Synthesis, Structural Studies, and Magnetic Properties of a New Mixed-Valence Diphosphate: Zn2+5Fe3+2(P2O7)4

A new mixed-valence diphosphate, Zn2+5Fe3+2(P2O7)4, was synthesized from FeII5FeIII2(P2O7)4 via the “solid way” route, by substitution of FeII by ZnII. The obtained X-ray data confirmed the crystallization of the compound in the C2221 symmetry space group (orthorhombic). Magnetic measurements were performed showing that, at room temperature, the compound is paramagnetic and that the Néel temperature is 15.44(20) K. The compound was also investigated by infrared and Raman spectroscopies, in particular to characterize the (P2O7)4− and M–O vibrations.


Introduction
Phosphates exhibit many useful properties for industrial applications, and they have been considered good candidates for encapsulation of nuclear wastes [1]. They have also demonstrated importance in catalysis [2,3]. However, according to the World Health Organization [4], the current processes in the phosphate industry generate significant amounts of solid waste containing dangerous substances, such as metallic trace elements (TMEs) that, beyond the authorized concentrations, are known to be very harmful for the environment and the health [5]. The need for novel phosphate-based materials and new production techniques that might reduce the environmental and health impact is then well recognized worldwide, and justifies the great attention given to these subjects in the last few decades. Morocco is one of the major producers of phosphates at world scale, and the present work is in line with the national strategic plan for valorization of phosphates by means of chemical recycling using phosphate-based matrices as useful carriers for metallic trace elements [6].
Derivatives of iron pyrophosphate, where the Fe II ion has been replaced by Cd II and the Fe III by V III , have been investigated previously [11,12]. The Cd 5 M 2 (P 2 O 7 ) 4 (M = Fe, V) [11] and Cd 4.12 Fe 0.83 Fe 2 (P 2 O 7 ) 4 [12] diphosphates were found to be isostructural with Fe 7 (P 2 O 7 ) 4  (P 2 O 7 ) 4 ) showed an antiferromagnetic behavior with a Neel temperature of 25 K. In the paramagnetic domain, above 40 K, the Curie-Weiss law is obeyed [16]. These later results encouraged us to prepare the zinc derivative of iron pyrophosphate where the Fe II ion is replaced by Zn II , Zn II 5 Fe III 2 (P 2 O 7 ) 4 , and characterize it structurally and magnetically.
Below, the synthetic procedure used to produce the new diphosphate is first described, followed by the characterization of the compound by X-ray diffraction and energy-dispersive X-ray analysis, complemented by infrared and Raman spectroscopic studies. Finally, magnetometry was used to investigate the magnetic properties of the synthesized new phosphate.

Synthesis
The procedure used to synthesize the Zn 5 Fe 2 (P 2 O 7 ) 4 pyrophosphate follows that described in refs. [13,14] and has been designated as "dry way" method. Briefly, stoichiometric amounts of ZnO, Fe 2 O 3 , and (NH 4 )HPO 4 were used as starting materials. After milling for 1 h, the solid solution was heat-treated at temperatures up to 850°C for 24 h, affording the desired material. The heat treatment was interrupted periodically for grinding, to ensure homogeneity.

Methods and Instrumentation
X-ray diffraction (XRD) was used to structurally characterize the sample powder at room temperature. A Cu-kα anode (1.541874 Å) was used, with a step size of 0.02°. The refinement of the lattice parameters was carried out by the Rietveld method using the HighScore Plus software [17], and the graphical representation of the crystalline structure was done using the VESTA software [18], based on the structure CIF file predicted by the program www.Materialsproject.com. This last program used generates the structures by substitution on known compounds using data-mining strategies from the ICSD database [19]. Crystallite size (Dsc) was calculated from the full-width-at-half-height (FWHM) of the diffraction pattern peaks, using the Scherrer equation [20].
Energy-dispersive X-ray analysis (EDXA) was performed with a QUANTA 200FEI microscope, operating at a voltage of 20 kV, and equipped with a Burker XFlagh 410M EDXA detector. These experiments allowed the identification of the sample's elemental composition. For the analysis, a small pellet of the compound under investigation was placed on a carbon tape.
The infrared spectroscopy experiments were done in the mid-IR range, at room temperature, using the KBr pellet technique, in a JASCOFTIR 4200 spectrometer, with 4 cm −1 resolution and 64 scans.
The Raman spectroscopy data were collected in the 400 to 1200 cm −1 Raman shift range with a Raman SENTERRA system (Bruker), using as excitation source a diode laser operating at 785 nm, and a laser power of 10 mW. The integration time selected was 20 s and the number of scans added was 16.
Magnetometry measurements were done in a vibrating sample magnetometer (VSM) fitted with a cryogen-free Physical Properties Measurement System (DynaCool PPMS), and operating at a vibration frequency of 40 Hz and an amplitude of 2 mm in the central area of the coils, where the powder sample was placed in a rod-shaped Perspex sample holder.

