Sol-gel synthesis and crystal chemical properties of the pigment Zn1.9Cu0.1SiO4 205 D O I: 1 0. 15 82 6/ ch im te ch .2 01 8. 5. 4. 05 Samigullina R. F., Rotermel M. V., Ivanova I. V., Krasnenko T. I. Chimica Techno Acta. 2018. Vol. 5, No. 4. P. 205–209. ISSN 2409–5613 R. F. Samigullina, M. V. Rotermel*, I. V. Ivanova, T. I. Krasnenko Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Sciences, 91 Pervomaiskaya St., Ekaterinburg 620990, Russian Federation *E-mail: rotermel@ihim.uran.ru Sol‑gel synthesis and crystal chemical properties of the pigment Zn1.9Cu0.1SiO4 The pigment Zn1.9Cu0.1SiO4 was obtained by the method of sol-gel synthesis. The crystallization temperature was set at 776 °C, ∆H ≈ –16.3 kJ / mol. Thermal expansion of the individual Zn2SiO4 and Zn1.9Cu0.1SiO4 solid solutions was studied by in situ high-temperature X-ray diffraction. It is shown that the substitution of Zn2+ → Cu2+ does not lead to significant changes in the lattice parameters; in the range from room temperature to 800 °C the structure expands monotoni- cally when heated. The coefficients of volumetric thermal expansion for Zn2SiO4 and Zn1.9Cu0.1SiO4 are αV = 8.05 · 10 –6 and 8.81 · 10–6 1 / K, respectively. The colorimetric coordinates in the RGB system are 71.8 % red, 72.9 % green and 79.6 % blue, which corresponds to the gray-blue pigment. Keywords: pigment; willemite; sol-gel synthesis; thermal expansion. Received: 06.12.2018. Accepted: 21.12.2018. Published: 31.12.2018. © Samigullina R. F., Rotermel M. V., Ivanova I. V., Krasnenko T. I., 2018 Introduction Divalent metal silicates are wide- ly used as luminescent, corrosion protect- ing, electrical insulating materials, cata- lysts, pigments. Thus, the emergence of the chemical manufacturing of synthetic pig- ments began with the production of dyed double silicates, known as egyptian blue CaCuSi4O10, han blue BaCuSi4O10 [1–3]. The continued interest to the silicate pig- ments is caused due to  the pure intense color, as  well as  by  their high thermal and chemical resistance. Therefore, it is possible to  use them for  dyeing ceramic products that exposed to high temperature calcination in the manufacturing process. Transition metal orthosilicates contain- ing copper, nickel, and cobalt ions are well known as blue pigments. Dopant M2+ ions (M = Cu, Ni, Co) in Zn2–2xM2xSiO4 solid solutions are coordinated by four oxygen atoms, which causes the blue color of the compounds due to  the splitting of  elec- tron levels of M2+ ions in the crystal field. Since the information about Zn2–2xM2xSiO4 (M = Cu, Ni, Co) is limited, the purpose of this work is a disclosure of sol-gel syn- thesis mechanism for  Zn1.9Cu0.1SiO4 and determination of main pigment character- istics, such as colorimetric parameters and thermal expansion coefficient. 206 Experimental The precursors used in  the sol-gel synthesis of  Zn1.9Cu0.1SiO4 were zinc ac- etate Zn(CH3COO)2·2H2O, copper acetate Cu(CH3COO)2·H2O, and tetraethyl or- thosilicate (TEOS) Si(OC2H5)4. The phase composition within the range from room temperature up to 800 °C was controlled in  situ by  the X-ray powder diffraction (XRPD) method (Shimadzu diffractom- eter, CuKa1 radiation, 2θ angle interval from 10 to 60° with a step of 0.02°), com- paring the XRD data with the X-ray char- acteristics of the possible impurity oxides and zinc silicates (PDF2 database, ICDD, USA, Release 2009). The temperature was controlled using an Anton Paar TTK-450 attachment. The unit cell parameters were refined by the Rietveld method using the Fullprof 2010 software. Thermogravimet- ric (TG) analysis together