 1. Therefore, the color restoration and reproduction of rockery is one of the important tasks in the preservation of historical gardens and cultural heritage [4]. Finding restoration pigments that are the same or similar to the original colors has become an urgent research task.

Fig. 1.

The verdant stone rockery with well-preserved colors in (a) Kanhua Corridor of Beihai Park and (b) the upright peak of the green stone rockery in Xiangshan Temple of Jinyi Garden. The green stone rockwork with color deterioration in (c) Zibishanfang of Yuanmingyuan and (d) the upright peak of the green stone rockery in Qiyun Building of Jinyi Garden.

To find the appropriate material for the restoration pigments, we focus on the rare-earth or transition metal elements doped inorganic materials [[5], [6], [7]]. Strontium magnesium melilites have been thoroughly investigated for rare-earth doped luminescence as wLED (white light emitting diode) or persistence phosphors [8,9]. Both structure and luminescence properties of this host material with Eu²⁺ dopant have been widely investigated [[10], [11], [12]]. Some general rules have been outlined which state that the substitution of Ca/Sr could create a continuous solid solution, in which the crystal filed strength changes successively, and the emission peak shifts to longer wavelengths from blue of Ca series to yellow of Sr series. This phenomenon could be successfully explained by the effects of the crystal field splitting of the d-manifold of Eu²⁺ and electron-phonon coupling effect (Stokes shift) originating from relaxation in the 4f⁶5d¹ excited state [[13], [14], [15]]. This is applicable to the end member components for a continuous solid solution. With Eu²⁺ and Dy³⁺ co-doping, (Sr,Ca)MgSi₂O₇ can present blue–green afterglow for more than 20 h under the ultra-violet-light excitation, and Eu³⁺/Dy²⁺ serve as the stable trapping centers [16,17]. Nevertheless, when doped with Eu²⁺ or Ce³⁺, the strong absorption and emission associated with parity-allowed f-d transition presents too vivid blueish or greenish color of the powders, making them inappropriate for rockery color-restoration pigments. On the contrary, for transition-metal ions, the low oscillator strength of the parity-forbidden d-d transitions could be explored for the potential color-restoration. For example, chromium doped inorganic compounds display relatively low visible light absorption [18,19]. It is therefore possible to explore the restoration pigments by doping Cr into strontium magnesium melilites.

In this work, we dope Cr into strontium melilites with varying Si/Ge ratio, and obtain light blue-green powders which have potential applications in the restoration pigments. We discuss in detail the site occupation of Cr and its valence states. The color change with compositional variation from Si to Ge is explained by the diffuse reflection spectra, and a crystal field analysis is conducted by the Tanabe-Sugano diagram of 3d² configuration in the tetrahedron environment. With this structure-property relationship, we hope to provide a useful guidance to tune the color of Cr doped strontium magnesium melilites.

2. Experimental

2.1. Materials and Synthesis Procedures

A series of Sr₂Mg_1-xSi₂O₇:xCr⁴⁺ (x = 0, 0.01, 0.03, and 0.05) and Sr₂Mg_1-xGe₂O₇:xCr⁴⁺ (x = 0, 0.01, 0.03, and 0.05) phosphors were synthesized using the traditional high-temperature solid-state reaction method. The raw materials of SrCO₃, MgO, SiO₂, GeO₂, and Cr₂O₃ were of high purity (99.99 %, Aladdin) and weighed in accordance with the stoichiometric ratio. Additionally, H₃BO₃ (99 %, Aladdin) was added with a mass fraction of 5 % to enhance the crystallinity. The raw materials were thoroughly mixed by grinding for 30 min in an agate mortar. Subsequently, the mixtures were placed in alumina crucibles and sintered at 1350 °C (Sr₂MgSi₂O₇:Cr⁴⁺) and 1300 °C (Sr₂MgGe₂O₇:Cr⁴⁺) for 4 h under air atmosphere. Finally, the resulting products were finely ground into powders after cooling down to room temperature for further characterization.

