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Mg0.5Ti2(PO4)3 — a new member of the NASICON family with low thermal expansion
Materials Letters I6 ( 1993) 96-10 1 North-Holland
M&.,Ti2 (PO& - a new member of the NASICON family with low thermal expansion S. Barth a, R. Olazcuaga b, P. Gravereau b, G. Le Flem b and P. Hagenmuller b ’ Institut ftir Anorganische and Analytische Chemie der Friedrich-Schiller-Universitiit Jena, August-Bebel-strasse 2, O-6900 Jena. Germany b Laboratoire de Chimie du Solide du CNRS, UniversitP de Bordeaux I, 351, cows de la Liberation, 33405 Talence Cedex, France
Received 24 December 1992
M&,,Ti*( P04)s has been prepared by a sol-gel technique and structure determination has been carried out by Rietveld retinement. It belongs to the NASICON-type structure, space group Rk, and is characterized by a low thermal expansion coeflicient of 3.7x 10-6K-‘.
Cu’M2(P0J~+~0,--*C~6f5M~(P0~)3+fC~0,
1. Introduction
withM=ZrorTi. The discovej by Goodenough et al. [ 1 ] in 1976 of the solid solution Nal+Jr,( PO,),_,( SiO.,),, by reason of its noteworthy high Na+ conductivity, has attracted many interest for the so-called NASICONtype structure. It is characterized by a three-dimensional network of corner shared octahedra and tetrahedra, offering a 3D system of interstitials for optimized ion transport, especially for alkaline ions. In the last 15 years a lot of new compounds were described, crystallizing in this structure type and confirming the great variability of this structure with reference to the composition. Yao and Fray [ 2 ] have shown that it is possible to substitute monovalent copper ions for the alkaline in order to get the compound Cu1Zr2(P04)3 with NASICON structure. A detailed structure determination of Cu’Til(P04)3 had been carried out by McCarron et al. [ 31 and Mbandza et al. [ 41. Moreover, CulZrz(PO,), is characterized by interesting luminescent properties [ 5 1. By using moderate oxidation processes, it is possible to transform these compounds into their analoga Cu&Ti2(P04)3 and Cu&Zr2 (PO.,) 3 with divalent copper, according to the following reaction [ 6,7]: 96
The nearly complete reversibility of this reaction at moderate temperatures makes these compounds interesting as promising catalysts for oxidative dehydrogenation reactions [ 8 1. Some other compounds of the general formula &.5M2 ( P04)3 with A = Ca2+, S? Cd2+ Pb2+ were directly prepared by sol-gel mehods \9]. These compounds are generally characterized by a low thermal expansion coefficient. That is why in the recent years a great deal of work was published on synthesis, structure, stability and thermal expansion of the so-called CTP ceramics [ lo]. It is the aim of this paper to describe the structure of a new member of this family: Mgo.5Ti2(PO,) 3.
2. Experimental 2.1. Preparation Mg,-,sTi2( PO.,) 3 has been prepared by a simple solgel method starting from Ti ( i-0C3H7 )4, NH.,H2P04 and freshly calcined MgO. In all raw materials the content of the desired components was carefully estimated to be sure to have perfectly stoichiometric
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MATERIALS LETTERS
Volume 16, number 2,3
which was sintered for 40 h at 900°C. Not any significant weight loss was detectable on heating until 12OO”C, followed by cooling down to 250°C and heating and cooling again. No additional thermal effects were observable in the same temperature cycle, indicating the thermal stability of this material up to 12oo”c.
proportions. MgO was dissolved in dilute acetic acid, NH,H,PO, in water and Ti (I-O&H,), was diluted with n-propanol and stabilized with acetic acid to reduce the viscosity. Firstly, the solution of magnesium acetate was added to the titanium-alkoxide, well stirred and at least the solution of ammonium dihydrogenphosphate was added, which causes immediately a white gel-like precipitation. The slurry was well stirred for 20 h and finally dried using a rotary evaporator. The residue was firstly heated to 200°C for 15 h to decompose the ammonium salts, grinded and further heated to 500°C to decompose the acetates. This amorphous powder was ball-milled in a planetary mill for ,5 h with ethanol, dried again and finally sintered at different temperatures.
2.3. Structure determination
2.2. Thermal analysis
The powder pattern of a material which was sintered for 40 h at 900°C was indexable within the space group R%. A first calculation of the hexagonal parameters yielded values for ao= 8.501 8, and co= 20.949 A. The X-ray diffraction data were collected at room temperature with a Philips PW 1820 diffractometer using a graphite monochromator. The experimental conditions are given together with the crystal structure data in table 1.
