In situ synthesis, structural chemistry and vibrational spectroscopy of Zn-doped Ca5Mg4(VO4)6

The phase formation of the solid solution Ca5Mg4–xZnx(VO4)6 (0≤x≤4) was studied in situ using differential scanning calorimetry and hightemperature X-Ray powder diffraction (XRPD). XRPD analysis shows the appearance of unavoidable secondary pyrovanadate phases using conventional synthesis methods. The local structure of the solid solution was verified by vibrational spectroscopy. The analysis of the infrared and Raman spectroscopy data allows establishing the main features between vanadate garnets and their isostructural analogs among natural silicates. Keywords


Introduction
A wide range of compounds with the general formula A3B2V3O12 that belong to calcium vanadate garnets is known nowadays. This system includes complex oxides with substitution of calcium ions in A position by alkaline, (Li, Na, K), alkaline-earth (Sr) and other metal cations like Cu, Ag, Cd, Pb [1,2]. Heterovalent substitution in B position in the garnet structure A3B2V3O12 (B = Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ ) has been analyzed by many research groups [1][2][3]. Ca5Mg3Zn(VO4)6 was first characterized as a garnet having the crystal symmetry of space group 4 ̅ 3 [4]. The lattice comprises the VO4 tetrahedra sharing edges with deformed CaO8 dodecahedrons. The crystal structure of most of garnet-related vanadates is described with the symmetry of space group 3 ̅ [1]. These compounds attract increasing attention due to a cation deficiency, where the vacancy concentration in A position varies from 1/10 to 3/10 [5][6][7][8]. The nonstoichiometry values in B position were evaluated for Ca5Co4(VO4)6 [9].
Different chemical routes are used to synthesize vanadates as single crystals as well and in polycrystalline forms [11-14, 20, 21]. Among them, solid-state method can be carried out in a quartz ampule upon annealing at 975 °C [4]. The melting point of V2O5 is, nevertheless, about 650 °C, and, typically, vanadium oxide melt interacts with quartz. Some single-crystal fiber methods are also employed for preparation of calcium vanadates [22,23]. However, most of the approaches for the synthesis of target compounds of this class of inorganic solids are highly time-consuming and require special equipment. The widely reported method implies a solid-state reaction [24][25][26]. For example, the synthesis of Ca5M4(VO4)6 (M = Mg and Zn) can be done taking MgCO3, ZnO, CaCO3 and either NH4VO3, or V2O5 as initial reagents with further annealing of the reaction mixture at 800-850 °C. The attempt to obtain Ca5Zn4−xMgx(VO4)6 (0≤x≤3) via a solid-state reaction results in the appearance of ZnO as a second phase [10]. The problems of synthesis of the single-phase powders are reported not only for the above-mentioned system but for other compositions of garnet-type vanadates [1,27,28].
Wet-chemical synthesis procedure provides high homogeneity of its products [29,30]. Many modified sol-gel methods using the corresponding nitrates as initial reagents are often applied to prepare vadanate-based solid solutions [15,31,32]. Other changes in the chemical routes to synthesize different inorganic solids include also mechanochemical treatment, variation of educts and raw materials, additional intermediate grindings and heat treatments, alternation in temperature of annealing. The observations listed above forces us to pay more attention to phase formation of the vadanate subclass of inorganic compounds. This can be accompanied by possible vaporization of vanadium oxides [33]. Ca5Mg4-xZnx(VO4)6 (0≤x≤4) is chosen as a model solid solution among garnet-related vanadates. Differential scanning calorimetry (DSC) analysis supplemented by gas emission analysis, in situ X-ray diffraction and evaluation of the local structure employing vibrational spectroscopy facilitate the solution of the problems in chemistry of vanadate-based materials more comprehensively than was reported earlier [4,5,10,20,31,32]. The present study is also relevant to understanding of the synthesis processes of other garnettype compounds such as silicates, hafnates, zirconates, germanates, etc.

