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Isothermal sections in the systems ZnOAO2Nb2O5 (A Ti, Zr, Sn) at 1473 K
Journal of Alloys and Compounds, 210 (1994) 75-81 JALCOM 1141
75
Isothermal sections in the systems ZnO-AO2-Nb205 (A--Ti, Zr, Sn) at 1473 K A. Baumgarte
and R. Blachnik*
Institut far Chemic, Barbarastrafle 7, D-49069 Osnabrfick (Germany)
(Received December 11, 1993)
Abstract Isothermal sections of the quasi-ternary systems ZnO-TiO2-Nb205, ZnO-ZrO2-Nb205 and ZnO--SnO2-Nb2Os at 1473 K are presented. Solid solutions are found on the sections TiO2(ZrO2, SnO2)-ZnNb206. The lattice parameters are given. In the system ZnO-TiO2-Nb205 a further solid solution is observed which extends from Zn2TiO4 into the ternary space.
1. Introduction
Nb2Os-based perovskites are promising materials with applications in optoelectronics. Their properties can be modified by extrinsic defects. The phase diagrams of such oxide systems containing Nb205 as one component are important for the determination of the solid solubilities in perovskites, the growth of single crystals and in the search for new materials. Studies of the systems ZnO-TiO2-Nb205, Z n O - Z r O 2 - N b 2 0 5 and Z n O - S n O 2 - N b 2 0 5 have been restricted to solid solutions based on the rutile structure [1, 2], therefore we started a preliminary investigation of these systems. Most of the seven binary systems have been published several times and those parts of the phase diagrams which are relevant for our work are well known. In the system Z n O - N b 2 0 s Dayal [3] proved the existence of the congruently melting compounds Zn3Nb2Os, and ZnNb206. ZnzNba4Os7 melts incongruently and exists at temperatures above 1358 K. The phase diagram for the system TiO2-Nb205 contains the congruently melting compounds TiNb207 and Ti2NbloO29; TiNb24062 decomposes peritectically [4, 5]. A high temperature phase of Ti2Nb6019 which forms above 1698 K was also reported [6], but it is not formed at the temperatures of this work. Sanghera and Williamson [7] determined the solubility of Nb205 in TiO2 at 1723 K to lie between 8 and 10 mol.%, whereas that of TiO2 in Nb205 is negligible [8]. A ZrO2-Nb205 phase diagram was proposed by Roth [8]. The author assumed a homologous series of com*Author to whom correspondence should be addressed.
pounds with the formula (Nb, Zr)nOz~+ 1 (n = 4, 6, 8, 10) between 60 and 85 mol.% ZrO2. Their appearance is due to various vernier structures. On the N b 2 0 : rich side of the system ZrNb24062 was identified by Allpress and Roth [9]. Trunov [10] obtained single crystals of ZrNbloO27 and ZrNb14037. However, the first one was not found by other authors; the second is, according to the X-ray data, the well-known ZrNb24062. The system SnO2-Nb20 5 is of the eutectic type. Solubilities of 4 mol.% Nb205 in SnO 2 and 5 mol.% SnO2 in Nb205 [11] are given at 1670 K. In the first investigation of the system ZnO-TiO2 Dulin and Rase [12] observed Zn2TiO4, probably congruently melting at 1822 K, and ZnTiO3. The previously mentioned Zn2Ti3Os [13] was identified as the low temperature form of ZnTiO3 [14], which decomposes peritectoidally between 1238 and 1283 K. The phase diagram of the system Z n O - Z r O 2 is eutectic. Von Wartenberg and Gurr [15] and Bartuska and Bartrakov [16] found no indications of the formation of compounds. In ZnO-SnO2 the compounds ZnSnO3, which decomposes below the temperature of our study [17], and ZnESnO4 [18] were reported.
2. Experimental details
The samples were made from ZnO (p.a., Riedel de Hahn), TiO2 (chem. pur, Riedel de Hahn), SnO2 (99.9%, Aldrich), ZrO2 (99.5%, Auer-Remy) and Nb205 (ceramic grade, H. C. Starck). Appropriate amounts of these compounds were mixed in an agate mortar. After addition of polyethylene glycol (400 g mol- 1, Chemische
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)01141-4
76
A: Baumgarte, R. Blachnik I Isothermal sections in ZnO-AO2-Nb205 at 1473 K
Ti02
80
",~
'._60
~xo.°
/
L
20
--i... - - -
"--4:','
I/ l/-. . . . . .
Nb205 Znzr~Nb~/30~20
/.,0 ZnNbzO6 60
Zn:'~08 80
ZnO
mol?/oZn 0 .--~ Fig. 1. The isothermal section ZnO-TiOz--Nb205 at 1473 K. Werke H01s) the mixtures were pressed into pellets under a hydrostatic pressure of 1.5 x 108 Pa. The pellets were fired at 1473 K for 48 h in open platinum crucibles. All products were reground and the organic binder added again. The specimens were then treated with a hydrostatic pressure of 4 X 109 Pa and annealed for 80 h at 1473 K to ensure complete reaction. After the thermal treatment the samples were rapidly cooled to room temperature. Phase analyses and characterizations (diffraction angles and intensities) were performed with the aid of an automatic Stoe powder diffractometer or a Guinier camera (Huber 620), both with Cu Kal radiation (A= 154.06 pm). Step scanning in 1° increments was performed with a 5 ° position sensitive detector over the range 20= 50-70° using 60 s counting time for each step. The lattice parameters were obtained from measurements in a Guinier camera (Huber 621) with Cr Kal radiation (h = 228.96 pm) and refined by the leastsquares method [19]. ct-SiO2served as internal standard. Boundaries of the single-phase regions were determined by extrapolating the composition dependence of the lattice parameters to the invariant values found in the two- or three-phase regions.
