# Study of thermochromic material and its properties for visual indicators

R Král, L Fortin, P Zemenová, M Akouibaa, M Poupon, A Nicaise, J P Šafrata, D D Šafrata, J M Chevalier, J Pokorný, et al.

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R Král, L Fortin, P Zemenová, M Akouibaa, M Poupon, et al.. Study of thermochromic material and its properties for visual indicators. Journal of Physics: Conference Series, 2024, 2931, 10.1088/1742-6596/2931/1/012022. [hal-05151245](https://hal-05151245)

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# Study of thermochromic material and its properties for visual indicators

To cite this article: R Král et al 2024 J. Phys.: Conf. Ser. 2931 012022

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# Study of thermochromic material and its properties for visual indicators

R Král¹, L Fortin², P Zemenová¹, M Akouibaa⁴, M Poupon¹, A Nicaise², J P Šafrata³, D D Šafrata³, J M Chevalier¹, J Pokorný¹, J Houdková¹, P Jiříček¹, M Lachkar⁴ and B El Bali⁴

¹ Institute of Physics of the Czech Academy of the Sciences, Prague, Czechia

² Polytech Nantes, Rue Christian Pauc, CS 50609, 44306 Nantes Cedex3, France

³ Gymnázium Evolution Sázavská, Sázavská 830/5, CZ-120 00 Prague, Czechia

⁴ Engineering Laboratory of Organometallic, Molecular Materials, and Environment (LIMOME), Faculty of Sciences, Sidi Mohamed Ben Abdellah University, 30000 Fez, Morocco

E-mail: kralr@fzu.cz

**Abstract.** This work deals with the preparation and characterization of inorganic hybrid phosphite salt (enH₂)[Ni(H₂O)₆](HPO₃)₂, where “en” stands for ethylenediamine (C₂N₂H₈). This material belongs to a broad family of chromogenic compounds, which may have wide applications as pigments for visual indicators due to their color change in response to external stimuli (e.g. temperature, pressure, radiation, etc.). Such pigment can be used to develop new smart textiles, smart windows, safety sensors for innovative packaging, etc. However, before that it is necessary to understand the origin of color transition and the impact of the transition on its environment. Therefore, the (enH₂)[Ni(H₂O)₆](HPO₃)₂ was studied by various thermoanalytical (OTM, TMA, DTA, TG, EGA), structural (HT XRPD, XPS), and optical (Raman spectroscopy) methods. Several color transitions were identified in the range from 67 to 215 °C, accompanied either by decomposition, recrystallization, or both. The first color change (from a translucent blue to an opaque light green) occurred at 67 °C due to a release of crystalline H₂O. The final color change (from a green to a dark blue) was at ca. 215 °C due to the formation of the new phases (NiₓP and Ni₂P). HT XRPD, Raman spectroscopy, and XPS observed the evolution of the crystallinity state and chemical bonding. Finally, the kinetic parameters (activation energy and reaction rate) of the first decomposition effect were determined via the Kissinger method.

## 1. Introduction

Thermochromic materials are compounds capable of reversible or irreversible alteration of their color in response to external temperature fluctuations, emerging as a pivotal class of functional materials [1]. In recent years, such materials have garnered significant attention as smart materials. For example, smart windows are based on thermochromic material (vanadium dioxide, perovskite, or hydrogel) that adjusts its optical properties in response to external heating stimuli, giving the windows a dynamic thermal regulator capability [2,3]. Besides, thermochromic materials such as leuco dye or liquid crystal can also be useful in developing smart textiles, offering an innovative approach to fashion and industrial purposes

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Content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
by creating thermally responsive clothing [4]. Furthermore, thermochromic materials can be used as safety sensors in innovative packaging to identify whether a product has been exposed to excessive heat or not [5]. However, before industrializing a new thermochromic material as a visual indicator, it is important to understand the response of these materials to external stimuli. In addition, the materials must be suitable for the desired temperature range, and their durability must be assessed. It is also important to understand the degradation of the materials over time or temperature to ensure that they will not become toxic to their environment and that the visual color indicator is not altered. In such cases, it is crucial to understand the color change mechanism of the materials [6,7].

In the case of inorganic or hybrid thermochromic materials, the color-changing behaviour of thermochromic materials can be due to different phenomena connected with the evolution within the physical and chemical interactions of the materials [7]. In addition, the color-changing mechanism can seriously impact the structure and microstructure of materials, especially in the event of dehydration or decomposition of the material. Combination of different analytical methods is essential to identify the origin of the color transition and the thermal stability of the materials.

