Chem. Mater. 1992,4,657-661 ficiencies drop rapidly below temperatures of 1800 K.
Thus low AlN yields are obtained at low temperatures even though higher conversion efficiency ratios may be obtained at low pressures and in the presence of excess hydrogen.
Conclusions Computerized thermodynamic analysis of the A1-N-C-H system indicates that pure A1N can be prepared by thermal nitridation of A 1 2 0 3 using ammonia and methane. Complete conversion of A 1 2 0 3 to AlN occurs at 1800 K. The reagent concentration ranges within which pure ALN is formed increase with increasing reaction temperature as well as at low pressure in the presence of excess
657
hydrogen. Low pressure together with an excess of hydrogen decreases the reaction temperature at which pure A1N is formed. Low pressure together with the introduction of excess hydrogen into the reaction system increases A1 and N conversion efficiency and retards C deposition. Acknowledgment. We thank Dr. Woo Lee of United Technologies Research Center for helpful discussions. This research has been supported by the State of Connecticut, Department of Higher Education, High Technology Grant Number 42. Registry NO. Al203,1344-28-1; NHs, 7664-41-7; CH4,74-82-8; AlN, 24304-00-5.
Ionic Conductivities in Solid Solutions of K5+nSb5-xTixP2020 and K5_,Sb5,M,P2020 (M = MoV1,Wvl) B. Wang and M. Greenblatt* Department of Chemistry, Rutgers-The State University of New Jersey, Piscataway, New Jersey 08855-0939 Received December 11, 1991. Revised Manuscript Received February 10, 1992 Solid solutions of K5+,Sb+,Ti,P20zo (0 Ix I0.2)and K5-rSb5-xMrP2020 (M = MoV1and Ww, 0 I r 0.15)were prepared by solid-state reaction. Ionic conductivity was investigated from 200 to 500 "C by ac complex impedance measurement. The ionic conductivityof the Ti-substituted phase increases with increasing x to the limit of substitution ( x = 0.2). This increase of conductivity is attributed to a larger bottleneck and an increase in the K+ concentrationdue to the Ti substitution. In the Mo/W-substituted phases, the conductivity increases with increasing x to the limit of substitution (x = 0.15)due to an increasing K+ vacancy concentration. I
Introduction Recently, a series of potassium phosphatoantimonates have been reported by Piffard et al.'+ The basic building units of these compounds are Sb06 octahedra and PO4 tetrahedra with one-, two-, and three-dimensional structures (lD, 2D, and 3D). Some of the potassium phosphatoantimonates also have good ion-exchange properties.&* Recently, the ionic conductivities of potassium phosphatoantimonates have been studied in our laboratory?JO Of all the potassium phosphatoantimonates studied, K5Sb5P2OZ0 has been found to exhibit the highest ionic conductivity? K5Sb5P2OZ0 has a framework structure in which Sb06 octahedra share both corners and edges and are also linked to PO4 tetrahedra via corners. The high ionic conductivity of K5Sb5P2Ozocompared to the other potassium phosphatoantimonate compounds with different structures is attributed to the skeleton structure (Figure 1)with 3D interconnected,large and mostly vacant tunnels that facilitate the motion of K+ ions. However, the small bottleneck in this structureg limits the fast motion of K+ ions. It was shown that the conductivity of this phase can be improved by ion exchange of K+ with smaller cations such as Li+ and Na+ or partial substitution of NbVor TaV for SbVto enlarge the bottleneck for K+ ion motion.1° In this work we investigated the effects of aliovalent substitution of antimony on the ionic conductivity of K5Sb$ O m by replacing some of the SbVwith TiN, Mow, and Wsi ions. Our objective here was 3-fold:
* To whom correspondence should be addressed.
