Exploring Brannerite-Type Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni) Oxides: Crystal Structure and Optical Properties

Abstract

Environmentally benign, highly stable oxides exhibiting desirable optical properties and high near-IR reflectance are being researched for their potential application as pigments. Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni) oxides with brannerite-type structures were synthesized by the conventional solid-state reaction method to study their optical properties. These series exhibit structural transitions from brannerite (C2/m) to distorted brannerite (P$\overline{1}$) and NiV₂O₆-type (P$\overline{1}$) structures. The average color of Mg_1−xM_xV₂O₆ compounds varies from reddish-yellow to brown to dark brown. The L*a*b* color coordinates reveal that Mg_1−xCu_xV₂O₆ and Mg_1−xNi_xV₂O₆ show more red hues in color with x = 0.4 and x = 0.5, respectively. The UV–Vis diffuse reflectance spectra indicate a possible origin for these results include the ligand-to-metal charge transfer (O²⁻ 2p-V⁵⁺ 3d), metal-to-metal charge transfer (from Mn²⁺ 3d/Cu²⁺ 3d/Co²⁺ 3d/Ni²⁺ 3d to V⁵⁺ 3d), band gap transitions, and d–d transitions. Magnetic property measurements revealed antiferromagnetic behavior for the compounds Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, and Ni), and an oxidation state of +2 for the M ions was deduced from their Curie–Weiss behavior. The system Mg_1−xMn_xV₂O₆ has a NIR reflectance in the range between 40% and 70%, indicating its potential to be utilized in the pigment industry.

1. Introduction

Transition metal oxides have attracted significant attention in the field of solid-state chemistry since the discovery of their various captivating colors, such as chrome green (Cr₂O₃) and cobalt blue (CoAl₂O₄), which are the currently used commercial inorganic pigments. However, there are concerns about Cr and Co due to their high toxicity to the environment and/or durability among commercial pigments [1,2,3]. The objective of the present study is to explore environmentally friendly, intense, durable, and cost-effective inorganic colorants for the pigment industry. Vanadium, a 3d transition metal, exhibits various oxidation states, +3, +4, and +5, in oxides such as V₂O₃, VO₂, and V₂O₅. Due to these oxidation states, vanadium oxides exhibit a diverse range of useful properties such as magnetic, catalytic, electrochemical energy storage in lithium-ion batteries, and optical properties [4,5,6]. The optical absorption of vanadium oxides lies in the near-UV-to-visible region, where ligand-to-metal charge transfer (LMCT) electronic transitions from O⁻² 2p to V⁺⁵ 3d occur.

The serendipitous discovery of the intense blue pigment Y(In,Mn)O₃ highlighted new directions for tuning pigments through structural–substitutional rationales, enabling structures amenable to substituting various transition metal ions to have their optical properties tuned accordingly [1]. For example, brannerite is a brown-colored uranium titanate mineral (UTi₂O₆), and the composition of this structure can differ extensively [7]. The general formula can be described as AB₂O₆, and this structure has larger A cations with divalent metal and small-sized B ions [8]. The structure of brannerite (AB₂O₆) is monoclinic with the space group C2/m in which A and B sites are octahedrally coordinated, forming chains of edge-sharing AO₆ connected to edge-sharing BO₆ octahedral units (Figure 1). The distance of A–A within the chains is 3.53 Å, and the angle of ∠A–O–A is 104.7°; the A–A distance between chains is 4.97 Å [9]. The divalent metal vanadium oxides, MV₂O₆ (M = Mg, Ca, Mn, Co, Ni, Cu, and Zn), have been widely studied for their magnetic, photocatalytic, fuel cell, battery applications, etc. [10,11,12]. Even as an inorganic–organic hybrid, the observed one-dimensional ferromagnetic interactions above the Néel temperature in the antiferromagnetic brannerite-type [{Mn(Bpy)_x}(VO₃)₂] ≈ (H₂O)_y [x = 0.5 or 1; y = 0.62 or 1.12] reveals the potential of brannerites as quantum materials [13]. Furthermore, solid solution systems can be realized in the brannerite types like MnV₂O₆–MoO₃, MgV₂O₆–CaV₂O₆, ZnV₂O₆–CaV₂O₆, and MnV₂O₆–CaV₂O₆ [9,14,15,16]. Lakshminarasimhan et al. recently reported the structure–optical property relationships of the solid solution Ca_1−xMn_xV₂O₆ and Zn_1−xCo_xV₂O₆ [17]. We also note that MgV₂O₆ exhibits a higher reflectance in the near-infrared (NIR) region compared to other divalent metal vanadium oxides. Therefore, MgV₂O₆, when combined with transition metals, could potentially exhibit strong reflectance in the NIR region, making these materials effective in energy-saving applications on rooftops and vehicles [2,3].

