# Patent

## Title
Preparation, characterization, ESR and PAS studies of Cu0.5NbAlP3O12 (CNP) and HNbAlP3O12 (HNP)

## Abstract

The present invention relates to the process of preparation and characterization of novel sodium superionic conductor (NASICON) type niobium aluminium phosphate of formula Cu0.5NbAlP3O12 (CNP), HNbAlP3O12(HNP) and to study its Electron Spin Resonance (ESR) and Photo Acoustic (PA) spectra.

## Background
This application claims the benefits Provisional Application No. 60/279,271, filed Mar. 28, 2001.


FIELD OF THE INVENTION
The present invention relates to the process of preparation, characterization, electron spin resonance (ESR) and photo acoustic (PA) studies of Cu0.5NbAlP3O12 (CNP) and HNbAlP3O12 (HNP).
BACKGROUND AND PRIOR ART TO THE INVENTION
Sodium super ionic conductor (NASICON) and related sodium zirconium phosphate NaZr2P3O12 (NZP) and niobium titanium phosphate NbTiP3O12 are well studied [1-6] due to their important properties of (i) low thermal expansion behavior [5-9], (ii) fast ionic conductivity [2] and (iii) high temperature stability [2]. They are used as heat exchangers and I mirror blanks for space technology, catalyst supports, high temperature fuel cells, possible host for radioactive waste and as humidity and gas sensors [10-15]. Their general formula is AMM′P3O12 where, ‘A’ can be alkali, alkaline earth or Cu2+ ion, M and M′ can be tri-, tetra- or pentavalent transition metal ion. The structure is characterized by corner sharing of PO4 tetrahedra with MO6, (and M′O6,) octahedrai [16,17]. The three dimensionally linked interstitial space can accommodate ions as H+ or as large as Cs+ This space can also remain vacant as in NbTiP3O12 [18]. The structure is flexible for substitution at A, M or M′ sites, giving rise to a large number of closely related compounds. Among the NASICONs, copper NASICONs are relatively less investigated [19-22]. In this paper, we report the preparation, characterization, electron spin resonance (ESR) and photo acoustic (PA) studies of Cu0.5—NbAlP3O12 (CNP) and HNbAlP3O12 (HNP).
REFERENCE
1. Vashishta, J. N. Mundy, O. K. Shenoy (Eds.). Proceedings of International Conference on Fast Ion Transport in Solids, Lake Geneva, North-Holland, Amsterdam, 1979.
2. J. B. Goodenough, H. Y. P. Hong, J. A. Kafalas. Mater. Res. Bull. 11 (1976) 203.
3. H. Y. P. Hong. Mater. Res. Bull. 11 (1976) 176.
4. J. Alamo, R. Roy, J. Am. Ceram. Soc. 67 (1984) C78.
5. T. Oota, I. Yamai, J. Am. Ceram. Soc. 69 (1986) 1.