The (enH₂)[Ni(H₂O)₆](HPO₃)₂ hybrid phosphite was initially synthesized to test its properties in water decontamination and water-splitting electrolysis [8]. The compound is built from three different units stacked together through a hydrogen bonding network: a phosphite anion [HPO₃]²⁻ a hexa-aqua nickel complex cation [Ni(H₂O)₆]²⁺ and an ethylenediamine cation (enH₂)²⁺. Akouibaa et al. [8] studied the thermal stability of the compound. They showed that the title phosphite has a phase transition around 65 °C, losing the water molecules bound to the aqua complex. No thermochromic properties were mentioned in the original paper. However, the change of chemical interaction in the transition metal Ni's coordination sphere should affect the materials color change. The work of Akouibaa et al. [8] demonstrated that the decomposition of the Ni-phosphite hybrid transforms the sample from a crystalline state to an amorphous state. However, the behaviour of the phosphite anion, the nickel cation, and the ethylenediamine cation in this second state is unknown, e.g. what is their chemical interaction? Does their oxidation state change?

If a phase transition at 65 °C is too high for the visual indicator of food packaging; a compound may be interesting as a pigment for the visual indication of overheated surface paint. Therefore, it is important to follow the evolution of the amorphous phase at high temperatures to determine whether recrystallization or decomposition occurs to be sure that the compound will not have any environmental impact. The origin of the color shift in the Ni-phosphite hybrid material is challenging to understand as the chemical interaction is present in two different states (crystalline and amorphous) as a function of temperature.

This study delves into the thermal properties of a hybrid Ni-phosphite compound ((enH₂)[Ni(H₂O)₆](HPO₃)₂). The aim is to elucidate its thermochromic behaviour using a comprehensive suite of experimental techniques, including thermoanalytical, structural, and optical methods. X-ray diffraction was used to confirm the structure and purity of the materials and follow the evolution of the crystallinity state as a function of the temperature. Raman spectroscopy was used to follow the evolution of chemical binding during the first phase transition. Thermal analysis, mass spectroscopy, and X-ray photoelectron spectroscopy were used to understand the mechanism and kinetics of the reaction(s). Finally, determined processes' kinetic parameters (activation energy and reaction rate) were evaluated.

## 2. Experimental

The hybrid Ni-phosphite of general formula (enH₂)[Ni(H₂O)₆](HPO₃)₂, where “en” stands for ethylenediamine (C₂N₂H₈), was synthesized, as already published in [8], by mixing following reactants in 1:2.3:7.5 molar ratio: Ni(OAc)₂·4H₂O (0.04977 g, 0.2 mmol), H₃PO₃ (0.0369 g, 0.45 mmol) and ethylenediamine (0.09015 g, 1.5 mmol) in 15 mL of distilled water. Ethylenediamine was selected as a structure-directing agent. The mixture was stirred for 3 h under ambient conditions and kept uninterrupted for 3 weeks at room temperature (RT). Afterwards, small hexagonal crystallites appeared, which were filtered out, washed with ethanol-water (80–20%), and dried.
The stereomicroscope Olympus SZX12 (objective 1.6, 63x magnification) equipped with a heating chamber Linkam THMS 600 and a Canon EOS5D digital camera for image capture was used to observe and determine color changes in Ni-phosphite. The computer-controlled regulator Linkam CI94 allowed temperature scanning in the 25-250 °C range with a heating rate of 5 °C.min⁻¹, further denoted as optical thermomicroscopy (OTM). Additionally, 3 thermocouples were placed in the heating chamber: one at the end, one in contact with the sample stage, and another above the sample stage. Each thermocouple was connected to an Almemo data logger (2690) and PC for temperature recording. Nitrogen 5N was used as a protective atmosphere. The experiments were conducted by placing crystallite samples in the heating chamber and focusing the stereomicroscope on a specific grain(s). Afterwards, the chamber's heating was initiated, and simultaneously, the temperature recording (Almemo) and image acquisition (digital camera) were started. The pictures of the sample were collected every 1 min allowing their further merging in QuickPHOTO software (version 2.4) and creating timelapse video. Since the color change was not instantaneous, capturing images in a shorter time frame was unnecessary. Our main focus was to observe a temperature range in which the color change occurred and correlate it with the results obtained from other analyses.

