Subsolidus phase relations and dielectric properties in the SrO–Al2O3–Nb2O5 system

International Journal of Inorganic Materials 2 (2000) 107–114

Subsolidus phase relations and dielectric properties in the SrO–Al 2 O 3 –Nb 2 O 5 system a a a, b a Julia Y. Chan , I. Levin , T.A. Vanderah *, R.G. Geyer , R.S. Roth a

National Institute of Standards and Technology, Gaithersburg, MD 20899, USA b National Institute of Standards and Technology, Boulder, CO 80303, USA Received 20 December 1999; accepted 23 December 1999

Abstract Subsolidus phase equilibria in the SrO–Al 2 O 3 –Nb 2 O 5 system were determined by synthesis of 75 compositions in air in the temperature range 1200–16008C. Phase assemblages were determined by X-ray powder diffraction at room temperature. Two new ternary ¯ compounds, Sr 4 AlNbO 8 and Sr 5.7 Al 0.7 Nb 9.3 O 30 , were observed to form in addition to the known double perovskite, Sr 2 AlNbO 6 (Fm3m, ˚ ˚ a57.7791(1) A). Sr 4 AlNbO 8 crystallizes with a monoclinic unit cell (P2 1 /c; a57.1728(2), b55.8024(2), c519.733(1) A; b 5 97.332(3)8) determined by electron diffraction studies; the lattice parameters were refined using X-ray powder diffraction data, which are given. This compound decomposes above 15258C; attempts to grow single crystals from neat partial melts, or using a strontium borate flux, were unsuccessful. The phase Sr 5.7 Al 0.7 Nb 9.3 O 30 (Sr 62x Al 12x Nb 91x O 30 , x50.3) forms with the tetragonal tungsten bronze structure ˚ melts incongruently near 14258C, and occurs essentially as a point compound, with little or no (P4bm; a512.374(1), c53.8785(1) A), range of x-values; indexed X-ray powder diffraction data are given. The tungsten bronze structure exhibits a narrow region of stability in the SrO–Al 2 O 3 –Nb 2 O 5 system, which is probably related to the small size of Al 31 . The existence of an extensive cryolite-type solid solution, Sr 3 (Sr 11x Nb 22x )O 923 / 2x , occurring between Sr 4 Nb 2 O 9 (x50) and Sr 6 Nb 2 O 11 (x50.5), was confirmed, with cubic lattice ˚ respectively. The dielectric properties of the three ternary compounds occurring in the parameters ranging from 8.268(2) to 8.303(1) A, system were measured using the specimen as a TE 011 or TE 0gd dielectric resonator: Sr 2 AlNbO 6 : ´r 525, tf 5 23 ppm / 8C, tan d 51.9310 23 (7.7 GHz); Sr 4 AlNbO 8 : ´r 527, tan d 52.8310 23 (10.5 GHz); Sr 5.7 Al 0.7 Nb 9.3 O 30 : ´r 5168, tan d 53.8310 22 (3.1 GHz). Sr 2 AlNbO 6 , when sintered in 1 atm oxygen, exhibited a reduced permittivity (´r 521) and a significantly improved dielectric loss tangent (tan d 55.2310 24 , 8.3 GHz), resulting in a four-fold increase in Q 3f as compared to the specimen sintered in air. Published by Elsevier Science Ltd. All rights reserved. Keywords: A. ceramics; electronic materials; C. X-ray diffraction; D. dielectric properties; phase equilibria

1. Introduction Dielectric oxide ceramics with high permittivity, low dielectric loss, and near-zero temperature dependence of dielectric properties are critical elements in components such as resonators, oscillators, and filters for wireless communications [1]. Relatively few ceramic systems are currently available with the properties needed for practical applications at various operating frequencies [2]. Niobiumcontaining oxides are known to display a wide range of dielectric properties, and therefore systematic studies of Nb 2 O 5 -based systems may reveal potentially useful materials as well as information needed for the development *Corresponding author. E-mail address: [email protected] (T.A. Vanderah) 1466-6049 / 00 / $ – see front matter PII: S1466-6049( 00 )00014-3

of structure–property correlations. Currently, Al 2 O 3 – Nb 2 O 5 -containing ternary systems are of interest as potential hosts of complex oxides with properties similar to the relatively costly tantalate ceramics (Ba 3 MTa 2 O 9 , M5 Zn, Mg [3–5]) that exhibit uniquely low dielectric losses. Al 2 O 3 –Nb 2 O 5 systems were chosen because the polar51 izability of Ta is apparently intermediate between that of 31 51 1 Al and Nb . The present report describes a systematic study of the subsolidus phase equilibria relations in the SrO–Al 2 O 3 –Nb 2 O 5 system, and the crystal chemistry and 51 ˚ 3 ) is larger Although the polarizability of Ta given in Ref. [6] (4.73 A 3 51 ˚ than that given for Nb (3.97 A ), considerable experimental data suggest that the reverse is true [7,8]. For example, the relative permittivities of isostructural Ba 3 ZnTa 2 O 9 and Ba 3 ZnNb 2 O 9 are |30 [2] and |40 [9], respectively. 1

Published by Elsevier Science Ltd. All rights reserved.