2.2. Characterization

The powder X-ray diffraction (XRD) patterns of all the samples were obtained using a diffractometer (TTR-III, Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV and 200 mA. The Rietveld whole pattern refinements were conducted using Fullprof Suite [20]. The diffuse reflection (DR) spectra were measured using a UV–Vis–NIR spectrophotometer (Hitachi High-Tech Science Corporation, UH4150), with BaSO₄ as the standard. The powders are loaded in a built-in integrated sphere. The scanning electron microscopy (SEM) and element mapping images were obtained using a JEOL JSM-6510A equipped with an energy dispersive X-ray analyzer. The sample photos were taken using a Huawei P30 mobile phone.

3. Results and discussion

3.1. Crystal structure

Both Sr₂MgSi₂O₇ (SMS) and Sr₂MgGe₂O₇ (SMG) exhibit tetragonal system crystallization with a Space group of P $\bar{4}$ 2₁ m (No. 113). The structure is layered, as depicted in Fig. 2a, with the Sr layer and Mg–Si/Ge layer interwoven along the a and b directions. The larger Sr cations occupy the Wyckoff site 4e, coordinated by eight oxygen ligands. The Si/Ge at 4e site and Mg at 2a site have a tetrahedral ligand environment. When viewed from the c direction, the Sr cations are situated in the columns of tetrahedron rings. The unique structure poses the question of which site the chromium activator may substitute for. As the raw material for chromium is Cr₂O₃ and the samples are sintered in air, the stabilized valence could be +3 or +4. However, given the larger ionic radius of Cr³⁺ and the valence difference between Cr³⁺ and Si⁴⁺/Ge⁴⁺, chromium has a higher likelihood of being stabilized in the +4 valence state.

Fig. 2.

Crystal structure of SMS/SMG viewed from (a) [010] direction, and (b) [001] direction.

3.2. Tetrahedron occupation by XRD analysis

The diffraction peaks of the synthesized Sr₂MgSi₂O₇/Sr₂MgGe₂O₇ correspond well with the standard patterns, as depicted in Fig. 3. In both series, the incorporation of Cr⁴⁺ results in a significant shift of diffraction peaks towards large angles, indicating cell contraction according to the Bragg equation. However, for Sr₂MgSi₂O₇:Cr⁴⁺, the peak shift displays a consistent trend towards larger angles, while for Sr₂MgGe₂O₇:Cr⁴⁺, the shift is postponed at low Cr content, as depicted in the inserts of Fig. 3 a and b. The difference in trend arises from the tetrahedral occupation of Cr⁴⁺ in Mg²⁺/Si⁴⁺/Ge⁴⁺. Since Mg²⁺ has larger ionic radius of 0.57 Å than the 0.26/0.39 Å of Si⁴⁺/Ge⁴⁺ in a tetrahedral environment, the cell contraction suggests that Cr⁴⁺ with an ionic radius of 0.41 Å substitutes the Mg²⁺ sites in the host [21,22]. In Sr₂MgSi₂O₇:Cr, the MgO4 tetrahedron serves as the only substitution site for Cr⁴⁺ due to the extreme small size of the Si site. On the other hand, Ge shares a similar ionic radius with Cr⁴⁺, and at low doping contents of Sr₂MgGe₂O₇, the substitution of Cr⁴⁺ for Ge⁴⁺ compensates for the cell shrinkage. Therefore, the co-substitution of MgO4-GeO4 tetrahedra in Sr₂MgGe₂O₇ explains the delayed diffraction peak shift. Further, we extract the cell parameters and volumes by whole pattern Rietveld refinements, as displayed in Fig. 4. It is obvious that the doping of Cr in Sr₂MgSi₂O₇ causes continuous shrinkage, but the shrinkage in Sr₂MgGe₂O₇ is postponed. Since the ionic radius of Cr⁴⁺ lies between Mg²⁺ and Si⁴⁺, it is then confirmed that Cr⁴⁺ mainly substitutes for Mg²⁺ not Si⁴⁺. It is likely that the valence imbalance caused by the aliovalent substitution between Cr⁴⁺ and Mg²⁺ could be compensated for by the creation of Mg²⁺ or Si⁴⁺/Ge⁴⁺ vacancies. The powder morphologies are measured by the scanning electron microscopy (SEM), and the images are displayed in Fig. 5. They have typical shapes of inorganic powders. Meanwhile, the energy dispersive X-ray spectroscopy (EDS) elemental mappings in Fig. 5 show uniform composition distribution of the selected particles.