The thermomechanical analysis (TMA) was conducted using a Setaram Labsys 1400 thermomechanical analyzer in the temperature range of 25-150 °C under the inert nitrogen atmosphere, with the heating rates of 2, 5, 10, 15, 20, and 25 °C.min⁻¹ and a load of 10 g. The crystallite samples (of height ca. 1 mm) were placed into an alumina crucible with an alumina piston on the top. The apparatus was calibrated with Al₂O₃, Al, and Ag cylinders (height ca. 7 mm). The blank run was performed before each TMA measurement. Obtained data were processed by the Calisto software. Various heating rates were selected as they were required for further kinetics analysis of the decomposition effect. Otherwise, 5 °C.min⁻¹ was used for standard TMA.

Simultaneous differential thermal analysis (DTA), thermogravimetry (TG), and evolved gas analysis (EGA) using mass spectrometry (MS) at non-isothermal conditions were performed using a simultaneous thermal analyzer Setaram Themys 24 and mass spectrometer Pfeiffer OmniStar GSD 320. The crystallite samples of weight ca. 10-30 mg were measured in an alumina crucible in the temperature range 25-350 °C or 25-600 °C under an argon atmosphere (flow of 60 ml.min⁻¹, purity 5.0N) at the heating rate of 5 °C.min⁻¹. The empty alumina crucible was used as a reference. Before each DTA-TG-MS measurement, the oxygen content had to be eliminated by introducing a vacuum and an inert gas refill. The apparatus was calibrated in 25-1600 °C using standards Al, Ag, Au, Pd, and Ni. The standard deviation of the performed calibration is around ± 0.8 °C. The blank run (baseline) was measured before a measurement.

The Pfeiffer OmniStar GSD 320 is a quadrupole mass spectrometer equipped with a controlled and heated anti-corrosive quartz capillary for coupling with Themys 24, yttria coated iridium filament as a source of ionization and fragmentation of evolved gases, secondary electron multiplier (SEM) as a detector (detection limit < 1 ppm of gases), and a turbo drag pumping system with an interstage turbo pump and diaphragm pump as a vacuum source. The spectrometer enables the detection of atomic masses in the 1-300 amu range. The measurements were conducted in multiple ion detection regime (MID) with a gas flow 1-2 cm³.min⁻¹, 850-1450 V at the SEM detector, mass a detection dwell of 50 ms, resolution 50, and the m/z range 1-100 amu. The all-measured data (DTA-TG-EGA) were processed by the program Calisto Processing.

Raman spectra were recorded in the back-scattering configuration using a Renishaw Raman spectrometer (System 1000) equipped with Bragg filters and a 514 nm Argon laser. The spectra were scanned in the range 15-4000 cm⁻¹, laser power ca. 1 mW, at a spot size of 5 µm, with 10 repeated acquisitions of 10 s/frame and total measuring time around 15 mins. The spectrometer was calibrated using the silicon F1g peak at 520.2 cm⁻¹. Laser parallel-polarization (VV) allowed to measure the phonon modes from the 15 to 4000 cm⁻¹ Raman shift range. The temperature measurements were performed in a temperature chamber Linkam HFS600E-PB4 stage at RT and 100 °C.

High-temperature X-ray powder diffraction (HT XRPD) was performed on a Rigaku Smartlab equipped with a high-temperature chamber Anton-Paar HTK 1200N in Bragg-Brentano geometry using
Cu Kα radiation, λ = 1.54051 Å, and detector Hypix-3000 in 2D mode scan. The sample was placed into the chamber, where the temperature was increased by 5 °C.min⁻¹ and kept at an isotherm for ca. 1 h to record the XRPD. The diffraction data were collected in angular (2θ) range from 2 to 90° with speed of 0.2° s⁻¹ and step width of 0.02°. This way, the XRPD was measured in the temperature range from 23 to 505 °C with a step of 30 °C.

X-ray photoelectron spectroscopy (XPS) spectra were recorded with the AXIS-Supra photoelectron spectrometer (Kratos Analytical Ltd.). Photoelectrons were excited by the monochromatized Al Kα radiation (1487 eV, 300 W, analyzed area of 0.7 × 0.3 mm²). The high-energy resolved C 1s, O 1s, N1s, P2p, and Ni2p spectra were recorded with a pass energy of 20 eV and a step of 0.1 eV. The resulting overall energy resolution of the spectrometer is 0.45 eV (measured on the width of the Ag 3d5/2 line). Binding energy calibration was performed concerning the C-C bond of the fitted C 1s lines that peaked at 285.0 eV. Quantification was performed using the integrated peak areas of the C1s, O 1s, N 1s, P2p, and Ni2p core level spectra after standard Shirley's electron inelastic background subtraction and using the atomic sensitivity factors given in ESCApe software (Kratos Analytical Ltd.). The high-energy resolved spectra were analyzed by peak fitting using the symmetric Voigt curves.