108

J.Y. Chan et al. / International Journal of Inorganic Materials 2 (2000) 107 – 114

dielectric properties of ternary compounds found in this system.

1.1. Overview of the boundary systems The compounds Sr 4 Al 2 O 7 , Sr 3 Al 2 O 6 , Sr 12 Al 14 O 33 , SrAl 2 O 4 , SrAl 4 O 7 , and SrAl 12 O 19 have been reported in various studies of the SrO–Al 2 O 3 system [10–16]. Under the conditions of the present study, Sr 12 Al 14 O 33 , which is unstable above 10508C [16] and probably contains hydroxide, and SrAl 4 O 7 , obtainable only from a liquid formed above 17808C [17,18], are not expected to occur. The crystal chemistries of the three other binary compounds highest in SrO-content (Sr 4 Al 2 O 7 [14,15], Sr 3 Al 2 O 6 [19,20], and SrAl 2 O 4 [21,22]) are dominated by tetrahedral [AlO 4 ] groups arranged in silicate-like motifs. At the lowest SrO-content, in magnetoplumbite-related SrAl 12 O 19 [23], Al 31 exhibits both tetrahedral and octahedral coordination. In several studies of the binary Al 2 O 3 –Nb 2 O 5 system [24–30], compounds have been reported to occur at molar ratios of 1:1, 1:9, 1:11, 1:25, and 1:49, respectively. However, the preponderance of experimental evidence suggests that only three binary compounds form at 1:1, 1:11, and 1:49. The structure of AlNbO 4 is built from distorted NbO 6 and AlO 6 octahedra sharing edges and corners, and linked together to give an infinite threedimensional network [27]. AlNb 11 O 29 [31] and AlNb 49 O 124 [30] are isostructural with their respective analogs Ti 2 Nb 10 O 29 [32] and TiNb 24 O 62 [33]; these compounds are members of homologous series with structures formed by crystallographic shear of ReO 3 -type blocks of metal–oxygen octahedra [34]. In the SrO–Nb 2 O 5 system, six compounds have been reported: Sr 6 Nb 2 O 11 , Sr 4 Nb 2 O 9 , Sr 5 Nb 4 O 15 , Sr 2 Nb 2 O 7 , SrNb 2 O 6 , and Sr 2 Nb 10 O 27 [35–38]. Sr 6 Nb 2 O 11 [39,40] and Sr 4 Nb 2 O 9 [41,42] adopt perovskite-related structures with cryolite-like [43] cation ordering; Sr 4 Nb 2 O 9 is reported to exhibit polymorphism [41,42]. Sr 5 Nb 4 O 15 adopts a structure very similar to that of Ba 5 Ta 4 O 15 [44,45], which features slabs of the perovskite structure cut along the h111j perovskite planes (i.e. n55 member of A n B n21 O 3n series). The structure of Sr 2 Nb 2 O 7 [46,47] is also built from slabs of the perovskite structure, but in this case the slabs are cut parallel to the h110j perovskite planes (i.e. n54 member of the A n B n O 3n12 series) [48]. SrNb 2 O 6 [35] adopts a framework structure similar to that of CaTa 2 O 6 [35] which is built from edge- and vertex-sharing octahedra. Sr 2 Nb 10 O 27 [49] adopts a tungsten bronze-type framework structure [50]. To our knowledge, studies of phase equilibria occurring in the ternary SrO–Al 2 O 3 –Nb 2 O 5 system have not been previously reported. One ternary compound is known to occur, namely Sr 2 AlNbO 6 [51,52], which exhibits a cubic ‘double-perovskite’ unit cell with ‘1:1’ NaCl-type ordering of Al 31 and Nb 51 in the octahedral sites. The physical and

electrical properties of this compound have been well characterized owing to its potential usefulness as a substrate material for high-temperature superconducting cuprate films [52].

