Neodymium disilicate powders were synthesized via a sol-precipitation method with different precipitants (i.e., ammonia, ammonium bicarbonate and urea) and subsequent sintering. The samples were characterized by X-ray diffraction. In addition, a theoretical relationship between neodymium ion concentrations in different precipitation equilibrium solutions was also investigated and analyzed based on thermodynamic equilibrium calculation. The results show that the precipitant type has an impact on the phase compositions of the precursors and the sintered products. In the synthesis, neodymium ions can be completely precipitated as amorphous neodymium hydroxide with ammonia as a precipitate at pH 8~9. For ammonium bicarbonate as a precipitate, the precursor type is related to both carbon concentration and pH value. The excessive hydroxide forms neodymium hydroxycarbonate in a heated urea solution. The possible formation mechanism of silicon sol in the presence of different precipitants was discussed.

Keywords: Precipitant, Precursor, Neodymium disilicate, Thermodynamis

Inorganic pigments are widely used in the field of ceramic decoration [1-3]. As is known, the coloring ions of ceramic pigments are from mainly transition metal elements. However, some ceramic pigments contain a certain amount of heavy metal elements, which are strictly controlled due to the corresponding environmental protection issues [4-7]. Due to special structure and low toxicity, rare-earth ions are also widely used as colorants in ceramic pigments [8, 9]. In addition, some functional ceramic pigments are reported, such as effect pigments [10, 11], luminescent pigments [12, 13], phosphorescent pigments [14, 15] and near-infrared reflective pigments [16-19]. Moreover, recent work [20, 21] revealed that neodymium disilicate (i.e., Nd₂Si₂O₇) powder can be applied as a functional ceramic pigment with allochroic effect under various illuminants. Nd₂Si₂O₇ powder has a violet red color under an incandescent lamp, while presents a blue color under a fluorescent lamp. The mechanism of color change under different illuminations was discussed in detail in a previous work [22].
Solid-state reaction and sol-gel methods are the main preparation routes for Nd₂Si₂O₇ pigment [20-22]. However, solid-state reaction usually requires harsh reaction con-       ditions, such as high temperatures and/or long treatment time [23, 24]. Although sol-gel method can achieve a low-temperature preparation, it needs long-reaction time and precursors with severe agglomeration during drying, leading to the formation of a poor dispersibility of the resultant powder. In a previous study [25], Nd₂Si₂O₇ powder was synthesized via a sol-precipitation method with different precipitants. This sol-precipitation method is a relatively simple way for the synthesis of Nd₂Si₂O₇ pigment. However, different precipitants have a greater impact on the phase composition of the precursor and the final product. The relevant reasonable explanations on the precipitation process of the precursor powder with different precipitants are not reported so far.
As is well known, thermodynamic equilibrium analysis is an effective method for describing the precipitation process of ceramic materials. Many studies [26-28] reported that the thermodynamic equilibrium calculation could establish the equilibrium diagram to provide a theoretical guide for the precipitation reaction. Li et al. [29] investigated the precipitation process of silicate species in (NH₄)₂WO₄-(NH₄)₂CO₃-NH₃-H₂O system by thermodynamic analysis. Their experimemtal results consisted with the theoretical calculation.
In this paper, neodymium disilicate powders were synthesized via a sol-precipitation method with different precipitants (i.e., ammonia, ammonium bicarbonate and urea) and subsequent sintering. The phase compositions of the precursors and the sintered products with different precipitants were discussed. In addition, the thermodynamic equilibrium parameters of precursors in different precipitation equilibrium solutions were also calculated and discussed based on the thermodynamic data and mass conservation. 