The DTA and TG investigations in the high-temperature region were performed with a Netzsch thermal analyser STA 429. The crystallization temperature in the lower part was determined with a Rigaku thermoanalytical system. A DTA investigation of the amorphous powder shows that with a heating rate of 10 K/min the crystallization begins at 766°C. Fig. 1 shows the TG curve of a well-crystallized powder of M&.,Ti* ( P04) 3
2.4. Refinement of the structure The initial atomic coordinates used to refine the structure of M&.,Ti2(P0,)3 were derived from those of NaZr, ( PO4)3 according to Hagman and Kirkegaard [ 12 1. Starting with these parameters, the re-
I .5
P -
-.5-
: 2
-I
-
ii .z e
-1.5
-
-2
-2.5
-
-
-3 0
~~.~‘..~~‘.~..‘~~~~‘~~..‘~.~.‘....’~~~~’..*~’~~~.~....l~~..I 200 400
600
a00
1000
Tsmpsraturd*C
I200
Fig. 1. Thermogravimetrical slope of Mgo.sTiz(PO4)3 on heating up to 1200°C.
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Table 1 Crystal structure data for Mg0.STi2(P0.,)S based on X-ray Rietveld refinement space group cell parameters: a0 (A) co (A) volume (A’) wavelength (A) angular range (deg 28) step width (deg 20) count time (s) refinement program background law for full width at half-maximum analytical function for profile shape no. of contributing reflections no. of variables R values: R,=~lyi-y,ill~~i [Cw,(~i-y,)~/~w~yiz]*‘~ Bragg R factor Ri= X [“&,“-I,(
R3c 8.4981(2) 20.9746(6) 1311.8 1.54184 18-120 0.02 23.7 DBW 3.2 S (Wiles and Young, 1981 [ 111) polynomial function (degree 5 in (26) ) (fwhm)r= Stan%+ Vtan 0+ W PV=nL+(l-n)G;n=0.43(3) 459 30 9.6 13.1 5.21 4.3
R,=
Rr=CIFo-
/l“Io”
IF,II/Y‘F,”
finement yielded after a few cycles to a satisfactory result, indicating the membership of M&.,Tiz(P04)3 to the NASICON family. In fig. 2 the calculated data
are compared with the experimental data. The refined parameters are given in table 2.
1.0
: c : u m 0 n
0.5
x % -3
0.0
n- n-+-~-tl-~-tl~-t-~+n-t~+~-~-~,~+~~,+,-~~~-~
10
15
20
25
30
35
10
45
50
55
b++t++w++*-70 60 65
75
80
85
90
95
100
105
110
2*THETAl'l
Fig. 2. Difference map for the experimental and calculated diffractogram of Mgr,.,Tiz( PO4)a
98
115
1
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Table 2 Reduced coordinates
of Mgc,,Tiz(P04)3
refined in space group Rk
March
LETTERS
with hexagonal
parameters
ac=8.4981
A and cc=20.9746
Atom
X
Y
z
B (A*)
Mg (6b)
0.00 0.00 0.2887(3) 0.1799(4) 0.1881(4)
0.00 0.00 0.00 0.9886(4) 0.1642(4)
0.00 0.1417(l) 0.25 0.1881(2) 0.0846(2)
0.60(2) 0.78(5) 1.42(5) 2.50( 15) 2.30( 15)
Ti (12~) P (18e) O(1) O(2)
(36f) (36f)
2.5. Dilatometry Bulk thermal expansion measurements were made on cylindrical bars ( 10 mm in diameter, 4 mm height ) utilizing a horizontal Netzsch dilatometer 402 E with a sample holder of A1203. The heating rate was adjusted to 1 K/min and the thermal expansion was measured on heating and cooling in the temperature range from room temperature to 500°C giving for the thermal expansion coefficient a value of 3.7x 1O-6 K-l.
