Crystal structure reinvestigation and spectroscopic analysis of tricadmium orthophosphate

The transition-metal orthophosphate, β-Cd3(PO4)2, was synthesized by a solid-state reaction and characterized by single-crystal X-ray diffraction and EDS spectroscopy. It crystallizes in the monoclinic system, space group P21/n.


Chemical context
Phosphates with transition metals have received significant attention due to their wide range of potential applications in different fields of technology such as farming, energy storage, and in the medical field, as medicines or for diagnosis.For instance, Ni 3 (PO 4 ) 2 was identified as a heat-sensitive pigment and a catalyst for breaking and dehydrogenating aliphatic hydrocarbons (Correcher et al., 2013), while the orthophosphates Zn 3 (PO 4 ) 2 and Cu 3 (PO 4 ) 2 have been applied in dentistry as a component of tooth fillings and environmental contamination control (Servais &Cartz, 1971 andRong et al. 2017), respectively.The aim of this paper is to provide a comprehensive discussion on the crystallographic arrangement of the �-Cd 3 (PO 4 ) 2 structure and to supply a full crystallographic description.Additionally, we will present the findings of our investigations into the compound's optical and morphological properties.

Structural study
On the basis of the single crystal X-ray diffraction data analysis, the tri-cadmium orthophosphate crystallizes in the monoclinic system, space group P2 1 /n.In this phosphate structure, all cadmium, phosphorus and oxygen atoms occupy the general Wyckoff positions 4e.The anisotropic refinement of all atoms belonging to the crystal structure of �-Cd 3 (PO 4 ) 2 leads to excellent merit factors {R[F 2 > 2�(F 2 )] = 0.023, wR(F 2 ) = 0.054 and S = 1.07}, which corroborate the adopted crystallographic model.network is formed by layers of PO 4 tetrahedra stacked nearly along the [101] direction and the cadmium polyhedra fill the remaining space as shown in Fig. 2. Furthermore, the description of the sequence of the cadmium polyhedra is not easy.Indeed the polyhedra surrounding the cations (Cd2, Cd4 and Cd9) and (Cd5, Cd6 and Cd8) form two successive layers parallel to the ab plane (Fig. 3a).The first layer of the framework consists of Cd2O 5 , Cd4O 5 and Cd9O 6 polyhedra, forming a ring of eight polyhedra two pairs of square-based prisms that share an edge and four pyramids linked by the vertices (Fig. 3b).The second layer is composed of two Cd5O 5 , Cd6O 5 pyramids and a deformed Cd8O 6 octahedron arranged to share corners (Fig. 3c).This layer is connected to the first layer to form a 3D framework with tunnels along the a-axis direction (Fig. 3d).The zigzag chain composed of the remaining cadmium polyhedra, namely two pyramids, Cd1O 5 , Cd3O 5 , and the Cd7O 6 prism fills the large tunnels of the framework (Fig. 3e).The polyhedra belonging to this chain Figure 1 Three-dimensional view the �-Cd 3 (PO 4 ) 2 crystal structure showing small tunnels along the a-axis direction.

Figure 2
The anionic network of �-Cd 3 (PO 4 ) 2 , formed by layers of PO 4 tetrahedra stacked along the [101] direction.share the edges or vertices and form a zigzag pattern in the tunnels, which consolidates the connection of the two layers.Moreover, a more laborious examination of the structure shows that the four groups of cadmium polyhedra Cd5O 5 -Cd7O 6 -Cd1O 5 -Cd6O 5 share edges to form slabs, which are linked together by the corners to build an infinite zigzag chain along the a-axis direction, as shown in Fig. 3f.

Powder X-ray diffraction
The single crystal diffraction analysis and refinement of �-Cd 3 (PO 4 ) 2 produced high-quality crystallographic data, which were then used to run a profile matching with a Le Bail approach for X-ray powder analysis.This study leads to a very good match (Fig. 4), confirming the unit-cell parameters and space-group symmetry of the compound.The obtained lattice parameters are a = 9.1861 (8) A ˚, b = 10.3349(8) A ˚, c = 21.689(2) A ˚, and � = 99.575(3) � , in the monoclinic system, space group P2 1 /n.This fact is corroborated by the good merit factors: R p = 8.1%, R wp = 11.7%,R exp = 8.5%, �2 = 1.904.
Fourier-transform infrared analysis Fig. 5 presents the FTIR spectra of �-Cd 3 (PO 4 ) 2 , displaying two distinct regions of bands that originate from the [PO 4 ] 3À groups.The first region, ranging from 1151 to 936 cm À 1 , correspond to the P-O fundamental vibrational modes, while the second region, spanning from 624 to 431 cm À 1 , indicates the bending modes of O-P-O.These two groups of bands exhibit similarities with those observed in the A 3 (PO 4 ) 2 family (Jin et al., 2014).Specifically, the bands located at 1051, 1026, 971, and 936 cm À 1 in �-Cd 3 (PO 4 ) 2 correspond to the fundamental vibrational modes of the symmetric P-O stretching, while the bands at 624, 597, 570, 558, 544, 531, and 431 cm À 1 are assigned to the bending modes of O-P-O.Table 1 summarizes the bands and their corresponding assignments.

