Phase Transition in Weberite-Type Gd3NbO7

Lu Cai,w,z Sava Denev,y Venkatraman Gopalan,y and Juan C. Ninoz

zDepartment of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611

yDepartment of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802

Gd3NbO7 has an orthorhombic weberite-type crystal structureand undergoes a phase transition at about 340 K. The phasetransition was confirmed by second harmonic generation, heatcapacity measurement, and high-resolution X-ray diffraction.Above 340 K, Gd3NbO7 has a centrosymmetric weberite-typestructure with space group Cmcm (No. 63) and transforms atabout 340 K into a non-centrosymmetric structure with spacegroup Cm2m (No. 38). The phase transition is mainly due tooff-center shifts of Nb

51ions within NbO6 octahedra, as well as

off-center shifts of one-third of the Gd31

ions within GdO8

polyhedra.

I. Introduction

THE Gd3NbO7 belongs to a weberite-type Ln3MO7 family(where Ln is a rare-earth element, and M is Os51, Re51,

Ru51, Re51, Mo51, Ir51, Sb51, Nb51, or Ta51).1–28 The char-acteristic of this family is that MO6 are corner linked to eachother and form chains of MO6 octahedra with parallel chains ofLnO8 distorted cubes. The crystal structure of Gd3NbO7 wasfirst determined by Rossell.23 Rossell assigned the space groupC2221 to Gd3NbO7 at room temperature (RT). However, thenon-polar space group C2221 was later questioned by Astafyevet al.29 as Gd3NbO7 exhibited a second harmonic generation(SHG) signal at RT. The SHG signal disappeared above 330 K,consistent with a non-centrosymmetric to centrosymmetric tran-sition. It was suggested then that the transition was betweenCmm2 and Cmmm without proof. The existence of a transitionwas further confirmed by heat capacity measurement by Asta-fyev et al.29 and Klimenko et al.30 and Raman spectroscopy byKovyazina et al.31 In addition, Gd3NbO7 exhibited a dielectricrelaxation and the relaxation temperature, where the maximumpermittivity occurs, was close to the transition temperature.4

It is important to note that phase transitions are commonlyobserved in the Ln3MO7 family and there have been numerousstudies especially focusing on the crystallographic aspects ofLn3IrO7,

21 Ln3MoO7,2,3,9,32 and Ln3RuO7.

12,17 In this study,high-resolution X-ray diffraction (XRD) was performed onGd3NbO7 powder before and after the reported phase transi-tion to solve the space group issue, and for the first time, deter-mine an unequivocal space group and crystal structure that isconsistent with all the experimental data available (dielectricproperty, heat capacity, SHG, XRD, etc.). In addition, heat ca-pacity and SHG were also conducted on Gd3NbO7 samples inthis study as the experimental procedures were not well stated byAstafyev et al.29 This indicated use of the same method as thatof Sirotinkin et al.33 However, two methods were used by Siro-

tinkin et al.33: furnace cooled (calcined from 1473 to 1673 K)and quenched from 1593 K. It is not clear which experimentalmethod was used for preparing the SHG sample.29 To avoidpossible discrepancy, all measurements in this study were con-ducted on Gd3NbO7 samples within the same batch.

II. Experimental Procedure

Polycrystalline specimens of Gd3NbO7 were synthesized by aconventional solid oxide reaction from a stoichiometric mixtureof Gd2O3 (Alfa Aesar, Ward Hill, MA; 99.9%) and Nb2O5 (AlfaAesar; 99.9985%). Gd2O3 and Nb2O5 were mixed by wet ballmilling for 24 h and then subsequently dried in an oven at 393 Kfor 16 h. The dried powder was then ground and sieved througha 212 mm mesh. The powder was then placed in an aluminacrucible for calcination at 1673 K. Multiple calcinations wereused and CuKa radiation XRD was collected after each calci-nation at RT to verify phase formation. Equilibrium was pre-sumed when no further changes in the XRD pattern could bedetected. High-resolution powder XRD with 30 keV and awavelength 0.4009 and 0.4142 A was performed at 11-BM Ad-vanced Photon Source, Argonne National Lab.