Table 3 Interatomic
1993
A
(A) of Mg,,,Ti, (PO.,) 3 at room tempera-
distances
ture MIS-W 1) Mg-O(2)
(6x
1
2.972 3.507
Mg-Ti Mg-P Ti-P Ti-P
3.599 2.328
3.343 3.277
(3~) (3~)
Ti-O(1) Ti-O(2)
(3x) (3x)
1.855 1.925
P-O(l) P-O(2)
(2X)
1.568 1.533
(2X)
3. Discussion The refined atom coordinates are given in table 2, the derived interatomic distances in table 3. It is evident that the structure of Mg,,Tiz( P04)S looks more like that of NaZr,(PO,), than that of Cu’Til( P04)S. The reason is the noble-gas-like nature of the cations Mg2+ and Na+, causing a highly ionic interaction between these cations and the remaining [M,(PO,),]“skeleton. Inversely Cu+, as a d’O cation, prefers a more covalent interaction, which is frequently expressed in the occurrence of CU’-O~,~ dump-bells with extraordinary short Cu*-0 or CuiCur distances in the crystal chemistry of many Cu+ compounds, especially in the delafossites [ 131 or in Cu’Zrz ( PO4)3 [ 14,15 1, causing a distortion of the oxygen surrounding as in Cu’Tiz( P04)J [ 3,4 1. In Mgc,sTi2( PO4)j the noble-gas-like nature of the cation Mg2+ favours a high symmetric coordination with oxygen, the Mg-0 distance of 2.33 A is about 10% longer than the sum of the ionic radii according to Shannon and Prewitt [ 161. This is an indication for the stability of the covalent [Ti2( PO,),] - skeleton, which accommodates the Mg*+ cations as guests. On the contrary, both Ti06,* octahedra and
0(1)-O(l) 0(1)-O(l) 0(1)-O(l) 0(1)-O(2) 0(1)-O(2) O( 1 )-O(2) O(1 )-O(2) 0(2)-O(2) 0(2)-O(2)
2.735 2.979 2.602 2.615 2.553 2.714 2.498 2.611 2.49
PO,,, tetrahedra, as in the structure of NaZr, ( P04)3, are slightly distorted. This distortion results from the NASICON structure, which is shown in fig. 3. In the case of NaZr, ( P04) 3, the sequence of the octahedra and tetrahedra along the c axis can be written as: 03Zr0,
* 03Zr03Na03Zr03
* 03Zr03Na03Zr03
* 03Zr0,.
The alternating arrangement of sodium ions and between the ZrO6/2 octahedra causes an asymmetrical field gradient in the c-direction around these Zr06/, octahedra. As a consequence the Zr atom is shifted outward of the centre of the octahedron, giving two different Zr-0 distances. In the VXatKiCS
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Mg2+ ions are able to occupy the Zr positions in the three-dimensional NASICON-network structure in a statistical manner. Thus, it should also be possible to take into account the occupation of the Ti sites by Mg2+ ions in the case of Mgo.sTi2(P04)3. The substitution of divalent magnesium for tetravalent titanium demands a simultaneous insertion of additional‘magnesium for charge balance. This is very easy to realize since in Mgo.5Ti2(P04)3 half of the M, sites is unoccupied. Hence, the above discussed substitution can be described by the general formula Such an enrichment with Mgo.,+,Mg,Ti,_,(PO,),. demands in a stoichiometric magnesium Mgc,.,Ti2( P04) 3 simultaneously a separation of TiP20, and Ti02, according to the following reaction: Fig. 3. The structure of Mgo.5Tiz(P04)3 viewed along [ 1IO]. Table 4 Additional, non-labelling reflexes for Mgo.,Ti2(P04)a within the space group R3c No.
28
d(A)
I(%)
Explanation
1
12.40 12.66 22.72 27.88
7.132 6.984 3.912 3.197
1.14 0.78 0.77 0.66
(003) (011) TiP*O, TiP207
2 3 4
case of NaZr,( PO4)J this difference is 1.7%, whereas in M&.,T& (PO*), for the difference in the Ti-0 bond distances a value of 3.7% was found. The reason for the stronger distortion is the nearly twofold field strength of the Mg2+ cation compared with the Na+ ion [ 171. An important problem concerns the homogeneity of the specimen. In spite of carefully prepared samples, four very weak reflections (I< 1.2O/6)were always observed, which are not compatible with the space group R3c. They were independently found in samples of the same composition, prepared by El Jazouli and co-workers [ 181. These distances (d) and intensities (I) are given in table 4. Two of them (samples 3 and 4) correspond to the main reflexes of TiP20,. The permanent occurrence of traces of TiP207 can be explained by a simple model of intrinsic disorder. In a recent paper [ 19 1, we have described an intensive “P-MAS NMR study of the solid solution Na ,+2xMgxZr2_-x(P04)3, showing that the 100