Morphology of the powders
In Fig. 6, the morphology of �-Cd 3 (PO 4 ) 2 powders is depicted, showing particulate structures of a pulverized powder.The micrographic analysis indicates that the grains possess a well-defined shape.Furthermore, the EDX analysis confirms the purity and composition of the compound, which was also reported by Rajasri et al. (2019), thereby verifying its high quality.
UV-Visible absorbance spectra of the �-Cd 3 (PO 4 ) 2 compound is presented in Fig. 7.The analysis was performed on a powder sample.An absorbance band is observed at 289 nm.The Kubelka-Munk analyses are required to determine the experimental band-gap energy.The band gap energy  Cd 3 (PO 4 ) 2 1157   Figure 4 Calculated and observed X-ray diffraction patterns for �-Cd 3 (PO 4 ) 2 .

Table 1
Infrared bands of �-Cd 3 (PO 4 ) 2 and their assignments.The symmetric P-O stretching corresponds to the undamental vibrational mode v1 971 The symmetric P-O stretching corresponds to the fundamental vibrational mode v1 1026 The triple-degenerate asymmetric P-O stretching mode corresponds to the v3 fundamental vibrational mode 1051 The triple-degenerate asymmetric P-O stretching mode corresponds to the v3 fundamental vibrational mode FT-IR spectra of �-Cd 3 (PO 4 ) 2 .
is the crossing point between the linear inclination of the absorption band and the energy axis.The estimated optical indirect band-gap energy is 3.85 eV.This energy value roughly places this phosphate in the class of semiconductors.

Database survey
The crystal structure of the tricadmium diorthophosphate, namely �-Cd 3 (PO 4 ) 2 , was determined by Stephens (1967) using X-ray diffraction data collected from Weissenberg photographs.Its corresponding high-temperature form crystallizes in the monoclinic system and presents structural similarities with the �-Mn 3 (PO 4 ) 2 graftonite type (Stephens & Calvo, 1969).In light of this literature, �-Cd 3 (PO 4 ) 2 adopts the monoclinic space group P2 1 /c with the following cell parameters: a = 9.221 (1) A ˚, b = 10.335(1) A ˚, c = 24.902(5) A ˚, and � = 120.7 (2) � (Stephens, 1967; see Table 2).However, the crystal structure details are not readily available in the published articles.Furthermore, during our research on transition-metal-based phosphates, we have synthesized �-Cd 3 (PO 4 ) 2 crystals that crystallize in the monoclinic system with the lattice parameters a 0 = 9.1895A ˚, b 0 = 10.3507A ˚, c 0 = 21.6887A ˚, � 0 = 99.64 � , space group P2 1 /n (see Table 2).In fact, these parameters are related to those found by Stephens through the following basis transformation a 0 = a, b 0 = b and c 0 = a + c.Although there is a relationship between the unitcell parameters, it is very difficult to compare the two structural models due to the low quality of the Stephens (1967) model.Thus, we cannot conclude that it is the same structure.
It is important to note that the present structural model is obtained from the resolution and least-squares refinement of single crystal X-ray diffraction data (8280 reflections), measured with high precision.The low values of the reliability factors R and Rw (see Table 3) show that this model is correct.Moreover, the precisions of the interatomic distances and angles calculated from the atomic positions are very satisfactory and the values are compatible with the P-O and Cd-O distances and the O-P-O and O-Cd-O angles given in the literature of this type of phosphate.In the recent model, the cadmium-oxygen (Cd-O) bonds vary between 2.1454 and 2.5935 A ˚.In contrast, Stephens' structure shows a more varied Cd-O bond length, ranging from 2.13 to 2.7 A ˚. Furthermore, the current structure portrays the phosphate tetrahedra with regular geometries, wherein phosphorusoxygen (P-O) bond distances are consistently between 1.527 and 1.556A ˚. Stephens' model, on the other hand, presents a wider P-O bond distance variation, from 1.44 to 1.63 A ˚.The irregularities observed in the Stephens' polyhedral units potentially stem from the aforementioned data resolution limitations of that study.
The rich crystal chemistry of the 3d transition-metal (II) orthophosphates attracts scientists to study their physicochemical properties.From the crystallographic point of view, the most commonly adopted symmetry for the M 3 (PO 4 ) 2 UV-Vis absorption spectra of �-Cd 3 (PO 4 ) 2 .The inset shows the plot of (�h�) 1/2 for determining the band-gap energy.
family is the monoclinic system, space group P2 1 /c.Table 2 summarizes the crystallographic data for a selection of compounds belonging to this family.It appears from analysis of this table that the structural study of practically all phosphates belonging to this family has long been carried out, except Cr 3 (PO 4 ) 2 .The latter phosphate crystal structure, constructed from CrO 5 , CrO 6 and PO 4 polyhedra, is closely related to the studied phosphate in the present work.However, a structural reinvestigation of some phosphates, such as Ni 3 (PO 4 ) 2 and Mn 3 (PO 4 ) 2 , has been undertaken in recent years, as shown in Table 2.