Fig. 3.

XRD patterns and selected diffraction peaks near 30° of (a) Sr₂Mg_1-xSi₂O₇:xCr⁴⁺ (x = 0, 0.01, 0.03 and 0.05), and (b) Sr₂Mg_1-xGe₂O₇:xCr⁴⁺ (x = 0, 0.01, 0.03 and 0.05).

Fig. 4.

(a) Represented Rietveld refinement of Sr₂Mg_1-xGe₂O₇ sample; (b) Variations of cell parameters a, c and volumes as a function of x in Sr₂Mg_1-xSi₂O₇:xCr⁴⁺ and Sr₂Mg_1-xGe₂O₇:xCr⁴⁺.

Fig. 5.

The SEM and mapping images: (a) and (b) for SMS, (c) and (d) for SMG.

3.3. Color rendering modulation of Cr⁴⁺ due to Si/Ge substitution

As depicted in the inserts of Fig. 6 a and b, the visual colors of Sr₂MgSi₂O₇:Cr⁴⁺ and Sr₂MgGe₂O₇:Cr⁴⁺ stem from the incorporation of Cr⁴⁺ into the host, as the undoped samples have a white color. The color of Sr₂MgSi₂O₇:Cr⁴⁺ appears as ice blue, while that of Sr₂MgGe₂O₇:Cr⁴⁺ presents as dark-sea green. To comprehend the origin of color, DR spectra are measured, as shown in Fig. 6 a and b. The undoped hosts exhibit nearly uniform reflection throughout the visible-light region, resulting in the aforementioned white color. The incorporation of Cr⁴⁺ results in a series of absorptions that arise from the intra-configurational transitions of 3d² electrons. The energy levels of 3d-block transition-metals could be analyzed using Tanabe-Sugano (T-S) diagrams, which, however, are derived under the octahedral environment. In this study, the energy levels of 3 d² electrons in a tetrahedral environment are required to analyze the color change caused by Si/Ge compositional modification. Although the T-S diagrams of 3d⁸ in octahedron could be employed as an approximation for 3d² in tetrahedron, it fails to quantify the crystal-field parameters of Dq/B, as evidenced in numerous Cr⁴⁺ doped near-infrared luminescent phosphors.[[23], [24]] Recently, the energy level diagram of 3d² electrons in the tetrahedron environment has been constructed using the irreducible tensor operator method [25], and it can be employed to conduct a crystal field analysis of Cr⁴⁺ in Sr₂Mg(Si/Ge)₂O₇. As shown in Fig. 6 c, the absorption peaks are ascribed to transitions from the ground state ³A₂(³F) to excited states ³T₁(³P) and ³T₁(³F). The ∼430 nm peak due to ³A₂(³F)→³T₁(³P) transition is overlapped with the host absorption band. The three peaks in the red region belong to ³T₁(³F). All those peak wavelengths denoted in Fig. 6 c are used to calculate the Dq/B value according to the following equations [25].

