Tetragonal CeNbO4 at 1073 K in air and in vacuo

Several authors have investigated the structure of CeNbO4, particularly under ambient conditions (Negas et al., 1977; Roth et al., 1977, 1978; Cava et al., 1978; Santoro et al., 1980; Thompson et al., 1999), and only recently has there been any attempt to characterize the structure at elevated temperatures, with one recent report of an in situ characterization of phase transformations on heating that presented minimal structural information (Skinner & Kang, 2003). The recent interest in this material has stemmed from the possible incorporation of oxygen interstitials that would make this material amenable to oxide ion conducting applications, such as solid electrolytes, sensors and separation membranes. The purpose of this study was to investigate the structure of the high-temperature polymorph under both static air and vacuum conditions in order to gather information regarding the likely ̄exibility of oxygen stoichiometry in CeNbO4. The low-temperature polymorph of CeNbO4 has been described previously as a monoclinic distortion of the tetragonal scheelite structure, adopting a fergusonite-type structure (Santoro et al., 1980; Thompson et al., 1999). It was also predicted that CeNbO4 will undergo oxidation on heating in air and a structural transition on heating above 847 K (Gingerich & Blair, 1964; Negas et al., 1977; Roth et al., 1977; Cava et al., 1978). Recently, in situ measurements have raised questions about the nature of these oxidation and transformation processes (Skinner & Kang, 2003). However, until the present work, there has been no determination of the structure of the tetragonal form of CeNbO4. From our initial X-ray diffraction results, it was immediately apparent that the data recorded at 1073 K conformed to a scheelite-type structure. As a starting model for the Rietveld re®nement of the neutron powder diffraction data, the structure of scheelite (CaWO4) was therefore used [space group I41/a; alternate setting with origin at (0, 14, 1 8); the Ce and Nb atoms in the 4b and 4a positions, respectively, and the O atom in the 16f position (Hazen et al., 1985)]. The re®nements were carried out using the GSAS package (Larson & Von Dreele, 1994) and provided good ®ts to the 90 and backscattered neutron diffraction data collected in air and in vacuo (10ÿ3 Pa) (Fig. 1). In common with other scheelite-type compounds, the Ce atoms in CeNbO4 are in an eightfold coordination environment, with two distinct bond lengths, while the Nb environment is tetrahedral, with equal NbÐO bonds (Fig. 2). The slightly larger unit-cell volume of the sample heated in static


Comment
Several authors have investigated the structure of CeNbO 4 , particularly under ambient conditions (Negas et al., 1977;Roth et al., 1977Roth et al., , 1978Cava et al., 1978;Santoro et al., 1980;Thompson et al., 1999), and only recently has there been any attempt to characterize the structure at elevated temperatures, with one recent report of an in situ characterization of phase transformations on heating that presented minimal structural information (Skinner & Kang, 2003). The recent interest in this material has stemmed from the possible incorporation of oxygen interstitials that would make this material amenable to oxide ion conducting applications, such as solid electrolytes, sensors and separation membranes. The purpose of this study was to investigate the structure of the high-temperature polymorph under both static air and vacuum conditions in order to gather information regarding the likely¯exibility of oxygen stoichiometry in CeNbO 4 .
The low-temperature polymorph of CeNbO 4 has been described previously as a monoclinic distortion of the tetragonal scheelite structure, adopting a fergusonite-type structure (Santoro et al., 1980;Thompson et al., 1999). It was also predicted that CeNbO 4 will undergo oxidation on heating in air and a structural transition on heating above 847 K (Gingerich & Blair, 1964;Negas et al., 1977;Roth et al., 1977;Cava et al., 1978). Recently, in situ measurements have raised questions about the nature of these oxidation and transformation processes (Skinner & Kang, 2003). However, until the present work, there has been no determination of the structure of the tetragonal form of CeNbO 4 . From our initial X-ray diffraction results, it was immediately apparent that the data recorded at 1073 K conformed to a scheelite-type structure. As a starting model for the Rietveld re®nement of the neutron powder diffraction data, the structure of scheelite (CaWO 4 ) was therefore used [space group I4 1 /a; alternate setting with origin at (0, 1 4 , 1 8 ); the Ce and Nb atoms in the 4b and 4a positions, respectively, and the O atom in the 16f position (Hazen et al., 1985)]. The re®nements were carried out using the GSAS package (Larson & Von Dreele, 1994) and provided good ®ts to the 90 and backscattered neutron diffraction data collected in air and in vacuo (10 À3 Pa) (Fig. 1).
In common with other scheelite-type compounds, the Ce atoms in CeNbO 4 are in an eightfold coordination environment, with two distinct bond lengths, while the Nb environment is tetrahedral, with equal NbÐO bonds (Fig. 2). The slightly larger unit-cell volume of the sample heated in static  air is associated with slightly longer NbÐO and CeÐO distances (Tables 1 and 2). The distorted tetrahedral environment for the Nb atom, with OÐNbÐO angles of 106.476 (19) and 115.64 (4) , is comparable to that in isostructural LaNbO 4 (David, 1983;Machida et al., 1995). The transition temperature in CeNbO 4 is signi®cantly higher than that in LaNbO 4 (773 K). From the results of both re®nements and from Fourier difference maps, it is apparent that the high-temperature CeNbO 4 polymorph is fully stoichiometric, and there is no evidence to suggest that heating it in air introduced any interstitial oxygen. Because the sample was heated in situ while packed in a vanadium can, it is conceivable that limited oxidation occurred at the surface only. Hence, it would be desirable either to perform a series of measurements at one temperature over a period of time to enable the study of possible oxygen incorporation into CeNbO 4 or to oxidize a sample before carrying out a set of measurements at temperatures up to 1073 K, allowing any oxygen stoichiometry variations on heating to be investigated.