The main ingredients include neodymium nitrate tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si, 98 wt.%, Guangzhou Chemical Reagent Factory, China) and hexahydrate (Nd(NO₃)₃·6H₂O, 99.5 wt.%, Ganzhou Ruihua Rare-earth Co. Ltd., China). Ammonia (NH₄OH, 25 wt.%), ammonium bicarbonate (NH₄HCO₃, analytical grade) and urea ((NH₂)₂CO, 99.0 wt.%) as precipitants were purchased from Tianjin Fuchen Chemical Reagent Factory, China. Polyethylene glycol (PEG-10000, analytical grade) was used as a surfactant. Absolute ethanol (C₂H₅OH, 99.7%) and deionized water were obtained from Guangzhou Qianghui Bose Instrument Co. Ltd., China.
Nd(NO₃)₃·6H₂O and TEOS with a molar ratio of 1:1 were dissolved in 400 ml alcohol-water solution. 2.5 g PEG-10000 was firstly added into the mixture solution above, and then the mixture was added in dropwise into the precipitant solution (2 mol/L NH₄OH or NH₄HCO₃) to keep the solution pH values 8-9 under vigorous stirring at room temperature. After 12 h, the resultant suspension was filtered and washed for three times with deionized water, and dried at 60 ^oC for 24 h. For (NH₂)₂CO as a precipitant, the concentration ratio of (NH₂)₂CO and the total metal ions was 15:1. The mixture solution was heated to 95 ^oC and kept at this temperature for 6 h. The precipitate was filtered, washed and dried by using the same procedure above. The dried precipitates were ground in an agate mortar and calcined at 1,200 ^oC for 5 h, respectively.
The phase compositions of the precursor and sintered product were determined by a model PW-1710 X-ray diffractometer (XRD, Philips Co. Ltd., The Netherlands), using Cu Kα radiation.

Fig. 1 shows the XRD patterns of the synthesized samples with different precipitants. The precursor obtained with NH₄OH is amorphous Nd(OH)₃ phase (JCPDS No. 83-2035). When the sintering temperature reaches 1,200 ^oC, the tetragonal Nd₂Si₂O₇ (JCPDS No. 22-1177) is the main crystalline phase for the sample synthesized with NH₄OH (see Fig. 1(a)). For the precursor obtained with NH₄HCO₃, there are some obvious diffraction peaks in the XRD pattern, which coincides with a unknown phase of Nd₂(CO₃)₃·xH₂O. In some previous studies [30, 31], the precursor obtained with (NH₄)₂CO₃ as a precipitant could be Nd₂(CO₃)₃· 2.5H₂O, Nd₂(CO₃)₃·4H₂O or Nd₂(CO₃)₃·8H₂O. After the precursor produced with NH₄HCO₃ is sintered at 1,200 ^oC, the main crystalline phases are Nd₄Si₃O₁₂ (JCPDS No. 42-0171), Nd₂SiO₅ (JCPDS No. 40-0284) and Nd₂O₃ (JCPDS No. 43-1023) (see Fig. 1(b)). The XRD pattern for the precursor obtained with (NH₂)₂CO is in agreement with that for NdOHCO₃ (JCPDS No. 27-1296). The main crystalline phases of the precipitate sintered at 1,200 ^oC are also Nd₄Si₃O₁₂, Nd₂SiO₅ and Nd₂O₃ (see Fig. 1(c)). After sintering, the phase compositions of the precursors prepared with different precipitants are inconsistent, indicating a poor element (i.e., Si and Nd) homogeneity of the precursors. In order to explain the result above, the reaction processes of the systems with different precipitants should be further analyzed based on thermodynamic equilibrium calculation.
For the prepared precursors, the neodymium element is precipitated as different precipitates. Two variables such as temperature and pressure are usually constants for the precipitation process in the solution. Therefore, in this study, the diagram of the precipitated ion concentration and pH value at different precipitation equilibrium solutions are drawn for further discussion.
In the system of Nd³⁺-NH₄OH-H₂O (NH₄OH as a precipitant), the relevant reactions are shown in Table 1. Based on the principle of mass balance, the mathematical model of lg[Nd]_T vs pH value can be deduced, where [Nd]_T is the total concentration of Nd in equilibrium solution. The concentration is used instead of the activity for the calculation. The total concentration of Nd can be obtained by
 

 
where [Nd ³⁺]_h and [Nd(OH)²⁺]_h are the concentration of Nd ³⁺ and Nd(OH)²⁺, respectively. The mathematical relationships of [Nd ³⁺]_h and [Nd(OH) ²⁺]_h can be expressed as follows:
 

 
where [OH^-] is the concentration of OH^- in the equilibrium solution. According to Eqs. (2) and (3), Eq. (1) can be re-written as
 


The diagram of lg[Nd]_T vs pH can be proposed according to Eq. (4), as shown in Fig. 2. Clearly, in the whole pH value range, the concentration of Nd in the equilibrium solution sharply decreases with the increase of pH value, which is attributed to the formation of Nd(OH)₃ (see Fig. 1(a)). Moreover, a high pH value of equilibrium solution is beneficial for the precipitation of Nd ions. When the pH value is 8~9, the concentration of Nd ions is the range of 10^-3~10^-6 mol/L. In this experiment, the pH value of the equilibrium solution is 8.72 before the precursor is filtered, which can be defined as an almost complete precipitation. Zhu et al. [33] found that Nd(OH)₃ nanoparticles with different morphologies can be synthesized using a microemulsion-precipitation method with NH₄OH as a precipitant in the presence of cetyltrimethyl ammonium bromide. Arunachalam et al. [34] also reported that a facile chemical precipitation method was used to prepare Nd(OH)₃ nanopowder at an ambient temperature without the addition of any surfactants.
In the system of Nd³⁺-NH₄HCO₃-H₂O (NH₄HCO₃ as a precipitant), the hydrolysis equation of NH₄HCO₃ can be expressed by
 

