# Influence of the NiII/MnIIratio on the physical properties of heterometallic Ni2xMn(2–2x)P2S6phases and potassium intercalates
## K0.8Ni2xMn(1.6–2x)P2S6×2H2O

C. Cortés, P. Fuentealba, J. Manzur, J. Pérez-Obando, C. Aliaga, Nathalie Audebrand, H. Aguilar-Bolados, E. Spodine

► To cite this version:

C. Cortés, P. Fuentealba, J. Manzur, J. Pérez-Obando, C. Aliaga, et al.. Influence of the NiII/MnIIratio on the physical properties of heterometallic Ni2xMn(2–2x)P2S6phases and potassium intercalates K0.8Ni2xMn(1.6–2x)P2S6×2H2O. New Journal of Chemistry, 2021, 45 (4), pp.2175-2183. 10.1039/d0nj04680a . hal-03157988

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# Influence of the Ni<sup>II</sup>/Mn<sup>II</sup> Ratio on the Physical Properties of Heterometallic Ni<sub>2x</sub>Mn<sub>(2-2x)</sub>P<sub>2</sub>S<sub>6</sub> Phases and Potassium Intercalates K<sub>0.8</sub>Ni<sub>2x</sub>Mn<sub>(1.6-2x)</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O

Received 00th January 20xx,

Catalina Cortes<fnref n="1" />,<fnref n="2" />, Pablo Fuentealba<fnref n="1" />,<fnref n="2" />*, Jorge Manzur<fnref n="1" />, Juliana Pérez-Obando<fnref n="1" />,<fnref n="2" />, Carolina Aliaga<fnref n="2" />,<fnref n="3" />, Nathalie Audebrand<fnref n="4" />, Hector Aguilar-Bolados<fnref n="1" />, Evgenia Spodine<fnref n="1" />,<fnref n="2" />*

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Bimetallic Ni<sub>2x</sub>Mn<sub>2-2x</sub>P<sub>2</sub>S<sub>6</sub> phases, where 2X = 0.4 (Ni<sub>0.4</sub>), 0.8 (Ni<sub>0.8</sub>) and 1.2 (Ni<sub>1.2</sub>), were synthesized by a microwave assisted method, starting from the corresponding potassium intercalates K<sub>0.8</sub>Mn<sub>1.6</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O, K<sub>0.8</sub>Ni<sub>0.4</sub>Mn<sub>1.2</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O, and K<sub>0.8</sub>Ni<sub>0.8</sub>Mn<sub>0.8</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O. The magnetic, optical, and electrical properties were recorded, in order to visualize the influence of the secondary nickel(II) ions on the physical properties of the pristine Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> phase. Both dc susceptibility and EPR data show that the antiferromagnetic interactions are attenuated in the layers of the bimetallic phases, while the critical temperature remains constant (ca. 80K) for all the bimetallic phases, similar to that of Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub>. Conversely, the absorption edge of the solid state UV- visible spectra shifted to lower energies as the percentage of nickel(II) increased, tending to that of the band gap value of the pristine Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub> phase. Moreover, the electric conductivity of Ni<sub>0.4</sub> was similar to that of the pristine Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> phase, while that of Ni<sub>0.8</sub> and Ni<sub>1.2</sub> approached the values of the electric conductivity of the pristine Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub> phase. On the other hand, the potassium intercalates K<sub>0.8</sub>Mn<sub>1.6</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O and K<sub>0.8</sub>Ni<sub>0.4</sub>Mn<sub>1.2</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O showed magnetization at 16 K, the latter being much weaker as compared to that of the first. The detrimental effect of the secondary nickel(II) ions in the layers of the bimetallic phases became evident for K<sub>0.8</sub>Ni<sub>0.8</sub>Mn<sub>0.8</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O, where the magnetization at low temperatures was not observed. The absorption edge shifted to higher energy values for K<sub>0.8</sub>Mn<sub>1.6</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O and K<sub>0.8</sub>Ni<sub>0.4</sub>Mn<sub>1.2</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O, as compared to Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> and Ni<sub>0.4</sub>, respectively, but the value remained close to that of Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub> for K<sub>0.8</sub>Ni<sub>0.8</sub>Mn<sub>0.8</sub>P<sub>2</sub>S<sub>6</sub>·2H<sub>2</sub>O. In Ni<sub>0.8</sub> and Ni<sub>0.4</sub>, the electrical conductivity at low frequencies stems from polarization processes such as Maxwell/Wagner/Sillars electrode polarization. Moreover, an additional polarization process is observed in the intercalates because K⁺ ions in the interlayer act as charge transport carriers. Consequently, the interacalates present higher electrical conductivity compared with that of the pristine Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> phase, by a factor of ca. 10².

Dedicated to Prof. Jean-Rene Hamon on his 65th birthday.

## Introduction

Many interesting properties of the two-dimensional M<sub>2</sub><sup>II</sup>P<sub>2</sub>S<sub>6</sub> compounds arise from their layered structure, where the ab planes are well defined by chemically bonded atoms, while the stacking of these planes along c depends on the weak van der Waals interactions <sup>[1,2]</sup>. Moreover, these interactions generate a van der Waals gap, which permits interesting reactions such as intercalation of alkaline ions and bulkier species into the inter-lamellar space <sup>[3-9]</sup>. Besides, substitution of M<sup>II</sup> sites in the layers by secondary metal ions can also be attained, thus generating bimetallic phases <sup>[10-14]</sup>. The most common method to obtain these bimetallic compounds is the ceramic synthesis, where the pure elements are heated at high

temperatures <sup>[1,15]</sup>. Both the intercalates and the bimetallic phases have their physical properties drastically modified, as compared to the pristine phases <sup>[12,16-18]</sup>. When analysing the magnetic properties of Fe<sub>2x</sub>Cd<sub>2-2x</sub>P<sub>2</sub>S<sub>6</sub> layered compounds (X = 0.1-0.8), magnetic interactions result predominantly ferromagnetic at low Fe<sup>II</sup> content and antiferromagnetic in the bimetallic phases with a high Fe<sup>II</sup> concentration. Léaustic et al. conclude that the secondary iron(II) ions are not randomly dispersed, forming clusters in the layers, and thus defining the magnetic behaviour <sup>[19]</sup>. On the other hand, pristine Fe<sub>2</sub>P<sub>2</sub>S<sub>6</sub> is known to have a rather complicated magnetic behaviour, where one iron(II) ion is antiferromagnetically coupled with one of its nearest neighbour, and ferromagnetically coupled with the other two neighbours. The result is the formation of antiferromagnetically coupled chains <sup>[20]</sup>.