**3. Results and discussion**

**3.1. Optical thermomicroscopy**

The OTM of selected hybrid Ni-phosphite crystals proved a transition occurring in range 65–85 °C with color changing from a translucent blue (figure 1a), opaque light green (figure 1b) to a green (figure 1c). A significant volume loss of ca. 18 % was observed throughout the heating until 175 °C. At the end of the heating cycle at ca. 200 °C, the sample showed surface bubbles and a dark blue color (figure 1d). These bubbles indicated a release of gas(es).

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Hybrid Ni-phosphite grains captured at distinct color transitions with designated temperatures: 30 °C (a), 65 °C (b), 85 °C (c), and 215 °C (d).</figcaption></figure>

**3.2. HT XRPD analysis**

Figure 2 shows the HT XRPD patterns of the hybrid Ni-phosphite measured at different temperatures. The patterns at the RT to 55 °C (blue patterns) confirmed a presence of pure (enH₂)Ni(H₂O)₆(HPO₃)₂ phase. The structure was fitted using the Lebail fit method with Jana2020 [9] software from the cif file obtained in the CCDC database (code #2220095). No unindexed diffraction peak was distinguished. Then, the sample underwent a transition resulting in an amorphous phase (green patterns), which is in temperature agreement with our observation in OTM. The diffraction peaks of the alumina holder started to appear due to the contraction of the sample, i.e. with temperature-forming cracks in its layer, thus
enabling the detection of the holder. The amorphous phase was stable until ca. 205 °C when weak and broad diffraction peaks ascribed to the Ni₂P phase appeared (black patterns). The broadness of the peaks would suggest the content of small-size crystallites or aggregates. Afterwards, their intensity increased due to higher recrystallization of the Ni₂P phase.

<figure><img src="image_7.png" /><figcaption>**Figure 2.** HT XRPD patterns of the raw hybrid Ni-phosphite were measured at different isotherms with a step of 30 °C.</figcaption></figure>

### 3.3. DTA-TG-MS analyses

The DTA-TG-MS of hybrid Ni-phosphite had to be measured twice due to an error occurring in the MS device, which led to an interruption of the measurement at ca. 130 °C. Unfortunately, at that moment, all hybrid Ni-phosphite samples were already used for other characterizations. Thus, we had to use a Ni-phosphite already measured by TMA (to 150 °C). The DTA-TG and dTG of both samples are shown in figures 3 and 4, respectively. The DTA curves showed the first endothermic effect with an onset temperature ($T_{onset}$) and peak maximum ($T_m$) at 67 and 82 °C, respectively, corresponding to HT XRPD and OTM, the blue curve in figure 3. Interestingly, this proves that color changes shown in figures 1b and 1c can be ascribed to the same effect – only to its beginning (onset) and end (peak maximum). Accordingly, this effect is accompanied by a color change from blue to green, see figures 1a and 1c. Further, the first DTA effect was also accompanied by mass loss of 24 and 5 % for raw and sample after TMA (at 150 °C), respectively. Apparently, the 24 % loss can be ascribed to the decomposition of the sample and the formation of an amorphous product. However, the effect seems to be reversible as 5 % loss was also present in the sample after TMA (at 150 °C), which was stored for ca. 1 month at ambient atmosphere prior to the DTA-TG-MS. Nevertheless, the sample showed no color change while stored.
<figure><img src="image_8.png" /><figcaption>**Figure 3.** DTA-TG curves of the hybrid Ni-phosphite raw and after TMA (at 150 °C).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 4.** The dTG curves of the hybrid Ni-phosphite raw and after TMA (at 150 °C).</figcaption></figure>

The EGA of the first effect (in the raw sample) is presented in figure 5, which shows that various fragments (15–80 amu) were recorded. The composition of evolved gases, performed by integrating the area under each MS peak, is summarized in table 1, showing that the most dominant fragments in the first effect belonged to 18 (H₂O), 17 (OH⁻), and 16 (O²⁻). Fragments 18 and 17 have an integral intensity ratio of 4:1 suggesting that 17 and 16 originate only from fragmentation of the mass 18. Other fragments were below 0.5 %. Thus, we can conclude that H₂O was dominantly released during the first transition. The EGA result is also supported by the fact that the observed mass loss of 24 % corresponds to a theoretical value of 5 H₂O (23 %) present in the Ni-phosphite (molar mass 388.9 g.mol⁻¹). The remaining 1 % could be ascribed to the other residuals (CH₃, O₂, etc.). The EGA of the first effect in the sample after TMA was almost identical to the raw one supporting the conclusion that this effect is reversible and originates in the release of H₂O.
<figure><img src="image_10.png" /><figcaption>**Figure 5.** DTA-TG-MS curves of the raw hybrid Ni-phosphite (only to 130 °C).</figcaption></figure>