3. Results
We performed phase equilibrium studies of the constituent binaries involved, namely ZnO-TiO2, ZnO-ZrO2, ZnO-SnO2, TiO/-Nb2Os, ZrO2-Nb2Os, SnOz-NbzO5 and ZnO-Nb205, with mixtures annealed at 1473 K for 4 days. These samples contained the compound ZnETiO4 in the ZnO-TiO2 binary, the compound Zn2SnO 4 in the ZnO--SnO2 binary, the compounds TizNbloO29, TiNb207 and an unidentified compound at about 15 tool.% TiO2 in the TiO2-Nb205 binary, the compounds ZrNb24062 and Zr6Nb2Oa7 in the ZrO:-Nb205 binary and the compounds Zn3Nb2Os, ZnNb206 and Zn2Nb34087 in the ZnO-Nb205 binary. No compounds were detected by X-ray analyses in the systems ZnO-ZrO2 and SnO2-Nb2Os. The isothermal sections at 1473 K were constructed from the X-ray phase analyses of ternary samples taking into account the phase relations of the constituent binaries. The results are shown in Figs. 1-3. The points give the composition of the samples. The direction and size of the arrows indicate the compounds and their amounts found in these samples. The hatched areas represent single-phase regions. Two-phase regions are indicated by rectangular and three-phase fields by tri-
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb205 at 1473 K
77
Zr,pz Zr .80
(~)or'
I", i
X
X-G°
\
t
,/
\ O
t
-<
NbgO~
Zn21(163~,08726
"'
L6
Zn~lbz06
6C)
Zn3Nb208 80
ZnO
mol.% Zn 0 --~
Fig. 2. The isothermal section ZnO-ZrOE-Nb205 at 1473 K. angular areas. The preliminary tie-lines in the twophase fields were constructed from the X-ray data. In the system ZnO-TiO2-Nb205 seven three-phase equilibria were observed: (1) TiO2 (solid solution, s.s.) + ixiolite (ix.) (S.S.) + Zn2TiO 4 (s.s.) (2) Zn2TiO4(s.s.) + ix. (s.s.) + Zn3Nb208 (s.s.) (3) Zn2Nb308 (s.s.) + ZnzTiO4 (s.s.) + ZnO (4) Zn2Nb308 (s.s.) + ix. (s.s.) + ZnNbzO6 (s.s.) (5) ZnNb206 (s.s.) + ix. (s.s.) + TizNb,oO29 (s.s.) (6) Ti2Nb,oO29 (s.s.) + ix. (s.s.) + TiNb207 (s.s.) (7) TiNb207 (s.s.) + ix. (s.s.) + TiO2 (s.s.). The phase diagram reveals the formation of a line compound TixZnl/3_ (1/3)~Nbz/3_(z/3~O2 (x = 0.73-0.85, ideal composition ZnTiNb2Os) and line-like regions of solid solution based on the futile or columbite structure, all along the section TiOz-ZnNb206. The details of the section TiO2-ZnNb206 have been discussed elsewhere [20]. In the system a solid solution range extends from the compound Zn2TiO4 into the ternary system. Zn2TiO4 exists in two modifications [21]. The a form with tetragonal symmetry is of the inverse spinel type with the lattice parameters a = 600.5(3) pm, c = 841.6(3) pm [22] and ordering of the cations on the B sites. Above 1043 K it transforms into the cubic/3-Zn2TiO4
with complete disorder of the cations. We observed no solid solubility of ZnO in ZnzTiO4, whereas up to 40 mol.% TiO2 can be dissolved in ZnzTiO 4. In this reaction two Zn atoms are substituted by a Ti atom and a vacancy. In the ZnO-TiO2 binary most solid solutions had the cubic structure as expected at 1473 K; in TiO2rich solid solutions partial cation ordering is indicated by the appearance of the a-Zn2TiO4 reflections in the X-ray results. The lattice parameters of the disordered ZnzTiO4 solid solutions at 1473 K are given in Table 1. They decrease with increasing TiO2 content. The homogeneity region of the spinel extends into the ternary ZnO-TiO2-NbzO5 system (Fig. 4). Comparable with the ZnO-TiO2 binary, the zinc content is limited to two Zn atoms per unit formula. Some typical lattice parameters of these solid solutions are given in Table 2. At the highest Nb205 concentrations we observed the tetragonal o/-Zn2TiO 4 structure with cation ordering. We established six three-phase equilibria in the system ZnO-ZrO2-Nb2Os: (1) ZrO2 +wolframite (wo.) (s.s.) + ZnO (2) ZnO + wo. (s.s.) + Zn3Nb208 (3) Zn3Nb208 + wo. (s.s.) + ZnNb206 (4) Z n N b 2 0 6-k wo. (s.s.) -t- ZnzNb34087 (s.s.)
78
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb205 at 1473 K
50.02
\
k .
/
J
+,j?,
Zn0.sSn0.sNbO~.~~
Nb205 Zn2Nb3~Oa7 20
"
40
ZnNb206 60
+--
__~
Zn3N~O8 80
'_20
ZnO
mol.%Zn 0---. Fig. 3. The isothermal section ZnO-SnOz-Nb205 at 1473 K.
TABLE 1. Lattice parameters of the Zn2TiO+ solid solutions in the binary ZnO-TiO2 system TiO2 (mol.%)
a (pm)
29 31
846.94(4) 846.93(3) 846.92(3)
32 32.6 33.3 35
37 39 41 43
\\ 0~S$
846.95(4) 847.00(5) 846.55(6) 845.98(6) 845.27(7) 844.74(7) 844.61(6)
(5) Zn2Nb34Oa7 (s.s.)+ wo. (s.s.)+ Zn6N-b2017 (s.s.) (6) Zr6Nb2017 (s.s.) +wo. (s.s.) + ZrO2. This system contains the line compound ZrxZnl,3_(lm,Nb2t3_(z~,Oz (x--0.67--0.82, ideal composition ZnZrNb2Os). The line shaped homogeneity region of ZnzNb34Os7 was constructed to explain the phase relations observed in the samples. Seven three-phase equilibria were determined in ZnO-SnO2-Nb2Os:
60
70
80
mol.%Zn O--,,Fig. 4. The homogeneity range of the inverse spinel Zn2TiO+ in the ternary ZnO-TiOz-Nb205 system at 1473 K.