Experimental
Samples were produced through the mixing of CeO 2 (99.9%, Sigma Aldrich) and Nb 2 O 5 (99.99%, Sigma Aldrich) in an agate mortar and pestle. These starting materials were mixed thoroughly under acetone and allowed to dry in air before the stoichiometric mixture was transferred to an alumina crucible for heat treatment. The mixture was pre®red at 1273 K for 10 h and allowed to cool. After further mixing, the material was heated at 1673 K for 16 h and quenched.
This procedure produced a bright-green material. X-ray powder diffraction data were recorded on a Philips PW1700 series diffractometer using Cu K 1 radiation and a graphite crystal as secondary monochromator. Neutron powder diffraction data were collected from 6 g samples contained in vanadium cans on the POLARIS diffractometer using both 90 and backscatter detectors at ISIS, CCLRC Rutherford Appleton Laboratory, Oxfordshire, England. Each data set was collected over a period of 90 min, and then normalized and corrected in the usual way. Two separate samples were examined during these measurements. The ®rst sample was heated in situ under vacuum with data recorded on heating and cooling. For the second sample, the same in situ heating regime was used but the can was exposed to static air. The data were collected at 296 K, every 100 K from 473 to 973 K, and every 50 K from 973 to 1123 K. On cooling, data were recorded at 1023, 823 and 623 K. Only the data recorded at 1073 K from the samples heated in air and in vacuo are included here.

In vacuo
Crystal data  A representation of the tetragonal scheelite-type structure of CeNbO 4 , showing the Nb and Ce coordination environments. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry code: (i) x + 1 2 , y + 1 2 , 1 2 À z.] Initial re®nement cycles showed no signi®cant deviation from stoichiometry, and therefore the occupancies of the Ce-and Nb-atom sites were ®xed to unity in the ®nal cycles for both the air and the vacuum data sets. The occupancy of the O-atom site was allowed to vary but did not deviate signi®cantly from unity either. Fourier difference maps indicated no signi®cant residual scattering density within the unit cell, with maxima of 0.49 A Ê À3 at (0.2465, 0.5004, 0.1234) for the vacuum data and 0.46 A Ê À3 at (0.1545, 0.9652, 0.1803) for the air data.
The authors thank the CLRC for funding this work through a beamtime grant (No. RB14118) and Dr Ron Smith, instrument scientist, for his invaluable contribution to the data collection.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: BC1031). Services for accessing these data are described at the back of the journal. Table 2 Selected interatomic distances (A Ê ) for air data.

Tetragonal CeNbO 4 at 1073 K in air and in vacuo
Stephen J. Skinner, Ian J. E. Brooks and Christopher N. Munnings

Computing details
For both compounds, program(s) used to solve structure: GSAS; program(s) used to refine structure: GSAS.