The following effect(s) were more difficult to identify as there was a mismatch in the DTA of raw Ni-phosphite and the sample after TMA, see blue and green DTA curves in figure 3. So, for better clarity, a derivation of TG curves (in figure 4) was applied for the identification of DTA peaks (both the raw sample and the sample after TMA) confirming other effects at 162 and 167 °C (weak endothermic), 273 and 269 °C (intensive exothermic), and broad exothermic effect with the onset at 353 °C, see figure 3. The exothermic effect in the raw sample occurring at ca. 110 °C (present only in the blue curve) is not accompanied by any mass loss, so, its origin is difficult to ascribe. Assumably, such effect could be related to recrystallization occurring in the amorphous phase. However, its absence in the sample after TMA does not support that. All effects in both samples (raw and after TMA) were followed by the mass losses of 3 and 4 %, 7 and 9 %, and 11 %, respectively, proving that both samples had similar thermal behaviour above 150 °C.

The MS curves of the second effect (at 162 °C) showed a similar EGA composition as in the first one proving its decompositional origin. The mass loss of 4 % could be ascribed dominantly to H₂O (due to the remaining theoretical content of 4.6 %) and also to a higher content of O₂ and PO₃, see figure 5 and table 1. This result suggests that a phosphite group started to be unstable and was partly released from the amorphous phase.

On the contrary, the third effect (at ca. 270 °C) could be ascribed to the crystallization of the Ni₂P phase, which well matched with the observed diffraction peaks in HT XRPD, see figure 2. The EGA composition of the 9 % mass loss showed a distinct release of 18 (H₂O, NH₃), 17 (NH₂, OH⁻), and 16 (NH, O²⁻) as well as of other fragments such as 15 (CH₃), 28 (CO), and 30 (NO), see figure 6 and table 1. The content of other fragments was below 0.5 %. Interestingly, the ratio between 18 and 17 amu was 1.4:1 indicating that the gas composition changed by releasing dominantly NH₂ rather than H₂O. The MS data would suggest that the decomposition of ethylenediamine was initiated and accompanied by the possible release of H₂O residuals. Noticeably, the phosphorus (P) valence state in phosphites is 3+. And 3⁻ in phosphides. Therefore, P is part of the reduction-oxidation (redox) reaction within the crystallization of Ni₂P. Based on this assumption, we conclude that the presence of CO and NO are not only a products of a limited oxidization from the released O₂ but also from such a redox reaction, i.e. reduction of P³⁺ to P³⁻ and oxidation of carbon and nitrogen from ethylenediamine to CO and NO.

The fourth broad DTA effect (at ca. 350–500 °C) is also accompanied by the continuing release of various gases related to the following fragments 17 (NH₃, OH⁻), 16 (NH₂, O²⁻), 15 (CH₃), 18 (H₂O), 28
(CO), and 79 (PO₃), see figure 6 and table 1. However, the MS curves contain at least 3 peaks suggesting that the gas release occurred continuously at ca. 350, 370, and 410 °C. The ratio of the peak integrated area for 18:17 amu was completely reversed to ca. 1:10 suggesting a full decomposition of ethylenediamine and a release of the NHₓ group instead of H₂O. This result is also supported by the ratio of the fragments 17:16 reaching 1.3:1, which is more equalized than in the third effect (17:16 = 1.9:1). Moreover, the initial molar ratio of Ni:P in (enH₂)[Ni(H₂O)₆](HPO₃)₂ is 1:2, however, in crystallized Ni₂P it is opposite 2:1. This could also explain the increase of the detected PO₃ group to 0.108 %.

The fifth effect is unnoticeable on DTA, only with a background slope increases above 500 °C. However, in this region, the effects on MS curves of 18 (H₂O), 17 (NH₃, OH⁻), 16 (NH₂, O²⁻), 15 (CH₃), 30 (NO), and 79 (PO₃) were observed. Their composition was almost identical to that detected in the third exothermal effect suggesting a higher release of H₂O than of NHₓ.