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb20 s at 1473 K TABLE 2. Lattice parameters of the Zn2TiO4 solid solutions in
ternary samples Sample composition (tool.%) ZnO
TiO2
Nb205
75 76 71 71.4 62.5
15 19 24 14.3 25.0
10 5 5 14.3 12.5
a (pm)
853.1(2) 852.0(1) 851.4(1) 604.4(2) 604.7(2)
TABLE 3. List of d values, intensities and indices of the ixiolite ZnTiNb2Os and the wolframites ZnZrNb208 and ZnSnNb208 at their stoichiometric composition
C (pm)
846.0(3) 843.6(3)
(1) S n O 2 + w o . + Z n 2 S n O a
(2)
79
Zn2SnO4 + wo. + Z n N b 2 0 6
(3) Z n N b 2 0 6 + Z n 2 S n O 4 + Z n 2 N b 3 0 8 (4) Z n 2 N b 3 O s + Z n 2 S n O 4 + Z n O
Zn2Nb34087 (6) ZneNb3aOs7 + wo. + SnO2 (5) Z n N b 2 0 6 + w o . +
(7) S n O 2 + Zn2Nb34087 + Yb205.
A ternary compound Zno.sSn0.sNbO4 with the wolframite structure was observed. Its homogeneity range was not determined. The three isothermal sections are separated into two subsystems by the quasi-binary sections M O 2 - Z n N b 2 0 6 . On these sections the formation of solid solutions by the reaction 3 M 4 + ~ 2 Z n 2 + + N b s+ is possible, thus allowing the substitution of the M 4+ ion by the two other metals ions without the introduction of defects. This substitution mechanism prevents the solid solutions from extending into the neighbouring ternary space and thus explains their line shape. "ZnTiNbzOs" has at this ideal composition the lattice parameters a=467.46(5) pm, b=566.21(5) pro, c = 501.37(4) pm, Z = 4, orthorhombic, space group Pbcn. The structure is related to the high pressure modification of TiO2 (a-PbO2 structure). The cations are statistically distributed on octahedral cation sites of the tetragonal close-packed O 2layers. In the system ZnO-ZrO2-Nb2Os a similar compound is found on the section Z r O 2 - Z n N b 2 0 6. Its structure is of the wolframite type, which is also derived from the a-PbO2 structure. The cations occupy the octahedral sites of the 0 2layers and are ordered in the sequence (Zn,Zr), Nb, (Zn,Zr), Nb. The stoichiometric composition ZnZrNb2Os has the lattice parameters a=480.48(6) pm, b=568.55(5) pm and c=507.66(4) pm, /3=91.311(8) °, monoclinic, space group P2/c. n~nr~o2us with the wolframite structure is found in the third system. The lattice parameters were determined to be a = 471.98(3) pm, b=571.07(4) pm, c=507.20(2) pm, /3= 90.451(4) °. The X-ray powder diffraction data of the three compounds are given in Table 3. Within our experimental conditions (annealing temperature 1473 K) no other compounds were observed in the ternary systems.
dobs (pm)
dc~l¢ (pro)
Int.
h k l
I/Io
ZnTiNb208 3.6082 2.9285 2.8332 2.5083 2.4670 2.3382 2.2107 2.1818 2.0589 1.8777 1.8034 1.7512 1.7105 1.6969 1.5168 1.5034 1.4642 1.4401
1.4360
1.4164 1.3631
1.3177 1.2893 1.2541 1.2261 1.2024 1.1846
1.1776 1.1692
1.1467 1.1178 1.1052 1.1015 1.0911
1.0841 1.0808 1.0757
1.0597 1.0567 1.0296 1.0196 1.0084 0.9925
0.9810 0.9666
0.9628 0.9457 0.9279 0.9197 0.9166 0.9076
3.6070 2.9285 2.8331 2.5081 2.4668 2.3385 2.2103 2.1819 2.0592 1.8779 1.8035 1.7513 1.7104 1.6971 1.5170 1.5031 1.4642 1.4400 1.4399 1.4359 1.4165 1.3632 1.3179 1.2893 1.2540 1.2262 1.2023 1.1845 1.1777 1.1693 1.1692 1.1467 1.1178 1.1052 1.1014 1.0910 1.0842 1.0808 1.0808 1.0757 1.0597 1.0566 1.0296 1.0196 1.0084 0.9926 0.9811 0.9665 0.9629 0.9457 0.9281 0.9198 0.9167 0.9077 0.9077
27.3 100 8.5 12.4 13.2 4.3 4.1 7.5 7.7 6.4 5.8 11.0 14.1 18.6 12.2 0.7 3.8 14.9 10.5 0.9 3.8 0.6 1.6 1.1 1.6 1.8 1.1 3.3 1.5 1.1 2.4 2.6 0.9 0.7 2.5 2.0 1.3 1.1 1.1 0.9 2.7 1.3 0.8 2.0 2.0 0.7 0.6 0.5 1.1 0.6 2.3
1 10 11 1 0 20 00 2 0 2 1 2 00 10 2 12 1 112 0 22 22 0 130 2 0 2 22 1 1 13 3 10 2 22 0 2 3 3 1 1 132 04 0 0 4 1 32 1 3 12 0 04 2 2 3 33 0 1 14 24 1 4 0 0 33 1 0 24 3 13 2 04 15 0 24 2 3 32 4 20 0 4 3 15 1 4 0 2 4 2 1 224 134 15 2 4 22 24 3 115 3 14 02 5 0 6 1 15 3 3 50 4 2 3 5 1 1
(continued)
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb20~ at 1473 K
80
TABLE 3. (continued)
TABLE 3. (continued)
dobs (pm) 0.9015
dcalc (pm) 0.9017 0.9017
Int.