In situ synthesis
DSC and thermal gravimetric analysis (TG) were performed to estimate the optimal temperature of heat treatment of the final product. Thermal analysis was conducted with a Netzsch STA 449C Jupiter simultaneous analyzer operating with the heating rate of 10 K/min. In situ phase formation was studied by collecting of the X-ray powder diffraction (XRPD) patterns upon heating of the reagents 5CaO:4MgO:3V2O5 and 5CaO:4ZnO:3V2O5 taken with the stoichiometric ratios. Room temperature XRPD patterns before heating and after the phase formation were recorded using a Shimadzu XRD-7000 Maxima diffractometer and Cu Kα radiation in the 2θ range between 10° and 90°. High temperature (HT) XRPD in the 2θ range between 15° and 60° was employed with Ni-filter instead of a graphite monochromator and a Shimadzu HA-1001 sample heating attachment (the scanning rate 1 °/min; the 2θ step of 0.02°). The temperature uncertainty was maintained at less than 1 °C. The xerogels obtained during the HT XRPD experiments were placed on a Pt plate. The samples were heated from 20 °C up to 700 °C at the rate <10 °C/min. Further heating was carried out from 700 °C to 785 °C in the steps of 15 °C and the rate <5 °C/min. The samples were maintained for 1 h prior to every measurement at the temperatures listed. Heat treatment at 785 °C was continued for over 12 h. Further heating was carried out on MgO-containing sample from 850 °C to 950 °C with the step 50 °C and the rate <5 °C/min with the dwell time of 1 h at every step. The scanning temperatures were selected according to the DSC curves of the precursor samples. The highest scanning temperature (820 °C) was chosen for Ca5Zn4(VO4)6.

Characterization
XRPD patterns were collected at room temperature using a Rigaku DMAX-2200 diffractometer operating with Cu Kα1 radiation over the angular range 15°≤2θ≤80° with the step increment Δ2θ = 0.02°. The XRPD patterns were compared with those in the ICSD-Web database (2021). Rietveld refinement of the crystal structures was performed using the XRPD data collected on a Bruker D8 ADVANCE diffractometer with position-sensitive detector VÅNTEC-1 operating with Cu Kα1 radiation (the angular range 10°≤2θ≤134°; the step increment Δ2θ=0.021°). The XRPD patterns were compared with those in the PDF4+ICDD (2018) [34]. The crystallographic computing system JANA2006 [35] was employed for Rietveld refinements (Figs. S1-S5). Raman spectra were recorded on a Renishaw Ramascope U1000. This setup comprised a confocal Leica DML microscope, 50× Olympus objective lens (the numerical aperture of 0.55), a notch filter, and a cooled charge-coupled device detector. A Renishaw HeNe laser operating at 632.8 nm and 4 mW at the sample was employed as an excitation source. Typical spectra acquisition time was 300 s and the resolution was 1 cm −1 . The Raman spectrum of silica was used for spectral calibration. Fourier transform infrared (FTIR) measurements were carried out on a Bruker vacuum spectrometer Tensor 27 using KBr pellets. The FTIR spectra were collected in a transmittance mode in the range from 400 cm -1 to 1000 cm -1 with the resolution of 2 cm −1 . The FTIR spectra in the range of 50-600 cm -1 were recorded using ATR technique with a diamond optical element employed at a Bruker Vertex 70v spectrometer. Vibrational spectroscopy was performed at room temperature.

Results and Discussion
Decomposition of the precursor colloidal solution upon heating in air is an exothermic process with the mass loss mainly between 200 °C and 730 °C (Fig. 1). The total mass loss of the precursor mixture of Ca5Mg4(VO4)6 is 61.02 wt%, whereas the value of 52.57 wt% was obtained in the case of Ca5Zn4(VO4)6 up to 730 °C. Following the DSC data of a dried mixture of reagents, one can observe the water desorption at ~220 °C ( Fig. 1) with the maximum of the DSC signal at 250 °C. It is accompanied by emission of СО2 and NO2 after decomposition of organics and (HN4) − groups with energy uptake. The slow exothermal process at 270 °C associated with the prolonged emission of H2O, СО2, NO2 and NO was reported in earlier papers [31]. However, the next stages depicted in the DSC plots differ due to the other precursors chosen for synthesis. At 350 °C, there is an exothermal stage with the ongoing emission of the gases listed above (СО2 and NO2). After the known exothermal effect at 372 °C [31], a well pronounced exothermal peak near 450 °C correlates with emission of СО2, NO2 and H2O. This can evidently be explained by the exhaust that forms during the decomposition of the available organic matrix. The energy output at 510-528 °C is accompanied by emission of СО2, NO and NO2. Extraction of СО2 and NO2 occurs up to 730 °C with energy consumption. The observed disagreement between the obtained results and those reported earlier [31] arises from two main reasons. First, this difference may be caused by the reactants used to synthesize the target compositions. Second, the DSC data are analyzed in this study, whereas only the DTA results were presented previously [31]. The DSC curves of the dried precursor of Ca5Zn4(VO4)6 demonstrate several exothermal peaks at 396 °C, 451 °C, 528 °C and 727 °C and one endothermal one at 689.5 °C (Fig. 1). Some of the processes listed above can be evaluated more accurately employing in situ X-ray diffraction as an additional method.
