Advanced Synthesis and 3D-AFM-Structural Features of Mono-Metalized Cyclotetraphosphates

Solution route applying the precursor of metaldihydrogen phosphate dihydrate was used to synthesize Mcyclophosphatesat ambient temperature (cobalt (II) was selected as model for metal in these investigations). The precursor was fired and sintered at different temperatures (600, 800, 1000 and 1100°C) respectively to optimize best conditions to obtain Co2P4O12 crystal form with high purity. The products were monitored by both of XRD, IR spectra by additional to accurate imaging via scanning electron microscope (SEM) and AFM-microscopeto analyzesurface topology and microstructural features of the metal cyclotetraphosphate. Structural investigations via XRD proved that the product obtained at 1100°C is the best and fine structure with monoclinic structure phase and C12/C1 space group with lattice parameter a=11.809(2), b=8.293(1), c=9.923(2) A respectively. A visualized investigations were performed to confirm structure validity and stability at temperature of sintering (1100°C). Visualization studies indicated that variations of bond distances between Co1, Co2, P1 and P2 and different six oxygen atoms (O1, O2, O3, O4, O5 and O6) inside crystal lattice are responsible for increasing lattice flexibility factor (by controlling in shrinkage and expansion coefficient) and consequently increase its bonds stability to break.


Introduction
Cyclic phosphates (or cyclophosphates) have a ring anionic unit, and have the general formula [PnO3n]n−, where n ≤ 3. In this geometry each phosphate tetrahedronshares 2 oxygen atoms with its neighbour, giving an O/P ratio of 3: 1. Ultraphosphates, in contrast to polyphosphates and cyclic phosphates, are branched: meaning the anionic unit contains phosphate tetrahedra that share 3 of their oxygen atoms, which can form various different geometries such as threedimensional networks, infinite ribbons and layers, [6] or finite groups.
Morphology influences not only the intrinsic chemical, optical, and catalytic properties of micro-/nanoscalemetal phosphates, but also theirrelevant applications in electronic, biocompartible and biodegradable intissue [2,4].
The difference of media (solvents) in the precipitation process leads to the obtaining different phosphates, asrevealed by XRD and FTIR data. Due to its solubility in water and its ability to associate with metal ions inmedia, solvent has been used as a binder cum gel for shaping materials (bulk, porous, micro-or nano-particles) and a matrix for entrapment of ions to generate a gelled precursor which resulted in obtaining different material or same material with different size and morphology after heat treatment. The results obtained are also in agreement with other phosphate group reported in literature [15,16].
The major goal of the present investigations is understanding the role of structural parameters within crystal lattice of M 2 P 4 O 12 that stabilize structure of crystal even at elevated temperatures. Furthermore understanding the structural parameters effects on the morphological and surface nature of metalized cyclotetraphosphates.

Synthesis of Metal-Cyclotetraphosphate
The cobalt cyclotetraphosphate was synthesized via three step reactions 1 st reaction is dissolving cobalt carbonate in few drops of concentrated nitric acid forming acidic cobalt nitrate then solution neutralized by conc. ammonia solution.2 nd step is the reaction with 70% phosphoric acid forming cobalt dihydrogen phosphate at temperature 230°C.3 rd step is firing followed by sintering process at 1100°C to form violet powder from pure cobaltcyclotetraphosphate. These steps are in patial agreement with [5].
Theviolet powder from pure cobaltcyclotetraphosphate was grounded in agate mortar for 15 min. then the resulted powder forwarded to perform the different structural measurements.

Structural Measurements
The X-ray diffraction (XRD): Measurements were carried out at room temperature on the fine ground samples using Cu-Kα radiation source, Ni-filter and a computerized STOE diffractometer.
Germany with two theta step scan technique. Rietveld andindexing of structure were made via Fullprof package and Gesasprogram.
A visualized studies of crystal structure were made by using Diamond Molecular Structure version 3.2 package, Germany and MERCURY-2.3 depending up on single crystal structural data of pure cobalt cyclotetraphosphatesincluding atomic coordinates of monoclinic phase supplied from ICSD-Karlsruhe-Germany. Scannig electron microscopy (SEM): measurements were carried out along ab-plane using a small pieces of the prepared samplesby using a computerized SEM camera with elemental analyzer unit Shimadzu (Japan). Atomic force microscopy (AFM): Highresolution Atomic Force microscopy (AFM) is used for testing morphological features and topological map (Veeco-di Innova Model-2009-AFM-USA). The applied mode was tapping non-contacting mode. For accurate mapping of the surface topology AFM-raw data were forwarded to the Origin-Lab version 6-USA program to visualize more accurate three dimension surface of the sample under investigation. This processis new trend to get high resolution 3D-mapped surface for very small area.
A visualization study made is concerned by matching and comparison of experimental and theoretical data of atomic positions, bond distances, oxidation states and bond torsion on the crystal structure formed. Some of these data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request @ccdc.cam.ac.uk, or by contacting ICSD-Fiz-Karlsruhe-Germany.