Table I. Effective Ionic Radii and Electronegatives of Relevant Ions ion'
rll, A
X12
SbV Ti" MoV1
0.74
1.763
0.75
WV'
0.74
1.577 2.025 2.132
0.73
"CN: 6. (1)To study the effect of changes in the K+ ion concentration and K+ ion vacancy concentration. When tetravalent titanium ions or hexavalent molybdenum or tungsten ions are substituted for pentavalent antimony ions, one expects K+ ion concentration and K+ ion vacancy concentration to increase respectively, with concomitant (1) Lachgar, A.;Deniard-Courant, S.; Piffard, Y. J.Solid State Chem. 1986. 63. 409. (2) Piffard, Y.; Oyetola, S.; Courant, S.; Lachgar, A. J. Solid State Chem. 1985, 60, 209. (3) Piffard, Y.;Lachgar, A.; Tournoux, M. J.Solid State Chem. 1985, 58,253. (4)Piffard, Y.;Lachgar, A.; Tournoux, M. Mater. Res. Bull. 1986,21, 1231. (5) Piffard, Y.;Lachgar, A.; Tournoux, M. Mater. Res. Bull. 1985,20, 715. (6) Piffard, Y.;Verbaere, A.; Oyetola, S.; Deniard-Courant, S.; Tournoux, M. Eur. J. Solid State Inorg. Chem. 1989,26, 113. (7) Piffard, Y.;Verbaere, A.; Lachgar, A.; Deniard-Courant, S.; Tournoux, M. Rev. Chim. Miner. 1986,23, 766. (8) Tournoux, M.;Piffard, Y. French Patent 85-10839. (9) Wang, E.;Greenblatt, M. Chem. Mater. 1991, 3, 542. (10) Wang, E.;Greenblatt, M. Chem. Mater. 1991, 3, 703. (11) Shannon, R.D. Acta Crystallogr. 1976, A32, 751. (12) Zhang, Y.Inorg. Chem. 1982,21, 3886.
0897-4756f 92f 2804-0657$03.00/0 0 1992 American Chemical Society
658 Chem. Mater., Vol. 4, No. 3, 1992
Wang and Greenblatt
7
23*65 23.60 n
5 a
23.40 0.0
0.1
0.2
18.35 0.0
0.1
0.2
0.0
0.1
0.2
n
5 Y
a
framework. Figure 1. [OOl]view of the (Sb5P20205-)
changes in the ionic conductivity. (2) To study the effect of "bottleneckn size on ionic conductivity; i.e., the ionic conductivity is expected to increase with enlargement in the bottleneck size when Sbv is replaced by the larger TiN ion (Table I). (3) To study the effect of substitution on the K+framework-oxygen bonds strength and its effect on the ionic conductivity. Since SbVis more electronegativethan TiIV (Table I), one expects the Sb-0 bond to be more covalent (less ionic) than the Ti-0 bond. Consequently, the K+-oxygen bond in K-0-Ti should be stronger than in K-0-Sb. Thus the electronegativity fador is expected to decrease the ionic conductivity in the Ti-substituted K5Sb5P20mphase. On the basis of similar reasoning, the ionic conductivityis expected to increase in the Mo/ Wsubstituted phases, because the electronegativity of the MoV1/WVT ions is greater than that of SbV.
Experimental Section
All samples were prepared by solid-state reactions from appropriate amounts of the stoichiometric starting materials of KN03 (Aldrich, 99.99%), Sb206(Aldrich,99.995%), NH4H2P04 (Aldrich, 99.999%), Ti02 (Aldrich, 99.99%), M03 (Aldrich, 99.995%), and W 0 3 (Aldrich,99.99%). Exact mole ratios of the readants were mixed thoroughly in an agate mortar. The mixtures were initially heated at 300 "C for 4 h to decompose ammonium monophosphate before final calcination at lo00 "C for 24 h. Powder X-ray diffraction (PXRD)patterns of resulting samples were obtained with a SCINTAG PAD V diffractometer with monochromatized Cu Kar radiation. Cell parameters were calculated with a least-squares program. Ionic conductivitieswere measured by an ac complex impedance technique with a Solartron Model 1250 frequency analyzer and 1186 electrochemical interface that were programmed by a Hewlett-Packard 9816 desktop computer for data collection and analysis. Samples were pelletized and sintered at 700 "C for a few hours before the surfaces of the pellets were coated with platinum paste. Before the conductivity measurement,samples were preheated to 600 "C in the conductivity cell under flowing helium and then cooled to room temperature in the same atmosphere. The frequency range 10 Hz to 65 kHz was employed at a heating rate of 2 "C/min over the temperature range 200-500 "C in flowing helium. Results and Discussion Powder x-ray diffraction (PXRD) measurements of K5+xSbS-xTixP2020 with x 5 0.2 showed a single-phase pattern of K5Sb5P2020type. There was no significant change in the lattice parameters for x > 0.2, and impurity lines appeared at x = 0.3. The cell parameters of Tiw
n
5 0
X
Figure 2. Change in the lattice parameters (a) a, (b) b, and (c) c as a function of Ti content in K5+xSb6-xTixP2020.