h k l
I/Io
2.1944 2.1052 2.0709 1.8936 1.8351 1.7638 1.7356 1.7254 1.7163 1.6702 1.6611 1.5474 1.5270 1.4997 1.4845 1.4752 1.4663 1.4542 1.4429 1.3694 1.3051 1.2695 1.2553 1.2337 1.2235 1.2164 1.2017 1.1929 1.1866 1.1592 1.1527 1.1331 1.1274 1.1121 1.1099
5.6875 4.8056 3.7879 3.6707 2.9999 2.9505 2.8438 2.5389 2.4812 2.4028 2.2664 2.2240 2.2148 2.2134 2.1947 2.1054 2.0712 1.8939 1.8354 1.7636 1.7358 1.7256 1.7165 1.6703 1.6616 1.5474 1.5270 1.4999 1.4845 1.4752 1.4664 1.4545 1.4542 1.4428 1.3692 1.3690 1.3052 1.2695 1.2553 1.2336 1.2335 1.2237 1.2236 1.2161 1.2017 1.1933 1.1928 1.1865 1.1592 1.1526 1.1332 1.1275 1.1120 1.1100 1.1099 1.1099
dc~lc (pro) 1.1074 1.0948 1.0887
3 2 2
-2 4 2 3 3 2 04 3
3.7942 3.6403 2.9664 2.9497 2.8567 2.5375 2.4894 2.3616 2.2426 2.2281 2.2057 2.1988 2.0875 2.0758 1.8971 1.8201 1.7663 1.7354 1.7221 1.7165 1.7100 1.5376 1.5306 1.5178 1.4832 1.4825 1.4748 1.4571 1.4556 1.4516 1.4512 1.4477 1.4283 1.3749 1.3332 1.3329 1.3324 1.3209 1.3069 1.2983 1.2687 1.2428 1.2354 1.2150 1.2134 1.1893 1.187 1.808
2 51 100 97 16 26 31 14 4 2 6 5 10 12 14 11 36 15 15 19 21 13 12 2 5
0 11 1 10 -1 1 1 1 1 1 0 20 0 0 2 0 2 1 2 00 -1 0 2 102 -1 2 1 12 1 -1 1 2 1 12 0 22 2 20 130 -2 0 2 2 02 -2 2 1 2 2 1 -1 1 3 1 13 -3 1 0 -2 2 2 -2 3 0 2 22 -3 1 1 0 23 -1 3 2 3 11 132 0 40 04 1 3 0 2 -321 2 13 -1 4 1 -3 1 2 3 12 0 04 -2 2 3 2 23 -3 2 2 3 30 -2 4 1 2 4 1 4 00
0.9
4 4 0 3 5 1
1.1075 1.0948 1.0888
2
0 10
ZnSnNb208
4 3 42 98 100 20 25 23 11 4 2 3
2 2
10 0 0 1 1 -1 1 0 -1 1 1 1 1 1 02 0 0 0 2 0 2 1 2 0 0 -1 0 2 10 2 -1 2 1 2 10 12 1 - 112 112 0 2 2 -2 2 0 13 0 -2 2 1 2 0 2 2 2 1 -1 3 1 13 1 -1 1 3 1 13 -2 2 2 -3 1 1 2 2 2 3 1 1 0 2 3 - 12 3 13 2 04 1 -3 0 2 3 12 0 0 4 -2 2 3 2 2 3 -3 2 2 24 0 3 3 0 13 3 4 0 0 1 14 -2 4 1 24 1 02 4 -313 -024
2
3 13
2 2
204 051 2 3 3 -3 3 2
ZnZrNb20s 5.6863 4.8042 3.7889 3.6699 2.9991 2.9505 2.8432 2.5386 2.4807 2.4023 2.2660 2.2240 2.2146
dobs (pro)
9 6 9 16 15 28 14 14 18 3 2 12 11 4 7 5 9 19 9 8 1 2 1 1 4 1 3 4 4 2
(continued)
3.7928 3.6408 2.9656 2.9493 2.8562 2.5370 2.4887 2.3611 2.2426 2.2282 2.2053 2.1987 2.0874 2.0759 1.8970 1.8201 1.7659 1.7356 1.7220 1.7165 1.7101 1.5375 1.5307 1.5179 1.4834 1.4749 1.4571 1.4557 1.4515 1.4478 1.4284 1.3750 1.3328
1.3210 1.3068 1.2983 1.688 1.2430 1.2354 1.2150 1.2134 1.1893 1.1871 1.1808
Int.
h k l
I/Io
5 10 16 20 13 2 7 1
2 2 3 2 3 2 3 5 5 4 4
Acknowledgments W e w i s h t o e x p r e s s o u r g r a t i t u d e t o S F B 225 o f t h e Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie.
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb20s at 1473 K
References 1 F. Hund, Angew. Chem., 93 (1981) 763. 2 J. Andrade, M.E. Villafuerte-Castrejon, R. Valenzuela and A.R. West, J. Mater Sci. Lett., 5 (1986) 147. 3 R.R. Dayal, J. Less-Common Met., 26 (1972) 381. 4 R.S. Roth and L.W. Coughanour, J. Res. NBS, 55 (1955) 209. 5 T.G. Babich, A.V. Zagorodnyuk, G.A. Teterin, M. Ya. Khodos and A.P. Zhirnova, Russ. J. Inorg. Chem., 33 (1988) 560. 6 A. Jongejan and A.L. Wilkins, J. Less-Common Met., 19 (1969) 185. 7 J.S. Sanghera and J. Williamson, J. Mater Sci. Lett., 6 (1987) 449. 8 R.S. Roth, Prog. Solid State Chem., 13 (1980) 159. 9 J.G. Allpress and R.S. Roth, J. Solid State Chem., 2 (1970) 366. 10 V.K. Trunov, lnorg. Mater, 1 (1965) 1760. 11 E.N. Isupova, T.I. Panova and E.P. Savchenko, Inong. Mater, 17 (1981) 836.