Results and Discussions
3.1 X-ray Diffraction and Energy-Dispersive X-ray Analysis Studies Figure 1 shows the X-ray diffraction pattern of the studied diphosphate, Zn 5 Fe 2 (P 2 O 7 ) 4 , obtained at room temperature. It was indexed to an orthorhombic structure belonging to the C222 1 space group. The data show good correspondence to the X-ray diffraction pattern of iron diphosphate, Fe II 5 Fe III 2 (P 2 O 7 ) 4 [10], thus following the same trend previously observed for Cd 5 M 2 (P 2 O 7 ) 4 (M = Fe, V) diphosphates [11], which were also found to be isostructural to Fe 7 (P 2 O 7 ) 4 [10]. The same was found for Cd 4.12 Fe 0.83 Fe 2 (P 2 O 7 ) 4 [12].
The Zn 5 Fe 2 (P 2 O 7 ) 4 diphosphate develops in an orthorhombic C222 1 symmetry space group crystal, which is formed by a series of polyhedra: ZnO 6 and FeO 6 octahedra, and PO 4 tetrahedra belonging to two different phosphate groups. Figure 2 shows a representation of the crystalline structure of Zn 5 Fe 2 (P 2 O 7 ) 4 , which was obtained as described in Section 2.2. The crystalline structure consists of layers composed of rows of FeO 6 and ZnO 6 octahedra (shown in red and blue colors, respectively, in Fig. 2). The threedimensional arrangement of these layers is provided through the diphosphate groups (in yellow).
The obtained average value of the crystallite size (Dsc = 24.30 nm) confirms that the prepared Zn 5 Fe 2 (P 2 O 7 ) 4 diphosphate material possesses nanometric dimension. According to the literature [24], this grains' size is appropriate to facilitate the chemisorption of oxygen.
The EDXA experiments were used to confirm the elemental composition of the studied compound. As expected, all peaks of Zn, Fe, P, and O are present in the EDXA spectrum presented in Fig. 3. This shows that all the elements are not volatile at the maximum temperature during the synthesis (850°C). The carbon peak around 0 keV is due to the carbon present in the support material of the sample used in the analysis (see Section 2.2).  4 , respectively. The comparison of these spectra with those previously reported for other diphosphates [13,14,[24][25][26][27][28][29] allowed characterization of the vibrations associated with the diphosphate fragment, as well as those associated with the Zn-O and Fe-O moieties.

Infrared and Raman Spectroscopic Studies
In the infrared spectrum of the compound (Fig. 4), no bands are observed between 1800 and 1400 cm −1 , allowing to conclude on the absence of water molecules in the sample and, thus, confirming that the synthesized material is anhydrous. In the 1300-600 cm −1 region, the infrared spectrum shows a considerably large number of broad bands of variable intensity. The stretching modes of the terminal PO 3 2− groups give rise to two broad intense pairs of bands, at 1273/1081 cm −1 and 1011/938 cm −1 , which are ascribed to the anti-symmetric and symmetric stretching modes of these groups, respectively. In turn, the symmetric P-O stretching mode of the bridges is observed at 850 cm −1 , while the bands at 576, 551, and 521 cm −1 are attributed to deformation modes of the PO 3 2− groups [13,14].

Conclusion
The substitution of Fe II in Fe II 5 Fe III 2 (P 2 O 7 ) 4 diphosphate by Zn II gave a new diphosphate, Zn II 5 Fe III 2 (P 2 O 7 ) 4 , which was structurally characterized and investigated in relation with its spectroscopic and magnetic properties. The refined X-ray diffraction pattern shows that the new phosphate crystallizes in the orthorhombic C222 1 symmetry space group, being  An antiferromagnetic-paramagnetic transition has been found before for Fe II 5 Fe III 2 (P 2 O 7 ) 4 diphosphate [10,16]. Then, the present study reveals that the substitution of Fe II by Zn II in this type of materials leads to isostructural diphosphates to the mixed-valence iron diphosphate, and also that the Zn II Fe III materials also exhibit predominant antiferromagnetic interactions, with a smaller T N . These results stress the relevance of further studying mixed zinc-iron phosphates, which appear as appropriate materials for magnetic applications.
Funding Information This work was partially supported by funds from FEDER (Programa Operacional Factores de Competitividade COMPETE) and from FCT-Fundação para a Ciência e a Tecnologia under the Project No. UID/FIS/04564/2016. Access to TAIL-UC (VSM measurements) was funded under QREN-Mais Centro Project ICT-2009-02-012-1890. The Coimbra Chemistry Center (CQC) is also supported by FCT (Project UI0313/QUI/2013) and COMPETE. R.F. also acknowledges the Portugal 2020 Project MATIS.