It is not surprising that not all MV₂O₆ compounds have this ideal brannerite-type structure, as variations in the structure can be expected due to differences in stoichiometry. For example, CuV₂O₆ is a slight distortion of the brannerite structure with the space group P$\overline{1}$ and consists of CuO₆ and VO₆ groups (Figure 2a). The Cu–Cu distance along the b-axis is 3.54 Å, and the ∠Cu–O–Cu bond angle is 103.99° within the chains, whereas the Cu–Cu distance along the a-axis is 4.97 Å between the chains [18]. α-CuV₂O₆ shows quasi-one-dimensional behavior with short-range magnetic ordering and on cooling towards a long-range antiferromagnetic transition at 24 K [19,20]. On the other hand, MV₂O₆ (M = Co and Ni) have NiV₂O₆-type structures with the space group ($P\overline{1}$) formed by MO₆ and corner-shared VO₄ and corner- and edge-shared VO₆ (Figure 2b). The M–M distance along the b-axis is 2.96 Å, and the ∠M–O–M bond angle is 92.67° within the chains, whereas the M–M distance along the a-axis is 7.13 Å between the chains in the NiV₂O₆-type structure [21]. Comparing these three brannerite-related structures, we have the ideal brannerite with a monoclinic angle greater than 110°, the distorted brannerite with a triclinic angle around 110°, and the NiV₂O₆-type with a triclinic angle approximately 93°. The structures of these compounds show octahedral coordination of the M-site ion by oxygen, often with distortion attributable to the varying magnitudes of the Jahn–Teller effect exhibited by each metal ion [22].

The valence states of transition metal ions and the crystal structure of the series played vital roles in the origin of the color of the samples reported here [2]. To further explore the optical properties of brannerite-type oxides, here, we report a new series Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni; x = 0.0 to 1.0) and their optical properties. The structures in the solid solution are also further investigated through powder X-ray diffraction (XRD), with refinements of the lattice parameters. The structure–optical property relationships are shown for the series Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni).

2. Experimental

2.1. Synthesis

We synthesized Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni) compounds, with x ranging from 0.0 to 1.0, using the conventional high-temperature solid-state reaction method. Stoichiometric amounts of MgO (Alfa Aesar, Ward Hill, MA, USA, 99.5%), MnCO₃ (Alfa Aesar, Ward Hill, MA, USA, 99.9%), Co(OH)₂ (Alfa Aesar, Ward Hill, MA, USA, 97%), CuO (Fluka, Buchs, Switzerland, 99.9%), NiO (Alfa Aesar, Ward Hill, MA, USA, 99.99%), and V₂O₅ (Johnson Matthey, London, United Kingdom, 99.9%) were ground thoroughly using an agate mortar and pestle. After the mixture was homogeneous and fine, the starting materials were pressed into pellets with an applied pressure of 0.8 psi and transferred into an alumina ceramic crucible and heated in air between 600 and 700 °C for 12 h in a muffle furnace. The solid solutions Mg_1−xCu_xV₂O₆ and Mg_1−xMn_xV₂O₆ series were sintered at least twice at 600 °C and 700 °C with intermediate grinding, respectively, while the synthesis temperature of Mg_1−xCo_xV₂O₆ and Mg_1−xNi_xV₂O₆ was 650 °C with repeated heating at least twice.