FT-Infrared Spectroscopy
The infrared spectra of the solid products obtained were recorded from KBr discs using a Shimadzu FT-IR Spectrophotometer in the range from 400 to 4000 cm -1 . Figure 1 displays different x-ray diffraction patterns of cobalt cyclotetraphosphate at different Sintering temperatures (600, 800, 1000 and 1100°C) respectively. The accurate analyses of these patterns were performed by using both of rietveld andindexing via Fullprof package and Gesasprogram. The analysis is focused on the main intense reflection peaks (Fingerprint of structure) and indicated that cobalt cyclotetraphosphateis mainly belong to single monoclinic phase with C12/c1 space group as symbolized by pink cycles in Fig. 1 and only very few percentage of cobalt oxide as secondary phase in minor. It was observed that the impurity phases are decreasing as sintering temperatures are increasing as shown in Fig. 1 where impurity phases are assigned by blue squares. The comparisons of most intense reflections peaks in all patterns (fingerprint reflections represent monoclinic -phase) indicated that cobalt cyclotetraphosphate which is sintered at 1100°C is the best fit one with high purity than others which sintered at temperatures 600,800 and 1000°C respectively.

Structural Identification
In the hypothesis of isostructural, due to existence of cobalt (II) and Cobalt (III) the spectrum peaks for the system of cobalt cyclotetraphosphate (solid solution) which is single metal cyclotetraphosphate (M 2 P 4 O 12 , M=Co) are quite similar because of the equivalent electronic charges and the close radii of cations. Consequently, all the diffraction peaks in the Fig. 1 are found to be in agreement with monoclinic M 2 P 4 O 12 and space group C12/c 1 without violation. Only few characteristic peaks of other impurities (e. g. Co-Oxide) was clearly observed at lower sintering temperatures (600, 800°C).
From XRD analysis (Figure 1), grain size evaluated and calculated according to the Scherrer's formula: D=Kλ/ (β cos θ), where D is particle diameter, K=0.89 (the Scherrer's constant), λ=1.5406 (wavelength of the X-ray used), β is the width of line at the half-maximum intensity and θ is the corresponding angle. The average crystallite size of product is estimated from the strongest three diffraction peaks below 40° for 2θ and found to be 98±11 nm. This crystallite size of the prepared cobalt cyclotetraphosphate is smaller than those data estimated from SEM and AFM-investigations in the present work which confirm that the powder mixture of cobalt cyclotetraphosphate is not unified grain sizes and grain sizes are variated in the bulk than surface's layers. The lattice parameters were calculated from the XRD spectra and found to be a =11.809(2), b=8.293(1), c=9.923 (2) A, which are very close to those of the standard data file (ICSD #300027) and the literatures [9][10][11]14].