(1 +4~)Mgo.5WPW3
+M&.,+,MgXTi2_,( PO,,)3 + 6xTiP20, + 3xTi02. Even if the range of x is very small, the amount of separated TiP20, should be detectable. A reason for the enrichment of Mgc,,Ti2 ( P04) 3 with magnesium could be, in spite of the use of a sol-gel route, the insufficient distribution of MgO in MgasTi2 ( P04)3, since the weight percentage of MgO in this compound is only 5.13%. The two other reflections, with d=7.132 8, and 6.984 8, respectively, are not to attach to any other known phase, but it is very remarkable that in NasZr(P04)3, where four Na atoms are substituted for one Zr atom, a similar value (d=7.518 A) for the longest distance was found, corresponding to the (011) reflection and indicating the reduction of symmetry from R3c to R32. This was caused at first by ordering of the Zr and Na atoms on the octahedra position of the network structure and moreover by an enhanced disordering of the remaining Na+ ions over the interstitials of the three-dimensional octahedra-tetrahedra host [ 201. In the structure of NaSTi (PO,) 3, nearly the same symmetry reduction has recently been observed, which is also due to a 22 ordered distribution of the TiOb12 and Na06,2 octahedra along the c axis [ 2 11. In our case, the intensity of these two additional reflections, which are compatible with a hexagonal cell without the glide mirror as (003) and (01 1 ), is however very small. This can be interpreted as an indirect hint for the occurrence of traces of an other
MATERIALS LETTERS
Volume 16, number 2,3
phase
with
a similar
cation
disordering
as
in
NaZr(POA, namely M&1.~+,Mg,Ti2-,(P0,)3, supporting the previously proposed reaction. Since these two reflections are furthermore broadened, it seems that Mg,. ,+,Mg,Ti,_,(PO,), does not exist really as an independent phase but must be understood more as domains in the three-dimensional network structure, caused by a nonhomogeneous distribution of the magnesium.
References [ 1 ] J.B. Goodenough, H.Y.-P. Hong and J.A. Kafalas, Mater. Res. Bull. 11 ( 1976) 203. [2] P.C. Yao and D.J. Fray, Solid State Ionics 8 (1983) 35. [ 31 E. McCarron, J.K. Calabrese and M.A. Submmanian, Mater. Res. Bull. 22 (1987) 1421. [4] A. Mbandza, E. Bordes, P. Courtine, A. El Jazouli, J.L. Soubeyroux, G. Le Flem and P. Hagenmuller, React. Solids 5 (1988) 315. [ 5 ] G. Le Polles, C. Parent, R. Olazcuaga, G. Le Rem and P. Hagenmuiler, Compt. Rend. Sci. (Paris) 306 II ( 1988) 765. [ 61A. El Jazouli, J.L. Soubeyroux, J.M. Dance and G. Le Flem, J. Solid State Chem. 65 (1986) 351.
March 1993
[ 71 I. Bussereau, R. Olazcuaga, G. Le Flem and P. Hagenmuller, Eur. J. Solid State Inorg. Chem. 26 (1989) 383. [ 8 ] A. Serghini, R. Brochu, M. Ziyad, M. Loukah and J.C. Vedrine, J. Chem. Sot. Faraday Trans. 87 ( 199 1) 2487. 191 A. El Jazouli, M. Alami, R. Brochu, J.M. Dance, G. Le Flem and P. Hagenmuller, J. Solid State Chem. 71 ( 1987) 444. lo] R. Roy, D.K. Agrawal, J. Alamo and R.A. Roy, Mater. Res. Bull. 19 (1984) 471. 1I ] D.B. Wiles and R. Young, J. Appl. Cryst. 14 ( 1981) 149. 121 L. Hagman and P. Kirkegaard, Acta Chem. Scan. 22 ( 1968) 1822. 131 M. Jansen, Angew. Chem. Intern. Ed. 26 (1987) 1098. 1411. Bussereau, MS. Belkhiria, P. Gravereau. A. Boireau, J.L. Soubeyroux, R. Olazcuaga and G. Le Flem, Acta Cryst. B, to be published. [ 15 ] E. Fargin, I. Bussereau, G. Le Rem, R. Olazcuaga, C. Cantier and H. Dexpert, Eur. J. Solid State Inorg. Chem., to be published. [ 161 R.D. Shannon and C.W. Prewitt, Acta Cry%. B 25 (1969) 925. [ 17) A. Dietzel, Z. Elektrochem. 48 ( 1942) 9. [ 181 A, El Jazouli, private communication. [ 191 C. Jlger, S. Barth, A. Feltz and G. Scheler, Phys. Status Sol. 102b (1987) 791. [20] J.P. Boilot, G. Collin and R. Comes, J. Solid State Chem. 50 (1983) 91. [21] S. Krimi, I. Mansouri, A. El Jazouli, J.P. Chaminade, P. Gravereau and G. Le Rem. in preparation.