2.2. Characterization

Phase formation was confirmed through powder X-ray diffraction (XRD) using a Rigaku Miniflex II X-ray Diffractometer (Rigaku, Tokyo, Japan) with Cu-Kα radiation. The lattice parameters were obtained by Le Bail fitting using GSAS/EXPGUI software First version [23,24]. An internal standard, NaCl (Fm$\overline{3}$m; Sigma Aldrich, St. Louis, MO, USA, 99.99%), was used while recording the XRD. Crystal structures were generated using VESTA 3.5.8 software [25]. Diffuse reflectance UV–Vis (DRUV–Vis) and NIR reflectance spectra (up to 2500 nm) were obtained using a Jasco V-670/V-770 spectrophotometer (JASCO, Tokyo, Japan). The data were converted to absorbance using the Kubelka–Munk equation. The L*a*b* color coordinates were measured using a Konica Minolta CM-700d spectrophotometer (Konica Minolta, Tokyo, Japan). The magnetization measurements were performed using a commercial magnetometer (MPMS3, Quantum Design, San Diego, CA, USA) in the temperature range 5–350 K.

3. Results and Discussion

3.1. Structural Analysis

The phase formation of Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni; x = 0.0–1.0) was analyzed by powder XRD. Figure S1, in the Supporting Information, shows the powder XRD patterns of solid-solution Mg_1−xMn_xV₂O₆ prepared at 700 °C in air. The end members, despite having the same space group (C2/m), the solid solution exhibited a mixture of phases, resulting in a miscibility gap (x = 0.4–0.8) due to slight variations in the unit cell edges for the end members. The lattice parameters of the samples were refined in order to further investigate the structures by Le Bail fitting. The lattice parameters and unit cell volume vs. x in the Mg_1−xMn_xV₂O₆ are shown in Figure S2. The lattice parameters and unit cell volume increase upon Mn substitution due to the larger ionic radii of Mn²⁺ (0.83 Å) when compared to Mg²⁺ (0.72 Å) in six coordination [26].

Similarly, Mg_1−xCu_xV₂O₆ (x = 0.0–1.0) samples were synthesized at 600 °C in air, and the powder XRD patterns are shown in Figure 3. The Mg_1−xCu_xV₂O₆ series shows a change in structure from monoclinic brannerite (C2/m) to brannerite with a slight triclinic distortion ($P\overline{1}$) [17]. We can see the sample has a mixed phase starting from x = 0.4 with impurities. Based on the structural analysis (Figure 4a,b), the lattice parameters a, b, and c; angle β; and the unit cell volume are presented for the ranges x = 0.0–0.4 and x = 0.8–1.0. These variations are attributed to a miscibility gap between the C2/m and $P\overline{1}$ space groups, leading to a mixture of phases with impurities at x = 0.6. The a and c parameters show a decrease in the lattice through the substitution of Mg²⁺ by Cu²⁺, while the b parameters exhibit an increase. Since the ionic radii of Mg²⁺ and Cu²⁺ are 0.72 Å and 0.73 Å in the 6-fold coordination, respectively, as the copper content increases, the lattice parameters also exhibit a marginal increase [26]. The a and c parameters, however, show a decrease, and this probably can be explained by the structural distortion leading to lattice contraction. We can observe that the lattice parameter c exhibits a larger decrease compared to the other parameters. The change in the angle β and decrease in volume with the increasing amount of Cu²⁺ in the brannerite-type structure are observed in Figure 4c,d. Therefore, we can conclude that the shrinking of the brannerite structure, accompanied by distortion upon Cu²⁺ substitution, is influenced by the strong Jahn–Teller effect of Cu²⁺.

In the series Mg_1−xCo_xV₂O₆ (x = 0.0–1.0) and Mg_1−xNi_xV₂O₆ (x = 0.0–1.0), solid solutions were formed with some miscibility gaps. It is to be noted that CoV₂O₆ and NiV₂O₆ have a triclinic structure ($P\overline{1}$). These compounds were synthesized at 650 °C in air, and a phase transition was clearly observed in the series through XRD analysis, as shown in Figure S3. In the Mg_1−xCo_xV₂O₆ series, the phase transition to a triclinic structure ($P\overline{1}$) occurred when the cobalt content was x = 0.8. For compositions with x ≤ 0.6, the major phase presented was a monoclinic structure (C2/m) with impurities. Similarly, in the series of Mg_1−xNi_xV₂O₆ samples, the major phase of the brannerite structure was found for compositions x ≤ 0.1, while for x ≥ 0.3, compounds predominantly showed a triclinic structure. Therefore, the presence of two distinct structures in the end members causes the series to exhibit a clear phase transition upon substitution.