2. Experimental methods Specimens were prepared by solid-state reaction of SrCO 3 (99.999%), Al 2 O 3 (0.3 micron, 99.99%) and Nb 2 O 5 (Kawecki, optical grade). Prior to each heat treatment, samples were ground with an agate mortar and pestle for 15 min, pelletized, and placed in an alumina boat on sacrificial powder of the same composition. After an initial overnight calcine at 9508C, multiple 2–4 day heatings (with intermediate grinding and re-pelletizing as above) were carried out in the temperature range 1200– 16008C, near the solidus temperatures (indicated by melting point experiments); samples were furnace-cooled to |7508C and then air-quenched on the bench top. The minimum solidus temperatures observed in the ternary system were 1300–13258C, in compositions above 50 mol% Nb 2 O 5 . Typically, three to six heatings were required to attain equilibrium, which was presumed when no further changes could be detected in the details of X-ray powder diffraction patterns. Quenching, melting point, and crystal growth experiments were carried out in sealed Pt capsules (2.5 mm diameter). Crystal growth was also attempted using a 4SrO:B 2 O 3 flux prepared from H 3 BO 3 and SrCO 3 . Selected samples were heated in flowing oxygen at 1 atm to determine the effect on dielectric properties. Phase assemblages were determined from X-ray powder diffraction data, which were obtained with a Philips 2 diffractometer equipped with incident soller slits, theta compensating slit and a graphite monochromator, and a scintillation detector. Samples were mounted in welled glass slides. Data were collected at ambient temperatures using Cu Ka radiation with a 0.028 2u step size and a 2 s count time; longer scans were taken (0.0158 2u step size, 4 s count time) to obtain data for least-squares refinement of lattice parameters. Intensity data measured as relative peak heights above background were obtained using the Siemens DIFFRAC5000 second derivative peak location program. The observed 2u line positions reported here have been corrected using SRM 660, LaB 6 [53], as an external calibrant. Lattice parameters were refined using the corrected powder diffraction data (2u values, Cu ˚ Ka 1 51.540562 A) with the least-squares program CELLSVD [54] or NBSLSQ [55]. Powder patterns were calculated with the GSAS program [56] when atomic 2

Certain commercial equipment is identified in order to adequately specify the experimental procedure; recommendation or endorsement by the National Institute of Standards and Technology is not therein implied.

J.Y. Chan et al. / International Journal of Inorganic Materials 2 (2000) 107 – 114

positions were available, and were used to assign indices when refining lattice parameters from experimental data. Transmission electron microscopy (TEM) studies were carried out using a Phillips EM-430 electron microscope operated at 200 kV. Specimens were prepared by sectioning, grinding, and polishing sintered pellets, followed by dimpling to a thickness of 30 mm. Thinning was carried out in a Gatan precision ion polishing system (PIPS) at 5 kV until perforation was attained. Dielectric properties were measured using sintered cylindrical pellets with diameters and heights of |8 mm. Microwave measurements were performed using the specimens as TE 011 or TE 0gd dielectric resonators in either a parallel plate waveguide or cylindrical cavity. Density was calculated from the mass and dimensions of the pellets and compared to the crystallographic density to estimate pore volume. Complex permittivity was measured at four temperatures (ambient, 50, 75, and 1008C) in the 3 to 10 (60.0001) GHz range. Permittivity was calculated from the measured frequency of the TE 011 or TE 0gd resonance mode and sample dimensions. Variable-temperature and

109

variable-frequency conductor losses were measured and accounted for in evaluation of dielectric loss tangents. The temperature coefficient of resonant frequency was calculated from a linear regression analysis of the data obtained at different temperatures. Permittivity and dielectric loss tangent values were corrected to theoretical density using two-phase effective-medium formalisms [57]. The uncertainties in the reported permittivity values are approximately 610%, and are dominated by the estimate of pore volume. The estimated uncertainty in measurement of loss tangent is 2310 25 .

3. Results and discussion The subsolidus phase relations found in the SrO– Al 2 O 3 –Nb 2 O 5 system are shown in Fig. 1. The ternary phase assemblages observed were consistent with the binary phase equilibria reported for the SrO–Al 2 O 3 , SrO– Nb 2 O 5 , and Al 2 O 3 –Nb 2 O 5 systems, as summarized previously. In addition to the double-perovskite Sr 2 AlNbO 6

Fig. 1. Subsolidus phase equilibria relations in the SrO–Al 2 O 3 –Nb 2 O 5 system determined in air with synthesis temperatures 1200–16008C. Three ternary compounds were confirmed; A: ‘Sr 4 AlNbO 8 ’, B: Sr 2 AlNbO 6 , and C: Sr 5.7 Al 0.7 Nb 9.3 O 30 (Sr 62x Al 12x Nb 91x O 30 , x50.3). Compounds A and C have not been previously reported. Compounds A and B exhibit close-packed perovskite-related structures, while compound C forms with a framework-type tetragonal tungsten bronze structure.

110

J.Y. Chan et al. / International Journal of Inorganic Materials 2 (2000) 107 – 114

¯ ˚ two new ternary compounds, (Fm3m, a57.7791(1) A), ‘Sr 4 AlNbO 8 ’ and Sr 5.7 Al 0.7 Nb 9.3 O 30 , were observed to form:

3.1. Crystal chemistry Sr 4 AlNbO 8 X-ray powder diffraction data indicated that this compound occurs on the SrO–Sr 2 AlNbO 6 composition line and is unstable above 15258C; the solidus temperature is |15758C at this composition. Attempts to grow crystals of Sr 4 AlNbO 8 from neat melts, or using a 4SrO:B 2 O 3 flux, were unsuccessful. Since a full singlecrystal structure determination was precluded, the composition Sr 4 AlNbO 8 should be considered approximate. The unit cell and symmetry of this compound were

determined by TEM. A set of representative electron diffraction patterns is shown in Fig. 2, and can be indexed according to a monoclinic unit cell with a5œ3a c , b5 œ2a c , c52œ6a c , b 5978, where a c denotes the lattice ˚ parameter for an ideal cubic perovskite (|4 A). The reflection conditions (00l): l52n, (h0l): l52n, and (0k0): k52n observed in the electron diffraction patterns are consistent with space group P2 1 /c (No. 14). All observed peaks in the X-ray powder diffraction pattern of Sr 4 AlNbO 8 could be indexed on the basis of this unit cell; the data and results of least-squares refinement of the lattice parameters are collected in Table 1. Sr 5.7 Al 0.7 Nb 9.3 O 30 X-ray powder diffraction data indicated that this compound forms with the tetragonal tung-