FT-IR-Spectroscopic Investigations
Figure 2 displays infrared spectra recorded for Co 2 P 4 O 12 (126-Phase =AB 2 X 6 structure type) after firing process and sintering at 1100°C. The most intense reflection pear of IRspectra are assigned by red cycles as clear in Figure 2. It is well known that the Co 2 P 4 O 12 structure is mainly characterized by a three-dimensional framework with MO 6 (M = Co) polyhedral linkedwith P 4 O 12 ringsby M-O-P. The basic structure unit is the centrosymmetriccyclotetraphosphatering P 4 [17][18][19].
The peaks splitting in theseregions is due to the different strength of the bond between cations (M=Co 2+ orCo 3+ ) and anion [P 4 O 12 ] 4− , which confirm the insertingdifferent cations in the skeletal aswell as the formation of multi-valence cobalt (II, III) cyclotetraphosphate as confirmed in the visualization studies part. The anion contains the [PO 2 ] 2− radical and theP-O-P bridge which differ in their bond strength and as result multi-splitting processes are occurred as shown in Figure. The asymmetric and symmetricstretching frequencies of the P-O-P bridge are observed in the regions of1000-900 and 800-700 cm −1 , respectively. The symmetric P-O-Pbridge stretching modes occur at 736 and 714 cm −1 . These observedbands are known to be the most striking feature of cyclotetraphosphatespectra, along with the presence of the νas -OPO− band. From X-raydiffraction data [12], it was shown that the crystal structure ismonoclinic (space group C2/c1) with a cyclic structure of the [P 4 O 12 ] 4− anion. This has been confirmed by the IRmeasurements. The bendingmodes are expected in the area 600-400 cm −1 [PO 2 ] 2− radical) and 400-370 cm −1 (P-O-P bride). Themetal-O stretching usually appears in thebending mode region as the bending modes of the P-O-P bridge andabsorption bands associated with these vibrations are usually veryweak. The weak IR band at 400 cm −1 is probably due to metal-Oxide ~ (Co-O) stretching mode.  Figure 3a shows scanning electron micrograph recorded for cobalt cyclotetraphosphate synthesized at 1100°C, it is so difficult to observe inhomogeneitiy within the micrograph due to that the powders used are very fine and the grain size estimated is too small. The average grain size was estimated from SE-micrograph and found to be ranged in between 3.2-3.78 µm which is relatively high in contrast with data estimated from XRD through Scherrer's equation (D=0.98 µm). This indicates that the actual grain size in the material bulk could be smaller than that detected on the surface morphology. Furthermore, in our EDX (energy disperse Xray) analysis as shown in Fig. 3b and Table. 1, the molar ratios of cobalt cyclotetraphosphate was detected qualitatively with very good fitting to the actual molar ratio (1: 2: 6) as shown in Table 1.

Scanning Electron Microscopy (SEM) and EDX-Elemental Analysis (EDX)
The EDX examinations were performed on random spots within the same sample to confirm accuracy of calculations molar ratios of cobalt cyclotetraphosphateas possible.

Atomic Force Investigations (AFM)
Figure. 4 shows 3D-AFM-micrograph tapping mode image captured for scanned area 0.1 µm 2 . Of cobalt cyclotetraphosphate (Co 2 P 4 O 12 ). The image was constructed by application tapping mode with slow scan rate and high resolution imaging with 1024 line per 0.1 nm. The tapping amplitude current was monitored as a function of line drawing heights. For more accurate surface analysis AFMraw data was forwarded to Origin Lab program version 7 and the data are converted into matrix then 3D-contour surface mapping is constructed as shown in Figure 5a. Figure 5a displays 3D-visualized-contour plot of AFM-micrograph surface imagingcaptured forscanned area 0.2 µm 2 of cobalt cyclotetraphosphate (Co 2 P 4 O 12 ). To increase the accuracy of analysis of this image the data were forwarded to plot Fig. 5b which is 2D-visualized-contour plot of the same image f cobalt cyclotetraphosphate (Co 2 P 4 O 12 ). The analysis of the surface nature and morphology enhance us to understand application of such these materialsmetal cyclotetraphosphate (M 2 P 4 O 12 ) as colorant materials in coating and ceramic industry.  The AFM-tapping mode captured image can be divided into three zones 1 st zone include (yellow, orange and red color) this zone represents ~21% of the whole scanned area which is equal ~ 0.042 µm 2 , the surface heights in this zone ranged in between 6.072-6.14 µm as clear in the key-image. The red zone represents 2% = 0.004µm 2 which processes the highest height on the scanned area with height max . = 6.14 µm. The second zone represents ~ [dark green zone (23%) + pale green zone (18%)] which represents ~ 41% (0.082µm 2 ) from the whole scanned area with heights gradient ranged in between 6.027-6.06 µm. The 3 rd zone occupies ~ 38% = 0.076 µm 2 from the whole scanned area with heights gradient lies in between 4.88 -6.06µm. The average grain size was estimated from AFM-analysis and found in between 56-80 nm which is nearly matched with that calculated from XRD through applying Scherrer's formula ~ (98 nm). The differences in the values of average grain sizes calculated via SEM, AFM and Scherrer's formula are good evidence for existence gradient in the grain sizes in the bulk which are completely different than those on the surface layers. Many researchers in the last did their best to understand the crystallographic structure of phosphates (open phosphates or cyclic poly phosphates) [20][21][22][23][24][25][26][27][28].