The variations in the unit cell edges in the Mg_1−xCo_xV₂O₆ series show different scales in two regions: x = 0.0–0.6 and x = 0.8–1.0 (Figure S4). The lattice parameters do not change much except for ‘c’, which shows a decrease with the increasing Co²⁺ composition from x = 0.0 to x = 0.6. It is possible that the Co²⁺ substitution leads to a decrease in the average distance between neighboring ions in unit cell c. For the triclinic structure (x = 0.8–1.0), the lattice parameters a and c increase as the cobalt content increases. This can be due to the larger ionic radius of Co²⁺ (0.735 Å) as compared to that of Mg²⁺ (0.72 Å) [26]. In the series of Mg_1−xNi_xV₂O₆, with both end members having different structures (brannerite and NiV₂O₆ types), a decrease in the a and c lattice parameters was observed as the Ni²⁺ content increases (Figure S5). This can be attributed to the smaller ionic radius of Ni²⁺ (0.69 Å) when compared to Mg²⁺ (0.72 Å) [26]. The lattice parameter ‘b’ also exhibits a decreasing trend upon the Ni²⁺ substitution in the NiV₂O₆ ($P\overline{1}$) phase (x ≥ 0.8). Contrarily, the C2/m phase (x < 0.8) shows an increase in the ‘b’ lattice parameter when Ni increases. To summarize, it is known that brannerite and NiV₂O₆-type structures have different segments within the morphotropic series and, hence, a miscibility gap between them.

3.2. Optical Properties

The samples in the Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni) series exhibit varying colors that can be understood by measuring the optical properties. The color of the Mg_1−xMn_xV₂O₆ samples changes from brown to dark brown and eventually to black as the amount of Mn²⁺ increases (Figure S6a). The colors originate from the combination of band gap, d–d transitions, and charge transfers, transitions which can be confirmed by diffuse reflectance UV–Vis spectra of Mg_1−xMn_xV₂O₆ samples, as shown in Figure S6b. The spectra show that, with the increasing concentration of Mn²⁺, the optical absorption edges are red-shifted. This can be understood by narrowing of the band gap with the increasing Mn²⁺ concentration, as the end members of the solid solution MgV₂O₆ (E_g = 2.3 eV) and MnV₂O₆ (E_g = 1.44 eV) have different band gaps [17]. The band gap in these compounds can be considered as the difference in energy levels between the filled O 2p band and empty V 3d band. The absorption edges not only shift from 580 to 1000 nm but also show a variation in intensity, and these are evidence for the influence of Mn²⁺ on the ligand-to-metal charge transfer (LMCT: O²⁻-V⁵⁺) in Mg_1−xMn_xV₂O₆. In addition to band gap (or LMCT) transition, the introduction of Mn²⁺ induces absorption bands in the visible region, as observed with humps in the region 450–550 nm for x = 0.1–0.7 and between 600 and 750 nm for x = 0.7. These absorptions are due to the d–d transition of Mn²⁺ in a distorted six-coordination environment. In general, Mn²⁺ in a perfect octahedral coordination with an inversion center does not exhibit any d–d transition [2]. However, the forbidden d–d transitions are enhanced in intensity because of the distorted octahedral coordination on Mn²⁺. Also, the metal-to-metal charge transfer from half-filled Mn 3d orbitals to empty V 3d orbitals influences the color of the samples. The corresponding L*a*b* color coordinates were calculated. The positive values of a* indicate red hues, which decrease in the Mg_1−xMn_xV₂O₆ series upon Mn²⁺ substitution. Similarly, the positive values of b*, which represent yellow, decrease as well in this series. Furthermore, the lightness (L*) decreases with the increase in the Mn²⁺ substitution, as listed in Table S1. The NIR reflectance spectra of the Mg_1−xMn_xV₂O₆ samples are measured (Figure S6c). The percentage reflectance of Mg_1−xMn_xV₂O₆ varies between 75% and 40% across the NIR region for x = 0.0 to x = 1.0, revealing a decrease in NIR reflectance with the increasing Mn²⁺.

