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Synthesis, structure and magnetic properties of a new hollandite: Sr0.75Rh4O8
Journal of Alloys and Compounds 314 (2001) 56–61
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Synthesis, structure and magnetic properties of a new hollandite: Sr 0.75 Rh 4 O 8 J.R. Plaisier*, A.A.C. van Vliet, D.J.W. IJdo Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Received 12 July 2000; accepted 25 July 2000
Abstract The synthesis, structure and magnetic properties of a new synthetic hollandite, Srx Rh 4 O 8 (x¯0.75) are discussed. The compound ˚ b53.0626(1) A, ˚ c59.2135(5) A, ˚ b 595.262(3)8, Z52. Electron crystallizes in the monoclinic space group I2 /m, a510.4399(5) A, diffraction patterns show satellites, indicative of an incommensurate distribution of the strontium cations in the channels. The modulation vector was found to be (20.5, |0.25, 0.5). The compound shows Pauli-paramagnetic behavior between 5 and 300 K. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Oxide materials; Chemical synthesis; Crystal structure; Magnetic measurements
1. Introduction Compared to the oxide chemistry of ruthenium and iridium, little is known about the rhodates. This is related to the fact that rhodium is usually present as diamagnetic Rh 31 , which is not expected to show any interesting electronic properties. Recently, the first magnetically ordered Rh 41 oxides have been reported in the strontium rich part of the Sr–Rh–O system [1,2]. In 1997 the phase relations in the Sr–Rh–O system were reported by Horyn et al. [3]. In the rhodium-rich part of this system they found two compounds, which they identified as SrRh 2 O 4 and SrRh 3 O 5.5 . The structure of the high temperature form of the first compound was recently determined by Hector et al. [4]. It consists of layers of RhO 6 octahedra, with the strontium cations statistically distributed over the interlayer-sites. However on examining the X-ray diffraction pattern of the second compound, suspicion arose about the composition of this phase. Clearly, the peaks found for SrRh 2 O 4 were also present in the diffraction pattern of ‘SrRh 3 O 5.5 ’ [3]. Close inspection of the remaining reflections revealed a close similarity to the diffraction patterns, which are obtained from compounds having the hollandite structure. A strontium rhodium hollandite, such as IV Sr x Rh 422x Rh III 2x O 8 , might have interesting electronic and *Corresponding author. E-mail address: [email protected] (J.R. Plaisier).
electrocatalytic properties, because of the high concentration of rhodium ions and the fact that these ions are of mixed valency. In the Ba–Rh–O system Siegrist et al. [5] reported already the existence of another rhodium based hollandite, Ba 0.86 Rh 4 O 8 . This compound has an unusually high value of x. Furthermore, the monoclinic distortion for this compound is large compared to other hollandites ( b 5 94.48). In the scheme of a larger project, dealing with the electrocatalytic activities of various noble metal oxides, Srx Rh 4 O 8 was studied. In this paper the structure determination and magnetic properties of this hollandite are presented.
2. Experimental Since the exact stoichiometry of the target compound was unknown, several samples were prepared by mixing stoichiometric amounts of chemically pure SrCO 3 and Rh metal corresponding to x50.67, 0.70, 0.75 and 0.80. The samples were heated in an Al 2 O 3 crucible at 8508C in air for 1 day and subsequently for 160 h at 10508C under flowing oxygen. This heating treatment resulted in black powders in all cases. The reaction products were examined with a Philips PW 1050 X-ray diffractometer using monochromated CuKa radiation. The data were collected digitally in steps of 0.02
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01208-1
J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
degrees 2Q and 14 s counting time in the range: 108# 2Q #908. All data were collected at room temperature. Since no single crystals were available, Rietveld’s method [6] was used to analyze the diffraction patterns. For this purpose the GSAS suite of programs [7] was used. A scale factor, six background parameters, the unit cell parameters, a zeropoint correction, the atomic fractional coordinates, four profile parameters, defining the pseudoVoigt shape of the reflections, one asymmetry parameter and one overall thermal parameter were refined in the final refinement cycles. As trial models the structural parameters of Ba 0.86 Rh 4 O 8 [4] and Ba x Sn 422x Cr 2x O 8 [8] were used. Electron diffraction was performed with the Siemens Elmiskop 102 electron microscope and with the Philips EM420 transmission electron microscope. Magnetic measurements were carried out in the temperature range of 5–300 K by means of a Quantum design MPMS-5S Squid on approximately |0.2 g of randomly oriented powder of the Sr 0.75 Rh 4 O 8 sample. The magnetic susceptibility was measured as a function of temperature in a magnetic field of 0.1 T.