M<sub>2</sub><sup>II</sup>P<sub>2</sub>S<sub>6</sub> have been classified as wide-range band gap energy semiconductors (1.3 to 3.5 eV <sup>[21]</sup> for bulk layered M<sub>2</sub><sup>II</sup>P<sub>2</sub>S<sub>6</sub> phases), which allow to consider these materials as potential materials for

<footnote n="1">¹Facultad de Ciencias Químicas y Farmacéuticas, U. de Chile</footnote>

<footnote n="2">²CEDENNA</footnote>

<footnote n="3">³Facultad de Química y Biología, Universidad de Santiago de Chile</footnote>

<footnote n="4">⁴Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) UMR 6226, F-35000 Rennes, France.</footnote>
optoelectronic applications[1]. For instance, MnPS₃ layers have been used as FET transistors, UV photodetectors[22] or spiropyran-MnPS₃ intercalate nanoparticles for photochromic response. [23].

Moreover, many other layered thiophosphate phases have been reported in literature containing the homometallic or heterometallic centres in the same oxidation state, M'' or containing heterometallic metal centres with different oxidation states, M' M''' . Examples of the homometallic compounds are Mn₂P₂S₆, Ni₂P₂S₆, Fe₂P₂S₆, Co₂P₂S₆, while for the heterometallic compounds, Fe₂ₓCd₂₂ₓP₂S₆, Mn₀.₅Cd₁.₅P₂S₆, Co₀.₅Cd₁.₅P₂S₆, Ni₀.₅Cd₁.₅P₂S₆, Zn₀.₅Cd₁.₅P₂S₆, AgBiP₂S₆, CuInP₂S₆, Ni₁.₂Fe₀.₈P₂S₆, Ni₁.₆₆Cu₀.₆₈P₂S₆, AgInP₂S₆, AgScP₂S₆ can be cited among others.[18,19,20,24,25]

On the other hand, heterometallic bimetallic phases based on
manganese(II) thiophosphate, M'₀.₄Mn₁.₆P₂S₆, have also been
reported [26]. These were generated by a two-step synthetic
procedure, starting with the preparation of a potassium intercalate,
K₀.₈Mn₁.₆P₂S₆·2H₂O, and followed by a second step using a
microwave assisted cationic exchange reaction. Since the transition
metal(II) ions have similar ionic radii, these replaced the potassium
ions existing in the interlamellar space and were further able to
migrate into the existing vacancies in the layers [26].

In this study, heterometallic phases with Ni₂ₓMn₂₂ₓP₂S₆ stoichiometry (2X = 0.4, 0.8, 1.2) and potassium intercalates K₀.₈Mn₁.₆P₂S₆·2H₂O, K₀.₈Ni₀.₄Mn₁.₂P₂S₆·2H₂O, and K₀.₈Ni₀.₈Mn₀.₈P₂S₆·2H₂O are reported, together with the analysis of the influence of the secondary nickel(II) ions on the magnetic, optical and electrical properties of the pristine Mn₂P₂S₆.

**Experimental**

**Synthesis and characterization**

Pure elements of manganese, phosphorous and sulphur were mixed in stoichiometric 1:1:3 Mn: P: S ratio, with a low proportion of pure iodine (ca. 30 mg per 2 g of mixture). The mixture was sealed in a quartz tube under an Argon atmosphere, which was heated at 750°C for one week. After that, the mixture was cooled down during 48 h to obtain a microcrystalline green powder of Mn₂P₂S₆ as product. The product was washed with CS₂ in order to remove the excess of iodine[¹]. To obtain the Ni₂P₂S₆ pristine phase, pure elements of nickel, phosphorous and sulphur were mixed and heated, using the same procedure as mentioned above.

1 g of Mn₂P₂S₆ powder was suspended in 100 mL of 2M KCl aqueous solution and stirred at room temperature for 24 h. The result of this reaction is the potassium intercalate with chemical formula K₀.₈Mn₁.₆P₂S₆·2H₂O [27].

The potassium K₀.₈Mn₁.₆P₂S₆·2H₂O phase is the precursor to obtain the first bimetallic phase, Ni₀.₄Mn₁.₆P₂S₆ (Ni0.4) This reaction was done by a microwave-assisted cation exchange method [26]. 300 mg of potassium precursor phase were suspended in a solution of 30 mL of methanol with 0.32 mmol of Ni(NO₃)₂·6H₂O in a 100 mL reactor. The suspension was irradiated with 800 W microwave radiation for six minutes. The bimetallic phase was washed with methanol and water for several times, in order to remove the remaining potassium ions. The obtained bimetallic phase was

Ni₀.₄Mn₁.₆P₂S₆; (Ni0.4). To insert a higher proportion of Ni'' ions into **Ni0.4** it was necessary to repeat the same reaction steps. A new potassium precursor of the bimetallic phase was obtained with a chemical formula K₀.₈Ni₀.₄Mn₁.₂P₂S₆·2H₂O, which was used in the microwave assisted reaction to obtain the second bimetallic phase, Ni₀.₈Mn₁.₂P₂S₆ (Ni0.8). Finally, using K₀.₈Ni₀.₈Mn₀.₈P₂S₆·2H₂O and the same synthetic procedure, Ni₁.₂Mn₀.₈P₂S₆ (Ni1.2) was obtained.