In the photographs of the samples, DRUV–Vis absorption and reflectance spectra of the Mg_1−xCu_xV₂O₆ samples are shown in Figure 5. The DRUV–Vis reflectance spectrum of TiO₂ is also shown in Figure 5c (also in Figures S6–S8) as a comparison since it has a high NIR reflectance due to its high refractive index and application as a white, reflective pigment. In the UV–Vis spectra, it is clear that, as the amount of Cu²⁺ increases, the optical absorption edge is red-shifted from 600 to 780 nm, which can be understood as due to the decreasing band gap, which is different for MgV₂O₆ (E_g = 2.3 eV) and CuV₂O₆ (E_g = 1.69 eV) [16]. The broad optical absorption bands in the visible region (400–600 nm) indicate Cu 3d to V 3d metal–metal charge transfer (MMCT) transitions [27]. In the wavelength range of 650–900 nm, the weak absorption bands arise from partially filled Cu 3d e_g orbitals in the octahedral coordination. In addition, the Mg_1−xCu_xV₂O₆ series with x = 0.2 to x = 1.0 exhibits a strong absorption in the near-IR region. The images of the samples and L*a*b* parameters showing the color changes in the Mg_1−xCu_xV₂O₆ series are presented in Figure 5a and

Table 1. The measured L*a*b* color coordinates of Mg_1−xCu_xV₂O₆.

	L*	a*	b*
x = 0.0	54.52	12.26	43.08
x = 0.1	45.23	16.20	32.47
x = 0.2	46.29	18.21	40.08
x = 0.4	36.18	20.79	21.89
x = 0.6	29.59	14.08	5.52
x = 0.8	28.29	8.43	2.42
x = 0.9	25.41	4.52	0.22
x = 1.0	21.61	3.88	0.08

, respectively. According to L*a*b* color measurement, the solid-solution series Mg_1−xCu_xV₂O₆ displays colors ranging from red to yellow hues, particularly the sample with x = 0.4, showing a more pronounced red hue. As the concentration of Cu²⁺ increases, the color changes from yellowish to brown to black, which makes an agreement with the color measurement.

In the series Mg_1−xCo_xV₂O₆, as in the previous cases, the color changes to dark, as shown in the photographs (Figure S7a), and the optical absorption edge shows a red shift with the increasing Co²⁺ content (Figure S7b). Also, the compositions with x = 0.2, 0.4, and 0.6 show the distinct d–d transitions in the wavelength regions 650–780 nm and 780–1000 nm due to Co²⁺ present in the octahedral coordination environment. The variation in the optical absorption (x < 0.8 and x ≥ 0.8) is attributed to the differences in the crystal structures (Brannerite vs. NiV₂O₆-type). This is similar to the observations of optical properties of Co²⁺ in Zn_1−xCo_xV₂O₆ with brannerite-type structures [17]. The NIR reflectance of the samples in the Mg_1−xCo_xV₂O₆ series exhibits a decrease from 70 to 20% with the increasing Co²⁺ concentration (Figure S7c). From the measured L*a*b* color coordinates for the Mg_1−xCo_xV₂O₆ samples, it is clear that the color shifts from yellowish to brown to dark brown and black as the cobalt composition increases (Table S2). For the sample with x = 0.4, the color has a more reddish tone as compared to the other compositions. Furthermore, it is evident that the yellow hue diminishes with the increasing cobalt content.

The color of the samples in the series Mg_1−xNi_xV₂O₆ shows a change from brown to reddish brown to a dark color with the increasing Ni²⁺ concentration in Figure S8. The DRUV–Vis absorption spectra of these samples exhibit a red shift in the optical absorption edge with the increasing Ni²⁺, and this is similar to the observed results with the introduction of transition metal ions in MgV₂O₆, as discussed above. The optical absorption occurs at 350 nm in the near-UV region due to the ligand–metal charge transfer (O²⁻ 2p $\to$ V⁵⁺ 3d). There are two absorption bands in the visible region located at ~ 450 nm and 750 nm, which indicate ³A_2g $\to$ ³T_1g (P) and ³A_2g $\to$ ³T_1g (F) transitions, respectively, of Ni²⁺ ions in the octahedral coordination [17,28]. In the NIR reflectance, the spectrum indeed shows an intense absorption from x = 0.2 to x = 1.0 and as the Ni²⁺ replacement increases, the percentage reflectance decreases. The L*a*b* color coordinates show a decreasing lightness (L*), which means the samples become darker. Meanwhile, the b* decreases, showing the decrease in yellow hue, and the composition with x = 0.5 has a more intense red color of a* that is 20.23 (Table S3).

