Abstract:
The present invention relates to the process of preparation and characterization of novel sodium superionic conductor (NASICON) type niobium aluminium phosphate of formula Cu0.5NbAlP 3 O 12  (CNP), HNbAlP 3 O 12 (HNP) and to study its Electron Spin Resonance (ESR) and Photo Acoustic (PA) spectra.

Description:
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 Cu 0.5 NbAlP 3 O 12  (CNP) and HNbAlP 3 O 12  (HNP). 
     BACKGROUND AND PRIOR ART TO THE INVENTION 
     Sodium super ionic conductor (NASICON) and related sodium zirconium phosphate NaZr 2 P 3 O 12  (NZP) and niobium titanium phosphate NbTiP 3 O 12  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′P 3 O 12  where, ‘A’ can be alkali, alkaline earth or Cu 2+  ion, M and M′ can be tri-, tetra- or pentavalent transition metal ion. The structure is characterized by corner sharing of PO 4  tetrahedra with MO 6 , (and M′O 6 ,) 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 NbTiP 3 O 12  [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 Cu 0.5 —NbAlP 3 O 12  (CNP) and HNbAlP 3 O 12  (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 Cu 00.5 NbAlP 3 O 12  (CNP) and HNbAlP 3 O 12  (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 Cu 0.5 NbAlP 3 O 12  (CNP) and HNbAlP 3 O 12  (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 Cu 0.5 NbAlP 3 O 12  (CNP) and HNbAlP 3 O 12  (HNP). 
     In one embodiment, the said compounds are isomorphous with NbTiP 3 O 12  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 Cu 2+  ion. 
     In another embodiment, CNP is prepared by mixing stoichiometric amounts of Nb 2 O 5 , Al 2 O 3 , NH 4 H 2 PO 4  and Cu(NO 3 ) 2 3H 2 O (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 Cu 0.5 NbAlP 3 O 12 (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 CuK α  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- 3 X-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 NbTiP 3 O 12  [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 Cu 2+  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 Cu 2+  and H + . 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Experimental and calculated d-values of CNP and HNP 
               
             
          
           
               
                   
                 CNP 
                   
                 HNP 
                   
               
             
          
           
               
                 h k1 
                 d obs   
                 d cal   
                 d obs   
                 d cal   
               
               
                   
               
             
          
           
               
                 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) ± 
                 d obs  (g 
                 d cal  (g 
                   
               
               
                 Compound 
                 0.02 
                 0.02 
                 cm −3 ) ± 0.02 
                 cm −3 ) 
                 Reference 
               
               
                   
               
             
          
           
               
                 Cu 0.5 Zr 2 P 3 O 12   
                 8.84 
                 22.77 
                  3.24 ± 
                 3.23 
                 [21] 
               
               
                   
                   
                   
                 0.20.02 
               
               
                 H 0.5 Cu 0.5 Zr 2 P 3 O 12   
                 8.84 
                 22.75 
                 — 
                 — 
                 [22] 
               
               
                 Cu 0.5 NbAlP 3 O 12   
                 8.522 
                 21.964 
                 4.035 ± 0.02 
                 4.05 
                 This work 
               
               
                 HNbAlP 3 O 12   
                 8.526 
                 21.966 
                 — 
                 3.90 
                 This work 
               
               
                 HZr2P 3 O 12   
                 8.80 
                 23.23 
                 — 
                 — 
                 [24] 
               
               
                 Cu 0.5 Ti 2 P 3 O 12   
                 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. 2   a ). This spectrum is similar to that of Cu 0.5 Ti 2 P 3 O 12  [20.21]. The g values are derived and shown in Table 3. Since g 11 &gt;g⊥, the unpaired electron is in d x   2 -y 2 , which corresponds to the John-Teller distortion involving an elongated octahedral configuration around Cu 2+  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. 2   b ). 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. 2   c ). The compound reduced at 550° C. did not give any ESR signal ( FIG. 2   d ). 
     In copper NASICON type phosphates, Cu +2  ion occupy the channels [21]. In reducing with hydrogen, CNP can give rise to (i) H 0.5 Cu 0.5 (I)NbAlP 3 O 12  or (ii) HNbAlP 3 O 12  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 H 0.5 Cu 0.5 (I)Zr 2 P 3 O 12  [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 H 0.5 Cu 0.5 (I)NbAlP 3 O 12 . Therefore, the compound formed when CNP is reduced at 550° C. is unambiguously identified at HNbAlPO 12 . 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 The g value of some NASICON type phosphates 
               
             
          
           
               
                 Sample 
                 g∥ 
                 g⊥ 
                 g av   
                 T(K) 
                 Reference 
               
               
                   
               
             
          
           
               
                 NaTi 2 P 3 O 12 :Cu 2+   
                 2.366 
                 2.072 
                 2.174 
                 300 
                 [20] 
               
               
                 CuTi 2 P 3 O 12 :Cu 2+   
                 2.36 
                 2.06 
                 2.164 
                 300 
                 [20] 
               
               
                 CaTi 2 P 3 O 12 :Cu 2+   
                 2.37 
                 2.06 
                 2.168 
                 300 
                 [20] 
               
               
                 Cu 0.5 Ti2P 3 O 12   
                 2.33 
                 2.067 
                 2.158 
                 300 
                 [20] 
               
               
                 Cu 0.5 NbAIP 3 O 12   
                 2.366 
                 2.10 
                 2.192 
                 300 
                 this work 
               
               
                 HNbAIP 3 O 12   
                 — 
                 — 
                 — 
                 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. 3   a ). PA spectrum of HNP is characterized by broad band around 300-450 nm. No band is observed above 600 nm ( FIG. 3   b ). 
     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 Cu 0.5 Zr 2 P 3  O 12  [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 Cu 2+  (3d 9 ) 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 Cu 2+  ions in HNP. 
     In a further embodiment, the possible mechanism for the formation of HNbAlP 3 O 12  is as follows. When CNP is heated with hydrogen at various temperature CU 2 +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 HNbAlP 3 O 12 . 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 HNbAlP 3 O 12 . 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 Nb 2 O 5 , Al 2 O 3 , NH 4 H 2 PO 4  and Cu(NO 3 ) 2 3H 2 O (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 Cu 1.5 NbAlP 3 O 12  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