substituted phases are expected to increase, because the effective radius of Tiw (0.75 A) is larger than that of the SbV(0.74 A) ion. The a cell parameter increases with x , while the b and c cell parameters only show very slight increase (Figure 2). This variation of the cell parameters is attributed to the structural features of K5Sb5P20m: along a the Sb06 octahedra and PO4 tetrahedra cornershare, whereas along b some of the Sb06 octahedra edgeshare. Moreover, the "ringlike" feature along c and the "chainlike" feature along b result in a more rigid connection than along a.1° Similar effects were also observed in the ion-exchanged K5Sb5P2020 compounds.1° For the Mo- and W-substituted phases the limit of substitution is x = 0.15, much smaller than that in K5SbSxMxP2Om (M = Nb; 0 Ix I2; Ta; 0 Ix I3; V; 0 Ix I1).lo This trend is consistent with the observation that K5Sb5P2020is stabilized by the larger cations.1° Least-squares refined cell parameters of the Mo- and W-substituted phases show no significant change of cell parameters, because Mow, Wm,and SbVhave almost the same effective ionic radii (Table I). Figure 3 shows the Arrhenius plots of ionic conductivity of the K5+xSb5-xTixP20m series. Figure 4 represents the conductivity isotherms of K5+xSbS-xTixP20m at 250, .350, and 450 "C as a function of x . Both Figures 3 and 4 indicate that the conductivity increases with increasing x. Figure 5 shows the variation of activation energy (E,) and preexponential factor (A) as a function of x (i.e., E, and A defiied by the Arrhenius equation t.r = A exp(-E,/RT)). A comparison of Figures 4 and 5 shows that the increase of conductivity for x < 0.1 is due to the combined effect of decreasing activation energy and increasing preexponential factor. The decrease of E, in this region can be ascribed to the increase in the bottleneck size, as evidenced
Chem. Mater., Vol. 4, No. 3, 1992 659
Ionic Conductivities in Solid Solutions
500
400
300
200
I
I
I
I
-2.5
-3.0-
l:M
0
x.0.2
0
x.o.1
0.50
x.0.05
0.45 0.0 0.40
x.0 r A
E
-3.5 -
0.1
0.2
0
eb
-m
-4.0-
0
-4.5 1 .o
4
-5.0
1
0.5
]
I error
-5.51 1.2
"
"
1.4
1.6
"
1.8
'
1
2.0
0.0 I
1
'
0.0
2.2
1OOOlT ( K 1 ) Figure 3. Arrhenius plot of conductivity in K5+,Sb5_,Ti,Pz020 in the temperature range 200-500 "C.
0.1
0.2
X
Figure 5. (a) Activation energy (E,) vs 1: in K5+,Sb5_,Ti,PzOm (b) Preexponential factor (A) vs x: in K5+,Sb5_,Ti,PzOm t ("C)
i 500 400 300 200 -2.5 -2.0 0 -2.0
450%
.
-3.0
-2.51
-3.5
-3.0-
-4.0
-3.51 -4.5 -5.0
-4.0 -
1
-4.5 -
-5.5
0.0
0.1
0.2
-5.0: X
Figure 4. Isotherms of the conductivity in K,+,Sb5_,Ti,PZOzo. by the increase in the unit-cell parameters with increasing x (Figure 2). Similar resulta were also observed when SbV was substituted by the larger NbV(0.78 A) and TaV(0.78 A) ions.'O For x > 0.1, the increase in E, is attributed to stronger K-O interactions in the structure with increasing concentration of the less electronegative TiN ions substituted for SbV(Table I). On the other hand, the increase in the preexponential factor (Figure 5b) is ascribed to the increasing K+ion concentration with increasing x . Thus, the observed increase of conductivity for x > 0.1 appears to be due to the increasing preexponential factor only. Arrhenius plots of the ionic conductivity for KkxSb6-xM~xP2020 series are shown in Figure 6. Figure 7 displays the conductivity isotherms of K&3bkMoxP20p at 250,350, and 450 "C as a function of x . The conductivity increases with increasing x to the substitution limit of x = 0.15 (Figures 6 and 7). Figure 8 shows the variation
-5.5 1.2
1.4
1.6
1.8
2.0
2.2
1000/T (K1)
Figure 6. Arrhenius plot of conductivity in K5_xSb5-xMoxP2020 in the temperature range 200-500
OC.