Fig. 2. A set of selected area diffraction patterns for the new compound Sr 4 AlNbO 8 . The reflections are indexed according to a monoclinic unit cell with ˚ The approximate angles between a5œ3a c , b5œ2a c , c52œ6a c , and b 5978, where a c denotes the lattice parameter for an ideal cubic perovskite (|4 A). different sections of reciprocal space are indicated. The reflection conditions observed are consistent with space group P2 1 /c (No. 14).

J.Y. Chan et al. / International Journal of Inorganic Materials 2 (2000) 107 – 114

Table 1 X-ray powder diffraction data for Sr 4 AlNbO 8 ; P2 1 /c; a57.1728(2), ˚ b 597.332(3)8 b55.8024(2), c519.733(1) A; h

k

l

0 1 21 1 1 21 0 21 0 2 1 21 21 22 2 2 0 0 2 21 22 1 22 21 23 1 2 2 22 3 0 22 21 22 1 21 22 22 2 22 3 2 23 24 1 22 1 24 24 24 0 2 22 0 0 4 0 21 1 3 22 4 0 0

0 0 0 0 1 1 1 1 0 0 1 0 1 0 0 1 2 1 1 1 1 2 0 0 0 2 0 2 2 1 2 0 2 2 1 1 1 2 0 0 0 2 2 0 1 1 3 1 1 1 3 3 3 0 2 0 3 1 2 2 1 2 4 2

2 0 2 2 0 3 4 4 6 2 4 6 5 4 210 22 0 6 2 6 4 0 6 8 2 4 6 0 2 0 6 8 6 4 8 9 8 6 8 10 6 6 4 4 10 10 4 2 0 4 6 2 4 12 10 4 7 12 10 6 12 2 0 12

2uobs

Iobs

2ucalc

9.025 12.433 14.402 16.320 19.727 23.065 23.777 25.763 27.335 27.775 28.014 28.559 28.962 29.036 29.441 29.843 30.792 31.450 31.841 32.530 32.952 33.325 34.783 37.269 37.830 39.012 39.843 40.083 40.394 41.089 41.631 41.980 42.496 42.832 43.536 44.748 44.917 47.138 47.783 50.156 50.427 51.191 51.319 52.363 52.493 52.734 53.076 53.509 53.893 54.886 55.174 55.415 56.146 56.390 56.705 57.376 57.933 58.556 59.787 60.259 61.441 63.040 64.147 65.642

6 1 1 4 8 2 6 5 1 1 15 11 16 53 46 7 76 100 84 15 16 1 12 8 8 10 27 4 4 10 4 5 18 64 12 3 5 24 22 6 7 15 6 5 9 10 3 21 8 18 32 26 6 8 3 4 16 8 12 6 7 20 12 16

9.033 12.437 14.398 16.328 19.731 23.063 23.774 25.763 27.330 27.772 28.024 28.562 28.964 29.030 29.438 29.841 30.794 31.446 31.836 32.536 32.952 33.327 34.787 37.256 37.835 39.012 39.848 40.080 40.389 41.101 41.635 41.980 42.498 42.829 43.531 44.747 44.916 47.133 47.778 50.157 50.429 51.189 51.320 52.366 52.488 52.739 53.074 53.507 53.894 54.875 55.169 55.418 56.141 56.391 56.705 57.369 57.937 58.562 59.789 60.262 61.449 63.045 64.148 65.636

D2u 0.008 0.004 20.004 0.008 0.004 20.002 20.003 0.000 20.005 20.003 0.010 0.003 0.002 20.006 20.003 20.002 0.002 20.004 20.005 0.006 0.000 0.002 0.004 20.013 0.005 0.000 0.005 20.003 20.005 0.012 0.004 0.000 0.002 20.003 20.005 20.001 20.001 20.005 20.005 0.001 0.002 20.002 0.001 0.003 20.005 0.005 20.002 20.002 0.001 20.011 20.005 0.003 20.005 0.001 0.000 20.007 0.004 0.006 0.002 0.003 0.008 0.005 0.001 20.006

d obs 9.7907 7.1136 6.1452 5.4270 4.4967 3.8530 3.7392 3.4553 3.2600 3.2094 3.1825 3.1230 3.0805 3.0728 3.0314 2.9915 2.9014 2.8422 2.8082 2.7503 2.7160 2.6865 2.5771 2.4107 2.3763 2.3069 2.2607 2.2477 2.2311 2.1950 2.1677 2.1398 2.1255 2.1096 2.0771 2.0236 2.0164 1.9265 1.9019 1.8174 1.8082 1.7830 1.7789 1.7459 1.7418 1.7344 1.7241 1.7111 1.6998 1.6714 1.6634 1.6567 1.6369 1.6304 1.6220 1.6047 1.5905 1.5751 1.5456 1.5346 1.5079 1.4734 1.4506 1.4212