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12 F.H. Dulin and D.E. Rase, J. Am. Ceram. Soc., 43 (1960) 125. 13 S.F. Bartram and R.A. Slepetys, J. Am. Ceram. Soc., 44 (1961)493. 14 O. Yamaguchi, M. Morimi, H. Kawabata and K. Shimizu, J. Am. Ceram. Soc., 70 (1987) C97. 15 H. von Wartenberg and W. Gurr, Z. Anorg. Allg. Chem., 196 (1931) 374. 16 M. Bartuska and N.A. Batrakov, Silikaty, 6 (1962) 341. 17 T. Dupuis, C. Duval and J. Lecomte, C.R. Acad. Sci. (Paris), 257 (1963) 3080. 18 M. Tr6mel, Z. Anorg. Allg. Chem., 387 (1972) 346. 19 H.T. Evans, D.E. Appleman and D.S. Handwerker, LSUCRE Computer Program, Program 42, American Crystallography Association, Cambridge, MA, 1963. 20 A. Baumgarte and R. Blachnik, ?dater. Res. Bull., 27 (1992) 1287. 2t H. Vincent, J.C. Joubert and A. Durif, Bull. Soc. Chim. Fr, 1 (1966) 246. 22 Y. Billiet, J. Morgenstern-Badarau, P. Poix and A. Michel, C.R. Acad. Sci. (Paris), 260 (1965) 4780.
75
Isothermal sections in the systems ZnO-AO2-Nb205 (A--Ti, Zr, Sn) at 1473 K A. Baumgarte
and R. Blachnik*
Institut far Chemic, Barbarastrafle 7, D-49069 Osnabrfick (Germany)
(Received December 11, 1993)
Abstract Isothermal sections of the quasi-ternary systems ZnO-TiO2-Nb205, ZnO-ZrO2-Nb205 and ZnO--SnO2-Nb2Os at 1473 K are presented. Solid solutions are found on the sections TiO2(ZrO2, SnO2)-ZnNb206. The lattice parameters are given. In the system ZnO-TiO2-Nb205 a further solid solution is observed which extends from Zn2TiO4 into the ternary space.
1. Introduction
Nb2Os-based perovskites are promising materials with applications in optoelectronics. Their properties can be modified by extrinsic defects. The phase diagrams of such oxide systems containing Nb205 as one component are important for the determination of the solid solubilities in perovskites, the growth of single crystals and in the search for new materials. Studies of the systems ZnO-TiO2-Nb205, Z n O - Z r O 2 - N b 2 0 5 and Z n O - S n O 2 - N b 2 0 5 have been restricted to solid solutions based on the rutile structure [1, 2], therefore we started a preliminary investigation of these systems. Most of the seven binary systems have been published several times and those parts of the phase diagrams which are relevant for our work are well known. In the system Z n O - N b 2 0 s Dayal [3] proved the existence of the congruently melting compounds Zn3Nb2Os, and ZnNb206. ZnzNba4Os7 melts incongruently and exists at temperatures above 1358 K. The phase diagram for the system TiO2-Nb205 contains the congruently melting compounds TiNb207 and Ti2NbloO29; TiNb24062 decomposes peritectically [4, 5]. A high temperature phase of Ti2Nb6019 which forms above 1698 K was also reported [6], but it is not formed at the temperatures of this work. Sanghera and Williamson [7] determined the solubility of Nb205 in TiO2 at 1723 K to lie between 8 and 10 mol.%, whereas that of TiO2 in Nb205 is negligible [8]. A ZrO2-Nb205 phase diagram was proposed by Roth [8]. The author assumed a homologous series of com*Author to whom correspondence should be addressed.
pounds with the formula (Nb, Zr)nOz~+ 1 (n = 4, 6, 8, 10) between 60 and 85 mol.% ZrO2. Their appearance is due to various vernier structures. On the N b 2 0 : rich side of the system ZrNb24062 was identified by Allpress and Roth [9]. Trunov [10] obtained single crystals of ZrNbloO27 and ZrNb14037. However, the first one was not found by other authors; the second is, according to the X-ray data, the well-known ZrNb24062. The system SnO2-Nb20 5 is of the eutectic type. Solubilities of 4 mol.% Nb205 in SnO 2 and 5 mol.% SnO2 in Nb205 [11] are given at 1670 K. In the first investigation of the system ZnO-TiO2 Dulin and Rase [12] observed Zn2TiO4, probably congruently melting at 1822 K, and ZnTiO3. The previously mentioned Zn2Ti3Os [13] was identified as the low temperature form of ZnTiO3 [14], which decomposes peritectoidally between 1238 and 1283 K. The phase diagram of the system Z n O - Z r O 2 is eutectic. Von Wartenberg and Gurr [15] and Bartuska and Bartrakov [16] found no indications of the formation of compounds. In ZnO-SnO2 the compounds ZnSnO3, which decomposes below the temperature of our study [17], and ZnESnO4 [18] were reported.
2. Experimental details
The samples were made from ZnO (p.a., Riedel de Hahn), TiO2 (chem. pur, Riedel de Hahn), SnO2 (99.9%, Aldrich), ZrO2 (99.5%, Auer-Remy) and Nb205 (ceramic grade, H. C. Starck). Appropriate amounts of these compounds were mixed in an agate mortar. After addition of polyethylene glycol (400 g mol- 1, Chemische
0925-8388/94/$07.00 © 1994 Elsevier Science S.A. All rights reserved SSDI 0925-8388(94)01141-4
76
A: Baumgarte, R. Blachnik I Isothermal sections in ZnO-AO2-Nb205 at 1473 K
Ti02
80
",~
'._60
~xo.°
/
L
20
--i... - - -
"--4:','
I/ l/-. . . . . .