with differential thermal analysis (DTA) were performed using a Setsys Evolution thermal analyz- er (Setaram) in air at a temperature scan rate of 10 ° / min in the temperature range 20–1100 °C, with alumina as a reference substance. Colorimetric analysis was per- formed using an SLR Olympus e-420 (light source temperature of 5400 K; ISO = 200; light camera parameters L×W×H = 35×25×32 cm) Photo Impact 12 program, through a calibrator monitor One-Eye Pro. The colorimetry results are given in  the RGB color coordinates system. Results and discussion The synthesis method used in our work allowed us to obtain the Cu2+ dopant con- centration equal to  5 at.%. Hydrolysis of  [Si(C2H5O)4] in  the mixture with the ratio H2O:TEOS = 1:1 took place within 30 min. The alcohol solutions of metal ac- etates and hydrolized TEOS were mixed. After the solutions were poured together the mixture was stirred on a magnetic stir- rer for 1 h. The precursor for the final stage of synthesis was obtained by evaporation of  the mixture for  2 hours at  65  °C.  The gel was formed after 2 days at room tem- perature. In order to determine the temperature range of Zn1.9Cu0.1SiO4 formation, thermo- gravimetric and differential thermal analy- ses of the obtained precursor were carried out (Fig. 1). The mass loss of 2–3 %, accompanied by a small endothermic effect at 100 °C, corresponds to the removal of water and ethanol residual. The weight loss of about 20 % with a simultaneous exothermic sig- nal on the DTA curve within the region of 250–410 °C is caused by the decompo- sition of  organic components. The DTA curve shows the sharp exothermic effect (∆H ≈ –16.3 kJ / mol) with a  maximum at  776  °C, while the mass of  the sample remains constant. The assignment of this effect was determined by  the thermal analysis of  Zn2SiO4 precursor, prepared from zinc acetate and TEOS (Fig. 2). The exothermic effect recorded on  the DTA curve in the temperature range 320–500 °C with simultaneous mass loss on  the TG curve corresponded to the decomposition of  organic components. The exothermic effect at 786 °C (∆H ≈ –15.5 kJ / mol) with a constant sample mass is similar to that observed on the DTA curve for the pre- cursor with the nominal composition Zn1.9Cu0.1SiO4 (Fig. 1). Consequently, one can conclude that the exothermal effects on  the compared DTA curves for  the Zn2SiO4 and Zn1.9Cu0.1SiO4 precursors are of the same nature, and both are related 207 to the process of a phase with the willemite structure formation. Thus, a comparative analysis of the thermal behavior of these samples showed that at temperatures above 776 °C the process of forming a long-range order at sol-gel synthesis of Zn1.9Cu0.1SiO4 was completed. XRD data of the Zn1.9Cu0.1SiO4 precur- sor annealed at 800 °C does indicate the formation of the phase with the willemite structure; however, an insignificant admix- ture of copper (II) oxide is present in the sample (Fig. 3). The single-phase product was obtained by firing the precursor sam- ple at 900 °C (Fig. 3). X-ray pattern of  the single phase Zn1.9Cu0.1SiO4 sample taken at room tem- perature was indexed in the willemite type structure with the trigonal space group R3. The refined unit cell parameters, unit cell volume and number of formula units are: a = 13.927(1) Å, c = 9.305(3) Å, V = 1563.03(8) Å3, Z = 18. One of the most important characteris- tics of the pigment is the volumetric ther- mal expansion coefficient (VTEC), which should be comparable to the thermal ex- pansion of  the coated material. Colored zinc orthosilicate doped with copper may be suitable as a