**Equipment**

To obtain the bimetallic phases, the assisted microwave reactions were carried out using a LAVIS-Multi-Quant microwave equipment with a frequency of 2459 MHz

The infrared data were collected using a Spectrum Two, Perkin Elmer Spectrometer. The atomic absorption spectroscopy analyses were carried out with a Pinacle 900AA, Perkin Elmer spectrophotometer, using an air-acetylene gas mixture.

Powder X- Ray diffraction data were collected using a Bruker D8 Advance diffractometer, with a Cu-Kα1 monochromatic radiation (λ=1.5406 Å) and equipped with a LynxEye detector. The Cu-Kα1 radiation was selected with an incident beam curved crystal germanium monochromator. The diffraction patterns were collected in the angle range between 5-100° (2θ), with 1s of counting time per step and with a step length of 0.008° (2θ). The powder pattern fittings were made using the FULLPROF program [28](pattern matching with Le Bail fit) and with the WinPLOTR interface. [29]

The magnetic measurements were carried out using a Quantum Design Dynacool Physical Properties Measurement System (PPMS), equipped with a Vibrating Sample Magnetometer (VSM). The dc data were collected under an external applied field of 0.1 kOe in the 2 – 300 K temperature range, and the magnetization data were collected between 0-70 kOe, with temperatures from 3 to 110 K.

Electron Paramagnetic Resonance (EPR) spectra were collected on a Bruker EMX-1572 with X band (9.0-9.9 GHz) with a temperature range of 5 to 250 K.

The solid-state absorption UV-vis data were collected with a Perkin Elmer, Lamba 1050 UV/Vis/NIR spectrophotometer, coupled to an integration sphere, in the 190-1100 nm range. The band gap energy values of the bimetallic phases, potassium precursors, and pristine Mn₂P₂S₆ and Ni₂P₂S₆ were calculated, using the Tauc function, from the solid state absorption spectra of the studied phases [30].

Electrical conductivity, dielectric permittivity and loss tangent data were collected at room temperature with a Broadband Dielectric Spectrometer BDS-40, Novo Control Technologies. Conductivity measurements were collected in a range of frequencies from 10⁻¹ Hz to 1·10⁷ Hz, with an electric field amplitude of 1.0 V, using a gold parallel electrode cell.

**Results and Discussion**

**General characterization**

In this work we report the synthesis by a microwave assisted method, and characterization of layered bimetallic phases of
<figure><img src="image_2.png" /><figcaption>**Fig.1.** SEM micrography of the bimetallic phases: a) **Ni0.4**, b) **Ni0.8** and c) **Ni1.2**, at 8.8k, 8.9k and 10k of magnification, respectively.</figcaption></figure>

Ni₂ₓMn₂₂ₓP₂S₆, where 2X = 0.4, 0.8 and 1.2. To verify the presence of all the elements, EDX analyses were performed on all the prepared phases (fig S1 a, b and c). Figure S1a shows the peaks of the EDX spectra, which correspond to Ni, Mn, S, and P. The EDX analyses permitted to conclude a stoichiometry close to Ni: Mn: P: S of 0.4, 0.8 or 1.2, 1.6, 1.2 or 0.8; 2; 6 respectively. Nickel(II) and manganese(II) contents were corroborated with analyses by atomic absorption. Thus, the following stoichiometry for the bimetallic phases were determined: Ni₀.₄Mn₁.₆P₂S₆ (**Ni0.4**); Ni₀.₈Mn₁.₂P₂S₆ (**Ni0.8**); and Ni₁.₂Mn₀.₈P₂S₆ (**Ni1.2**). Therefore, it is possible to assess that nickel(II) ions replaced manganese(II) ions from the layers of the pristine phase, by the cationic exchange reaction, using the potassium intercalates, as the signal corresponding to potassium is absent in the corresponding EDX spectra (Fig.S1 a, b and c). Figure 1(a, b and c) shows the SEM images, which clearly reveal that the layered structure of the heterometallic phases is preserved. Using infrared spectra, the migration of the Ni phenolate anions into the vacancies of the corresponding potassium precursors was verified, permitting to characterize the corresponding bimetallic phases (Fig. S2). All the potassium precursors present a splitted band, due to the P₂S₆ group at 557 and 608 cm⁻¹. The presence of the splitted band arises from the P₂S₆ groups with different environments, generated by the vacancies around the P₂S₆ groups in the corresponding layers of the phase[31]. These vacancies are formed, during the intercalation of the potassium ions into the interlamellar space, when the ejection of some manganese(II) ions from the layers is required, for the purpose of charge balance.

After the cation exchange reaction, the corresponding bimetallic phases (Ni₂ₓ, 2X = 0.4, 0.8, 1.2) present a single absorption band at 575 cm⁻¹ due to the P₂S₆ group, similar to that of the pristine Mn₂P₂S₆ and Ni₂P₂S₆ phases[32] (Fig. S2 and S3). The insertion of the secondary metal ions in the vacancies of the corresponding potassium precursor is due to the fact that the octahedral Ni phenolate anion (83 pm) is smaller as compared with that of the Mn phenolate anions (97 pm)[33]. For the bimetallic phases the formation of the potassium precursor takes place by the removal of manganese(II) ions from

the layers, whose bonds to the sulphur atoms of the P₂S₆ groups are ionic in nature, while those of Ni-S have a lower ionic character[34,35]. Moreover, manganese(II), having d5 spherical electronic configuration, is subject to a null-ligand field stabilization energy and therefore the corresponding manganese(II) ions are very labile, as compared to nickel(II). Thus, the nickel(II) ions remain unaltered in the layers, permitting the isolation of bimetallic phases with a larger content of these ions.