6. R. Roy, D. K. Agrawal, R. A. Roy. Mater. Res. Bull. 19 (1984) 471.
7. D. K. Agrawal, V. S. Stubican, Mater. Res. Bull. 20 (1985) 99.
8. G. E. Lenain, H. A. McKinstry, S. Y. Limaye, A. Woodword, Mater. Res. Bull. 19 (1984) 1451.
9. S. Y. Limaye, D. K. Agrawal, H. A. Mckinstry. J. Am. Ceram. Soc. 70(1987) C232.
10. D. K. Agrawal, J. H. Adair, J. Am. Chem. Soc. 71(1990) 2153.
11. R. Roy. E. R. Vance. J. Alamo. Mater. Res. Bull. 17 (1982) 585.
12. L. Fred, Y. Akihiri, M. Norrio, Y. Nobni, Chem. Lett. 49 (1994) 1173.
13. Y. Hideaki, S. Takehiko, Sens. Actuators, B 5(1991) 135.
14. Y. Sheng, S. Youichi, M. Norrio, Y. Nobru, Chem. Lett. 47; (1992) 587.
15. Y. Sheng, H. Sanchio, S. Youichi, M. Norrio, F. Hozumi, Y. I. Nobru, Chem. Lett. 46 (1991) 2069.
16. L. O. Hagman, P. Kierkegaard, Acta Chem. Scand. 22 (1968) 1822.
17. M. Sljukic, B. Matkovic, B. Prodic, Z. Kristallogr, 130 (1968) 1872.
18. R. Masse, A. Durif, J. C. Guitel, I. Tordjman. Bull. Soc. Fr. Mineral. Cristallogr. 95 (1972) 47.
19. P. C. Yao, D. J. Fray. Solid State Ionics 8 (1983) 35.
20. A. El Jazouli, J. L. Soubeyroux, J. M. Dance, G. Le Flem, J. Solid State Chem. 65 (1986) 351.
21. A. El Jazouli, M. Alami, R. Brochu, J. M. Dance, G. Le Flem, P. Hagenmuller. J. Solid State Chem. 71 (1987) 444.
22. G. Le Polles, A. El Jazouli, R. Olazcuaga, J. M. Dance, G. Le Flem, P. Hagenmuller, Mater. Res. Bull. 22 (1987) 1171.
23. J. R. Pilbrow, Transition Ion Electron Paramagnetic Resonance, Oxford Science Publications. 1990, Chap 5.
24. A. Clearfield, B. D. Roberts, M. A. Subramanian, Mater. Res. Bull. 19(1984) 219.
OBJECTS OF THE INVENTION
The main object of the present invention is to provide a process of preparation, characterization, electron spin resonance (ESR) and photo acoustic (PA) studies of Cu00.5NbAlP3O12 (CNP) and HNbAlP3O12 (HNP).
Another object of the present invention relates to a novel sodium superionic conductor (NASICON) type niobium aluminum phosphate and a method for preparation of the same.
Still another object of the present invention is to characterize the above said compound.
Yet another object of the present invention is to study the electron spin resonanace (ESR) and photoacoustic (PA) characters of the said compound.
SUMMARY OF THE INVENTION
The present invention relates to the process of preparation, characterization, electron spin resonance (ESR) and photo acoustic (PA) studies of compounds Cu0.5NbAlP3O12 (CNP) and HNbAlP3O12 (HNP).