101
M&.,Ti2 (PO& - a new member of the NASICON family with low thermal expansion S. Barth a, R. Olazcuaga b, P. Gravereau b, G. Le Flem b and P. Hagenmuller b ’ Institut ftir Anorganische and Analytische Chemie der Friedrich-Schiller-Universitiit Jena, August-Bebel-strasse 2, O-6900 Jena. Germany b Laboratoire de Chimie du Solide du CNRS, UniversitP de Bordeaux I, 351, cows de la Liberation, 33405 Talence Cedex, France
Received 24 December 1992
M&,,Ti*( P04)s has been prepared by a sol-gel technique and structure determination has been carried out by Rietveld retinement. It belongs to the NASICON-type structure, space group Rk, and is characterized by a low thermal expansion coeflicient of 3.7x 10-6K-‘.
Cu’M2(P0J~+~0,--*C~6f5M~(P0~)3+fC~0,
1. Introduction
withM=ZrorTi. The discovej by Goodenough et al. [ 1 ] in 1976 of the solid solution Nal+Jr,( PO,),_,( SiO.,),, by reason of its noteworthy high Na+ conductivity, has attracted many interest for the so-called NASICONtype structure. It is characterized by a three-dimensional network of corner shared octahedra and tetrahedra, offering a 3D system of interstitials for optimized ion transport, especially for alkaline ions. In the last 15 years a lot of new compounds were described, crystallizing in this structure type and confirming the great variability of this structure with reference to the composition. Yao and Fray [ 2 ] have shown that it is possible to substitute monovalent copper ions for the alkaline in order to get the compound Cu1Zr2(P04)3 with NASICON structure. A detailed structure determination of Cu’Til(P04)3 had been carried out by McCarron et al. [ 31 and Mbandza et al. [ 41. Moreover, CulZrz(PO,), is characterized by interesting luminescent properties [ 5 1. By using moderate oxidation processes, it is possible to transform these compounds into their analoga Cu&Ti2(P04)3 and Cu&Zr2 (PO.,) 3 with divalent copper, according to the following reaction [ 6,7]: 96
The nearly complete reversibility of this reaction at moderate temperatures makes these compounds interesting as promising catalysts for oxidative dehydrogenation reactions [ 8 1. Some other compounds of the general formula &.5M2 ( P04)3 with A = Ca2+, S? Cd2+ Pb2+ were directly prepared by sol-gel mehods \9]. These compounds are generally characterized by a low thermal expansion coefficient. That is why in the recent years a great deal of work was published on synthesis, structure, stability and thermal expansion of the so-called CTP ceramics [ lo]. It is the aim of this paper to describe the structure of a new member of this family: Mgo.5Ti2(PO,) 3.
2. Experimental 2.1. Preparation Mg,-,sTi2( PO.,) 3 has been prepared by a simple solgel method starting from Ti ( i-0C3H7 )4, NH.,H2P04 and freshly calcined MgO. In all raw materials the content of the desired components was carefully estimated to be sure to have perfectly stoichiometric
0167-577x/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
March 1993
MATERIALS LETTERS
Volume 16, number 2,3
which was sintered for 40 h at 900°C. Not any significant weight loss was detectable on heating until 12OO”C, followed by cooling down to 250°C and heating and cooling again. No additional thermal effects were observable in the same temperature cycle, indicating the thermal stability of this material up to 12oo”c.
proportions. MgO was dissolved in dilute acetic acid, NH,H,PO, in water and Ti (I-O&H,), was diluted with n-propanol and stabilized with acetic acid to reduce the viscosity. Firstly, the solution of magnesium acetate was added to the titanium-alkoxide, well stirred and at least the solution of ammonium dihydrogenphosphate was added, which causes immediately a white gel-like precipitation. The slurry was well stirred for 20 h and finally dried using a rotary evaporator. The residue was firstly heated to 200°C for 15 h to decompose the ammonium salts, grinded and further heated to 500°C to decompose the acetates. This amorphous powder was ball-milled in a planetary mill for ,5 h with ethanol, dried again and finally sintered at different temperatures.
2.3. Structure determination
2.2. Thermal analysis
The powder pattern of a material which was sintered for 40 h at 900°C was indexable within the space group R%. A first calculation of the hexagonal parameters yielded values for ao= 8.501 8, and co= 20.949 A. The X-ray diffraction data were collected at room temperature with a Philips PW 1820 diffractometer using a graphite monochromator. The experimental conditions are given together with the crystal structure data in table 1.