Phase-pure powders were then uniaxially pressed at 150 MPainto cylindrical-shaped pellets (13 or 7 mm in diameter and ap-proximately 1 mm in thickness) using polyvinyl alcohol (B1–3wt%) as a binder. A binder burn-out step at 723 K for 2 h wasfollowed by sintering in air at 1873 K for 8 h. Heat capacitymeasurement on the Gd3NbO7 pellet was conducted using adifferential scanning calorimeter (DSC, Seiko Instrument Inc.,Chiba, Japan) following ASTM E 1269-05.34 The measurementwas performed in a nitrogen atmosphere using a synthetic sap-phire disk as a standard from 160 to 673 K. As for the SHGmeasurement, the pellet was polished progressively with smallersizes of alumina powder and finally submicrometer-size colloidalsilica. The laser source was an amplified Ti:Sapphire laser with a1 kHz repetition rate, 800 nm wavelength, and 130 fs pulsewidth. The signal was collected using a photomultiplier tube anda lock-in amplifier to reduce noise. The laser was incident at 451to the sample surface.

III. Results and Discussion

(1) SHG and Heat Capacity Measurements

The SHG of Gd3NbO7 was measured twice, the second timewith a higher power yielding a higher intensity and it is shown inFig. 1. Both measurements confirmed that the polar order dis-appears at about 340 K. Figure 1 also shows the dielectric con-stant at 1 MHz.4 This clearly indicates that the dielectricrelaxation temperature, at which the maximum of the dielectricconstant occurs, matches well with the temperature at which thepolar order disappears. Before the transition, Gd3NbO7 shouldhave a polar space group. There are only three point groupscorresponding to an orthorhombic lattice: 222, mm2, and mmm,and the only polar group is mm2. It is thus proposed that thetransition is between mm2 and mmm.29 The heat capacity ofGd3NbO7 has been re-measured and the measuring temperature

A. Feteira—contributing editor

This work was financially supported by the U.S. National Science Foundation throughthe CAREER award (DMR-0449710), CBET 0730900 grant, and NSF grant numbersDMR-0820404, DMR-0602986, and DMR-0908718.

wAuthor to whom correspondence should be addressed. e-mail: lclisa3@.ufl.edu

Manuscript No. 26704. Received August 17, 2009; approved October 16, 2009.

Journal

J. Am. Ceram. Soc., 93 [3] 875–880 (2010)

DOI: 10.1111/j.1551-2916.2009.03494.x

r 2009 The American Ceramic Society

875

has been reduced down to 160 K in Fig. 2. It shows a phasetransition between 310 and 340 K. This result matches previousstudies.29,30

(2) High-Resolution XRD

High-resolution XRD at different temperatures was performedon Gd3NbO7 as shown in Fig. 3. The wavelength used was0.4009 A for the pattern at 100 K and 0.4142 A for all highertemperatures. For better visual comparison, the 2y at 100 K isconverted using the wavelength 0.4142 A in Fig. 3. Diamondsymbols indicate low-intensity peaks that can be associated withthe structure. It is seen that with increasing temperature from100 to 345 K, there is increased peak splitting of the first twostrongest reflections (220) and (022) as shown in Fig. 3(b). UsingGaussian peak fitting, at 100 K, the splitting is negligible, at 295K the peaks split by 0.0101, and at 345 K, they split by 0.0161.There is no obvious change in peak splitting between 345 and400 K. A similar splitting trend is observed for the (440) and(044) peaks; increasing from 100 to 345 K, with no obvious in-crease between 345 and 400 K. This is consistent with the ex-pected anisotropic lattice expansion. By contrast, it is alsointeresting to note that both (022) and (044) peaks shift to theright from 295 to 345 K, which indicates contraction acrossthose planes as a function of temperature. Figure 4(a) shows asummary of lattice parameters at the four different tempera-tures. While lattice parameters b and c increase with increasingtemperature, lattice parameter a exhibits an anomalous decreasefrom 100 to 345 K, which is also indicative of a possible phasetransition. This type of lattice parameter shrinkage also occursnear the phase transition temperature in Gd3RuO7, which alsohas a weberite-type structure.17 It is worth remembering that theweberite-type structure is a superstructure of fluorite, andthe lattice parameters of the weberite type can be viewed asO2af, 2af, andO2af (af is the lattice parameter of fluorite,B5 A).Figure 4(b) shows af calculated from lattice parameters a, b, c,and the unit cell volume at four different temperatures. Thisclearly shows that although a exhibits an anomalous lattice con-traction from 100 to 345 K, the overall effect is still lattice ex-pansion.