DESCRIPTION OF THE ACCOMPANYING DRAWINGS
In the drawings accompanying the specification,
FIG. 1 represent the Powder X-ray diffractogram of (a) CNP and (b) HNP.
FIG. 2 represent the room temperature electron spin resonance spectra of (a) CNP, (b) simulated ESR spectra of CNP, (c) CNP heated at 450° C. and (d) HNP.
FIG. 3 represent the PA spectra of (a) CNP and (b) HNP.



DETAILED DESCRIPTION OF THE INVENTION
In an embodiment, the present invention relates to the process of preparation, characterization, electron spin resonance (ESR) and photo acoustic (PA) studies of a novel sodium super ionic conductor (NASICON) type niobium aluminum phosphates of the composition Cu0.5NbAlP3O12 (CNP) and HNbAlP3O12 (HNP).
In one embodiment, the said compounds are isomorphous with NbTiP3O12 and unit cell parameters are evaluated. Copper and hydrogen occupy the channels. Reduction of CNP gives rise of HNP. Electron spin resonance (ESR) and photo acoustic (PA) spectral data are consistent with elongated octahedral configuration around Cu2+ ion.
In another embodiment, CNP is prepared by mixing stoichiometric amounts of Nb2O5, Al2O3, NH4H2PO4 and Cu(NO3)23H2O (all are analytical reagents obtained from SD chemicals) in a mortar with spectral grade acetone (SD chemicals). The resultant powder was sequentially heated at 500° C. (3 h), 750° C. (3 h) and finally 1075° C. (5 h). This process resulted in Cu0.5NbAlP3O12(CNP) as a light green compound.
In yet another embodiment HNP is prepared from CNP by placing it in a ceramic boat and reducing at 400° C. and 550° C. (4 h each) by passing hydrogen gas in a tube furnace. The compound obtained by reducing at 550° C. was HNP and was light black in color.
In a yet another embodiment, density of CNP was experimentally determined using xylene as the immersion liquid. Powder X-ray diffractograms were recorded on Siemens D-4000 using CuKα radiation (λ=1.506 Å). The unit cell parameters are derived by using a computer programme by providing d and hkl values as input parameters.
In still yet another embodiment, room temperature ESR spectra were recorded on a JEOL PE-3X-band spectrometer equipped with a 100 kHz field modulation unit. DPPH was used as standard. The powder ESR spectrum of CNP was simulated using a computer programme [23 ]. The “g”, “a” and line width are given as input parameters. Room temperature emission spectra were recorded by Hitachi-3010 spectrofluorometer.
In still yet another embodiment, PA spectra were recorded on an extensively modified OAS-400 PA spectrometer (EDT Research, London). The light beam from a 300-W xenon lamp was intensity-modulated using a continuously variable mechanical chopper (HMS 222, NY) operating at a chopping frequency of 40 Hz. The signals were recorded using a B and K 4165 microphone (Bruel and Kjaer, Naerum, Denmark) coupled to a power supply (B and K 2804). The sensitivity of the microphone was 40 mV/Pa. The signal was processed through a preamplifier (EG and G 113); Princeton applied Research, Princeton, N.J. and a lock-in analyzer EG and G 7620. Normalization of the spectra to constant input light intensity was achieved by using the PA spectrum of carbon black.
Powder XRD of CNP was found to be free from impurities. It is found to be isomorphous with NbTiP3O12 [18]. All the d-lines are indexed and the unit cell parameters are evaluated using a computer programme. The observed and calculated d-values are given in Table 1. The unit cell parameters are shown in Table 2 along with the unit cell parameters of related systems. The powder XRD of HNP was found to be similar to that of CNP, except for the intensities. The reflections of various hkl planes of HNP were less intense than CNP (FIG. 1). All the d-lines are indexed and the unit cell parameters are derived (Tables 1 and 2). From Table 2, it is clear that the unit cell parameters of CNP and HNP are very close to each other. Since both Cu2+ and H+ occupy the channels, the basic skeleton remains the same and, hence, the “a” and “c” parameters are close to each other. The observed change in the intensities may be due to the difference in the number of electrons present in Cu2+ and H+.







TABLE 1







Experimental and calculated d-values of CNP and HNP












CNP

HNP












h k1
dobs
dcal
dobs
dcal














102
6.096
6.125
6.127
6.127


104
4.383
4.405
4.407
4.406


110
—
4.261
4.283
4.262


113
3.678
3.682
3.691
3.683


204
3.147
3.063
3.065
3.063


116
2.770
2.776
2.778
2.777


108
—
2.573
—
2.573


214
2.480
2.486
2.484
2.487


300
—
2.460
—
2.461


208
2.239
2.202
2.215
2.203


119
2.108
2.117
2.120
2.118


217
2.089
2.084
2.078
2.085


223
2.046
2.045
—
2.046


306
2.014
2.041
—
2.042


312
2.014
2.012
2.013
2.013


218
1.954
1.956
1.962
1.957


314
1.918
1.917
1.920
1.918


2010
1.895
1.887
—
1.887


226
1.841
1.841
1.845
1.841


402
—
1.819
—
1.820


2110
1.721
1.725
1.726
1.726


317
—
1.714
—
1.714


1112
1.679
1.681
1.679
1.681


318
1.642
1.641
1.640
1.641


324
1.616
1.617
1.617
1.618


410
1.590
1.610


325
1.575
1.579


413
—
1.572


408
1.528
1.531


3110
1.496
1.497
















TABLE 2







Unit cell parameters of CNP, HNP and related systems













a (A) ±
c(A) ±
dobs (g
dcal (g



Compound
0.02
0.02
cm−3) ± 0.02
cm−3)
Reference















Cu0.5Zr2P3O12
8.84
22.77
 3.24 ±
3.23
[21]