The XRPD patterns of dried sol-gel precursors show the phase formation of CaCO3 at 360 °C which is stable up to 785 °C ( Fig. 2 and 3). Its appearance is accompanied by the emission of СО2 from the decomposition of an organic matrix. The pyrovanadates CaMgV2O7 and Mg2V2O7 are formed at 360 °C along with calcium carbonate (Fig. 2). The exothermal process near 450 °C is assigned to the formation of the intermediates α-Zn2V2O7 and Ca2V2O7 whose crystal structures are characterized by monoclinic symmetry (Fig. 1-3). The energy output at 510 °C (or 566 °C in Ref. [31]) corresponds to the phase with a cubic structure (space group 3 ̅ ). Extraction of СО2 observed up to 730 °C with energy consumption is caused by the start of calcite decomposition, which ends at 785 °C, according to the XRPD data ( Fig. 1-3).
The mixture of phases is observed from 460 °C to 950 °C in the magnesium-containing powder sample (Fig. 2), whereas the zinc-containing sample is characterized by coexistence of the phases like CaCO3, pyrovanadates, garnet-type vanadate in the temperature range 460-820 °C (Fig. 3). In turn, the phase transition in Zn2V2O7 implies the change of space group C2/c→C2/m at 620 °C [36]. Ca2V2O7 undergoes also the phase transition P1 ̅ →P21/c above 900 °C. CaMgV2O7 melts incongruently at 885 °C, whereas Ca2V2O7 and Mg2V2O7 are thermally stable even at 950 °C.
The coexistence of Ca5Mg4(VO4)6 (space group Ia3 ̅ d) and CaMgV2O7 (space group P21/c) is detected at room temperature in the reaction products (Fig. 2). In this way, one can carry out synthesis at 850 °C and, after shortterm heat treatment, can obtain the target product with contamination of Ca2V2O7. The presence of satellite pyrovanadate phases was revealed in many earlier studies on evaluation of the most efficient chemical route to synthesize garnet-type vanadates [3,5,15]. The elimination of these satellite pyrovanadates is a complex problem which is often dismissed when the target chemical products of Ca5M4(VO4)6 (M = Mg, Zn, Co, etc.) are synthesized. Fortunately, the impurity-free samples can be obtained. However, this usually requires a long duration of thermal treatment [4,5]. Indeed, the heat treatment at 980 °C for 150 h allows a single-phase sample of Ca5Mg4(VO4)6 to be prepared. This temperature of the final annealing during the synthesis procedure corresponds to the melting point of Mg2V2O7. The XRPD analysis shows that polycrystalline Ca5Mg4-xZnx(VO4)6 (0≤x≤4) with some quantity (2-3 wt%) of pyrovanadate phases is formed via the sol-gel process (Fig. 4). The crystal structures of Ca5Mg4-xZnx(VO4)6 (0≤x≤4) were refined using the Rietveld method (Table  S1; Supporting Information). The solid solution Ca5Mg4-xZnx(VO4)6 (0≤x≤4) crystallizes in the cubic space group Ia3 ̅ d, Z = 8 (Supporting Information: Fig. S1-S5).
The fractional atomic coordinates and refinement parameters are listed in Table 1. Magnesium and zinc cations have close radii that promotes the formation of solid solutions [38]. Both Mg and Zn ions occupy the octahedral position (16a) and the formation of the solid solutions is proved by a linear change in the cell parameter and volume when zinc concentration increases (Fig. 4). The lattice parameter a refined for Ca5Mg3Zn(VO4)6 deviates from the linear dependence because of a relatively large amount of CaMgV2O7 impurity (Supporting Information: Table S1).