3. Results The X-ray diffraction patterns of all samples clearly showed a large resemblance to those of hollandite-type phases. The patterns did not change upon longer heating at the same temperature. However, heating at 11008C resulted in dissociation of the products into b-SrRh 2 O 4 and Rh metal in all samples. Refinement of the structures of these compounds went smoothly and yielded reasonably good agreement factors. After these inititial refinements some samples were found to contain a small amount of Rh 2 O 3 , which was added as a second phase in the refinements. In the case of ‘Sr 0.75 Rh 4 O 8 ’ the phase fraction of Rh 2 O 3 refined to a negligible value, and no change in the agreement factors was observed upon removal of this second phase from the refinement. In the strontium sample for x50.80 a small peak was observed at 2Q ¯15.58, which might be ascribed to b-SrRh 2 O 4 . This sample did not show any traces of Rh 2 O 3 . Refinements using the structural model of Ba x Sn 422x Cr 2x O 8 were found to yield better results than those using the structure of Ba 0.86 Rh 4 O 8 as a trial model. The difference between these models is the positioning of the A cation, which is statistically distributed on the 2(a) position (0,0,0) in the latter case and on the 4(g) (0,1 / 2 y,0) position in the former one. The results of the refinement of ‘Sr 0.75 Rh 4 O 8 ’ are presented in Table 1. The atomic positional parameters are given in Table 2. Table 3 shows the relevant interatomic distances. The observed and calculated diffraction patterns of ‘Sr 0.75 Rh 4 O 8 ’ are shown in Fig. 1. The electron diffraction patterns of the [1] zone, which
57
Table 1 Results of the Rietveld refinement of Sr 0.75 Rh 4 O 8 R wp Rp x2 Nvar x
17.55 12.63 4.27 33 0.75
˚ (A) ˚ (A) ˚ (A) (8) ˚ 3) (A
a b c b V
10.4402(5) 3.0646(1) 9.2137(5) 95.263(3) 293.55(3)
Table 2 Atomic parameters of Sr 0.75 Rh 4 O 8
Sr Rh(1) Rh(2) O(1) O(2) O(3) O(4)
(4g) (4i) (4i) (4i) (4i) (4i) (4i)
x
y
z
Fraction
0 0.3383(2) 0.8622(2) 0.305(1) 0.042(1) 0.623(1) 0.657(2)
0.205(2) 0 0 0 0 0 0
0 0.1604(3) 0.3260(2) 0.387(2) 0.272(2) 0.034(2) 0.331(2)
0.375 1.0 1.0 1.0 1.0 1.0 1.0
are usually shown when discussing the structure of hollandite-type structures, did not show any satellite reflections. However, satellites were observed in the diffraction patterns of other zones. The patterns of the [1021] and the [0221] zones are shown in Fig. 2. The satellite reflections could be indexed using a modulation vector of (20.5, |0.25, 0.5). Fig. 3 shows an indexed schematic representation of these zones. The molar magnetic susceptibility of ‘Sr 0.75 Rh 4 O 8 ’ is given in Fig. 4. The susceptibility remains constant at |9?10 24 emu / mole over nearly the complete temperature range. At temperatures below 30 K a very small increase of the susceptibility is observed with decreasing temperature.