of activation energy (E,) and preexponential factor (A) as a function of x . Both the activation energy and preexponential factor increase with increasing x . A comparison of Figures 7 and 8 suggests that the conductivity is more strongly affected by the preexponential factor than by the activation energy in the K,_xSb,_xMo,PzOzo series. The increase of E, with x in the Mo-substituted phase is probably due to the Coulombic attraction between cations and cation vacancies.13 This effect is stronger than (13) Shewmon, P.Diffusionin Solids; J. Williams Book Co.: Jenks,
OK,1989;p 161.
Wang and Greenblatt
660 Chem. Mater., Vol. 4, No. 3, 1992 -2.0
i 4 5 0 9
-2.5
500
300
400
200
-2.0 r h
-3.0
1
E
%s.i
0 -3.5 ", D
-
o
x4.15
0
xd.1 x.0.05
-4.0
0
-4.5 -5.0
1
-5.5
0.10
0.05
0.00
0.15
X
-5.0*
Figure 7. Isotherms of the conductivity in K,,SbS_,MoxP20w
O"O 0.65
1 error
I -5.5 1.2
1
,
1.4
'
I
1.6
'
I
1.8
'
l
2.0
2.2
1000/T ( K ' ) Figure 9. Arrhenius plot of conductivity in K&~b~~W~P2020 in the temperature range 200-500 O C .
0.45 0.40
L
0.00
0.05
0.10
-2.5
0.15
-3.0 r h
kp
-3.5
2
-k
-4.0 -4.5
0.0 i -5.0 -5.5 0.00 0.05 0.10 0.15 X
i
0.00
0.10
0.15
0.05
Figure 8. (a) Activation energy (E,) vs x in KS_xSbkMo,P20m (b) Preexponential factor ( A ) vs x in K~xSbS_xMo,P202~
X Figure 10. Isotherms of the conductivity in KS_xSb~xWrP202~
the electronegativity effect (i.e., the electronegativity of MoV1is greater than that of SbV(Table I), and the K+0-Mo bond is expected to be weaker than the K+-0-Sb bond, resulting in a decrease in E,) and results in the increase of E,. However, x is small, and this effect is weak. E, increases only slightly. In contrast, even the small amount of cation vacancies (x = 0.15) introduced by the substitution of Mow for SbVhas a larger positive effect on A. Thus the large increase in A with increasing x leads to the dramatic increase of the conductivity observed (Figures 7 and 8). Figure 9 shows Arrhenius plots of conductivity in the K,-,Sb,-xW,P2020 series. Similar to that in the K5-,Sb5-,Mo,P2020 series, the conductivity of K,SbkW,P20m series increases with increasing x to the substitution h u t of x = 0.15 (Figure 10). The E, remains constant for 0 1 x I 0.05 and then decreases with further
increase in x (Figure lla). The decrease of E, may be ascribed to the weaker K+-oxygen network interactions with increasing concentration of the more electronegative Wvl ions substituted for SbV(Table I). The preexponential factor of K6-xSb5-xWxP2020 series increases with increasing x for 0 I x I 0.05 and then essentially remains constant with further increase of x . The constant preexponential factor for x > 0.05 may be explained if we assume that some of the carriers are bound at specific defect sites, so that essentially only a small fraction is free to diffuse,14i.e., there is a fixed fraction of free charge carriers which is independent of x , and thus the preexponential factor of K,Sb5,WXP202o virtually (14) Nowick, A. S.;Lee, W.-K. Superionic Solids and Solid Electrolytes, Recent Trends; Laskar, A. L., Chandra, S.,Ed.; Academic Press: New York, 1990; p 381.