Upon detailed inspection of the XRD patterns, perhaps thestrongest evidence of phase transition is revealed in that the(201) reflection appears at 2yB71 at 100 and 295 K but does notoccur above 345 K (Fig. 3(c)). The disappearance of (201) re-flection adds an extra reflection condition above 345 K: both h

Fig. 1. Dielectric constant and SHG of Gd3NbO7.

Fig. 2. Heat capacity of Gd3NbO7.

Fig. 3. (a) High-resolution XRD of Gd3NbO7, l5 0.414201 A. (b) High-resolution XRD showing details of the increasing peak split of (220) and (022),and (044) (440). (c) High-resolution XRD showing details of the (201) reflection.

876 Journal of the American Ceramic Society—Cai et al. Vol. 93, No. 3

and l are 2n (n is an integer) for (h0I). At all measured temper-atures, the reflection conditions also have to satisfy h1k5 2nfor (hkl) and l5 2n for (00l). The software Checkcell was usedto search the space groups that may be consistent with theseobservations.35 The initial search resulted in eight space groupswith an mm2 point group for the low-temperature (LT) phaseand three space groups with an mmm point group for the

high-temperature (HT) phase. The similarity of XRD patternssuggests that the transition is displacive, with only straight-forward changes in symmetry: e.g. symmetry operations aregained or lost. Therefore, it is safe to assume that the spacegroups of the LT phase and the HT phase should have asub-group and super-group relationship. Previously, Astafyevet al.29 proposed that the transition is between Cmm2 and itssuper-group Cmmm. However, (201) reflection is allowed in theCmmm space group. Among the eight candidate space groupsfor the LT phase and three for the HT phase, only space groupsCm2m (No. 38) and Cmcm (No. 63) have a sub-group and su-per-group relationship. In the light of the above information,Cm2m and Cmcm were selected for the LT structure and the HTstructure, respectively, and utilized as the basis for the patternrefinement described below.

The initial structural model was based on Na2NiInF7

as Ln3MO7 are extreme cases of Na2NiInF7 in the weberitefamily (see Fig. 5).36 The space group of Na2NiInF7 is Pnma.37

A space group transformation between Pnma and Cm2m wasapplied using Powdercell38 through their common subgroupPmc21 to obtain the initial atomic positions of Gd3NbO7 in aC-centered unit cell. The powder diffraction patterns were re-fined using GSAS software.39,40 Table I shows a summary of therefinement parameters. There are a total of 71 refined parame-ters for LT phases and 54 parameters for HT phases includingcrystal structure (lattice, atomic positions, and isotropic atomicdisplacement), diffractometer constants (zero shift and polar-ization), scale factor, background, and profile function. The ob-served intensities, calculated intensities, and their difference at100 K are shown in Fig. 6. Tables II and III list the atomic po-sitions and isotropic atomic displacement parameters ofGd3NbO7 at 400, 345, 295, and 100 K after Rietveld refine-ment. It is important to note that due to the limitations of X-ray,the atomic displacement of oxygen ions may not be accurate.The Gd3NbO7 structure has an arrangement of NbO6–Gd1O8

layers and VII coordinated Gd2 between layers (see Fig. 7(a)). Itcan be seen that the Gd2 (Wyckoff position 8g) splits into twoWyckoff positions, 4e (Gd2(1)) and 4d (Gd2(2)). Also, the fouroxygen atomic positions in Cmcm split into eight types inCm2m.

At 345 and 400 K, Nb51 ions occupy Wyckoff position 4awith site symmetry 2/m in the center of NbO6 polyhedra (dis-torted octahedra). Each Nb51 ion is bonded to two O1 and fourO4, with O1 being corner shared by neighboring NbO6 poly-hedra. The bond angle of O1–Nb–O1 is 180o. The NbO6 poly-hedra align in a zig-zag manner along [001] and the Nb51 ions

Fig. 4. (a) Lattice parameters of Gd3NbO7 at different temperatures.(b) Lattice parameters of af.

Fig. 5. (a) In classic weberite (AVIII2B

VI2X7), half of the B ions (B-1, the ionic radius is larger than the rest of the B ions (B-2) if not the same) participate

in the formation of chains of BX6 octahedra oriented in an anti-phase motif. (b) In Na2NiInF7 (when the ionic radius of B-1 is smaller than B-2), theadjacent B-1X6 chains cannot hold anti-phase orientations; as a result, half of the A ions cannot hold eight coordination and the B-2 ions maintain sixcoordination with another anion relatively close to them (the dashed line). (c) The extreme case of Ln3MO7 when Ln31 sit in a position similar to B-2.The adjacent MO6 chains in the weberite type are in phase and the Ln31 ions between the two chains (accounts for 2/3 of the total Ln31) result in sevencoordination.