111

sten bronze (TTB) structure, analogous to Ba 3 TiNb 4 O 15 [50], and thus can be represented by the general formula Sr 62x Al 12x Nb 91x O 30 , with x50.3. However, in contrast to Ba 3 TiNb 4 O 15 , which forms the solid solution Ba 62x Ti 222x Nb 812x O 30 , with 0,x,0.5 [58,59], essentially no range in x-values was observed for Sr 62x Al 12x Nb 91x O 30 . The TTB structure exhibits limited stability in the SrO–Al 2 O 3 –Nb 2 O 5 system, both in composition and temperature: the phase was not observed to form below 13758C, was difficult to purify despite numerous re-heatings, and melted incongruently at 14258C. Indexed X-ray powder diffraction data and refined lattice parameters for TTB-type Sr 5.7 Al 0.7 Nb 9.3 O 30 are given in Table 2. The tetragonal tungsten bronze-type structure is illustrated in Fig. 3. In the SrO-rich part of the SrO–Nb 2 O 5 diagram, as shown in Fig. 1, the solid solution Sr 3 (Sr 11x Nb 22x )O 923 / 2x was observed to extend from Sr 4 Nb 2 O 9 (x50) to Sr 6 Nb 2 O 11 (x50.5), which is in agreement with previous reports [39,60]. Samples that were furnace-cooled from 15008C exhibited X-ray powder patterns that could be completely indexed with a cryolite-type, F-centered cubic ˚ for x50 (Sr 4 Nb 2 O 9 ) to a5 unit cell (a58.268(2) A ˚ for x50.5 (Sr 6 Nb 2 O 11 )). The cryolite structure 8.303(1) A is a superstructure of perovskite [61] with 1:1-type (NaCl) ordering on the B-cation sites; in the Sr 3 (Sr 11x Nb 22x )O 923 / 2x solid solution, the structure apparently forms with up to 8.3% oxygen vacancies at the limiting composition of x50.5 (Sr 6 Nb 2 O 11 ). X-ray powder diffraction data clearly indicated dissolution of Al 31 in the Sr 3 (Sr 11x Nb 22x )O 923 / 2x solid solution, up to about 1 mol% Al 2 O 3 , as indicated in Fig. 1. Sr 4 Nb 2 O 9 exhibited polymorphism upon annealing at temperatures below 12508C, as observed by others [41]. A more detailed study of the structural behavior and properties of Sr 4 Nb 2 O 9 is in progress and will be reported elsewhere. In the present study, superstructure peaks observed in the X-ray diffraction pattern for tungsten bronze-type Sr 2 Nb 10 O 27 could be indexed by tripling the b-dimension ˚ all data could be of the parent TTB cell (a|12, c|4 A); accounted for with an orthorhombic unit cell a5 ˚ This result 12.394(2), b536.869(8), and c53.9484(7) A. is slightly different from the unit cell reported previously [49] with tripling of, and hence superstructure along, both the a- and b-dimensions. Sr 2 Nb 10 O 27 was observed to dissolve |2 mol% Nb 2 O 5 , as indicated in Fig. 1, according to X-ray diffraction analysis. Crystals of Sr 2 Nb 10 O 27 were obtained by slow cooling of the melt, and a crystal structure refinement is in progress.

3.2. Dielectric properties Dielectric properties obtained for the ternary compounds

J.Y. Chan et al. / International Journal of Inorganic Materials 2 (2000) 107 – 114

112

Table 2 X-ray powder diffraction data for tetragonal tungsten bronze-type Sr 5.7 Al 0.7 Nb 9.3 O 30 (Sr 62x Al 12x Nb 91x O 30 , x50.3). P4bm; a512.3648(6), ˚ c53.8751(3) A h

k

l

2uobs

Iobs

2ucalc

1 2 3 0 1 3 2 2 4 2 4 3 3 4 4 5 5 6 6 6 0 4 5 6 6 5 6 3 5 3 6 4 4 7 3 5 4 7 8 7 8 5 6 8 5 7 8 8 6 8 7

1 0 1 0 1 2 0 1 1 2 2 1 2 0 1 2 3 0 1 2 0 4 3 3 0 5 2 1 4 2 3 0 1 3 3 5 2 2 0 4 2 2 6 3 3 4 2 4 2 3 5

0 0 0 1 1 0 1 1 0 1 0 1 1 1 1 0 0 0 0 0 2 1 1 0 1 0 1 2 1 2 1 2 2 0 2 1 2 1 0 0 0 2 0 0 2 1 1 0 2 1 1