Structural Visualization Studies
The initial analysis of structural parameters inside visualized crystal lattice of cobalt cyclotetraphosphate indicated that there are two different types of cobalt namely (Co1 and Co2), Two types of phosphorous atoms (P1 and P2) and finally six different types of oxygen atoms namely (O1, O2, O3, O4, O5 and O6).
The comparison between visualized XRD-profile Figure 7 and the experimental XRD-pattern sintered at 1100°C Figure   1 indicated that there is type of fitting coupled with high figure of merit between both patterns specially on the point of view positions of most intense reflection peaks on both patterns. The shifts on some intense reflection peak position within limits of two theta values ~ 2 degree could due to impurity phases interactions with the main monoclinic structure of cobalt cyclotetraphosphate on the experimental pattern. Figure 8 displays the regular distribution of PO 3 -polyhera throughout the unit cell of cobalt cyclotetraphosphate. The analysis of these polyhedron indicated that the phosphorous atom as central ion was surrounding by oxygen atoms, three oxygen atoms represents the triangle base lie at ~ nearly the same distance from phosphorous (central metal ion) while the forth one at distance longer than the others three oxygen of triangle base.
The accurate analysis of bond lengths, torsion on angles inside the crystal lattice of cobalt cyclotetraphosphate (Tables 2-11) can enhance us understand what is the structural factors responsible for lattice stability.    Cobalt type two (Co2) has similar behavior to cobalt type one but the oxygen atoms that represent triangle base are recommended to be O2, O5 and O6 with bond lengths 2.5737,2.3208 and 2.1694A respectively while axial oxygen atoms could be occupied by O1, O3 and O4 with bond distances 3.5271,3.9043 and 3.5398 A respectively.
The cobalt type two (Co2) is also linked with the two different types of phosphorous atoms namely (P1 and P2) with bond distances 3.1577 and 3.7464 A which confirm that cobalt has more than one oxidation state within the crystal lattice. Similar behavior of existence multi oxidation states was reported in references [20,21] in which the conditions of synthesis at elevated temperatures in air or oxygen were responsible.
With respect to phosphorous atoms (P1 and P2) it were observed that phosphorous type one (P1) was linked inside crystal lattice with all oxygen atoms recording bond lengths 1.2222, 1.3334 and 1.9623 A correspond to P1-O5, P1-O6 and P1-O4 respectively these bond distances are suitable to be base triangle of PO 3 -while the rest three oxygen atoms O1, O2 and O3 recorded bond distances 3.4635, 3.5532 and 3.3910 which are suited to be axial atoms.
In conclusion, one can conclude that variations of bond distances between Co1, Co2, P1 and P2 and different six oxygen atoms (O1, O2, O3, O4, O5 and O6) inside crystal lattice are responsible for increasing lattice flexibility factor (by controlling in shrinkage and expansion coefficient) and consequently increase itsbonds stability to break. These facts can be attributed to three main factors inside lattice 1 st oxidation state of cobalt takes values between Co 2+ , Co m+ , Co 3+ (where m fractions between 2,3 and 2≤ m≤ 3). 2 nd effect of coupling of charges due to environmental neighboring groups effects.3 rd the six oxygen atoms are liable to replace each other throughout the lattice to compensate any lattice defects could break bonds (evidence is exchanging positions of triangle base with axial positions).

Conclusions
Advanced solution route was successfully applied to synthesize cobalt cyclophosphates at ambient temperature. The products were examined by both of XRD, IR. Structural investigations via XRD proved that the product obtained at 1100°C is the best and fine structure with monoclinic structure phase and belongs to C12/C1 space group with lattice parameter a=11.809 (2), b=8.293(1), c=9.923(2) A respectively. A visualized investigations confirmed structure validity and stability at temperature of sintering (1100°C). Visualization studies indicated that variations of bond distances between Co1, Co2, P1 and P2 and different six oxygen atoms (O1, O2, O3, O4, O5 and O6) inside crystal lattice are responsible for increasing lattice flexibility factor (by controlling in shrinkage and expansion coefficient) and consequently increase its bonds stability to break.