It is important to highlight that the ordered distribution of the vacancies in the K₀.₈Mn₁.₆P₂S₆·2H₂O phase, based on neutron diffraction data, has been previously reported [36]. This distribution is assumed to be the same for the inserted Ni phenolate anions in the **Ni0.4** phase (Fig. 2)[26] Following this same idea, and assuming that the vacancies created in the K₀.₈Ni₀.₄Mn₁.₂P₂S₆·2H₂O phase follow the same order as in K₀.₈Mn₁.₆P₂S₆·2H₂O, a proposed scheme of the secondary Ni phenolate anions in the layered bimetallic phase of **Ni0.8** and **Ni1.2** compared with the initial bimetallic phase of **Ni0.4** is shown in Figure 2.

The obtained powder X-Ray diffractograms have been fitted in order to calculate the unit cell parameters of the pristine phases and the bimetallic phases, considering a monoclinic cell and C2/m space group. The plots and cell parameters are attached as supplementary material (Fig. S4, S5 and Table S1). The values of the cell parameters for the Ni₂P₂S₆ pristine phase were compiled from literature[2]. Pristine Mn₂P₂S₆ and Ni₂P₂S₆ have a monoclinic crystal structure, with a space group symmetry C2/m, and room temperature unit cell parameters of, *a* = 6.077, *b* = 10.524, and *c* = 6.796 Å, and β= 107.35°, and *a* = 5.812, *b* = 10.070, *c* = 6.632 Å, and β = 106.98° respectively[2]. Analysing the V, Mn, Fe, Co, Ni, Zn thiophosphate series, it becomes evident that Mn₂P₂S₆ has the biggest *β* angle, while Ni₂P₂S₆ the smallest of the series. Moreover, it has been shown that the *c* parameter increases near the magnetic ordering temperature of Mn₂P₂S₆ [37]. The obtained unit cell parameters for the bimetallic phases **Ni0.4**, **Ni0.8** and **Ni1.2** are compiled in table S1. These bimetallic phases also have a monoclinic crystal structure, with space group C2/m similar to the pristine phases. The unit cell parameters of the bimetallic phases are closer to those of Mn₂P₂S₆, which demonstrates that by introducing a greater proportion of nickel(II) cations in the lamellar system, the structure of the bimetallic phase is not modified, as compared to the pristine phase.

The interlayer distance of all the bimetallic phases was calculated using the Bragg law, considering the 2θ value of the 001 reflection from the powder X-ray diffractograms (table S2). Thus, the interlayer distances of all the bimetallic phases were shown to be close to 6.5 Å, a value which is similar to that of the Mn₂P₂S₆ interlayer distance, as compared to that of Ni₂P₂S₆ of 6.3 Å (table S2). The 00l peaks of the powder X-ray patterns of the bimetallic phases are slightly wider than those of the pristine phases, indicating that the crystallinity decreases in the materials obtained with the subsequent microwave assisted reactions, which permit to increase the percentage of Ni phenolate anions in the bimetallic phases (Fig S4.).

On the other hand, the interlayer distance of all potassium intercalated phases is 9.4 Å, due to the presence of hydrated potassium ions in the interlayer space. The smaller interlayer space
of 6.5 Å in the bimetallic phases, permits to conclude that the potassium cations were displaced from the interlayer space by the $Ni^{II}$ ions; these finally migrate from the interlayer space into the vacancies (table S2), in agreement with the SEM-EDX results described above.

## Magnetic Properties

### Magnetic Susceptibility

The magnetic data of the bulk phases were recorded as dc susceptibility measurements, under an external field of 100 Oe, in the 2 to 300 K temperature range. As shown in Figure S6 the thermal dependence of $χ_M$T of the bimetallic phases has a global antiferromagnetic behaviour. The effective magnetic moments ($μ_{eff}$) at 300 K of all bimetallic phases are given in table 1.

From the extrapolation of the linear part of the $χ_M^{-1}(T)$ curves (150-300 K), the Weiss constants of the bimetallic phases were calculated (Fig.S7). The bimetallic phases show a Curie-Weiss behaviour, with a negative $θ$ value which is indicative of bulk antiferromagnetic interactions. The obtained values of the Weiss constant of the bimetallic phases permit to conclude that when $Ni^{II}$ ions are introduced in the layers of the pristine $Mn_2P_2S_6$ phase, the antiferromagnetic interactions between the manganese(II) ions, as compared to those of the pristine phase, are attenuated (table 1).

A detailed analysis of the magnetic data of the bimetallic phases becomes difficult, as both pristine phases have different magnetic properties. In both magnetic structures each centre presents three first nearest-neighbours, six second nearest-neighbours and three third nearest neighbours. However, literature reports that the Neel temperature for $Mn_2P_2S_6$ is 78 K, and for $Ni_2P_2S_6$ is 155 K [38]. Furthermore, $Mn_2P_2S_6$ is classified as a Heisenberg antiferromagnet, [39] while $Ni_2P_2S_6$ as a quasi-two dimensional XY antiferromagnet [40]. $Mn_2P_2S_6$ is known to present the three nearest neighbour magnetic dipoles aligned within a layer in an antiferromagnetic manner. The magnetic dipoles have been described as perpendicular to the layers, or slightly tilted with an angle of $ca. 8°$ [20]. On the other hand, in the $Ni_2P_2S_6$ honeycomb lattice, there is a ferromagnetic interaction with two of the first neighbour nickel(II) ions, forming chains that are parallel to the a axis. These chains interact in an antiferromagnetic way (third intraplanar nearest-neighbour interaction). Literature reports that in this system the magnetic dipoles of the nickel(II) ions are parallel to the planes [41].