 
The other relevant reactions are listed in Table 2. Based on the principle of mass balance, the mathematical model of lg[Nd]_T vs pH value is deduced, where [Nd]_T is the total concentration of Nd ions in the equilibrium solution. The concentration is used instead of the activity for the calculation. The total concentration of Nd ions can be expressed by
 

 
where [Nd ³⁺]_c and [Nd(OH)²⁺]_c are the concentration of Nd ³⁺ and Nd(OH)²⁺, �respectively. The mathematical relationships of [Nd ³⁺]_c and [Nd(OH) ²⁺]_c can be obtained as follows:
 

 
where [OH^-] and [CO₃^2-] are the concentrations of OH^- and CO₃^2- in the equilibrium solution, �respectively. According to the mass balance of the system, the concentrations of OH^- and CO₃^2- can be established by
 

 
where [C]_T is the total concentration of C in the equilibrium solution. Moreover, [CO₃^2-], [HCO₃^-] and [H₂CO₃] are the concentrations of CO₃^2-, HCO₃^- and H₂CO₃, respectively. The mathematical relationships of [HCO₃^-] and [H₂CO₃] can be obtained as follows:
 

 
Based on Eqs. (10) and (11), Eq. (9) can be rearranged by
 

  
According to Eqs. (7), (8) and (12), Eq. (6) can be deduced as
 
 

 
For the given values of [C]_T at a certain pH value, the diagram of lg[Nd]_T vs pH value can be obtained from Eq. (13) by a computer software named Origin 8.0.
The lg[Nd]_T vs pH value diagram of the Nd³⁺- NH₄HCO₃-H₂O system with different total carbon concentrations at equilibrium is shown in Fig. 3. Clearly, the patterns of the lg[Nd]_T vs pH curves with different total carbon concentrations are similar. The concentration of Nd ions sharply decreases with the increase of pH value in the range of 7~10, which is attributed to the formation of Nd₂(CO₃)₃. A further increase of pH value results in the increase of Nd ion concentration due to the partial dissolution of Nd₂(CO₃)₃. In fact, precipitates with a low solubility are preferentially precipitated in the equilibration solution [29]. That is, the lower position of the curve is beneficial to the formation of the precipitate in the lg[Nd]_T vs pH value diagram. According to the theoretical calculation, the precipitate may be Nd₂(CO₃)₃ or Nd(OH)₃, when NH₄HCO₃ is used as a precipitant. When the [C]_T value is 10^-6 mol/L, the precipitate is Nd₂(CO₃)₃ in the pH value range of 7.0~8.3. The precipitate is Nd(OH)₃ in the pH value range of 8.3~14.0. Moreover, the pH value range of Nd₂(CO₃)₃ presence becomes greater with the increase of [C]_T value. In this experiment, the [C]_T value exceeds 0.01 mol/L. The precipitate is thus Nd₂(CO₃)₃ for the equilibrium solution in the pH value range of 8~9. Due to the special structure of Nd element, neodymium carbonate generally exists as hydrated carbonates. Liu et al. [36] found that neodymium carbonate with an orthorhombic system was synthesized using NH₄HCO₃ as a precipitant, which contained a certain amount of water. Zhu et al. [37] reported that Nd₂(CO₃)₃·   8H₂O with various morphologics and sizes was synthesized using a microemulsion-assisted solvothermal method in the presence of Na₂CO₃. In addition, there also can be formed poorly-ordered nanoparticulate precursors, as termed amorphous amorphous neodymium carbonate [38].
In the system of Nd³⁺-CO(NH₂)₂-H₂O (CO(NH₂)₂ as a precipitant), the hydrolysis of CO(NH₂)₂ can be expressed as follows:
 