Synthesis and crystallization
Single crystals of �-Cd 3 (PO 4 ) 2 were synthesized by a hydrothermal process using the following protocol.In a Teflon beaker of 23 mL, cadmium nitrate (0.567 g, 99%) and phosphoric acid (1.09 mL of a solution of 14.615 M) were mixed in the molar ratio Cd(NO 3 ) 2 :H 3 PO 4 = 3:2, and 12 mL of distilled water were added to the mix.The Teflon beaker was placed in the autoclave, carefully sealed, and heated at 473 K for two days.The resulting product constituted two singlecrystal types with different shapes.Binocular observations allowed us to estimate the percentage of the two different crystal forms at 50% each.Single-crystal X-ray analysis revealed that the first one corresponds to the well-known compound Cd 5 (PO 4 ) 3 OH (Hata et al., 1978), a prism-shaped phosphate, while the second type, which is parallelepiped shaped, is the subject of the present work and was identified as �-Cd 3 (PO 4 ) 2 .
The powder of the studied phosphate was synthesized by means of solid-state reaction carried out in air.Cadmium nitrate (99%), and di-ammonium hydrogen-phosphate (99%) were weighed at a molar ratio of 3:2 and ground thoroughly in an agate mortar.The mixture was pre-heated at 423 K, 623 K, and 823 K.The resulting powder was then ground thoroughly and heated to 1273 K for 24 h to obtain pure �-Cd 3 (PO 4 ) 2 .
Experimental details X-ray powder diffraction data were collected at room temperature using a Shimadzu diffractometer model LABXRD-6100, equipped with a secondary monochromator and Cu K� radiation (� = 1.54056A ˚).The X-ray diffraction data were collected at 40 kV in the interval 10 � � 2� � 70 � with a step of 0.04 in 2� and a counting time of 1.2 s per step.The collected XRD pattern was fitted using JANA2006 software (Petr ˇı ´c ˇek et al., 2014).The morphology and composition of the synthesized material were characterized using a JEOL JSM-IT 100 scanning electron microscope (SEM) equipped with an EDX at an accelerating voltage of 20 kV.Fouriertransform infrared spectroscopy (FTIR) was performed using a Bruker Platinum-ATR instrument.UV-Visible absorbance measurements were performed on powder samples using a JASCO instrument in the range of 190 to 900 nm at room temperature.The crystal structures were visualized using DIAMOND crystal and molecular structure software (Bergerhoff et al., 1996).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.Refinement.The crystal structure of β-Cd 3 (PO 4 ) 2 was investigated using single-crystal X-ray diffraction data collected at room temperature with a Bruker D8 Venture Super DUO Diffractometer equipped with a PHOTON100 CMOS areadetector and monochromatic MoKα radiation (λ=0.71073Å).APEX3 (Bruker,APEX3 (Version 5.054), SAINT (Version 6.36A), SADABS.(Bruker, 2016) software was used for data reduction and the absorption correction was performed by multi-scan semi-empirical method using SADABS program (Krause, et al. 2015).The crystal structure was solved using dual space algorithm as implemented in SHELXT program (Sheldrick, 2015a), completed by a Difference Fourier map and refined by least-squares using SHELXL program (Sheldrick 2015b) integrated into the WinGX interface (Farrugia, 2012).Geometric parameters (Å, º)

Figure 3
Figure 3 Schematic representation of the three dimensional coordination of the �-Cd 3 (PO 4 ) 2 structure; (a) two successive layers parallel to the ab plane of the cations (Cd2, Cd4 and Cd9) and (Cd5, Cd6 and Cd8), (b) Cd2O 5 , Cd4O 5 and Cd9O 6 polyhedra forming a ring of eight polyhedra, (c) two pyramids (Cd5O 5 , Cd6O 5 ) and a deformed octahedron (Cd8O 6 ) arranged to share corners building the second layer, (d) first and second layer connected, (e) a zigzag chain composed of the two pyramids Cd1O 5 and Cd3O 5 and the Cd7O 6 prism fills the large tunnels of the framework, (f) the sequence of the three pyramids Cd1O 5 , Cd5O 5 , Cd6O 5 and the square-based prism Cd7O 6 .

Table 3
Experimental details.