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Proposed scheme of secondary $Ni^{II}$ ions distribution in the bimetallic phases a) **Ni0.4**, b) **Ni0.8**, c) **Ni1.2**. The blue spheres (●) are $Ni^{II}$ ions and grey ones (●) $Mn^{II}$ ions.</figcaption></figure>

<table><caption>**Table 1.** Calculated and experimental magnetic moments ($μ_{SO}$ and $μ_{eff}$) and Weiss constant ($θ$) of bimetallic phases **Ni0.4**, **Ni0.8**, **Ni1.2**, and Pristine phases $Mn_2P_2S_6$, $Ni_2P_2S_6$.</caption><thead><tr><th>Phase</th><th>*μ<sub>SO</sub> (μB)</th><th>μ<sub>eff</sub> 300 K (μ<sub>B</sub>)</th><th>θ (K)</th><th>Ref.</th></tr></thead><tbody><tr><td>Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub></td><td>5.9</td><td>5.7</td><td>-250</td><td>[37]</td></tr><tr><td>Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub></td><td>2.8</td><td>2.9</td><td>-241<sup>**</sup><br>-254<sup>***</sup></td><td>[37]</td></tr><tr><td>Ni0.4</td><td>5.3</td><td>4.5</td><td>-213</td><td>t.w</td></tr><tr><td>Ni0.8</td><td>4.7</td><td>4.1</td><td>-211</td><td>t.w</td></tr><tr><td>Ni1.2</td><td>4.1</td><td>3.7</td><td>-208</td><td>t.w</td></tr></tbody></table>

*Calculated with spin values for $Mn^{II}$ and $Ni^{II}$ considering the corresponding fraction of each ion. Reported values for measurements on a single crystal; **$χ_∥$ and ***$χ_⊥$

If we take the model shown in Fig.2a it is possible to assess that for **Ni0.4**, the nickel(II) ions are located at different sites from those of the manganese(II) ions, and far away from each other to have some direct influence on each other. Their influence on the magnetic properties of $Mn_2P_2S_6$ will be related to the lowering of the number of unpaired electrons from five ($Mn^{II}$) to two ($Ni^{II}$), and their consequent contribution to the magnetic moments. In figures 2b and Fig.2c, it becomes evident that the nickel(II) ions are surrounded by a less amount of manganese(II) ions. For **Ni1.2** each manganese(II) ion should be surrounded by three nickel(II) ions, and vice versa. However, a drastic modification of the bulk magnetic properties is not observed. (Fig. S7). Finally, from first derivative of $χ_M$T(T) it is not possible to observe a maximum that indicates a critical temperature for **Ni0.8** and **Ni1.2** (Fig S13). **Ni0.4** was reinvestigated[42] and the better quality of the susceptibility data permitted to observe the corresponding critical temperature at $ca. 80K$.

On the other hand, the observed spontaneous magnetization for $K_{0.8}Mn_{1.6}P_2S_6*2H_2O$ is greatly reduced for $K_{0.8}Ni_{0.4}Mn_{1.2}P_2S_6*2H_2O$, as can be seen in figure 3. This spontaneous magnetization is completely lost for the corresponding potassium intercalate $K_{0.8}Ni_{0.8}Mn_{0.8}P_2S_6*2H_2O$. It becomes evident that the greater amount of nickel(II) in the layers deranges the ferromagnetic orientation of the magnetic dipoles of the manganese(II) ions of $K_{0.8}Mn_{1.6}P_2S_6*2H_2O$.

## Magnetization

Magnetization curves as a function of the magnetic field (M(H)) at different temperatures were collected. The M(H) curves and first derivative of the magnetization curves (dM/dH) of the bimetallic phases **Ni0.4**, **Ni0.8** and **Ni1.2** are depicted in figures S8 a, b and c respectively. The magnetization curves of **Ni0.4**, **Ni0.8** and **Ni1.2** show a linear behaviour at high temperatures, which disappears for temperatures lower than 40K. When the dM/dH curves below 40 K are analysed, it becomes evident that for these temperatures, the curves dM/dH show a non-linear behaviour, which is typical for spin flop transition systems (SF) [43]. The dM/dH curves for **Ni0.4** [42] **Ni0.8** show a maximum at 3K, 13K and 20 K. By extrapolation of the $H_{SF}(T)$ plot, $H_{SF}$ at 0 K can be calculated. The maxima are less defined for **Ni1.2**, preventing the estimation of $H_{SF}$ for **Ni1.2**. Table 2
shows the H<sub>SF</sub> values at 0 K for Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub>, Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub> and the bimetallic phases Ni0.4, Ni0.8 and Ni1.2.

The obtained values are indicative that the H<sub>SF</sub> values decrease as the percentage of Ni<sup>II</sup> increases. Thus, the insertion of a greater proportion of Ni<sup>II</sup> in the Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> phase generates a decrease of the anisotropy of the Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> layered material <sup>[42]</sup>. Literature reports that Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub> does not present a SF transition, <sup>[40]</sup> which demonstrates that inserting a greater proportion in Ni<sup>II</sup> into the pristine Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> phase, leads to a behaviour becoming similar to that of Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub>.

Electron Paramagnetic Resonance Spectroscopy (EPR)

In order to have a better understanding of the magnetic properties of the studied heterometallic phases, these were also studied by EPR spectroscopy. Since nickel(II) ions are EPR silent under the experimental conditions <sup>[44]</sup>, the recorded spectra show the influence of the secondary ions on the EPR bands of the manganese(II) ions. The EPR spectra for Ni0.4, Ni0.8 and Ni1.2 are given as figures S9, S10 and S11, respectively. No hyperfine structure is observed in the spectra in all the temperature range, for any of the bimetallic phases. This behaviour can be explained by an exchange narrowing process, which implies a suppression of the hyperfine lines of the Mn<sup>II</sup> bands due to spin-spin interactions.

By the analysis of the temperature dependence of the observed band, linewidth (ΔH) and I<sub>DIN</sub> (I<sub>DIN</sub> being the double integrated EPR spectra, parameter which can be related to the dc magnetic susceptibility) it is possible to obtain the transition temperature. Figure 4 (a, b and c) shows the plots of line width as a function of temperature of the spectra of the bimetallic phases Ni0.4, Ni0.8 and Ni1.2. When the temperature decreases, the line width increases for all the bimetallic phases, and ca. 80 K a maximum is observed for all the bimetallic phases, indicating a magnetic phase transition.