The DTA and TG investigations in the high-temperature region were performed with a Netzsch thermal analyser STA 429. The crystallization temperature in the lower part was determined with a Rigaku thermoanalytical system. A DTA investigation of the amorphous powder shows that with a heating rate of 10 K/min the crystallization begins at 766°C. Fig. 1 shows the TG curve of a well-crystallized powder of M&.,Ti* ( P04) 3
2.4. Refinement of the structure The initial atomic coordinates used to refine the structure of M&.,Ti2(P0,)3 were derived from those of NaZr, ( PO4)3 according to Hagman and Kirkegaard [ 12 1. Starting with these parameters, the re-
I .5
P -
-.5-
: 2
-I
-
ii .z e
-1.5
-
-2
-2.5
-
-
-3 0
~~.~‘..~~‘.~..‘~~~~‘~~..‘~.~.‘....’~~~~’..*~’~~~.~....l~~..I 200 400
600
a00
1000
Tsmpsraturd*C
I200
Fig. 1. Thermogravimetrical slope of Mgo.sTiz(PO4)3 on heating up to 1200°C.
97
Volume 16, number 2,3
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March 1993
Table 1 Crystal structure data for Mg0.STi2(P0.,)S based on X-ray Rietveld refinement space group cell parameters: a0 (A) co (A) volume (A’) wavelength (A) angular range (deg 28) step width (deg 20) count time (s) refinement program background law for full width at half-maximum analytical function for profile shape no. of contributing reflections no. of variables R values: R,=~lyi-y,ill~~i [Cw,(~i-y,)~/~w~yiz]*‘~ Bragg R factor Ri= X [“&,“-I,(
R3c 8.4981(2) 20.9746(6) 1311.8 1.54184 18-120 0.02 23.7 DBW 3.2 S (Wiles and Young, 1981 [ 111) polynomial function (degree 5 in (26) ) (fwhm)r= Stan%+ Vtan 0+ W PV=nL+(l-n)G;n=0.43(3) 459 30 9.6 13.1 5.21 4.3
R,=
Rr=CIFo-
/l“Io”
IF,II/Y‘F,”
finement yielded after a few cycles to a satisfactory result, indicating the membership of M&.,Tiz(P04)3 to the NASICON family. In fig. 2 the calculated data
are compared with the experimental data. The refined parameters are given in table 2.
1.0
: c : u m 0 n
0.5
x % -3
0.0
n- n-+-~-tl-~-tl~-t-~+n-t~+~-~-~,~+~~,+,-~~~-~
10
15
20
25
30
35
10
45
50
55
b++t++w++*-70 60 65
75
80
85
90
95
100
105
110
2*THETAl'l
Fig. 2. Difference map for the experimental and calculated diffractogram of Mgr,.,Tiz( PO4)a
98
115
1
Volume
16, number
MATERIALS
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Table 2 Reduced coordinates
of Mgc,,Tiz(P04)3
refined in space group Rk
March
LETTERS
with hexagonal
parameters
ac=8.4981
A and cc=20.9746
Atom
X
Y
z
B (A*)
Mg (6b)
0.00 0.00 0.2887(3) 0.1799(4) 0.1881(4)
0.00 0.00 0.00 0.9886(4) 0.1642(4)
0.00 0.1417(l) 0.25 0.1881(2) 0.0846(2)
0.60(2) 0.78(5) 1.42(5) 2.50( 15) 2.30( 15)
Ti (12~) P (18e) O(1) O(2)
(36f) (36f)
2.5. Dilatometry Bulk thermal expansion measurements were made on cylindrical bars ( 10 mm in diameter, 4 mm height ) utilizing a horizontal Netzsch dilatometer 402 E with a sample holder of A1203. The heating rate was adjusted to 1 K/min and the thermal expansion was measured on heating and cooling in the temperature range from room temperature to 500°C giving for the thermal expansion coefficient a value of 3.7x 1O-6 K-l.