10.087 14.305 22.699 22.908 25.099 25.941 27.119 28.073 29.739 30.782 32.328 32.472 34.874 37.141 37.876 39.172 42.565 43.876 44.499 46.365 46.809 47.693 48.881 49.361 50.051 52.231 52.333 52.633 52.903 54.283 55.084 55.917 56.446 56.592 56.987 57.750 58.031 59.313 59.737 60.259 61.775 62.632 63.772 64.282 65.108 65.334 66.788 67.667 68.011 69.184 69.656

1 1 12 34 2 47 2 61 100 26 71 91 29 5 1 10 21 3 3 28 65 3 5 30 21 46 29 4 31 16 17 5 38 4 4 66 30 3 1 4 17 4 7 14 11 3 2 5 13 13 2

10.101 14.304 22.706 22.911 25.094 25.941 27.113 28.072 29.745 30.783 32.329 32.477 34.880 37.146 37.875 39.174 42.567 43.864 44.501 46.371 46.808 47.693 48.883 49.366 50.052 52.231 52.331 52.631 52.890 54.290 55.086 55.914 56.448 56.601 56.980 57.752 58.033 59.314 59.737 60.250 61.773 62.633 63.771 64.265 65.107 65.334 66.791 67.669 68.011 69.186 69.661

D2u 0.014 20.001 0.007 0.003 20.005 0.000 20.006 20.001 0.006 0.001 0.001 0.005 0.006 0.005 20.001 0.002 0.002 20.012 0.002 0.006 20.001 0.000 0.002 0.005 0.001 0.000 20.002 20.002 20.013 0.007 0.002 20.003 0.002 0.009 20.007 0.002 0.002 0.001 0.000 20.009 20.002 0.001 20.001 20.017 20.001 0.000 0.003 0.002 0.000 0.002 0.005

d obs 8.7622 6.1866 3.9143 3.8790 3.5451 3.4320 3.2855 3.1760 3.0017 2.9024 2.7670 2.7551 2.5706 2.4187 2.3735 2.2979 2.1222 2.0618 2.0344 1.9568 1.9392 1.9053 1.8618 1.8448 1.8209 1.7500 1.7468 1.7375 1.7293 1.6886 1.6659 1.6430 1.6289 1.6250 1.6147 1.5952 1.5881 1.5568 1.5468 1.5346 1.5005 1.4820 1.4583 1.4479 1.4315 1.4271 1.3995 1.3835 1.3773 1.3568 1.3488

found in the SrO–Al 2 O 3 –Nb 2 O 5 system are given in Table 3. The relative permittivity of 25 obtained for a Sr 2 AlNbO 6 pellet sintered in air is significantly higher than the value of 21 obtained for a pellet that was sintered in 100% oxygen. The dielectric loss tangent was also significantly decreased by oxygen treatment, resulting in a

Fig. 3. View along the c-axis of the tetragonal tungsten bronze (TTB) structure (after Stephenson [50]) of Sr 5.7 Al 0.7 Nb 9.3 O 30 (Sr 62x Al 12x Nb 91x O 30 , x50.3). Large spheres indicate the Sr channel sites with an occupancy factor of 0.95, oxygens are represented as small spheres at octahedral vertices; Al and Nb are mixed within the octahedra. In this framework structure, metal–oxygen octahedra share vertices to create four- and five-fold channels that accommodate the larger A-cations. At x50, these channels are filled; however, the structure is often observed with a large range of x-values in other chemical systems. In the SrO–Al 2 O 3 –Nb 2 O 5 system, in contrast, a narrow stability region is observed – the TTB structure forms with difficulty only at x50.3, in the temperature region 1375–14258C.

four-fold increase in the Q 3f product, as seen in Table 3. The reasons for the differences in dielectric properties are not clear; oxygen treatment did not cause observable changes in sample color. The properties obtained for oxygen-treated Sr 2 AlNbO 6 are comparable to those reported elsewhere for this phase at comparable measuring frequencies (´r 518; tan d 59310 24 (6.1 GHz) [52]). As seen in Table 3, the new compound Sr 4 AlNbO 8 exhibits dielectric properties that are similar to those of (air-sintered) Sr 2 AlNbO 6 . In contrast, tungsten bronze-type Sr 5.7 Al 0.7 Nb 9.3 O 30 exhibited a high relative permittivity of 168, which is consistent with the frequent observation of ferroelectricity in compounds with this structure type.