4. Discussion The results described above clearly indicate that the compound described by Horyn et al. [3] is in fact a mixture of SrRh 2 O 4 and a strontium rhodium hollandite. It was possible to prepare the latter at atmospheric pressure. The previously reported barium rhodium hollandite single crystal had been obtained from a mixture of phases, which was produced by a high pressure reaction. Hollandites can be described as oxides of composition A IIx M IV 42x N y O 8 [8]. The M and N cations are octahedrally coordinated. The structure consists of corner sharing Table 3 ˚ for Sr 0.75 Rh 4 O 8 Selected atomic distances (A) Rh(1)–O(1) Rh(1)–O(1) Rh(1)–O(2) Rh(1)–O(3) Sr–O(1) Sr–O(2) Sr–O(4)
(1x) (2x) (2x) (1x) (2x) (2x) (2x)
2.14(2) 2.16(1) 2.04(1) 1.87(2) 2.38(1) 2.59(2) 2.53(2)
Rh(2)–O(2) Rh(2)–O(3) Rh(2)–O(4) Rh(2)–O(4)
(1x) (2x) (2x) (1x)
1.99(1) 2.00(1) 2.10(1) 2.15(2)
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J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
Fig. 1. Observed (crosses) and calculated (line) X-ray diffraction pattern of Sr 0.75 Rh 4 O 8 ; the difference (Iobs 2Icalc ) curve at the bottom of the figure. Tick marks indicate the position of the Bragg reflections included in the calculation. The inset shows an enlarged portion of the patterns including the indexing of the satellites.
double chains of edge sharing octahedra, thus forming square tunnels, which are occupied by large A ions. The difference between the charges of M and N determines the amount (x) of A, necessary to obtain electroneutrality [9]. Usually, the repetition unit of the A cations within the tunnels is different from that of the surrounding lattice, leading to an incommensurate superstructure (see e.g. [10– 12]). Generally, the ordering of the A-cations in a given channel is not strongly correlated to the ordering in any other channel. Any correlation depends on the nature of the
ions in the octahedral framework. The structure is tetragon˚ and an a-axis of about al with a short c-axis of about 3 A ˚ The tunnels are parallel to the short axis. In the case 10 A. that the space for the A ions is too big, the structure becomes monoclinic with space group I2 /m [8]. In most reported hollandites with divalent A cations barium ions are present in the channels. Strontium containing hollandites only exist when small cations, such as Ti or Ge, are present within the octahedral framework [9]. The strontium rhodium hollandite described in this paper
Fig. 2. Electron diffraction images of Sr 0.75 Rh 4 O 8 showing the [1021] and [0221] zones.
J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
59
Fig. 3. Schematic representation of the electron diffraction images of the [1021] and [0221] zones of Sr 0.75 Rh 4 O 8 .
crystallizes in the monoclinic distorted hollandite structure. The monoclinic angle, however, is significantly larger than those reported for other synthetic hollandites. This feature was also observed for the barium containing compound earlier reported. The value for x obtained from the chemistry is well out of the range for synthetic hollandites, as established by electron diffraction studies by Zandbergen et al. [10]. The second phase (Rh 2 O 3 ) disappears, when a starting mixture is used corresponding to 0.75,x,0.80. It should be noted, that refinement of the value of x from the X-ray powder diffraction refinements,
is unreliable because of the correlation of this parameter with the unknown thermal parameter of Sr, and the fact that the modulation is not incorporated in the refinement. The structure obtained from these refinements is in fact an average structure. The interatomic distances (Table 2), however, are in good agreement with the ionic radii as given by Shannon [13]. In order to gain more insight in the distribution of the strontium cations within the channels (see Fig. 5), one has to turn to electron diffraction studies. Satellites were observed in the [2101] and [0221] zones. The spots could
Fig. 4. Molar magnetic susceptibility of Sr 0.75 Rh 4 O 8 as a function of temperature.
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J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
Fig. 5. Projection of the structure of Sr 0.75 Rh 4 O 8 along the b-axis. The strontium atoms are located in the canals, while the rhodium atoms are positioned inside the oxygen octahedra.