Table 3 Interatomic
1993
A
(A) of Mg,,,Ti, (PO.,) 3 at room tempera-
distances
ture MIS-W 1) Mg-O(2)
(6x
1
2.972 3.507
Mg-Ti Mg-P Ti-P Ti-P
3.599 2.328
3.343 3.277
(3~) (3~)
Ti-O(1) Ti-O(2)
(3x) (3x)
1.855 1.925
P-O(l) P-O(2)
(2X)
1.568 1.533
(2X)
3. Discussion The refined atom coordinates are given in table 2, the derived interatomic distances in table 3. It is evident that the structure of Mg,,Tiz( P04)S looks more like that of NaZr,(PO,), than that of Cu’Til( P04)S. The reason is the noble-gas-like nature of the cations Mg2+ and Na+, causing a highly ionic interaction between these cations and the remaining [M,(PO,),]“skeleton. Inversely Cu+, as a d’O cation, prefers a more covalent interaction, which is frequently expressed in the occurrence of CU’-O~,~ dump-bells with extraordinary short Cu*-0 or CuiCur distances in the crystal chemistry of many Cu+ compounds, especially in the delafossites [ 131 or in Cu’Zrz ( PO4)3 [ 14,15 1, causing a distortion of the oxygen surrounding as in Cu’Tiz( P04)J [ 3,4 1. In Mgc,sTi2( PO4)j the noble-gas-like nature of the cation Mg2+ favours a high symmetric coordination with oxygen, the Mg-0 distance of 2.33 A is about 10% longer than the sum of the ionic radii according to Shannon and Prewitt [ 161. This is an indication for the stability of the covalent [Ti2( PO,),] - skeleton, which accommodates the Mg*+ cations as guests. On the contrary, both Ti06,* octahedra and
0(1)-O(l) 0(1)-O(l) 0(1)-O(l) 0(1)-O(2) 0(1)-O(2) O( 1 )-O(2) O(1 )-O(2) 0(2)-O(2) 0(2)-O(2)
2.735 2.979 2.602 2.615 2.553 2.714 2.498 2.611 2.49
PO,,, tetrahedra, as in the structure of NaZr, ( P04)3, are slightly distorted. This distortion results from the NASICON structure, which is shown in fig. 3. In the case of NaZr, ( P04) 3, the sequence of the octahedra and tetrahedra along the c axis can be written as: 03Zr0,
* 03Zr03Na03Zr03
* 03Zr03Na03Zr03
* 03Zr0,.
The alternating arrangement of sodium ions and between the ZrO6/2 octahedra causes an asymmetrical field gradient in the c-direction around these Zr06/, octahedra. As a consequence the Zr atom is shifted outward of the centre of the octahedron, giving two different Zr-0 distances. In the VXatKiCS
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Mg2+ ions are able to occupy the Zr positions in the three-dimensional NASICON-network structure in a statistical manner. Thus, it should also be possible to take into account the occupation of the Ti sites by Mg2+ ions in the case of Mgo.sTi2(P04)3. The substitution of divalent magnesium for tetravalent titanium demands a simultaneous insertion of additional‘magnesium for charge balance. This is very easy to realize since in Mgo.5Ti2(P04)3 half of the M, sites is unoccupied. Hence, the above discussed substitution can be described by the general formula Such an enrichment with Mgo.,+,Mg,Ti,_,(PO,),. demands in a stoichiometric magnesium Mgc,.,Ti2( P04) 3 simultaneously a separation of TiP20, and Ti02, according to the following reaction: Fig. 3. The structure of Mgo.5Tiz(P04)3 viewed along [ 1IO]. Table 4 Additional, non-labelling reflexes for Mgo.,Ti2(P04)a within the space group R3c No.
28
d(A)
I(%)
Explanation
1
12.40 12.66 22.72 27.88
7.132 6.984 3.912 3.197
1.14 0.78 0.77 0.66
(003) (011) TiP*O, TiP207
2 3 4
case of NaZr,( PO4)J this difference is 1.7%, whereas in M&.,T& (PO*), for the difference in the Ti-0 bond distances a value of 3.7% was found. The reason for the stronger distortion is the nearly twofold field strength of the Mg2+ cation compared with the Na+ ion [ 171. An important problem concerns the homogeneity of the specimen. In spite of carefully prepared samples, four very weak reflections (I< 1.2O/6)were always observed, which are not compatible with the space group R3c. They were independently found in samples of the same composition, prepared by El Jazouli and co-workers [ 181. These distances (d) and intensities (I) are given in table 4. Two of them (samples 3 and 4) correspond to the main reflexes of TiP20,. The permanent occurrence of traces of TiP207 can be explained by a simple model of intrinsic disorder. In a recent paper [ 19 1, we have described an intensive “P-MAS NMR study of the solid solution Na ,+2xMgxZr2_-x(P04)3, showing that the 100