4. Conclusions Subsolidus phase equilibria in the SrO–Al 2 O 3 –Nb 2 O 5 system have been determined by synthesis of 75 compositions between 1200 and 16008C in air. Three ternary compounds were observed; i.e. the known double-perovskite Sr 2 AlNbO 6 , and two new phases Sr 4 AlNbO 8 and Sr 5.7 Al 0.7 Nb 9.3 O 30 . Sr 4 AlNbO 8 is unstable above 15258C and forms with a monoclinic unit cell (P2 1 /c; a5 ˚ b 597.332(3)8) 7.1728(2), b55.8024(2), c519.733(1) A; elucidated by electron diffraction and refined using X-ray

J.Y. Chan et al. / International Journal of Inorganic Materials 2 (2000) 107 – 114

113

Table 3 Dielectric properties of ternary compounds found in the SrO–Al 2 O 3 –Nb 2 O 5 system Sample Sr 2 AlNbO 6 (air) Sr 2 AlNbO 6 (O 2 ) Sr 4 AlNbO 8 c (air) Sr 5.7 Al 0.7 Nb 9.3 O 30 d (Sr 62x Al 12x Nb 91x O 30 , x50.3) (air)

% Densification

Sintering temp.a (8C)

99

16008

94

15508

85 80

´r obs

Q 3f b (GHz)

´r corr

tan d

25

25

1.9310 23

4100

23

8.3

19

21

5.2310 24

16,000

25

15258

10.5

22

27

2.8310 23

3700

13758

3.1

118

168

3.8310 22

80

f (GHz) 7.7

tf (61 ppm / 8C)

a

Samples were cooled by turning the furnace off. Where Q51 / tan d. c Sample contains trace amounts (by XRD) of cryolite-like Sr 3 (Sr 11x Nb 22x )O 923 / 2x ss and Sr 2 AlNbO 6 . d Sample contains trace amounts (by XRD) of Sr 2 Nb 2 O 7 and SrNb 2 O 6 . b

powder diffraction data; attempts to grow single crystals of this phase for complete structure determination were unsuccessful. Sr 5.7 Al 0.7 Nb 9.3 O 30 (Sr 62x Al 12x Nb 91x O 30 , x50.3) forms with the tetragonal tungsten bronze structure ˚ (P4bm; a512.374(1), c53.8785(1) A), melts incongruently near 14258C, and occurs as essentially a point compound, with little or no range of x-values. Difficulties encountered preparing this compound and its limited composition region indicate that the tungsten bronze structure has a narrow stability region in this chemical system, probably resulting from the relatively small size of Al 31 . The existence of an extensive cryolite-like solid solution, Sr 3 (Sr 11x Nb 22x )O 923 / 2x , occurring between Sr 4 Nb 2 O 9 (x50) and Sr 6 Nb 2 O 11 (x50.5), was confirmed, with a cubic lattice parameter ranging from 8.268(2) to ˚ respectively. Dielectric properties were mea8.303(1) A, sured for the three ternary compounds at frequencies between 3 and 10 GHz. Sr 2 AlNbO 6 and Sr 4 AlNbO 8 exhibited relative permittivities of 25 and 27, respectively, in contrast to tetragonal tungsten bronze-related Sr 5.7 Al 0.7 Nb 9.3 O 30 (Sr 62x Al 12x Nb 91x O 30 , x50.3) with a permittivity of 168. Sintering in 100% oxygen resulted in a |15% reduction of the permittivity of Sr 2 AlNbO 6 , a decrease in the dielectric loss tangent of nearly an order of magnitude, and a four-fold increase in the Q 3f product.

References

[9] [10]

[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

[1] Wersing W. Curr Opin Solid State Mater Sci 1996;1:715–31. [2] Davies PK. In: Negas T, Ling H, editors, Materials and processes for wireless communications, vol. 53, Westerville: American Ceramic Society, 1995, pp. 137–51. [3] Kawashima S, Nishida M, Ueda I, Ouchi H. J Am Ceram Soc 1983;66:421–3. [4] Desu SB, O’Bryan JM. J Am Ceram Soc 1983;68:546–51. [5] Chai L, Davies PK. J Am Ceram Soc 1997;80:3193. [6] Shannon RD. J Appl Phys 1993;73:348–66. [7] Colla EL, Setter N. Mater Res Soc Symp Proc 1997;453:437–42. [8] Geselbracht MJ. Probing relative acidities in solid-acid-layered

[30] [31] [32] [33] [34]

[35] [36]