0.20.02


H0.5Cu0.5Zr2P3O12
8.84
22.75
—
—
[22]


Cu0.5NbAlP3O12
8.522
21.964
4.035 ± 0.02
4.05
This work


HNbAlP3O12
8.526
21.966
—
3.90
This work


HZr2P3O12
8.80
23.23
—
—
[24]


Cu0.5Ti2P3O12
8.41
21.88
 3.05 ± 0.01
3.07
[20]









Room temperature X-band powder ESR spectrum of CNP and its reduced product at 400° C. and 550° C. are shown in FIG. 2. The ESR spectrum of CNP is characterized by a broad unresolved band at lower magnetic field and a sharp one on the higher field side (FIG. 2a). This spectrum is similar to that of Cu0.5Ti2P3O12 [20.21]. The g values are derived and shown in Table 3. Since g11>g⊥, the unpaired electron is in dx2-y2, which corresponds to the John-Teller distortion involving an elongated octahedral configuration around Cu2+ ion. Similar results were obtained for other related systems [20,21]. The room temperature powder ESR spectrum of CNP is simulated using a computer programme (FIG. 2b). The ESR spectrum of the sample of reduced product at 400° C. was found to be identical to that of CNP, except for the intensities (FIG. 2c). The compound reduced at 550° C. did not give any ESR signal (FIG. 2d).
In copper NASICON type phosphates, Cu+2 ion occupy the channels [21]. In reducing with hydrogen, CNP can give rise to (i) H0.5Cu0.5(I)NbAlP3O12 or (ii) HNbAlP3O12 or a mixture of both. The samples reduced at 400° C. and 550° C. were subjected to fluorescence measurement by exciting at 287 nm. No signal was observed around 450-700 nm unlike H0.5Cu0.5(I)Zr2P3O12 [22]. Copper (I) hydrogen NASICONs, when heated in air, are known to give copper (II) NASICONs expelling the hydrogen [22]. When the sample (CNP) reduced at 550° C. is heated in air and subjected to ESR, no signal was observed. This rules out the formation of H0.5Cu0.5(I)NbAlP3O12. Therefore, the compound formed when CNP is reduced at 550° C. is unambiguously identified at HNbAlPO12.







TABLE 3







The g value of some NASICON type phosphates












Sample
g∥
g⊥
gav
T(K)
Reference















NaTi2P3O12:Cu2+
2.366
2.072
2.174
300
[20]


CuTi2P3O12:Cu2+
2.36
2.06
2.164
300
[20]


CaTi2P3O12:Cu2+
2.37
2.06
2.168
300
[20]


Cu0.5Ti2P3O12
2.33
2.067
2.158
300
[20]