be indexed using a modulation vector of (20.5, |0.25, 0.5). This also explains the fact that satellites are not observed in the [1] zone, since in this zone only second order satellites would be present, which have a very low intensity. Mijlhoff et al. [10] showed that in case of satellites appearing in the 010-direction, the value of x can be determined from the modulation of these spots using the formula x512m, in which m is the modulation distance in the direction of the channels. Applying the same rule in our case would lead to a value of x of 0.75, in good agreement with the experimental results. From these results, it is concluded that the value of x is 0.75. This value was, therefore, also used in the final cycles of the Rietveld refinement of the X-ray diffraction data. This value is still significantly higher than those observed for other synthetic hollandites. The phasewidth of the hollandite under investigation appears to be small. The modulation vector (20.5, |0.25, 0.5) and the fact that the satellites appear as very bright spots, both indicate that the correlation between the strontium cations in different channels is very high. The X-ray diffraction patterns also show some extra lines, which are present in all samples, and which did not show any significant change upon changing the starting composition. It is therefore believed that these lines are part of the diffraction pattern of the hollandite compound, and should be attributed to the superstructure formed by the strontium cations in the channels. The lines could in fact be indexed using the modulation vector obtained from the electron diffraction studies, as indicated in the inset of Fig. 1. Because of the difference in size between the strontiumand barium cations, it is likely that the value for x in the case of Ba x Rh 4 O 8 is slightly smaller than that of
Sr x Rh 4 O 8 . However, the value reported for the high pressure barium hollandite is significantly larger that the one found here for Sr x Rh 4 O 8 . This could be the result of the high pressure which was applied during the synthesis. It should be noted, that the Ba atom was also reported to have a very large temperature factor in the direction of the channels, and the authors do not mention any constraints used to take into account the existing correlations between the temperature factor and the fractional parameter. Furthermore, no additional techniques were applied to determine the composition of this phase. The larger monoclinic distortion in the strontium rhodium hollandite compared to the barium analogue is in agreement with the smaller size of the strontium cations. A larger distortion would be necessary in order to accommodate these smaller cations in the channels. The results obtained from the experiments which are described in this paper give a better insight into the phase relations in the Sr–Rh–O system. They clearly show that the phase diagram published by Horyn et al. [3] has to be revised in the rhodium rich part. It should be noted, that in the strontium rich parts two structures have been published (Sr 4 RhO 6 [1] and Sr 6 Rh 5 O 15 [2]), which are also not present in this phase diagram. Therefore, redetermination of the phase relations of the system Sr–Rh–O seems necessary. The magnetic susceptibility of the strontium rhodium hollandite is nearly constant between 5 and 300 K and has a very low value (9.5 10 24 emu / mole). This Pauli paramagnetic behavior is indicative of large delocalisation of the electrons (or holes) and metallic electrical conductivity. The small increase of the susceptibility with decreasing temperature below 30 K, could be an indication for a small conduction band. It could, however, also be caused by a minor paramagnetic impurity.
5. Conclusions It was shown to be possible to prepare Sr x Rh 4 O 8 at atmospheric pressures. This compound has not been reported before. Its composition and structure yields an improved understanding of the phase relations existing in the Sr–Rh–O systems. It crystallizes in the hollandite structure, showing a larger monoclinic distortion and a larger value for x than have been reported earlier. The value for x for Sr x Rh 4 O 8 was determined to be around 0.75. A large correlation between the distributions of the strontium cations within the channels is observed. Electron diffraction patterns showed satellite spots corresponding to a modulation vector of (20.5, |0.25, 0.5), which has also not been observed before. The results, therefore, broadens the knowledge on these types of compounds. The compound shows Pauli paramagnetic behavior between 5 and 300 K, typical of a metallic conductor.
J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
Acknowledgements We wish to thank Mr. G.H. Renes and Mr. J. van der Meulen for the electron microscopy experiments.
References [1] J.F. Vente, J.K. Lear, P.D. Battle, J. Mater. Chem. 5 (11) (1995) 1785. [2] J.B. Claridge, H.C. zur Loye, Chem. Mater. 10 (1998) 2320. [3] R. Horyn, Z. Bukowski, M. Wolcyrz, A.J. Zaleski, J. Alloys Comp. 262–263 (1997) 267. [4] A.L. Hector, W. Levason, M.T. Weller, Eur. J. Solid State Inorg. Chem. 35 (1998) 679.