Nb205 Znzr~Nb~/30~20
/.,0 ZnNbzO6 60
Zn:'~08 80
ZnO
mol?/oZn 0 .--~ Fig. 1. The isothermal section ZnO-TiOz--Nb205 at 1473 K. Werke H01s) the mixtures were pressed into pellets under a hydrostatic pressure of 1.5 x 108 Pa. The pellets were fired at 1473 K for 48 h in open platinum crucibles. All products were reground and the organic binder added again. The specimens were then treated with a hydrostatic pressure of 4 X 109 Pa and annealed for 80 h at 1473 K to ensure complete reaction. After the thermal treatment the samples were rapidly cooled to room temperature. Phase analyses and characterizations (diffraction angles and intensities) were performed with the aid of an automatic Stoe powder diffractometer or a Guinier camera (Huber 620), both with Cu Kal radiation (A= 154.06 pm). Step scanning in 1° increments was performed with a 5 ° position sensitive detector over the range 20= 50-70° using 60 s counting time for each step. The lattice parameters were obtained from measurements in a Guinier camera (Huber 621) with Cr Kal radiation (h = 228.96 pm) and refined by the leastsquares method [19]. ct-SiO2served as internal standard. Boundaries of the single-phase regions were determined by extrapolating the composition dependence of the lattice parameters to the invariant values found in the two- or three-phase regions.
3. Results
We performed phase equilibrium studies of the constituent binaries involved, namely ZnO-TiO2, ZnO-ZrO2, ZnO-SnO2, TiO/-Nb2Os, ZrO2-Nb2Os, SnOz-NbzO5 and ZnO-Nb205, with mixtures annealed at 1473 K for 4 days. These samples contained the compound ZnETiO4 in the ZnO-TiO2 binary, the compound Zn2SnO 4 in the ZnO--SnO2 binary, the compounds TizNbloO29, TiNb207 and an unidentified compound at about 15 tool.% TiO2 in the TiO2-Nb205 binary, the compounds ZrNb24062 and Zr6Nb2Oa7 in the ZrO:-Nb205 binary and the compounds Zn3Nb2Os, ZnNb206 and Zn2Nb34087 in the ZnO-Nb205 binary. No compounds were detected by X-ray analyses in the systems ZnO-ZrO2 and SnO2-Nb2Os. The isothermal sections at 1473 K were constructed from the X-ray phase analyses of ternary samples taking into account the phase relations of the constituent binaries. The results are shown in Figs. 1-3. The points give the composition of the samples. The direction and size of the arrows indicate the compounds and their amounts found in these samples. The hatched areas represent single-phase regions. Two-phase regions are indicated by rectangular and three-phase fields by tri-
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb205 at 1473 K
77
Zr,pz Zr .80
(~)or'
I", i
X
X-G°
\
t
,/
\ O
t
-<
NbgO~
Zn21(163~,08726
"'
L6
Zn~lbz06
6C)
Zn3Nb208 80
ZnO
mol.% Zn 0 --~
Fig. 2. The isothermal section ZnO-ZrOE-Nb205 at 1473 K. angular areas. The preliminary tie-lines in the twophase fields were constructed from the X-ray data. In the system ZnO-TiO2-Nb205 seven three-phase equilibria were observed: (1) TiO2 (solid solution, s.s.) + ixiolite (ix.) (S.S.) + Zn2TiO 4 (s.s.) (2) Zn2TiO4(s.s.) + ix. (s.s.) + Zn3Nb208 (s.s.) (3) Zn2Nb308 (s.s.) + ZnzTiO4 (s.s.) + ZnO (4) Zn2Nb308 (s.s.) + ix. (s.s.) + ZnNbzO6 (s.s.) (5) ZnNb206 (s.s.) + ix. (s.s.) + TizNb,oO29 (s.s.) (6) Ti2Nb,oO29 (s.s.) + ix. (s.s.) + TiNb207 (s.s.) (7) TiNb207 (s.s.) + ix. (s.s.) + TiO2 (s.s.). The phase diagram reveals the formation of a line compound TixZnl/3_ (1/3)~Nbz/3_(z/3~O2 (x = 0.73-0.85, ideal composition ZnTiNb2Os) and line-like regions of solid solution based on the futile or columbite structure, all along the section TiOz-ZnNb206. The details of the section TiO2-ZnNb206 have been discussed elsewhere [20]. In the system a solid solution range extends from the compound Zn2TiO4 into the ternary system. Zn2TiO4 exists in two modifications [21]. The a form with tetragonal symmetry is of the inverse spinel type with the lattice parameters a = 600.5(3) pm, c = 841.6(3) pm [22] and ordering of the cations on the B sites. Above 1043 K it transforms into the cubic/3-Zn2TiO4
with complete disorder of the cations. We observed no solid solubility of ZnO in ZnzTiO4, whereas up to 40 mol.% TiO2 can be dissolved in ZnzTiO 4. In this reaction two Zn atoms are substituted by a Ti atom and a vacancy. In the ZnO-TiO2 binary most solid solutions had the cubic structure as expected at 1473 K; in TiO2rich solid solutions partial cation ordering is indicated by the appearance of the a-Zn2TiO4 reflections in the X-ray results. The lattice parameters of the disordered ZnzTiO4 solid solutions at 1473 K are given in Table 1. They decrease with increasing TiO2 content. The homogeneity region of the spinel extends into the ternary ZnO-TiO2-NbzO5 system (Fig. 4). Comparable with the ZnO-TiO2 binary, the zinc content is limited to two Zn atoms per unit formula. Some typical lattice parameters of these solid solutions are given in Table 2. At the highest Nb205 concentrations we observed the tetragonal o/-Zn2TiO 4 structure with cation ordering. We established six three-phase equilibria in the system ZnO-ZrO2-Nb2Os: (1) ZrO2 +wolframite (wo.) (s.s.) + ZnO (2) ZnO + wo. (s.s.) + Zn3Nb208 (3) Zn3Nb208 + wo. (s.s.) + ZnNb206 (4) Z n N b 2 0 6-k wo. (s.s.) -t- ZnzNb34087 (s.s.)
78
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb205 at 1473 K
50.02
\
k .