pigment for ceramics made of porcelain, earthenware, majolica. Manu- facturing and operation proceeds in wide temperature range, therefore, the coinci- dence VTEC of the matrix and the pigment will allow to avoid cracking of the coating. Fig. 1. TG and DTA curves (on heating) of the Z1.9Cu0.1SiO4 precursor Fig. 2. Heating TG and DTA curves of the Zn2SiO4 precursor Fig. 3. X-ray diffraction profiles of Zn1.9Cu0.1SiO4 powder annealed at different temperatures 0 200 400 600 800 1000 t, °C 0 2 4 6 8 10 H ea tF lo w ,W /g -50 -40 -30 -20 -10 0 T G ,% exo 776°C 0 200 400 600 800 1000 t, °C -1 0 1 2 3 4 H ea tF lo w ,W /g -40 -30 -20 -10 0 T G ,% exo 786°C 10 20 30 40 50 60 Zn2SiO4 ICDD 00-079-2005 2Θ, degree 900oC 800oC In te ns ity (a .u .) 600oC CuO ICDD 00-041-0254 208 The values of  VTEC were calculated from the experimental results for  the Zn2–2xCu2xSiO4 unit cell parameters (x = 0; 0.05) in the range from room temperature up to  800  °С  (Fig. 4). It was shown that the sizes of the unit cell for the zinc ortho- silicate Zn2SiO4 and Zn1.9Cu0.1SiO4 solid solution monotonically expanded with increasing temperature. Doping of  zinc orthosilicate with the cations with similar size, like Cu2+ (for c.n.= 4 r Zn2+ = 0.74 Å, r Cu2+ = 0.71 Å) does not lead to  signifi- cant differences in the polyterms of unit cell parameters. A comparison of VTEC for Zn2–2xCu2xSiO4 (x = 0; 0.05) with that for the ceramic substrate [4], most often used as a coated material (Table 1), shows their proximity. Colorimetric coordinates of blue-gray Zn1.9Cu0.1SiO4 in the RGB color space con- sists of 71.8 % red, 72.9 % green and 79.6 % blue (the percentages are relative to pure color), the color saturation is 16.1 %. Conclusion The gray-blue pigment Zn1.9Cu0.1SiO4 was obtained by  the sol-gel synthesis method. The consequence of phase trans- formations during the synthesis of  the Zn1.9Cu0.1SiO4 solid solution was disclosed with the help of X-ray diffraction and ther- mal analysis. In situ high-temperature X-ray study for Zn2–2xCu2xSiO4 (x = 0; 0.05) in the range of 25–800 °C showed that the volume thermal expansion of the ceramic pigment reveals monotonic character. The calculated VTEC value for Zn2–2xCu2xSiO4 is close to VTEC reported for porcelain, earthenware, majolica. Acknowledgements The work was supported by UB RAS (project 18-10-3-32). References 1. Berke H. The invention of blue and purple pigments in ancient times. Chem. Soc. Rev. 2007;36:15–30. DOI: 10.1039 / B606268G. Fig. 4. The unit cell parameters and unit cell volume for Zn2–2xCu2xSiO4 (x = 0, 0.05) versus temperature 0 200 400 600 800 1566 1569 1572 1575 1578 9,31 9,32 9,33 9,34 9,35 9,36 13,92 13,93 13,94 13,95 13,96 13,97 V ,Å 3 t, o C c, Å Zn2SiO4 Zn 1,9 Cu 0,1 SiO 4 a, Å Table 1 Volume thermal expansion coefficients for the often used ceramics and Zn2–2xCu2xSiO4 Material αV∙10 –6, 1 / K Zn2SiO4 8.05 Zn1.9Cu0.1SiO4 8.81 Porcelain 5.5–7.0 Earthenware 7.0–8.1 Majolica 8.5–10.0 209 2. Pozza G., Ajo` D., Chiari G., De Zuane F., Favaro M. Photoluminescence of the inorganic pigments Egyptian blue, Han blue and Han purple. J. Cultural Heritage. 2000;1:393–8. DOI: 10.1016 / S1296-2074(00)01095–5. 3. Sidorov V. I., Malayvskyi N. I., Pokid`ko B. V. Poluchenie nizkoosnovnikh silika- tov nekotorikh perekhodnikh metallov metodom osadzenizya. Vestnik MGSU. 2007;1:163–6. Russian. 4. Khleborodova O. A. Tablica sootvetstviya keramicheskikh mass I bazovikh glazurei [Internet]. 2017. Russian. Available from: https://www.ceramistam.ru / blog / Keram- icheskie_massy / tablitsa-sootvetstviya-keramicheskikh-mass-i-bazovykh-glazurey / .