Figure 5 (a, b and c) shows the temperature dependence of I<sub>DIN</sub>T(T) and that of the first derivative (dI<sub>DIN</sub>T/dT) for each bimetallic phase.

<figure><img src="image_4.png" /><figcaption>**Fig.3.** χ<sub>M</sub>T(T) curves of potassium precursors K<sub>0.8</sub>Mn<sub>1.6</sub>P<sub>2</sub>S<sub>6</sub>•2H<sub>2</sub>O ■, K<sub>0.8</sub>Ni<sub>0.4</sub>Mn<sub>1.2</sub>P<sub>2</sub>S<sub>6</sub>•2H<sub>2</sub>O ▲ and K<sub>0.8</sub>Ni<sub>0.8</sub>Mn<sub>0.8</sub>P<sub>2</sub>S<sub>6</sub>•2H<sub>2</sub>O ●.</figcaption></figure>

<table><caption>**Table 2:** H<sub>SF</sub> values in kOe of pristine phases and bimetallic phases.</caption><thead><tr><td>Phase</td><td>H<sub>SF</sub> (kOe)</td><td>Ref.</td></tr></thead><tbody><tr><td>Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub></td><td>38.0</td><td>[42]</td></tr><tr><td>Ni0.4</td><td>9.4</td><td>t.w. <sup>[42]</sup></td></tr><tr><td>Ni0.8</td><td>8.5</td><td>t.w.</td></tr><tr><td>Ni1.2</td><td>Not estimated</td><td>t.w.</td></tr><tr><td>Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub></td><td>*</td><td>[40]</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig.4.** Line width as a function of temperature for a) Ni0.4, b) Ni0.8 and c) Ni1.2.</figcaption></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Fig.5.** I<sub>DIN</sub>T(T) (solid symbol) and dI<sub>DIN</sub>/dT(T) (open symbol) for a) Ni0.4, b) Ni0.8 and c) Ni1.2.</figcaption></figure>

All three plots show a critical temperature ca. 80 K; these results agreeing with those observed in figures 5a, b and c. The above data clearly indicate that the critical temperature for the bimetallic phases can be estimated from the EPR spectra. Since the critical temperature of the pristine Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub> is ca. 80 K, it can be concluded that the critical temperature is dominated by the manganese(II) ions present in the bimetallic phases, and apparently is not influenced by the increasing concentration of nickel(II) ions.

<footnote>*The pristine Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub> does not present the spin flop phenomenon.</footnote>
# Optical Properties

## Solid State Absorption Spectroscopy

To study the optical properties, UV-vis absorption data were collected in a wavelength range between 190 - 1100 nm (Fig.S12). The band gap energy values were obtained by an extrapolation of the linear zone from the Tauc function of the solid-state absorption spectra, and these values are given in table 3.

The band gap values of the bimetallic phases are lower than those of the pristine phase Mn₂P₂S₆ (2.5 eV), but closer to the band gap value of Ni₂P₂S₆ phase (1.65 eV). This fact shows that by introducing Ni¹¹ ions in the lamellar system of Mn₂P₂S₆, the optical properties will be dominated by those of the secondary metal ion, following a similar behaviour as that of the pristine Ni₂P₂S₆ phase.

Similar behaviour was observed when Ni¹¹ cations were introduced into the K[Cd₁.₅P₂S₆] phase, reducing the energy band gap from 2.95 to 1.23 eV. [24] Wang et al. studied layered sulphide phases, whose stoichiometry was reported to be Kₓ[Bi₄ₓMnₓS₆], and the corresponding intercalates with secondary metal ions. These authors attributed the smaller band gap values recorded for the latter phases to the interaction of these secondary metal ions with the sulphur atoms [35].

For the potassium precursors of the pristine phase (K₀.₈Mn₁.₆P₂S₆•2H₂O) the band gap shows an increase in the value, as compared to the pristine phase that generates the precursor (from 2.5 eV for Mn₂P₂S₆ to 2.7 eV for K₀.₈Mn₁.₆P₂S₆•2H₂O). This indicates that the presence of vacancies, formed by introducing potassium ions in the lamellar system, modifies the optical behaviour of the pristine phase. This same behaviour is observed in the bimetallic potassium precursor K₀.₈Ni₀.₄Mn₁.₂P₂S₆•2H₂O (1.87 eV), where the band gap value increases in comparison with **Ni0.4** (1.69 eV). Conversely, K₀.₈Ni₀.₈Mn₀.₈P₂S₆•2H₂O has a band gap value of 1.52 eV, identical to that of **Ni0.8** 1.52 eV. In other words, the displacement of the band gap of the potassium precursors to higher values, as compared to the phases that generate them, can be related to the influence of the existing vacancies, only in the case of the bimetallic phases with a low content of nickel(II) ions. The situation for K₀.₈Ni₀.₈Mn₀.₈P₂S₆•2H₂O is different, and the vacancies become less important in defining the optical properties.

<table><caption>**Table 3.** Band gap energy values of Mn₂P₂S₆, Ni₂P₂S₆, bimetallic phases and potassium precursor phases.</caption><thead><tr><th>Phase</th><th>“Band gap” (eV)</th></tr></thead><tbody><tr><td>Mn<sub>2</sub>P<sub>2</sub>S<sub>6</sub></td><td>2.50</td></tr><tr><td>Ni<sub>2</sub>P<sub>2</sub>S<sub>6</sub></td><td>1.65</td></tr><tr><td><strong>Ni0.4</strong></td><td>1.69</td></tr><tr><td><strong>Ni0.8</strong></td><td>1.52</td></tr><tr><td><strong>Ni1.2</strong></td><td>1.53</td></tr><tr><td>K<sub>0.8</sub>Mn<sub>1.6</sub>P<sub>2</sub>S<sub>6</sub>•2H<sub>2</sub>O</td><td>2.70</td></tr><tr><td>K<sub>0.8</sub>Ni<sub>0.4</sub>Mn<sub>1.2</sub>P<sub>2</sub>S<sub>6</sub>•2H<sub>2</sub>O</td><td>1.83</td></tr><tr><td>K<sub>0.8</sub>Ni<sub>0.8</sub>Mn<sub>0.8</sub>P<sub>2</sub>S<sub>6</sub>•2H<sub>2</sub>O</td><td>1.52</td></tr></tbody></table>

The presence of nickel(II) ions with a greater covalent character of the Ni-S bonds, as compared to the Mn-S bonds, produces a dominant influence on the energy of the valence and conduction bands, thus defining the band gap energy of the composites.