perovskites. Contribution 269 at the 54th Northwest Regional Meeting of the American Chemical Society, Portland, OR, June 22, 1999. Endo K, Fujimoto K, Murakawa K. J Am Ceram Soc 1987;70:C215–8. Phase Equilibria Diagrams (formerly Phase Diagrams for Ceramists). The American Ceramic Society Figs. 1:294, 1:295, 2:2349, 3:4362, 3:4379, 4:5191, 6:6427, 6:6453, 92-006. Lagerqvist VK, Wallmark S, Westgren A. Z Anorg Allg Chem 1937;234:1. Carlson ET. J Res Natl Bur Stand 1955;54:329–34. Massazza F. Chim Indus 1959;41:108–15. Appendino P. Rev Int Hautes Temper Refract 1972;9:297–302. Ganits F, Chemekova TY, Udalof YP. Russ J Inorg Chem 1979;24:260–3. Yamaguchi O, Narai A, Shimizu K. J Am Ceram Soc 1986;69:C36– 7. Boyko ER, Wisnyi LG. Acta Crystallogr 1958;11:444–5. Lindop AJ, Goodwin DW. Acta Crystallogr 1972;B28:2625–6. Swanson HE, McMurdie HF, Morris MC, Evans EH, Paretzkin B. Natl Bur Stand Monogr 1972;25:52–3. Mondal P, Jeffery JW. Acta Crystallogr 1975;B31:689–97. Morris MC, McMurdie HF, Evans EH, Paretzkin B, Parker HS, Pyrros NP. Natl Bur Stand Monogr 1984;25:91. Horkner W, Muller-Buschbaum H. J Inorg Nucl Chem 1976;38:983– 4. Wagner TR. J Solid State Chem 1998;136:120–4. Yamaguchi O, Tomihisa D, Shimizu K. Z Anorg Allg Chem 1989;569:177–82. Muller EK, Nicholson BJ. J Am Ceram Soc 1962;45:250–1. Goldschmidt HJ. Metallurgia 1960;62:211–8. Pedersen BF. Acta Chem Scand 1962;16:421–30. Godina NA, Isuprova EN, Keller EK. Izv Akad Nauk SSSR Neorg Mater 1970;6:1465. Trunov VK, Kovba LM, Pol’shchilova ZY. Zh Neorg Khim 1968;13:1494. Toropov NA, Fedrov IF, Andreev IF, Kasparyan RM. Izv Akad Nauk SSSR Neorg Mater 1971;7:643. Norin R. Acta Chem Scand 1969;23:1210–4. Wadsley AD. Acta Crystallogr 1961;14:664–70. Roth RS, Wadsley AD, Gatehouse BM. Naturwiss 1964;51:262. Wadsley AD, Andersson S. In: Dunitz JD, Ibers JA, editors, Perspectives in structural chemistry, vol. III, New York: John Wiley, 1970, pp. 1–58. Whiston CD, Smith AJ. Acta Crystallogr 1967;23:82–4. Carruthers JR, Grasso M. J Electrochem Soc 1970;117:1426–30.

114

J.Y. Chan et al. / International Journal of Inorganic Materials 2 (2000) 107 – 114

[37] Leshchenko PP, Shevchenko AV, Lykova LN, Kovba LM, Ippolitova EA. Inorg Mater 1982;18:1013. [38] Averkova OE, Evdokimov AA, Sirotinkin VP. Russ J Inorg Chem 1987;32:1183–4. [39] Lecomte J, Loup JP, Hervieu M, Raveau B. Phys Status Solidi (a) 1981;65:743–52. [40] Leshchenko PP, Lykova LN, Kovba LM, Ippolitova EA. Russ J Inorg Chem 1982;27:721–3. [41] Hervieu M, Raveau B, Lecomte J, Loup JP. Rev Chim Min 1985;22:44–57. [42] Hervieu M, Raveau B, Lecomte J, Loup JP. Chem Scripta 1985;25:206–11. [43] Steward EG, Rooksby HP. Acta Crystallogr 1953;6:49–52. [44] Galasso F, Katz L. Acta Crystallogr 1961;14:647–50. [45] Roth RS, Waring JL. J Res Natl Bur Std A Phys Chem 1961;65A:337–44. [46] Ishizawa N, Marumo F, Kawamura T, Kimura M. Acta Crystallogr 1975;B31:1912–5. [47] Scheunemann K, Mueller-Buschbaum HK. J Inorg Nucl Chem 1975;37:1679. [48] Levin I, Bendersky LA, Vanderah TA, Roth RS, Stafsudd OM. Mater Res Bull 1998;33:501–17. [49] Leshchenko PP, Kalinina ON, Lykova LN, Kovba LM. Russ J Inorg Chem 1980;25:1101–2.

[50] Stephenson NC. Acta Crystallogr 1965;18:496–501. [51] Goodenough JB, Longo JM. Landolt Bornstein 1970;4a:126. [52] Guo R, Bhalla AS, Sheen J, Ainger FW, Erdei S, Subbarao EC, Cross LE. J Mater Res 1995;10:18–25. [53] Hubbard CR, Zhang Y, McKenzie RL, editors, Certificate of analysis, S.R.M. 660, Gaithersburg, M.D: National Institute of Standards and Technology, 1989, p. 20899. [54] Program CELLSVD by Lowe-Ma. Naval Air Warfare Center Weapons Division Technical Publication 8128, September 1993. [55] Hubbard CR, Lederman S, Pyrros NP. A version of the geological survey lattice parameter least-squares program, National Bureau of Standards, 1985, Compiled for PC. [56] Larson AC, Von Dreele RB. GSAS, General Structure Analysis System, PC-GSAS, 1998. [57] Geyer RG, Baker-Jarvis J. Effective medium theory for ferriteloaded materials. NIST technical note 1371, 1994. [58] Feltz VA, Langbein HL. Z Anorg Allg Chem 1976;425:47–56. [59] Millet JM, Roth RS, Ettlinger LD, Parker HS. J Solid State Chem 1987;67:259–70. [60] Animitsa IE, Averkova OE, Neiman AY, Sirotinkin VP. Inorg Mater 1988;24:701–4. [61] Wells AF, editor, Structural inorganic chemistry, 4th edition, Oxford: Clarendon Press, 1975, p. 389.