Cu0.5NbAIP3O12
2.366
2.10
2.192
300
this work


HNbAIP3O12
—
—
—
300
this work









In yet another embodiment, the PA spectra of CNP and HNP are recorded in the range 210-800 nm (FIG. 3). The PA spectrum of CNP consists of broad band around 250-400 nm and of another broad band above 600 nm (FIG. 3a). PA spectrum of HNP is characterized by broad band around 300-450 nm. No band is observed above 600 nm (FIG. 3b).
In still another embodiment, the optical spectrum of copper NASICONs is characterized by a broad band of around 200-400 nm and of another between 600 and 1100 nm [20-22]. The broad band (200-400 nm) is due to phosphate group and its location is independent of the nature of the cation. The broad band in the region 60-1100 nm is due to d—d transitions [20-22]. The PA spectrum of CNP is identical to the reflectance spectrum of Cu0.5Zr2P3 O12 [21]. We could record PA spectrum up to 800 nm only due to instrumental constraints. However, the profile of the spectrum shows the transitions due to phosphates group and d-d transitions of Cu2+ (3d9) in high spin octahedral configuration. The broad band observed in the PA spectrum of HNP in the region 300-450 nm is due the phosphate group. The absence of band above 600 nm indicates the absence of Cu2+ ions in HNP.
In a further embodiment, the possible mechanism for the formation of HNbAlP3O12 is as follows. When CNP is heated with hydrogen at various temperature CU2+ions are reduced to elemental copper and are coming out of the channels. Once the reduction is complete, the compound becomes light black, since it is a mixture of Cu and HNbAlP3O12. This black mixture, when dissolved in dilute nitric acid, gives a blue color solution and white solid. The powder XRD of both black mixture and white solid are identical. Thus, the white solid is HNbAlP3O12. The sequence of reactions can be written as 




The invention is described by the following examples, which should not be construed as limitations to the scope of the invention.
EXAMPLE 1
In another embodiment, CNP is prepared by mixing stoichiometric amounts of Nb2O5, Al2O3, NH4H2PO4 and Cu(NO3)23H2O (all are analytical reagents obtained from SD chemicals) in a mortar with spectral grade acetone (SD chemicals). The resultant powder was sequentially heated at 500° C. (3 h), 750° C. (3 h) and finally 1075° C. (5 h). This process produced Cu1.5NbAlP3O12 light green compound.
EXAMPLE 2
In yet another embodiment HNP is prepared from CNP by placing it in a ceramic boat and reducing at 400° C. and 550° C. (4 h each) by passing hydrogen gas in a tube furnace. The compound obtained by reducing at 550° C. was HNP and was light black in color

## Claims

1. A compound as a sodium super ionic conductor (NASICON) type selected from compounds of the formula Cu0.5NbAlP3O12 (CNP) and HNbAlP3O12 (HNP).

2. A compound as claimed in claim 1, characterized by experimental and calculated d-values of Table 1.

3. A compound as claimed in claim 1 characterized by the unit cell parameters of Table 2.

4. A compound as claimed in claim 1 characterized by the g values of Table 3.

5. The CNP compound as claimed in claim 1, wherein the density of CNP is determined using Xylene immersion liquid.

6. Compounds as claimed in claim 1, wherein powdered XRD of CNP and HNP are similar except for intensities of one or more hkl planes.

7. Compounds as claimed in claim 1, wherein the unit cell parameters of CNP and HNP are very close to each other.

8. The CNP compound as claimed in claim 1 wherein photoacoustic spectra of CNP is characterized by a broad band around a wave length of 250-400 nm, a broad band around a wave length of 250-400 nm and a broad band above 600 nm.

9. The HNP compound as claimed in claim 1 wherein the photo acoustic spectrum of HNP is characterized by a broad band around a wavelength of 300-450 nm.

10. The compounds as claimed in claim 1 wherein the CNP is light green colored and HNP obtained is light black or white colored after treatment with nitric acid.

11. A process for preparation of the compounds of claim 1, said process comprises the steps of:
a) mixing stoichiometric amounts of Nb2O5, Al2O3, NH4H2PO4 and Cu (NO3)23H2O in the presence of spectral grade organic solvent to obtain a powder, 
b) heating the powder of step (a) sequentially to 500° C. for a period of 3-4 hrs, followed by 750° C. for a period of 5-6 hrs to obtain Cu0.5NbAlPO3O12 (CNP) as a light green solid, 
c) reducing the solid of step (b) in a ceramic boat using a stream of hydrogen gas in a tube furnace by heating to 400° C. for a period of 3-4 hrs and finally to 550° C. for 3-4 hrs to obtain HNbAlP3O12 (HNP) as a light black solid; 
d) optionally treating the solid of step (c) with nitric acid to obtain a solution containing a second solid, and 
e) separating the second solid of step (d) from the solution to obtain white colored HNbAlP3O12 (HNP).