<figure><img src="image_11.png" /><figcaption>**Figure 6.** DTA-TG-MS curves of the hybrid Ni-phosphite after TMA.</figcaption></figure>

<table><caption>**Table 1.** Composition of evolved gases in the raw hybrid Ni-phosphite and after TMA.</caption><thead><tr><th rowspan="2">m/z</th><th rowspan="2">effect</th><th colspan="2">raw</th><th colspan="3">after TMA</th></tr><tr><th>1</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th></tr></thead><tbody><tr><td></td><td>fragment</td><td>[%]</td><td>[%]</td><td>[%]</td><td>[%]</td><td>[%]</td><td>[%]</td></tr><tr><td>15</td><td>CH<sub>3</sub></td><td>0.23</td><td>0.17</td><td>0.11</td><td>1.59</td><td>3.43</td><td>3.34</td></tr><tr><td>16</td><td>O, NH<sub>2</sub></td><td>1.85</td><td>1.80</td><td>1.23</td><td>16.98</td><td>37.44</td><td>17.82</td></tr><tr><td>17</td><td>OH, NH<sub>3</sub></td><td>19.91</td><td>19.04</td><td>15.89</td><td>32.28</td><td>48.30</td><td>30.16</td></tr><tr><td>18</td><td>H<sub>2</sub>O</td><td>77.20</td><td>77.69</td><td>81.08</td><td>45.29</td><td>4.57</td><td>45.99</td></tr><tr><td>28</td><td>N<sub>2</sub>, CO</td><td>0.20</td><td>0.46</td><td>0.31</td><td>1.79</td><td>4.28</td><td>0</td></tr><tr><td>30</td><td>NO</td><td>0</td><td>0</td><td>0</td><td>1.34</td><td>0.63</td><td>2.67</td></tr><tr><td>31</td><td>P</td><td>0.03</td><td>0.04</td><td>0.04</td><td>0.03</td><td>0.07</td><td>0</td></tr><tr><td>32</td><td>O<sub>2</sub></td><td>0.53</td><td>0.76</td><td>1.04</td><td>0.34</td><td>0.06</td><td>0</td></tr><tr><td>44</td><td>CO<sub>2</sub></td><td>0.01</td><td>0</td><td>0</td><td>0.27</td><td>0.97</td><td>0</td></tr><tr><td>79</td><td>PO<sub>3</sub></td><td>0.001</td><td>0.002</td><td>0.28</td><td>0.006</td><td>0.108</td><td>0.008</td></tr><tr><td>80</td><td>SO<sub>3</sub></td><td>0.01</td><td>0.04</td><td>0</td><td>0.07</td><td>0.16</td><td>0</td></tr></tbody></table>
### 3.4. Raman spectroscopy

Figure 7 presents the Raman spectra of (enH₂)[Ni(H₂O)₆](HPO₃)₂ from 0 to 900 cm⁻¹ obtained at RT and 100 °C. The Raman spectra are in very good agreement with those already reported in our previous work [8]. The observed peaks at RT can be separated into three regions: The first one, from 15 to 400 cm⁻¹, corresponds mainly to Ni-O and Ni-H₂O vibration modes. The second one, from 400 to 500 cm⁻¹, concurs with δs(PO₃) modes. The last one, from 650 to 900 cm⁻¹, corresponds to p(NH₃⁺, NH₂) bands of combination vibration modes.

The Raman spectrum of (enH₂)[Ni(H₂O)₆](HPO₃)₂ heated to 100 °C reports that a large part of the above-observed vibrations were significantly weakened or completely vanished. Broad peaks of weakly crystallized materials can be observed at around 30, 220, 440, 830, and 880 cm⁻¹. These vibration modes may correspond to Ni-O at 30 cm⁻¹, NiₓP and/or PO₃ for 220 and 440 cm⁻¹, and NiₓP at 830 and 880 cm⁻¹, respectively [8,10]. This result confirms the degradation of the (enH₂)[Ni(H₂O)₆](HPO₃)₂ to Ni-based phosphide materials when thermally treated.

<figure><img src="image_12.png" /><figcaption>**Figure 7.** Raman spectra of the raw hybrid Ni-phosphite at RT (in black) and 100°C (in red).</figcaption></figure>

### 3.5. XPS analysis

The XPS allowed analysis of raw hybrid Ni-phosphite and the one heated to 120 °C, further denoted as "Ni-phosphite_120°C". Their spectra regarding the binding energies of Ni and P photoelectron lines are depicted in figure 8. The fitting of the Ni photoelectron lines showed that Ni was present in the form of hydroxide NiOH₂. The valence state before and after heating treatment remained the same in the form of Ni²⁺ as shown by the difference of Ni 2p photoelectron lines, see figure 8a. Similarly, the P 2p photoelectron lines of also both samples also resembled, see figure 8b. They contributed to the following functional groups: (PO₄)³⁻ and (HPO₄)²⁻.