Chem. Mater. 1992,4,661-665
cancy concentration than by the increasing K+ ion concentration. The low K+ occupancy of unsubstituted K6Sb$20m phase4was ascribed as one of the major factors responsible for the high conductivity of the KsSbsP20zo phase compared to the other potassium phosphatoantimonate compounds with different structures.1°
0.70 0.65
-
0.60 -
0.55
I
0.50
2-
661
0
0.45 0.40
1
a
0.0 0.50.00
0.05
0.10
0.15
Conclusions The ionic conductivity of K5Sb6P20zocan be enhanced by substituting small amounta of Ti", Mom, and Wn ions for Sbv ions. The increase of conductivity with increasing x in KHxSbkTi,P20m is due to both the effecta of enlarged bottleneck and the increasing K+ ion concentration. The increase in the preexponential factor with increasing K+ vacancy concentration is attributed to the increase of ionic conductivity in the K6xSb5-xM~xP20m series. In the K6xSb5_xW,P2020series, the effects of increasing preexponential factor and the decreasing activation energy with increasing x are ascribed to the observed increase of conductivity. A comparison of the preexponential factor among the three series K5+,Sb5-,Ti,Pz020, K6-xSb6,M~,P2020,and K5-xSb5xW,P20zosuggests that the preexponential factor is more strongly affected by the increasing K+ vacancy concentration than by the increasing K+ ion concentration.
X Acknowledgment. This work was supported by the Figure 11. (a) Activation energy (E,) vs x in Kb-xSbb-xWxP2020.National Science Foundation - Materials Chemistry Pro(b)Preexponential factor ( A ) vs x in Kb-xSbS-xW,P2020. gram DMR 88-08234. Registry No. K5Sb5PzOzo, 98597-35-4;K5.mSb4.gsT&.mPzOzo, remains constant with increasing x for 0.05 I x I 0.15. 140225-43-0; K5.1Sb4.gTiolP20~, 140225-44-1; K5.zSb4,sTb.2PzOm, A comparison of the preexponential factor among the 140225-45-2; K,,g5Sb4,g5Moo.mP2020, 140360-30-1; K4.gSb&00.1three K5+xSb6-xTixP2020, K5-,Sb5-,Mo,P2020, and P2020, 140225-46-3;K4,ssSb~.ssMoo.l5P2020, 140225-47-4;K4.95K+,sb+,WXP2020 series suggests that the preexponentd Sb4,g5WO,O5P2020, 140225-48-5; K4.gSbp.gW0.1P2020, 140225-49-6; K,.,5Sb4,aWoo,l5PzOzo, 140225-50-9. factor is more strongly affected by the increasing K+ va-
Hydrothermal Synthesis and Characterization of Two Vanadium Organophosphonates: VO(CGHSP03)1-y(CH3P03)y~1.5H20, y = 0.50 and 0.75 G. Huan,? A. J. Jacobson,$ J. W. Johnson,* and D. P. Goshorn Corporate Research Laboratories, Exxon Research & Engineering Company, Annandale, New Jersey 08801 Received December 27, 1991. Revised Manuscript Received March 5, 1992 Synthesis of phases in the VO(C6HSP03)ly(CH3P03).xH20 system has been investigated by reaction of VzO3 + C6H6Po3H2+ CH3P03H2under hydrothermd conditions at 200 OC. Two discrete compounds with compositions, VO(C6H5P03~o.50(CH3P03)0.50.1.5H,0 and VO(C6HpP03)o.25(CH3P03~o.,~~1~5H20, have been isolated. A continuous solid solution series of the type found in some similar zirconium organophosphonate systems is not observed under the synthesis conditions used here. Both intermediate compositions are layered compounds with interlayer separations of 11.2 and 19.4 A for y = 0.5 and 0.75 respectively. On the basis of the compositions, the X-ray powder diffraction data, and the reaulta of magnetic susceptibility measurements, structural models for the two compounds are proposed. The data indicate that the y = 0.75 phase is an unusual example of a regularly interstratified inorganic-organiclayered compound.
Introduction The metal organophosphonates and organophosphates form a group of layered compounds with alternating or'Present address: Carus Chemical Co., 1001 Boyce Memorial Drive, Ottawa, IL 61350. Present address: Department of Chemistry, University of Houston, Houston, TX 77204-5641.
*
ganic and inorganic layers. Several systems have been studied in detail because of their interesting structural chemistry, sorption and catalytic properties. For example, zirconium1-8 and the vanadiumg organophosphonates, (1) Mikulski, C. M.; Karayannis, N. M.; Minkiewicz, J. V.; Pytlewski, L. L.; Labes, M. M. Inorg. Chim. Acta 1969,3,523-526.
0 1992 American Chemical Society