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[5] T. Siegrist, E.M. Larson, B.L. Chamberland, J. Alloys Comp. 210 (1994) 13. [6] H.M. Rietveld, J. Appl. Crystallogr. 2 (1969) 65. [7] A.C. Larson, R.B. von-Dreele, General Structure Analysis System (GSAS), in: Report LAUR 86-748, Los Alamos National Laboratories, 1990. ´ G.C. Verschoor, Acta Crystallogr. 24 (1982) 1968. [8] M.C. Cadee, ¨ A.A. Bystrom, ¨ Acta Crystallogr. 3 (1950) 146. [9] A. Bystrom, [10] H.W. Zandbergen, P.L.A. Everstijn, F.C. Mijlhoff, G.H. Renes, D.J.W. IJdo, Mater. Res. Bull. 22 (1987) 431. [11] F.C. Mijlhoff, D.J.W. IJdo, H.W. Zandbergen, Acta Crystallogr. B41 (1985) 98. [12] L.A. Bursill, G. Grzinic, Acta Crystallogr. B36 (1980) 2902. [13] R.D. Shannon, Acta Crystallogr. B26 (1970) 447.
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Synthesis, structure and magnetic properties of a new hollandite: Sr 0.75 Rh 4 O 8 J.R. Plaisier*, A.A.C. van Vliet, D.J.W. IJdo Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands Received 12 July 2000; accepted 25 July 2000
Abstract The synthesis, structure and magnetic properties of a new synthetic hollandite, Srx Rh 4 O 8 (x¯0.75) are discussed. The compound ˚ b53.0626(1) A, ˚ c59.2135(5) A, ˚ b 595.262(3)8, Z52. Electron crystallizes in the monoclinic space group I2 /m, a510.4399(5) A, diffraction patterns show satellites, indicative of an incommensurate distribution of the strontium cations in the channels. The modulation vector was found to be (20.5, |0.25, 0.5). The compound shows Pauli-paramagnetic behavior between 5 and 300 K. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Oxide materials; Chemical synthesis; Crystal structure; Magnetic measurements
1. Introduction Compared to the oxide chemistry of ruthenium and iridium, little is known about the rhodates. This is related to the fact that rhodium is usually present as diamagnetic Rh 31 , which is not expected to show any interesting electronic properties. Recently, the first magnetically ordered Rh 41 oxides have been reported in the strontium rich part of the Sr–Rh–O system [1,2]. In 1997 the phase relations in the Sr–Rh–O system were reported by Horyn et al. [3]. In the rhodium-rich part of this system they found two compounds, which they identified as SrRh 2 O 4 and SrRh 3 O 5.5 . The structure of the high temperature form of the first compound was recently determined by Hector et al. [4]. It consists of layers of RhO 6 octahedra, with the strontium cations statistically distributed over the interlayer-sites. However on examining the X-ray diffraction pattern of the second compound, suspicion arose about the composition of this phase. Clearly, the peaks found for SrRh 2 O 4 were also present in the diffraction pattern of ‘SrRh 3 O 5.5 ’ [3]. Close inspection of the remaining reflections revealed a close similarity to the diffraction patterns, which are obtained from compounds having the hollandite structure. A strontium rhodium hollandite, such as IV Sr x Rh 422x Rh III 2x O 8 , might have interesting electronic and *Corresponding author. E-mail address: [email protected] (J.R. Plaisier).
electrocatalytic properties, because of the high concentration of rhodium ions and the fact that these ions are of mixed valency. In the Ba–Rh–O system Siegrist et al. [5] reported already the existence of another rhodium based hollandite, Ba 0.86 Rh 4 O 8 . This compound has an unusually high value of x. Furthermore, the monoclinic distortion for this compound is large compared to other hollandites ( b 5 94.48). In the scheme of a larger project, dealing with the electrocatalytic activities of various noble metal oxides, Srx Rh 4 O 8 was studied. In this paper the structure determination and magnetic properties of this hollandite are presented.