# Electrical Properties

The electrical conductivity (σ'), dielectric permittivity (ε') and loss tangent (tanδ) measurements were recorded at room temperature in the frequency range between 10⁻¹ and 10⁶ Hz of the electric field. The plots of the real part (σ') of the complex electrical conductivity as a function of frequency of the pristine and bimetallic phases are depicted in figure 6. As seen in Figure 6a, the values at low frequencies of σ' (ν = 10⁻¹ Hz) are in a narrow interval, where the highest value is registered for Mn₂P₂S₆ (4.2•10⁻⁹ Scm⁻¹) and the lowest for Ni₂P₂S₆ (4.7•10⁻¹¹ Scm⁻¹). Moreover, the bimetallic phases present higher electrical conductivity, as compared with that of the pristine phase Ni₂P₂S₆. Furthermore, the Ni₂P₂S₆ phase presents a σ' curve with a “plateau” at lower frequencies (ν < 10¹ Hz), while at higher frequencies (ν > 10¹ Hz) the conductivity increases linearly. By contrast, in the Mn₂P₂S₆ curve a “plateau” is not clearly evidenced, but at higher frequencies (ν > 10⁴ Hz) σ' has a positive linear correlation. The bimetallic phases **Ni0.8** and **Ni1.2** present σ' with similar behaviour compared to that of Ni₂P₂S₆ but shifted to higher values of conductivity. By considering the permittivity (ε') of the Ni₂P₂S₆ curve, which has a low frequency dependence above ν > 10³ Hz, it is possible to infer that the increase of the conductivity at high frequencies corresponds to a pure ohmic process [45]. On the other hand, the increase of ε' with decreasing frequencies (ν < 10³ Hz) indicates the occurrence of polarization phenomena, i.e. electrode/sample polarization and inner boundary polarization at mesoscopic scale [45], the latter is the so-called Maxwell/Wagner/Sillars polarization (MWS)[46]. This behaviour was also registered for the bimetallic phases **Ni0.8** and **Ni1.2**. However, these phases present an additional polarization, evidenced by the increase of ε', probably produced from the electronegativity difference between Ni¹¹ and Mn¹¹ neighbour centres.

These increased ε' values, registered at low frequencies, indicate that the nature of the electric conductivity of these bimetallic phases stems from additional polarization. As a result, a non-ohmic conduction takes place [45]. On the other hand, **Ni0.4** presents σ', ε' and tanδ curves, whose behaviour is similar to that of Mn₂P₂S₆ (Fig.6). This fact suggests that the content of Ni¹¹ centres produces negligible changes on the electric properties of this bimetallic phase, compared to the pristine Mn₂P₂S₆ phase. As seen in figure 6a and 6b, σ' and ε' values of **Ni0.4** and Mn₂P₂S₆ are higher than that of the other phases. Moreover, their tanδ curves present a maximum at v ca. 10² Hz (Figure 6c). Fluctuation of mobile charge carriers associated to the ionic character of the Mn-S bonds contributes to the charge transport through the phase. Probably, other effects such as MWS and electrode/sample polarization also take place.

The Jonscher equation was used in order to estimate the dc conductivity (σ₀) by the fit of the conductivity data of the bimetallic phases, **Ni0.4**, **Ni0.8** and **Ni1.2**, as well as that of the pristine phases, Mn₂P₂S₆ and Ni₂P₂S₆:
$$ \sigma'(\omega) = \sigma_0 + A\omega^n \quad (1) $$

where $\sigma'$ is expressed in terms of $\sigma_0$ (dc conductivity), A corresponds to a temperature dependent factor, $\omega$ is the angular frequency of the electric field (2$\pi\nu$), and n is an exponent, related to the interactions between the dipoles and the environment.

The increase of the n values for Ni0.8 and Ni1.2 (table 4) respect to that of Ni₂P₂S₆ suggests that the nature of the Mn" and Ni" interactions through the layers favours the frequency dependence of the conductivity, which could imply a more efficient ohmic conduction [47]. However, Ni0.4 and Mn₂P₂S₆ do not show a behaviour which can be fitted with Jonscher equation. In the case of the potassium intercalates, the frequency dependence of $\sigma'$, $\epsilon'$ and $\tan\delta$ is shown in figure 7.

<figure><img src="image_8.png" /><figcaption>**Fig.6.** Electrical conductivity (a), dielectric permittivity (b) and loss tangent (c) as a function of frequency for Ni0.4 ■, Ni0.8 ● Ni1.2 ▲, and the pristine phases Mn₂P₂S₆ ▲, and Ni₂P₂S₆ ■.</figcaption></figure>