3.3. Magnetic Properties

To aid in understanding the microscopic origin of the color changes, the valence states of the transition metal ions can be deduced from the magnetic behavior of the compounds discussed above. It is known that MnV₂O₆ exhibits antiferromagnetic (AFM) behavior with non-frustrated Mn²⁺ ordering [29]. To understand the influence on the magnetic behavior upon the substitution of Mn²⁺ with Mg²⁺, the magnetic properties of Mg_1−xMn_xV₂O₆ (x = 0.9 and 1.0) were measured in the temperature range between 5 and 350 K, and the results are shown in Figure S9. It shows antiferromagnetic (AFM) ordering of Mn²⁺ in octahedral coordination, and the Néel temperatures (T_N) are 17.0 and 20.0 K, respectively. These values are in close agreement with the reported T_N of 20 K [29]. The results show that the decrease in T_N is consistent with the simple mean field dependence of the ordering temperature on the coordination number.

Table 2. Observed magnetic moments μ_eff (μ_B), and Curie and Weiss constants of Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, and Ni). The theoretical spin-only magnetic moments are calculated for Mn²⁺ (d⁵), Cu²⁺ (d⁹), Co²⁺ (d⁷), and Ni²⁺ (d⁸).

Composition	Curie Constant (emu K/mol)	Weiss Constant (K)	Obs. Mag. Moment (μB)	Th. Mag. Moment (μB)
Mg0.1Mn0.9V2O6	4.69	−9.36	6.13	5.92
MnV2O6	4.77	−8.51	6.18	5.92
Mg0.2Cu0.8V2O6	0.58	−52.24	2.16	1.73
CuV2O6	0.57	−86.19	2.14	1.73
Mg0.2Co0.8V2O6	3.47	−12.22	5.27	3.87
CoV2O6	3.43	−11.35	5.24	3.87
Mg0.1Ni0.9V2O6	1.45	5.43	3.40	2.83
NiV2O6	1.42	6.80	3.37	2.83

lists the calculated magnetic moments (μ_B), Curie constant, C (emu K/mol), and Weiss constant, Θ_CW (K). In order to prove the oxidation states of Mn in Mg_1−xMn_xV₂O₆, we used the Curie–Weiss law to fit ${\mathsf{𝒳}}_{mol}^{-1}$ vs. T in the temperature range between 250 and 300 K, and from the linear region, the slope represents 1/C and y-intercept Θ_CW/C [29]. The negative values of Θ_W (Table 2) confirm the AFM nature of the nearest neighbor interaction, and the downturn in 1/χ on cooling below 50 K indicates the likelihood of next-nearest-neighbor interactions. The calculated effective spin-only magnetic moments of the samples with x = 0.9 and 1.0 are 6.13 and 6.18 μ_B, respectively, and these values are in good agreement with the theoretical magnetic moment (5.92 μ_B) of the divalent Mn²⁺ ion. Thus, the oxidation state of manganese in Mg_1−xMn_xV₂O₆ is confirmed as 2+.