The general elemental composition of both samples is summarized in table 2. Interestingly, the content of key elements (P, C, N, and O, apart of Ni), of which the (C₂N₂H₁₀)[Ni(H₂O)₆](HPO₃)₂ is composed, remained similar in both samples. This result suggests that the sample after thermal treatment remained unchanged, which contradicts our previous observation. A possible explanation is that the XPS collects data from the sample's surface area, i.e. from a few nanometers beneath the surface. This result can heavily influence the results if there are differences between the surface and bulk of the sample. The XRPD (figure 2) and the DTA-TG (figure 3) showed that Ni-phosphite remained in an amorphous state up to ca. 205 and ca. 270 °C, respectively. So, when the sample was heated to ca. 120 °C, the full
crystallization of the Ni₂P phase has not occurred yet. However, the Ni content showed the opposite conclusion as the Ni concentration dropped after heat treatment 5 times than in the initial sample, see table 2. This result would suggest a depletion of Ni from the surface to the bulk due to the possible formation of stable nuclei and aggregates of Ni₂P or NiₓP phases as shown by the Raman spectra (vibrational modes of NiₓP) and the XRPD (presence of broad diffraction peaks at ca. 205 °C), respectively.

<figure><img src="image_13.png" /><figcaption>**Figure 8.** XPS spectra of the raw hybrid Ni-phosphite (blue) and Ni-phosphite_120 °C (red) of nickel (a) and phosphorus (b).</figcaption></figure>

<table><caption>**Table 2.** Elemental composition of the raw hybrid Ni-phosphite and after 120 °C based on the XPS measurement.</caption><thead><tr><th>Sample</th><th>Units</th><th>P</th><th>C</th><th>N</th><th>O</th><th>Ni</th></tr></thead><tbody><tr><td>Ni-phosphite</td><td>[at%]</td><td>13.10</td><td>24.40</td><td>12.80</td><td>42.30</td><td>7.30</td></tr><tr><td>Ni-phosphite_120 °C</td><td>[at%]</td><td>11.90</td><td>35.50</td><td>16.20</td><td>35.10</td><td>1.20</td></tr><tr><td>Ni-phosphite</td><td>stoich. coef. [1]</td><td>1</td><td>1.86</td><td>0.98</td><td>3.23</td><td>0.56</td></tr><tr><td>Ni-phosphite_120 °C</td><td>stoich. coef. [1]</td><td>1</td><td>2.98</td><td>1.36</td><td>2.95</td><td>0.10</td></tr></tbody></table>

### 3.6. TMA analysis and decomposition kinetics

The TMA of raw hybrid Ni-phosphite is compared with DTA-TG showing reasonable similarity of the curves as well as of T-onset 67 (DTA, TMA) and 71 °C (TG) for the heating rate 5 °C.min⁻¹, see figures 3 and 9 (grey curve). Interestingly, the TMA outlined two effects instead of one with onsets at 67 and 109 °C and displacement of -12 and -10 %, respectively. Contraction of a material confirms previous results that a decomposition was present. Their total value of -22 % agrees with grain dimensions' change (ca.
−18 %) captured by OTM. The TMA also allowed a determination of the coefficient of thermal expansion (CTE) at 25 °C reaching 25 × 10⁻⁶ K⁻¹.

However, the undetection of both effects by DTA-TG is of unknown reason. Possibly, it might be due to overlap of both effects. Further, based on previous EGA results, we can confirm that both effects were related to the release of H₂O as all the MS curves contain broad peaks within the same temperature region (55–130 °C), see figures 3 and 6. Thus, we can conclude that both effects lead to the release of ca. 2.5 H₂O as the displacement in both effects is almost equal, which would follow the TG loss of 23 %, i.e. five crystalline H₂O.

<figure><img src="image_14.png" /><figcaption>Figure 9. TMA curves of the raw hybrid Ni-phosphite heated with 2–25 °C.min⁻¹ rates.</figcaption></figure>