<table><caption>**Table 4.** Fitting parameters $\sigma_0$, n, and A, and $r^2$ for Ni0.8 and Ni1.2 and Ni₂P₂S₆, using Jonscher equation.</caption><thead><tr><th>Phase</th><th>σ<sub>0</sub> [Scm<sup>-1</sup>]</th><th>N</th><th>A</th><th>r<sup>2</sup></th></tr></thead><tbody><tr><td>Ni0.8</td><td>7.00·10<sup>-10</sup></td><td>0.70</td><td>3.90·10<sup>-12</sup></td><td>0.996</td></tr><tr><td>Ni1.2</td><td>3.20·10<sup>-9</sup></td><td>0.65</td><td>7.20·10<sup>-12</sup></td><td>0.998</td></tr><tr><td>Ni₂P₂S₆</td><td>4.10·10<sup>-11</sup></td><td>0.56</td><td>6.40·10<sup>-12</sup></td><td>0.998</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>**Fig.7.** Electrical conductivity (a), dielectric permittivity (b) and loss tangent (c)-as a function of frequency the potassium intercalates: K₀.₈Mn₁.₆P₂S₆·2H₂O ■, K₀.₈Ni₀.₄Mn₁.₂P₂S₆·2H₂O ● and K₀.₈Ni₀.₈Mn₀.₈P₂S₆·2H₂O ▲.</figcaption></figure>

The $\sigma'$ of the potassium intercalates describe a clear tendency: the increase of nickel(II) content in the precursor favours the increase of $\sigma'$ at low frequencies (Fig.7). Concomitantly, the increase of $\epsilon'$ with the decrease of frequency is depicted in figure 7b. This indicates that the nature of conduction is non-ohmic, namely, the conduction promoted by polarization phenomena. This probably can be explained by the increase of Ni" content in the intercalate, which produces MWS polarization. While, the K⁺ ions not only would impart additional polarization, but also act as charge transport carriers in the interlayer of the intercalates [48]. The increase recorded in $\tan\delta$ at high frequencies suggests that a dipole fluctuation process takes place, due to the interaction between K⁺ ions and the layers.

Silipigni *et al.* studied Li₄xMn₂-2xP₂S₆ films, which were obtained by exfoliation, and therefore maintained the lithium ions in the interlayer space. The *ac* conductivity, together with permittivity data, permitted to conclude that $\sigma_{ac}$ originated from the migration of the partially mobile Li⁺ charge carriers by hopping, which eventually gives rise to a dc conductivity at the lowest frequency value [49]. This model should be considered as possible for the bimetallic potassium precursors, studied in this work.

With the obtained conductivity data it is possible to conclude that the pristine Mn₂P₂S₆ phase loses its conducting properties as nickel(II) is added to the layers. As the percentage of nickel(II) increases the bimetallic phases acquire a more insulating character, and tend to have a behaviour similar to that of the Ni₂P₂S₆ phase.
Even though the concentration of potassium ions remains constant in all these composites the conductivity increases as the content of nickel(II) increases in the layers, thus revealing the importance of the secondary nickel(II) ions on the conductivity of these bimetallic phases.

The results of the electrical properties of the bimetallic NiₓMn₂₋ₓP₂S₆ phases have shown that it is possible to tune the electrical properties, depending on the ratio of the metal centres in the phases. Likewise, the versatility of these materials also stems from the facile procedure to obtain them [26]. This indicates that these bimetallic phases can be used for different applications, such as, the development of sensor devices [50]. As known, there is a widespread variety of 2D layered inorganic materials, such as transition metal dichalcogenides [51,52]. To the best of our knowledge, the natural abundance of P, S and manganese, the ionic character of Mn-S bonds, and the easy synthetic approach may impart to the studied bimetallic thiophosphate phases advantages over other 2D-layered materials.

## Conclusions

The magnetic, optical and conductivity properties of the bimetallic phases are influenced by the presence of the secondary nickel(II) ions. The bulk antiferromagnetic behaviour (Θ) of the bimetallic phases is lower than that of Mn₂P₂S₆, but this magnetic behaviour is not drastically affected by the presence of the secondary nickel(II) ions. This is evidenced by the fact that EPR data permit to calculate a similar critical temperature ca. 80 K for all bimetallic phases, which is similar to the one reported for Mn₂P₂S₆. On the other hand, the band gap energy values for these phases are strongly dominated by the presence of nickel(II) ions. Moreover, Ni0.8 and Ni1.2 phases showed non-ohmic conduction at low frequencies, because of different contributions such as MWS and sample/electrode polarizations. Additionally, Ni0.4 and Mn₂P₂S₆ exhibited higher electrical conductivity attributed to the contribution of Mn-S bonds, which favours the charge transport through the phase.

Regarding the magnetic properties of the potassium intercalates, there is a detrimental effect of the observed spontaneous magnetization for K₀.₈Mn₁.₆P₂S₆·2H₂O, that is χT is reduced in K₀.₈Ni₀.₄Mn₁.₂P₂S₆·2H₂O, and is not observed for K₀.₈Ni₀.₈Mn₀.₈P₂S₆·2H₂O. The optical properties of the potassium intercalates are influenced by the presence of vacancies and nickel(II) cations. Regarding the electrical properties of the potassium intercalates, dipole fluctuation processes are favoured due to the interaction between K⁺ ions and the layers. This contributes to the charge transport; consequently, higher values of conductivity are registered compared with the pristine and bimetallic phases.

## Author contribution statement

Conceptualization: E.S. and P.F.; Investigation and Data Curation: C.C., J.P.-O. and N.A.; Formal Analysis: J.M., H.A.-B. and P.F.; Writing – original draft: C.C. and P.F; Writing – review & editing: E.S., J. M., H.A.-B. and P.F.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

CC thanks the CONICYT doctoral grant, 21160369. ES, PF and CC acknowledge the financial support of FONDECYT 1160106 Project, Basal financing FB0807 and AFB180001 (CEDENNA), CONICYT-FONDEQUIP: EQM130086 and EQM140060 projects, and the international collaborative project, International Associate Laboratory "Molecules and Multifunctional Materials" (LIA M3 – CNRS n°1027).

## Notes and references

Electronic supplementary information (ESI) available: [EDX spectra, FT-IR spectra, PDRX diffractograms an simulated patterns, Temperature dependence Susceptibility curves, M(H) and dM/dH curves, EPR spectra, Solid State UV-Visible spectra, d(χT)/dT curves, cell parameters and interlayer distances tables].

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