2. Experimental Since the exact stoichiometry of the target compound was unknown, several samples were prepared by mixing stoichiometric amounts of chemically pure SrCO 3 and Rh metal corresponding to x50.67, 0.70, 0.75 and 0.80. The samples were heated in an Al 2 O 3 crucible at 8508C in air for 1 day and subsequently for 160 h at 10508C under flowing oxygen. This heating treatment resulted in black powders in all cases. The reaction products were examined with a Philips PW 1050 X-ray diffractometer using monochromated CuKa radiation. The data were collected digitally in steps of 0.02
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01208-1
J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
degrees 2Q and 14 s counting time in the range: 108# 2Q #908. All data were collected at room temperature. Since no single crystals were available, Rietveld’s method [6] was used to analyze the diffraction patterns. For this purpose the GSAS suite of programs [7] was used. A scale factor, six background parameters, the unit cell parameters, a zeropoint correction, the atomic fractional coordinates, four profile parameters, defining the pseudoVoigt shape of the reflections, one asymmetry parameter and one overall thermal parameter were refined in the final refinement cycles. As trial models the structural parameters of Ba 0.86 Rh 4 O 8 [4] and Ba x Sn 422x Cr 2x O 8 [8] were used. Electron diffraction was performed with the Siemens Elmiskop 102 electron microscope and with the Philips EM420 transmission electron microscope. Magnetic measurements were carried out in the temperature range of 5–300 K by means of a Quantum design MPMS-5S Squid on approximately |0.2 g of randomly oriented powder of the Sr 0.75 Rh 4 O 8 sample. The magnetic susceptibility was measured as a function of temperature in a magnetic field of 0.1 T.
3. Results The X-ray diffraction patterns of all samples clearly showed a large resemblance to those of hollandite-type phases. The patterns did not change upon longer heating at the same temperature. However, heating at 11008C resulted in dissociation of the products into b-SrRh 2 O 4 and Rh metal in all samples. Refinement of the structures of these compounds went smoothly and yielded reasonably good agreement factors. After these inititial refinements some samples were found to contain a small amount of Rh 2 O 3 , which was added as a second phase in the refinements. In the case of ‘Sr 0.75 Rh 4 O 8 ’ the phase fraction of Rh 2 O 3 refined to a negligible value, and no change in the agreement factors was observed upon removal of this second phase from the refinement. In the strontium sample for x50.80 a small peak was observed at 2Q ¯15.58, which might be ascribed to b-SrRh 2 O 4 . This sample did not show any traces of Rh 2 O 3 . Refinements using the structural model of Ba x Sn 422x Cr 2x O 8 were found to yield better results than those using the structure of Ba 0.86 Rh 4 O 8 as a trial model. The difference between these models is the positioning of the A cation, which is statistically distributed on the 2(a) position (0,0,0) in the latter case and on the 4(g) (0,1 / 2 y,0) position in the former one. The results of the refinement of ‘Sr 0.75 Rh 4 O 8 ’ are presented in Table 1. The atomic positional parameters are given in Table 2. Table 3 shows the relevant interatomic distances. The observed and calculated diffraction patterns of ‘Sr 0.75 Rh 4 O 8 ’ are shown in Fig. 1. The electron diffraction patterns of the [1] zone, which
57
Table 1 Results of the Rietveld refinement of Sr 0.75 Rh 4 O 8 R wp Rp x2 Nvar x
17.55 12.63 4.27 33 0.75
˚ (A) ˚ (A) ˚ (A) (8) ˚ 3) (A
a b c b V
10.4402(5) 3.0646(1) 9.2137(5) 95.263(3) 293.55(3)
Table 2 Atomic parameters of Sr 0.75 Rh 4 O 8
Sr Rh(1) Rh(2) O(1) O(2) O(3) O(4)
(4g) (4i) (4i) (4i) (4i) (4i) (4i)
x
y
z
Fraction
0 0.3383(2) 0.8622(2) 0.305(1) 0.042(1) 0.623(1) 0.657(2)
0.205(2) 0 0 0 0 0 0
0 0.1604(3) 0.3260(2) 0.387(2) 0.272(2) 0.034(2) 0.331(2)
0.375 1.0 1.0 1.0 1.0 1.0 1.0
are usually shown when discussing the structure of hollandite-type structures, did not show any satellite reflections. However, satellites were observed in the diffraction patterns of other zones. The patterns of the [1021] and the [0221] zones are shown in Fig. 2. The satellite reflections could be indexed using a modulation vector of (20.5, |0.25, 0.5). Fig. 3 shows an indexed schematic representation of these zones. The molar magnetic susceptibility of ‘Sr 0.75 Rh 4 O 8 ’ is given in Fig. 4. The susceptibility remains constant at |9?10 24 emu / mole over nearly the complete temperature range. At temperatures below 30 K a very small increase of the susceptibility is observed with decreasing temperature.