(1 +4~)Mgo.5WPW3
+M&.,+,MgXTi2_,( PO,,)3 + 6xTiP20, + 3xTi02. Even if the range of x is very small, the amount of separated TiP20, should be detectable. A reason for the enrichment of Mgc,,Ti2 ( P04) 3 with magnesium could be, in spite of the use of a sol-gel route, the insufficient distribution of MgO in MgasTi2 ( P04)3, since the weight percentage of MgO in this compound is only 5.13%. The two other reflections, with d=7.132 8, and 6.984 8, respectively, are not to attach to any other known phase, but it is very remarkable that in NasZr(P04)3, where four Na atoms are substituted for one Zr atom, a similar value (d=7.518 A) for the longest distance was found, corresponding to the (011) reflection and indicating the reduction of symmetry from R3c to R32. This was caused at first by ordering of the Zr and Na atoms on the octahedra position of the network structure and moreover by an enhanced disordering of the remaining Na+ ions over the interstitials of the three-dimensional octahedra-tetrahedra host [ 201. In the structure of NaSTi (PO,) 3, nearly the same symmetry reduction has recently been observed, which is also due to a 22 ordered distribution of the TiOb12 and Na06,2 octahedra along the c axis [ 2 11. In our case, the intensity of these two additional reflections, which are compatible with a hexagonal cell without the glide mirror as (003) and (01 1 ), is however very small. This can be interpreted as an indirect hint for the occurrence of traces of an other
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phase
with
a similar
cation
disordering
as
in
NaZr(POA, namely M&1.~+,Mg,Ti2-,(P0,)3, supporting the previously proposed reaction. Since these two reflections are furthermore broadened, it seems that Mg,. ,+,Mg,Ti,_,(PO,), does not exist really as an independent phase but must be understood more as domains in the three-dimensional network structure, caused by a nonhomogeneous distribution of the magnesium.
References [ 1 ] J.B. Goodenough, H.Y.-P. Hong and J.A. Kafalas, Mater. Res. Bull. 11 ( 1976) 203. [2] P.C. Yao and D.J. Fray, Solid State Ionics 8 (1983) 35. [ 31 E. McCarron, J.K. Calabrese and M.A. Submmanian, Mater. Res. Bull. 22 (1987) 1421. [4] A. Mbandza, E. Bordes, P. Courtine, A. El Jazouli, J.L. Soubeyroux, G. Le Flem and P. Hagenmuller, React. Solids 5 (1988) 315. [ 5 ] G. Le Polles, C. Parent, R. Olazcuaga, G. Le Rem and P. Hagenmuiler, Compt. Rend. Sci. (Paris) 306 II ( 1988) 765. [ 61A. El Jazouli, J.L. Soubeyroux, J.M. Dance and G. Le Flem, J. Solid State Chem. 65 (1986) 351.
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[ 71 I. Bussereau, R. Olazcuaga, G. Le Flem and P. Hagenmuller, Eur. J. Solid State Inorg. Chem. 26 (1989) 383. [ 8 ] A. Serghini, R. Brochu, M. Ziyad, M. Loukah and J.C. Vedrine, J. Chem. Sot. Faraday Trans. 87 ( 199 1) 2487. 191 A. El Jazouli, M. Alami, R. Brochu, J.M. Dance, G. Le Flem and P. Hagenmuller, J. Solid State Chem. 71 ( 1987) 444. lo] R. Roy, D.K. Agrawal, J. Alamo and R.A. Roy, Mater. Res. Bull. 19 (1984) 471. 1I ] D.B. Wiles and R. Young, J. Appl. Cryst. 14 ( 1981) 149. 121 L. Hagman and P. Kirkegaard, Acta Chem. Scan. 22 ( 1968) 1822. 131 M. Jansen, Angew. Chem. Intern. Ed. 26 (1987) 1098. 1411. Bussereau, MS. Belkhiria, P. Gravereau. A. Boireau, J.L. Soubeyroux, R. Olazcuaga and G. Le Flem, Acta Cryst. B, to be published. [ 15 ] E. Fargin, I. Bussereau, G. Le Rem, R. Olazcuaga, C. Cantier and H. Dexpert, Eur. J. Solid State Inorg. Chem., to be published. [ 161 R.D. Shannon and C.W. Prewitt, Acta Cry%. B 25 (1969) 925. [ 17) A. Dietzel, Z. Elektrochem. 48 ( 1942) 9. [ 181 A, El Jazouli, private communication. [ 191 C. Jlger, S. Barth, A. Feltz and G. Scheler, Phys. Status Sol. 102b (1987) 791. [20] J.P. Boilot, G. Collin and R. Comes, J. Solid State Chem. 50 (1983) 91. [21] S. Krimi, I. Mansouri, A. El Jazouli, J.P. Chaminade, P. Gravereau and G. Le Rem. in preparation.
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