$$\begin{array}{l}\mathrm{E}\left[{{}^{3}T}_1\left({}^3\mathrm{F}\right)\right]‐\mathrm{E}\left[{{}^{3}A}_2\left({}^3\mathrm{F}\right)\right]=\mathrm{m}\\ \mathrm{E}\left[{{}^{3}T}_1\left({}^3\mathrm{P}\right)\right]‐\mathrm{E}\left[{{}^{3}T}_1\left({}^3\mathrm{F}\right)\right]=\mathrm{n}\\ \frac{\text{Dq}}{\mathrm{B}}=\frac{27\left(9\mathrm{m}+9\mathrm{n}+\sqrt{‐4{\mathrm{m}}^2‐8\text{mn}+81{\mathrm{n}}^2}\right)}{8\left(7\mathrm{m}+7\mathrm{n}‐3\sqrt{‐4{\mathrm{m}}^2‐8\text{mn}+81{\mathrm{n}}^2}\right)}\end{array}$$

Fig. 6.

DR spectra and the visual colors of (a) SMS:xCr⁴⁺ (x = 0, 0.01, 0.03 and 0.05), and (b) SMG:xCr⁴⁺ (x = 0, 0.01, 0.03 and 0.05). (c) DR spectra of SMS:0.01Cr⁴⁺ and SMG:0.01Cr⁴⁺. (d) T-S energy level diagram for Cr⁴⁺ ion in tetrahedral coordination environment in SMS:Cr⁴⁺ and SMG:Cr⁴⁺ phosphors.

The Dq/B values for Sr₂MgSi₂O₇:Cr and Sr₂MgGe₂O₇:Cr are 1.40 and 1.36, respectively. The two values are located in the weak field region as shown in Fig. 6 d. The variation in crystal field splitting could be interpreted by the Si/Ge substitution in the two hosts. The relationship between crystal field splitting Dq and center-vertex distance R could be characterized by the following equation [15].

$$Dq=\frac{Z{e}^2⟨r^4⟩}{6R^5}$$

In this model, the ligand has charge Ze, and r stands for the radial part of the electron wavefunction. As a result, the smaller ionic size of Si over Ge leads to shorter center-vertex distance R, and consequently larger crystal field splitting, as reflected by Fig. 6 d. Therefore, the color change and the subtle difference in DR spectra arise from the crystal field strength modulated by Si/Ge variation in the host composition. It needs to be noted that in Sr₂MgGe₂O₇, Cr⁴⁺ in the GeO4 tetrahedron could have a smaller vertex-distance R than that in MgO4 of Sr₂MgSi₂O₇. Considering the overall shrinking trend in Fig. 4, however, it still supports a smaller average Cr⁴⁺-O bond length in Sr₂MgSi₂O₇, and thus larger Dq/B value. Generally, the modulation of Dq/B values by compositional modulation has been widely studied in Cr³⁺ doped phosphors. For example, substituting Cr³⁺ for larger cations could reduce the Dq/B value, and change the luminescence profile from sharp peaks to the wide-band shape [26].

4. Conclusion

Cr doped Sr₂MgSi₂O₇ and Sr₂MgGe₂O₇ strontium melilites were synthesized by high-temperature solid-state reaction method. All the samples are single phased and crystallized in the melilite structure. By analyzing the diffraction peak shift, it is concluded that Cr⁴⁺ substitute for Mg site in the Sr₂MgSi₂O₇ host, and both Mg/Ge sites in the Sr₂MgGe₂O₇ site. Diffuse reflectance spectra explain the color difference caused by Si/Ge variation. The Tanabe-Sugano diagram is constructed to perform the crystal field analysis. This work will be helpful in applying spectroscopic technologies to the fields of cultural heritage protection.

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Fang Wen: Writing – review & editing, Writing – original draft, Investigation, Conceptualization. Chenghang Li: Software, Methodology, Investigation. Bo Zhang: Resources, Funding acquisition, Conceptualization. Ke Qin: Investigation, Data curation. Xin Li: Methodology, Investigation. Mingyue Chen: Writing – original draft, Visualization, Funding acquisition. Zhen Song: Validation, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.