4. Discussion The results described above clearly indicate that the compound described by Horyn et al. [3] is in fact a mixture of SrRh 2 O 4 and a strontium rhodium hollandite. It was possible to prepare the latter at atmospheric pressure. The previously reported barium rhodium hollandite single crystal had been obtained from a mixture of phases, which was produced by a high pressure reaction. Hollandites can be described as oxides of composition A IIx M IV 42x N y O 8 [8]. The M and N cations are octahedrally coordinated. The structure consists of corner sharing Table 3 ˚ for Sr 0.75 Rh 4 O 8 Selected atomic distances (A) Rh(1)–O(1) Rh(1)–O(1) Rh(1)–O(2) Rh(1)–O(3) Sr–O(1) Sr–O(2) Sr–O(4)
(1x) (2x) (2x) (1x) (2x) (2x) (2x)
2.14(2) 2.16(1) 2.04(1) 1.87(2) 2.38(1) 2.59(2) 2.53(2)
Rh(2)–O(2) Rh(2)–O(3) Rh(2)–O(4) Rh(2)–O(4)
(1x) (2x) (2x) (1x)
1.99(1) 2.00(1) 2.10(1) 2.15(2)
58
J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
Fig. 1. Observed (crosses) and calculated (line) X-ray diffraction pattern of Sr 0.75 Rh 4 O 8 ; the difference (Iobs 2Icalc ) curve at the bottom of the figure. Tick marks indicate the position of the Bragg reflections included in the calculation. The inset shows an enlarged portion of the patterns including the indexing of the satellites.
double chains of edge sharing octahedra, thus forming square tunnels, which are occupied by large A ions. The difference between the charges of M and N determines the amount (x) of A, necessary to obtain electroneutrality [9]. Usually, the repetition unit of the A cations within the tunnels is different from that of the surrounding lattice, leading to an incommensurate superstructure (see e.g. [10– 12]). Generally, the ordering of the A-cations in a given channel is not strongly correlated to the ordering in any other channel. Any correlation depends on the nature of the
ions in the octahedral framework. The structure is tetragon˚ and an a-axis of about al with a short c-axis of about 3 A ˚ The tunnels are parallel to the short axis. In the case 10 A. that the space for the A ions is too big, the structure becomes monoclinic with space group I2 /m [8]. In most reported hollandites with divalent A cations barium ions are present in the channels. Strontium containing hollandites only exist when small cations, such as Ti or Ge, are present within the octahedral framework [9]. The strontium rhodium hollandite described in this paper
Fig. 2. Electron diffraction images of Sr 0.75 Rh 4 O 8 showing the [1021] and [0221] zones.
J.R. Plaisier et al. / Journal of Alloys and Compounds 314 (2001) 56 – 61
59
Fig. 3. Schematic representation of the electron diffraction images of the [1021] and [0221] zones of Sr 0.75 Rh 4 O 8 .
crystallizes in the monoclinic distorted hollandite structure. The monoclinic angle, however, is significantly larger than those reported for other synthetic hollandites. This feature was also observed for the barium containing compound earlier reported. The value for x obtained from the chemistry is well out of the range for synthetic hollandites, as established by electron diffraction studies by Zandbergen et al. [10]. The second phase (Rh 2 O 3 ) disappears, when a starting mixture is used corresponding to 0.75,x,0.80. It should be noted, that refinement of the value of x from the X-ray powder diffraction refinements,
is unreliable because of the correlation of this parameter with the unknown thermal parameter of Sr, and the fact that the modulation is not incorporated in the refinement. The structure obtained from these refinements is in fact an average structure. The interatomic distances (Table 2), however, are in good agreement with the ionic radii as given by Shannon [13]. In order to gain more insight in the distribution of the strontium cations within the channels (see Fig. 5), one has to turn to electron diffraction studies. Satellites were observed in the [2101] and [0221] zones. The spots could
Fig. 4. Molar magnetic susceptibility of Sr 0.75 Rh 4 O 8 as a function of temperature.