 
The hydrolysate is similar to that of Eq. (5). However, the hydrolysis process of CO(NH₂)₂ requires auxiliary heating. In this experiment, the hydrolysis temperature is kept at 95 ^oC. At this temperature, the relevant equilibrium constants are difficult to obtain. According to the XRD result in Fig. 1(c), the precipitate obtained in the presence of (NH₂)₂CO is neodymium hydroxycarbonate (i.e., NdOHCO₃). The main reactions for the formation of NdOHCO₃ can be expressed as follows:
 


From Table 2 (see No. 3 and No.7), the equilibrium constant of Nd₂(CO₃)₃ is smaller than that of Nd(OH)₃, indicating that Nd₂(CO₃)₃ is preferentially precipitated. However, in the hydrolysis solution of (NH₂)₂CO, hydroxide concentration is much higher than that of carbon dioxide. The excessive hydroxide may thus cause the formation of NdOHCO₃ in the heated equilibrium solution. In fact, a ultrafine NdOHCO₃ powder was also synthesized using (NH₂)₂CO as a precipitant by a hydrothermal method [39]. Some studies showed reaction temperature and pressure both have important effects on the formation of NdOHCO₃. Tahara et al. [40] synthesized hexagonal and orthorhombic Nd(CO₃)OH via a hydrothermal reaction at 220 ^oC and different pressures. Dentritic NdOHCO₃ nanostructures were prepared using a facile hydrothermal approach [41].
For the prepared precursors, the silicon element is aggregated in the form of silicon sol, which is the result of hydrolysis and condensation of TEOS. The hydrolysis reaction of TEOS in abbreviated form can be given as
 

 
The condensation of silicon oxide can be expressed as
 

 
In the sol process of silicon alkoxide, generally, a homogeneous catalyst is used to facilitate the hydrolysis reaction since the reaction rate of hydrolysis is slow. Brinker et al. [42] proved that all the hydroxyl groups of Si(OH)₄ in hydrolysis reaction of TEOS are derived from water. Iler et al. [43] also proposed the mechanism for the basic hydrolysis of TEOS. The hydrolysis process of TEOS can be affected by some parameters like pH value, time and H₂O/Si molar ratio as well [44-46]. Noted that in general, the condensation reaction of silicon hydroxide is much faster than the hydrolysis reaction of silicon alkoxide.
In alkaline solution, the hydroxyl anions attack Si directly, leading to the abatement of Si-O bond and the acceleration of -OC₂H₅ group cleavage [43]. In this work, three reagents (i.e., NH₄OH, NH₄HCO₃ or (NH₂)₂CO) are selected as catalysts to promote the reaction of hydrolysis of TEOS, respectively. When a catalyst is NH₄OH, the hydroxide ions formed by the ionization of ammonia water can effectively attack Si, which is beneficial to the complete hydrolysis of TEOS. However, the other reagents (i.e., NH₄HCO₃ and (NH₂)₂CO) need to be hydrolyzed to form ammonia and then partially ionized, leading to the incomplete hydrolysis reaction. Therefore, there is a small amount of residual Nd₂O₃ that are not reacted due to the lack of silicon element, when NH₄HCO₃ or (NH₂)₂CO is a catalyst.

Neodymium disilicate powders were prepared by a sol-precipitation method with different precipitants (i.e., ammonia, ammonium bicarbonate and urea) and subsequent sintering. The reaction processes of the systems with different precipitants were analyzed based on thermodynamic equilibrium calculation.
When NH₄OH was used as a precipitate, the precursor was amorphous Nd(OH)₃. The precursor obtained with NH₄HCO₃ was a unknown phase of Nd₂(CO₃)₃·xH₂O. NdOHCO₃ was the crystal phase of the precursor obtained in the presence of (NH₂)₂CO. In the system of Nd³⁺-NH₄OH-H₂O, a high pH value for the equilibrium solution was beneficial for the precipitation of Nd ions. In the system of Nd³⁺-NH₄HCO₃-H₂O, the precursor type was related to both carbon concentration and pH value. A low pH value was beneficial to Nd₂(CO₃)₃ formation, while a high pH value was beneficial to Nd(OH)₃ formation. In the system of Nd³⁺-CO(NH₂)₂-H₂O, the excessive hydroxide could form NdOHCO₃ in the heated equilibrium solution. Only a precursor with NH₄OH could transform to a single Nd₂Si₂O₇ phase after it was sintered at 1,200 ^oC for 5 h, which was due to the incomplete hydrolysis of TEOS in the presence of NH₄HCO₃ or (NH₂)₂CO.

This work was supported by the China Postdoctoral Science Foundation (No. 2019M650196) and the Major Scientific and Technological Projects of Foshan (No. 2016AG101415).