/
J
+,j?,
Zn0.sSn0.sNbO~.~~
Nb205 Zn2Nb3~Oa7 20
"
40
ZnNb206 60
+--
__~
Zn3N~O8 80
'_20
ZnO
mol.%Zn 0---. Fig. 3. The isothermal section ZnO-SnOz-Nb205 at 1473 K.
TABLE 1. Lattice parameters of the Zn2TiO+ solid solutions in the binary ZnO-TiO2 system TiO2 (mol.%)
a (pm)
29 31
846.94(4) 846.93(3) 846.92(3)
32 32.6 33.3 35
37 39 41 43
\\ 0~S$
846.95(4) 847.00(5) 846.55(6) 845.98(6) 845.27(7) 844.74(7) 844.61(6)
(5) Zn2Nb34Oa7 (s.s.)+ wo. (s.s.)+ Zn6N-b2017 (s.s.) (6) Zr6Nb2017 (s.s.) +wo. (s.s.) + ZrO2. This system contains the line compound ZrxZnl,3_(lm,Nb2t3_(z~,Oz (x--0.67--0.82, ideal composition ZnZrNb2Os). The line shaped homogeneity region of ZnzNb34Os7 was constructed to explain the phase relations observed in the samples. Seven three-phase equilibria were determined in ZnO-SnO2-Nb2Os:
60
70
80
mol.%Zn O--,,Fig. 4. The homogeneity range of the inverse spinel Zn2TiO+ in the ternary ZnO-TiOz-Nb205 system at 1473 K.
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb20 s at 1473 K TABLE 2. Lattice parameters of the Zn2TiO4 solid solutions in
ternary samples Sample composition (tool.%) ZnO
TiO2
Nb205
75 76 71 71.4 62.5
15 19 24 14.3 25.0
10 5 5 14.3 12.5
a (pm)
853.1(2) 852.0(1) 851.4(1) 604.4(2) 604.7(2)
TABLE 3. List of d values, intensities and indices of the ixiolite ZnTiNb2Os and the wolframites ZnZrNb208 and ZnSnNb208 at their stoichiometric composition
C (pm)
846.0(3) 843.6(3)
(1) S n O 2 + w o . + Z n 2 S n O a
(2)
79
Zn2SnO4 + wo. + Z n N b 2 0 6
(3) Z n N b 2 0 6 + Z n 2 S n O 4 + Z n 2 N b 3 0 8 (4) Z n 2 N b 3 O s + Z n 2 S n O 4 + Z n O
Zn2Nb34087 (6) ZneNb3aOs7 + wo. + SnO2 (5) Z n N b 2 0 6 + w o . +
(7) S n O 2 + Zn2Nb34087 + Yb205.
A ternary compound Zno.sSn0.sNbO4 with the wolframite structure was observed. Its homogeneity range was not determined. The three isothermal sections are separated into two subsystems by the quasi-binary sections M O 2 - Z n N b 2 0 6 . On these sections the formation of solid solutions by the reaction 3 M 4 + ~ 2 Z n 2 + + N b s+ is possible, thus allowing the substitution of the M 4+ ion by the two other metals ions without the introduction of defects. This substitution mechanism prevents the solid solutions from extending into the neighbouring ternary space and thus explains their line shape. "ZnTiNbzOs" has at this ideal composition the lattice parameters a=467.46(5) pm, b=566.21(5) pro, c = 501.37(4) pm, Z = 4, orthorhombic, space group Pbcn. The structure is related to the high pressure modification of TiO2 (a-PbO2 structure). The cations are statistically distributed on octahedral cation sites of the tetragonal close-packed O 2layers. In the system ZnO-ZrO2-Nb2Os a similar compound is found on the section Z r O 2 - Z n N b 2 0 6. Its structure is of the wolframite type, which is also derived from the a-PbO2 structure. The cations occupy the octahedral sites of the 0 2layers and are ordered in the sequence (Zn,Zr), Nb, (Zn,Zr), Nb. The stoichiometric composition ZnZrNb2Os has the lattice parameters a=480.48(6) pm, b=568.55(5) pm and c=507.66(4) pm, /3=91.311(8) °, monoclinic, space group P2/c. n~nr~o2us with the wolframite structure is found in the third system. The lattice parameters were determined to be a = 471.98(3) pm, b=571.07(4) pm, c=507.20(2) pm, /3= 90.451(4) °. The X-ray powder diffraction data of the three compounds are given in Table 3. Within our experimental conditions (annealing temperature 1473 K) no other compounds were observed in the ternary systems.
dobs (pm)
dc~l¢ (pro)
Int.
h k l
I/Io
ZnTiNb208 3.6082 2.9285 2.8332 2.5083 2.4670 2.3382 2.2107 2.1818 2.0589 1.8777 1.8034 1.7512 1.7105 1.6969 1.5168 1.5034 1.4642 1.4401
1.4360
1.4164 1.3631
1.3177 1.2893 1.2541 1.2261 1.2024 1.1846
1.1776 1.1692
1.1467 1.1178 1.1052 1.1015 1.0911
1.0841 1.0808 1.0757
1.0597 1.0567 1.0296 1.0196 1.0084 0.9925
0.9810 0.9666
0.9628 0.9457 0.9279 0.9197 0.9166 0.9076
3.6070 2.9285 2.8331 2.5081 2.4668 2.3385 2.2103 2.1819 2.0592 1.8779 1.8035 1.7513 1.7104 1.6971 1.5170 1.5031 1.4642 1.4400 1.4399 1.4359 1.4165 1.3632 1.3179 1.2893 1.2540 1.2262 1.2023 1.1845 1.1777 1.1693 1.1692 1.1467 1.1178 1.1052 1.1014 1.0910 1.0842 1.0808 1.0808 1.0757 1.0597 1.0566 1.0296 1.0196 1.0084 0.9926 0.9811 0.9665 0.9629 0.9457 0.9281 0.9198 0.9167 0.9077 0.9077
27.3 100 8.5 12.4 13.2 4.3 4.1 7.5 7.7 6.4 5.8 11.0 14.1 18.6 12.2 0.7 3.8 14.9 10.5 0.9 3.8 0.6 1.6 1.1 1.6 1.8 1.1 3.3 1.5 1.1 2.4 2.6 0.9 0.7 2.5 2.0 1.3 1.1 1.1 0.9 2.7 1.3 0.8 2.0 2.0 0.7 0.6 0.5 1.1 0.6 2.3
1 10 11 1 0 20 00 2 0 2 1 2 00 10 2 12 1 112 0 22 22 0 130 2 0 2 22 1 1 13 3 10 2 22 0 2 3 3 1 1 132 04 0 0 4 1 32 1 3 12 0 04 2 2 3 33 0 1 14 24 1 4 0 0 33 1 0 24 3 13 2 04 15 0 24 2 3 32 4 20 0 4 3 15 1 4 0 2 4 2 1 224 134 15 2 4 22 24 3 115 3 14 02 5 0 6 1 15 3 3 50 4 2 3 5 1 1
(continued)
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb20~ at 1473 K
80
TABLE 3. (continued)
TABLE 3. (continued)
dobs (pm) 0.9015
dcalc (pm) 0.9017 0.9017
Int.