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Fig. 5. Projection of the structure of Sr 0.75 Rh 4 O 8 along the b-axis. The strontium atoms are located in the canals, while the rhodium atoms are positioned inside the oxygen octahedra.
be indexed using a modulation vector of (20.5, |0.25, 0.5). This also explains the fact that satellites are not observed in the [1] zone, since in this zone only second order satellites would be present, which have a very low intensity. Mijlhoff et al. [10] showed that in case of satellites appearing in the 010-direction, the value of x can be determined from the modulation of these spots using the formula x512m, in which m is the modulation distance in the direction of the channels. Applying the same rule in our case would lead to a value of x of 0.75, in good agreement with the experimental results. From these results, it is concluded that the value of x is 0.75. This value was, therefore, also used in the final cycles of the Rietveld refinement of the X-ray diffraction data. This value is still significantly higher than those observed for other synthetic hollandites. The phasewidth of the hollandite under investigation appears to be small. The modulation vector (20.5, |0.25, 0.5) and the fact that the satellites appear as very bright spots, both indicate that the correlation between the strontium cations in different channels is very high. The X-ray diffraction patterns also show some extra lines, which are present in all samples, and which did not show any significant change upon changing the starting composition. It is therefore believed that these lines are part of the diffraction pattern of the hollandite compound, and should be attributed to the superstructure formed by the strontium cations in the channels. The lines could in fact be indexed using the modulation vector obtained from the electron diffraction studies, as indicated in the inset of Fig. 1. Because of the difference in size between the strontiumand barium cations, it is likely that the value for x in the case of Ba x Rh 4 O 8 is slightly smaller than that of
Sr x Rh 4 O 8 . However, the value reported for the high pressure barium hollandite is significantly larger that the one found here for Sr x Rh 4 O 8 . This could be the result of the high pressure which was applied during the synthesis. It should be noted, that the Ba atom was also reported to have a very large temperature factor in the direction of the channels, and the authors do not mention any constraints used to take into account the existing correlations between the temperature factor and the fractional parameter. Furthermore, no additional techniques were applied to determine the composition of this phase. The larger monoclinic distortion in the strontium rhodium hollandite compared to the barium analogue is in agreement with the smaller size of the strontium cations. A larger distortion would be necessary in order to accommodate these smaller cations in the channels. The results obtained from the experiments which are described in this paper give a better insight into the phase relations in the Sr–Rh–O system. They clearly show that the phase diagram published by Horyn et al. [3] has to be revised in the rhodium rich part. It should be noted, that in the strontium rich parts two structures have been published (Sr 4 RhO 6 [1] and Sr 6 Rh 5 O 15 [2]), which are also not present in this phase diagram. Therefore, redetermination of the phase relations of the system Sr–Rh–O seems necessary. The magnetic susceptibility of the strontium rhodium hollandite is nearly constant between 5 and 300 K and has a very low value (9.5 10 24 emu / mole). This Pauli paramagnetic behavior is indicative of large delocalisation of the electrons (or holes) and metallic electrical conductivity. The small increase of the susceptibility with decreasing temperature below 30 K, could be an indication for a small conduction band. It could, however, also be caused by a minor paramagnetic impurity.
5. Conclusions It was shown to be possible to prepare Sr x Rh 4 O 8 at atmospheric pressures. This compound has not been reported before. Its composition and structure yields an improved understanding of the phase relations existing in the Sr–Rh–O systems. It crystallizes in the hollandite structure, showing a larger monoclinic distortion and a larger value for x than have been reported earlier. The value for x for Sr x Rh 4 O 8 was determined to be around 0.75. A large correlation between the distributions of the strontium cations within the channels is observed. Electron diffraction patterns showed satellite spots corresponding to a modulation vector of (20.5, |0.25, 0.5), which has also not been observed before. The results, therefore, broadens the knowledge on these types of compounds. The compound shows Pauli paramagnetic behavior between 5 and 300 K, typical of a metallic conductor.
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Acknowledgements We wish to thank Mr. G.H. Renes and Mr. J. van der Meulen for the electron microscopy experiments.
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