For Mg_1−xCu_xV₂O₆ (x = 0.8 and 1.0) in Figure 6, as the Cu concentration decreases, we observed a decrease in the magnetic ordering temperature. Additionally, at x = 1.0, antiferromagnetic ordering with T_N = 22 K is observed, consistent with previous studies [27]. In the inset in Figure 6, the molar susceptibility of CuV₂O₆ is displayed, and we observed that the magnitude of the ferromagnetic signal was around 3 emu/mol, which is very small compared to a FM saturation moment. This signal, which accounts for less than 0.1% of the compound, suggests the presence of uncompensated spins. We can conclude that the magnetic coupling is relatively weak in the structures. Mg_0.2Cu_0.8V₂O₆ exhibits magnetic interactions along the b-axis, with an edge-sharing octahedra contributing to ferromagnetic ordering. Based on the Curie–Weiss law in the temperature range between 250 and 300 K, a negative Curie temperature (Θ) suggests that the Cu²⁺ magnetic moments might overall exhibit antiferromagnetic correlations. The calculated effective moments for the samples with x = 0.8 and 1.0 are 2.16 and 2.14 μ_B, respectively (Table 2). The results show a slightly larger magnetic moment than the theoretical value (μ_eff = 1.73 μ_B); however, the values fall within the observed range and are consistent with the literature [27]. The enhanced effective magnetic moment is attributed to the partial orbital contribution resulting from Jahn–Teller distortion. This is further supported by the discussion of lattice parameters in the Structural Analysis (Section 3.1), with the structure of CuV₂O₆ shown in Figure 2a. The measurements confirm that the oxidation state of copper is indeed +2 in Mg_1−xCu_xV₂O₆ (x = 0.8 and 1.0) within the structure.

Similarly, the samples of Mg_1−xCo_xV₂O₆ (x = 0.8 and 1.0) and Mg_1−xNi_xV₂O₆ (x = 0.9 and 1.0) clearly exhibit antiferromagnetic ordering, as shown in Figures S10 and S11. When the concentrations of Co and Ni are decreased, these samples show a corresponding decrease in the ordering temperature. In the Mg_1−xCo_xV₂O₆ (x = 0.8 and 1.0) series, the AFM transition with Néel temperatures (T_N) was observed at 2.8 and 7.0 K, respectively, while the samples presented T_N at 14.5 and 15.0 K, respectively, in Mg_1−xNi_xV₂O₆ (x = 0.9 and 1.0). It is worth noting that the susceptibility decreases by 10% in Mg_0.1Ni_0.9V₂O₆ compared to NiV₂O₆, which implies the sample was successfully synthesized as a solid solution. The observed magnetic moments in Mg_1−xCo_xV₂O₆ (x = 0.8 and 1.0) are 5.27 and 5.24 μ_B, respectively, which are much larger than the expected spin-only moment (3.87 μ_B) of high-spin Co²⁺ due to orbital contribution in the temperature range between 50 and 150 K. For the Mg_1−xNi_xV₂O₆ (x = 0.9 and 1.0) samples, the observed magnetic moments of the Ni spins with S = 1 are 3.40 and 3.37 μ_B, respectively, in the temperature range between 50 and 200 K. According to the Curie–Weiss law results, the oxidation states of Co and Ni were confirmed as +2 in Mg_1−xCo_xV₂O₆ and Mg_1−xNi_xV₂O₆. Thus, the magnetic measurements of the studied brannerite samples helped in probing the oxidation states of the transition metal ions, which, in turn, explains the observed optical properties originating from d–d transitions, LMCT, and/or MMCT.

4. Conclusions

New brannerite-type solid solutions Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni) were synthesized, and their optical properties were studied. The results showed a miscibility gap between two segments of morphotropic transitions in solid solutions, causing phase transitions within the brannerite structure from C2/m to $P\overline{1}$. The color and optical properties of these series originate from a combination of band gap, LMCT, MMCT, and d–d transitions. It was observed that Mg_1−xCu_xV₂O₆ and Mg_1−xNi_xV₂O₆ show more red hues in color with x = 0.4 and x = 0.5, respectively. Few compositions in the series Mg_1−xMn_xV₂O₆ exhibit a high reflectance, which shows potential for their application as a ‘cool’ pigment. Furthermore, Mg_1−xM_xV₂O₆ (M = Mn, Cu, Co, or Ni) display antiferromagnetic behavior in the temperature range between 5 and 350 K. Further, the magnetic moments deduced from the measurements confirmed the presence of Mn²⁺, Cu²⁺, Co²⁺, and Ni²⁺ in these series. It is worth exploring brannerite-type oxides as pigments due to their flexible compositional tuning with less expensive and non-toxic elements and relatively low temperature of synthesis.