h k l
I/Io
2.1944 2.1052 2.0709 1.8936 1.8351 1.7638 1.7356 1.7254 1.7163 1.6702 1.6611 1.5474 1.5270 1.4997 1.4845 1.4752 1.4663 1.4542 1.4429 1.3694 1.3051 1.2695 1.2553 1.2337 1.2235 1.2164 1.2017 1.1929 1.1866 1.1592 1.1527 1.1331 1.1274 1.1121 1.1099
5.6875 4.8056 3.7879 3.6707 2.9999 2.9505 2.8438 2.5389 2.4812 2.4028 2.2664 2.2240 2.2148 2.2134 2.1947 2.1054 2.0712 1.8939 1.8354 1.7636 1.7358 1.7256 1.7165 1.6703 1.6616 1.5474 1.5270 1.4999 1.4845 1.4752 1.4664 1.4545 1.4542 1.4428 1.3692 1.3690 1.3052 1.2695 1.2553 1.2336 1.2335 1.2237 1.2236 1.2161 1.2017 1.1933 1.1928 1.1865 1.1592 1.1526 1.1332 1.1275 1.1120 1.1100 1.1099 1.1099
dc~lc (pro) 1.1074 1.0948 1.0887
3 2 2
-2 4 2 3 3 2 04 3
3.7942 3.6403 2.9664 2.9497 2.8567 2.5375 2.4894 2.3616 2.2426 2.2281 2.2057 2.1988 2.0875 2.0758 1.8971 1.8201 1.7663 1.7354 1.7221 1.7165 1.7100 1.5376 1.5306 1.5178 1.4832 1.4825 1.4748 1.4571 1.4556 1.4516 1.4512 1.4477 1.4283 1.3749 1.3332 1.3329 1.3324 1.3209 1.3069 1.2983 1.2687 1.2428 1.2354 1.2150 1.2134 1.1893 1.187 1.808
2 51 100 97 16 26 31 14 4 2 6 5 10 12 14 11 36 15 15 19 21 13 12 2 5
0 11 1 10 -1 1 1 1 1 1 0 20 0 0 2 0 2 1 2 00 -1 0 2 102 -1 2 1 12 1 -1 1 2 1 12 0 22 2 20 130 -2 0 2 2 02 -2 2 1 2 2 1 -1 1 3 1 13 -3 1 0 -2 2 2 -2 3 0 2 22 -3 1 1 0 23 -1 3 2 3 11 132 0 40 04 1 3 0 2 -321 2 13 -1 4 1 -3 1 2 3 12 0 04 -2 2 3 2 23 -3 2 2 3 30 -2 4 1 2 4 1 4 00
0.9
4 4 0 3 5 1
1.1075 1.0948 1.0888
2
0 10
ZnSnNb208
4 3 42 98 100 20 25 23 11 4 2 3
2 2
10 0 0 1 1 -1 1 0 -1 1 1 1 1 1 02 0 0 0 2 0 2 1 2 0 0 -1 0 2 10 2 -1 2 1 2 10 12 1 - 112 112 0 2 2 -2 2 0 13 0 -2 2 1 2 0 2 2 2 1 -1 3 1 13 1 -1 1 3 1 13 -2 2 2 -3 1 1 2 2 2 3 1 1 0 2 3 - 12 3 13 2 04 1 -3 0 2 3 12 0 0 4 -2 2 3 2 2 3 -3 2 2 24 0 3 3 0 13 3 4 0 0 1 14 -2 4 1 24 1 02 4 -313 -024
2
3 13
2 2
204 051 2 3 3 -3 3 2
ZnZrNb20s 5.6863 4.8042 3.7889 3.6699 2.9991 2.9505 2.8432 2.5386 2.4807 2.4023 2.2660 2.2240 2.2146
dobs (pro)
9 6 9 16 15 28 14 14 18 3 2 12 11 4 7 5 9 19 9 8 1 2 1 1 4 1 3 4 4 2
(continued)
3.7928 3.6408 2.9656 2.9493 2.8562 2.5370 2.4887 2.3611 2.2426 2.2282 2.2053 2.1987 2.0874 2.0759 1.8970 1.8201 1.7659 1.7356 1.7220 1.7165 1.7101 1.5375 1.5307 1.5179 1.4834 1.4749 1.4571 1.4557 1.4515 1.4478 1.4284 1.3750 1.3328
1.3210 1.3068 1.2983 1.688 1.2430 1.2354 1.2150 1.2134 1.1893 1.1871 1.1808
Int.
h k l
I/Io
5 10 16 20 13 2 7 1
2 2 3 2 3 2 3 5 5 4 4
Acknowledgments W e w i s h t o e x p r e s s o u r g r a t i t u d e t o S F B 225 o f t h e Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie.
A. Baumgarte, R. Blachnik / Isothermal sections in ZnO-AO2-Nb20s at 1473 K
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