Kinetics of decomposition in the first effect (DTA curve, see figure 3) required heating of the sample in a wide range of rates (2–25 °C.min⁻¹), see their TMA curves in figure 9. Expectedly, the T_onset shifted towards higher temperatures (from 65 to 75 °C) with increasing heating rate (from 2 to 25 °C.min⁻¹), their summary in table 3. Similar behaviour was observed also for the second TMA effect. These data were used to calculate the kinetics parameters of the effects by Kissinger method (equation 1) and Arrhenius equation (equation 2):

$$ \ln \ln \left( \frac{\beta}{T_{\text{onset}}^2} \right) = \ln \ln \left( -\frac{AR}{E_a} f(\alpha_p) \right) - \frac{E_a}{RT_{\text{onset}}} \quad (1) $$

$$ k = A e^{-\frac{E_a}{RT_{\text{onset}}}} \quad (2) $$

where T_onset is the onset temperature (K) of the effect, β is the heating rate (°C.min⁻¹), E_a is the activation energy (kJ.mol⁻¹), α_p is the degree of conversion at peak maximum, A is the pre-exponential factor, R is the gas constant (8.314 J.K⁻¹.mol⁻¹), and k is the reaction rate (s⁻¹). The applicability of TMA data for the model was based on the similarity assumption of displacement (i.e. volume contraction while decomposition) and the degree of conversion.
<table><caption>**Table 3.** Values of the Tonset determined from TMA of raw hybrid Ni-phosphite and calculated kinetics parameters E_a and k.</caption><thead><tr><th rowspan="3">effect</th><th colspan="6">Heating rate [°C.min<sup>-1</sup>]</th><th colspan="2">Kinetics parameters</th></tr><tr><th>2</th><th>5</th><th>10</th><th>15</th><th>20</th><th>25</th><th rowspan="2">E<sub>a</sub> [kJ.mol<sup>-1</sup>]</th><th rowspan="2">k [s<sup>-1</sup>]</th></tr><tr><th colspan="6">T<sub>onset</sub> [°C]</th></tr></thead><tbody><tr><td>1</td><td>65</td><td>67</td><td>71</td><td>73</td><td>74</td><td>75</td><td>191</td><td>2.5</td></tr><tr><td>2</td><td>100</td><td>109</td><td>111</td><td>113</td><td>115</td><td>116</td><td>181</td><td>1.6</td></tr></tbody></table>

Calculations showed that both processes had very similar E_a of 191 and 181 kJ.mol⁻¹ for the first and second effects (table 3). These values are consistent, for example, with E_a (149 kJ.mol⁻¹) of kaolinite dehydroxylation [11], confirming that the first and second TMA effects were ascribed to the release of H₂O. Similarly, the reaction rate of both effects reached 2.5 and 1.6 for the first and second effects, respectively, confirming that the reaction was truly going towards decomposition products.

## 4. Conclusion

In this work, we dealt with a complex analysis of decomposition in (enH₂)[Ni(H₂O)₆](HPO₃)₂ using various thermal (OTM, TMA, DTA, TG, EGA), structural (HT XRPD, XPS), and optical (Raman spectroscopy) analyses to determine color transitions occurring with temperature. The OTM identified color changes from blue to green at 67 °C and green to dark blue at 215 °C. The DTA-TG-EGA analysis showed complex decomposition processes that accompany the color transitions. However, at low temperatures (at 67 °C), the dominant process was dehydroxylation, i.e., the release of crystalline H₂O. HT XRPD and Raman spectroscopy confirmed structural transition of (enH₂)[Ni(H₂O)₆](HPO₃)₂ into an amorphous state and remained stable until ca. 200 °C. At this temperature, new phases of Ni₂P or NiₓP started to appear (possibly as aggregates), which led to the depletion of Ni content from the surface towards its bulk already occurring at ca. 120 °C, as was shown by XPS and HT XRPD. Full crystallization of Ni₂P/NiₓP was observed at ca. 270 °C (DTA-TG-EGA), accompanied by the release of multiple gaseous fragments such as H₂O, NH₃, NH₂, OH⁻, CH₃, CO, and NO originating in the decomposition of specific functional groups of (enH₂)[Ni(H₂O)₆](HPO₃)₂. Further, the TMA actually suggested that the first effect contained two subsequent ones originating in the release of 2.5 H₂O at each one. The kinetics analysis of decomposition based on TMA showed that the determined activation energy (around 190 kJ.mol⁻¹) of dehydroxylation is in agreement with the values of similar processes in different systems. Based on the described analyses, we suggested following scheme of phase transition mechanism in (enH₂)[Ni(H₂O)₆](HPO₃)₂, see figure 10. This study showed the importance and necessity of understanding the origin of phase and color transition mechanism if the Ni-phosphite should be considered as a candidate for visual indicators.

<figure><img src="image_15.png" /><figcaption>**Figure 10.** Scheme of phase transition mechanism in (enH₂)[Ni(H₂O)₆](HPO₃)₂ based on performed analyses.</figcaption></figure>
**Acknowledgments**

The work is supported by Operational Programme Johannes Amos Comenius, financed by European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project No. SENDISO - CZ.02.01.01/00/22_008/0004596).

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