The site occupancy factor of Ca 2+ positions is chosen to be about 5/6 that is close to the data reported by Ronniger and Mill [6]. Cation deficiency from 1/10 to 3/10 is not surprising and was also observed in other garnet-type vanadates [6,39]. Therefore, a study of the local structure of these compounds becomes highly relevant in view of high deficiency in the cation sublattice.
In Ca2.5M II 2(VO4)3 (M II = Mg, Zn), the symmetry of the VO4 tetrahedron changes due to the site symmetry and crystal field effects, and all the internal modes are split. The latter are summarized in Table 3.
FTIR and Raman spectroscopy techniques are employed to study possible structural changes when Mg 2+ cations are substituted for Zn 2+ in the solid solution Ca5Mg4−xZnx(VO4)6. Both types of vibrational spectra of Ca5Mg4−xZnx(VO4)6 (0≤x≤4) are similar and do not indicate any significant changes in the crystal structure (Fig. 5). Whereas 25 Raman and 17 infrared modes are active according to the group theory, not all of them can actually be detected [10]. This discrepancy originates from the negligible changes in polarizability and dipole moment, which do not give rise to observable Raman and infrared modes [43]. According to the previous studies on garnet compounds [41,44,45], the Raman lines between 910 cm −1 and 950 cm −1 and infrared bands in the range of 700-950 cm −1 correspond to the internal anti-symmetric (ν3) stretching vibrations of [VO4] 3− ions, whereas the symmetric (ν1) stretching modes of [VO4] 3− are revealed only in the Raman spectra between 690 cm −1 and 875 cm −1 . Typical weak Raman lines at ~450 cm −1 with the shoulder at ~430 cm −1 are expected for the VO4 bending vibrations. An observed general order of mode frequencies, i.e., L(VO4) > T′ (the metal cation) > T′(VO4) is also presented in most garnet-type silicates. A very strong Raman peak (as well as some weak ones) at ~320 cm −1 is attributed to the librational VO4 modes which can be also distinguished as the infrared band in the range of 340-380 cm −1 (Fig. 5).  The Raman line at ~245 cm −1 is assigned to translation modes of Ca 2+ . Meanwhile, T′(Ca 2+ ) and T′(Mg 2+ /Zn 2+ ) modes are associated with the infrared band at 190-250 cm −1 . Very weak Raman lines below 200 cm −1 correspond to external translations of VO4 tetrahedrons. The latter stand in the line with the T′(SiO4) modes analyzed previously for A3B2(SiO4)3 (A = Mg, Ca, Mn, Fe; B = Al, Cr, Fe) [44,45]. Substitution of Mg 2+ by Zn 2+ in Ca5Mg4−xZnx(VO4)6 leads to the monotonic increase of the unit cell parameter (Fig. 4b) and a slight shift (not exceeding ~10 cm −1 ) of most of the vibrational bands towards lower wavenumbers (Fig. 5). The similar dependencies of vibrational frequencies on the lattice parameter were earlier found in silicate garnets [44,45]. Given that the density of modes is relatively high to allow a clear assignment of all the lines, it is obviously not complete. Therefore, first-principle calculations in the density functional theory (DFT) framework based on the geometry optimization of Ca5M4(VO4)6 (M = Mg, Zn) are needed to perform an accurate assignment of vibrational bands and will be discussed elsewhere.

Conclusions
High-temperature X-Ray diffraction supported by DSC technique allowed studying Ca5Mg4-xZnx(VO4)6 (0≤x≤4) so as to provide more detailed and reliable data on the phase formation of garnet-type vanadates. The synthesis of Zndoped Ca5Mg4(VO4)6 is accompanied by the appearance of Ca2V2O7, Mg2V2O7 or Zn2V2O7. These satellite pyrovanadate phases are stable up to 950 °C and disappear only above their melting point. The single-phase samples of Ca5Mg4(VO4)6 and Ca5Zn4(VO4)6 can be obtained following heat treatment for 150 h at 980 °C and at 750 °C, respectively.
The formation of the solid solution Ca5Mg4-xZnx(VO4)6 (0≤x≤4) was confirmed by XRPD and vibrational spectroscopy. The findings in structural chemistry contribute to understanding of the impact of synthesis procedures on crystal engineering of vanadates and other garnet-type oxides.