March 2010 Phase Transition in Weberite-Type Gd3NbO7 877

are separated by a constant distance (c/2, c being the lattice pa-rameter). When cooling down to the transition temperature,Nb51 ions shift slightly away from the twofold axis parallel tothe [100] direction and occupy the 4c site with mirror symmetry(see Figs. 7(b) and (c)). The distances between two neighboringNb51 ions are alternatively c/21d or c/2 �d (d/2 is the projectedshifting distance along [001] of Nb51 ions; see Figs. 7(d) and (e)).Nb51 ions are displaced by 0.02 A at 295 K and by 0.15 A at 100K from the geometrical center of NbO6 polyhedra.

In the anion sublattice, due to the loss of the twofold sym-metry, the O1 position (multiplicity: 4) splits into O1(1) (multi-plicity: 2) and O1(2) (multiplicity: 2). The neighboring NbO6

octahedra are corner shared alternatively through the O1(1) orthe O1(2) ions along the [001] direction. The bond angle ofO1(1)-Nb–O1(2) is 177.41 at 295 K and 171.41 at 100 K.

Focusing on the rare earth, in the HT phase, one-third of theGd31 ions (Gd131) occupy a position with symmetry 2/m in thecenter of LnO8 polyhedra. However, in the LT phase, they shiftslightly away from the twofold axis that is parallel to [100] to

Table I. Crystal Data and Refinement Parameters

Temperature 100 K 295 K 345 K 400 K

Lattice 7.5393(3) A10.6108(3) A7.5327(3) A

7.5324(1) A10.6185(2) A7.5476(1) A

7.5294(2) A10.6202(2) A7.5522(1) A

7.5321(2) A10.6239(2) A7.5554(2) A

Z 4 4 4 4Space group Cm2m Cm2m Cmcm Cmcml (A) 0.400919 0.414201 0.414201 0.414201Refined 2y range (deg) 0.5B29.999 0.5B29.999 0.5B29.999 0.5B29.999Total no. of reflections 29501 29501 29501 29 501No. of peaks 432 404 371 373No. of parameters refined 71 71 54 54Rwp (%) 12.91 10.39 10.28 11.06Rp (%) 9.16 7.93 8.10 8.69GOF (X2) 8.100 5.169 4.989 5.808

Fig. 6. Observed and calculated high-resolution powder X-ray diffrac-tion pattern for Gd3NbO7 at 100 K.

Table II. Lattice Parameters and Atomic Positions at 345 and 400 K (space group Cmcm)

345 K 400 K

Wyckoff x y z Uiso x y z Uiso

Gd1 4b 0 0.5 0 0.0205 (1) 0 0.5 0 0.0261 (1)Gd2 8g 0.7337 (3) 0.7330 (2) 0.25 0.01303 (5) 0.7341 (3) 0.73335 (3) 0.25 0.01871 (6)Nb 4a 0 0 0 0.0067 (1) 0 0 0 0.0115 (2)O1 4c 0 0.0648 (5) 0.25 0.018 (2) 0 0.0648 (6) 0.25 0.025 (2)O2 4c 0 0.6288 (5) 0.25 0.006 (1) 0 0.6289 (6) 0.25 0.012 (1)O3 4c 0 0.3640 (5) 0.25 0.008 (1) 0 0.3642 (6) 0.25 0.015 (1)O4 16 h 0.1918 (3) 0.8794 (3) 0.0317 (3) 0.0202 (9) 0.1922 (4) 0.8794 (3) 0.0319 (4) 0.028 (1)

Table III. Lattice Parameters and Atomic Positions at 100 and 295 K (Space Group Cm2m)

100 K 295 K

Wyckoff x y z Uiso x y z Uiso

Gd1 4c 0 0.5060 (4) 0.2440 (1) 0.0291 (1) 0 0.4943 (5) 0.2546 (2) 0.0291 (1)Gd2 (1) 4e 0.7734 (2) 0.2327 (4) 0.5 0.0455 (2) 0.7681 (2) 0.2275 (5) 0.5 0.0348 (2)Gd2 (2) 4d 0.2661 (1) 0.2678 (4) 0 0.00242 (5) 0.2664 (1) 0.2618 (5) 0 0.00733 (6)Nb 4c 0 0.9996 (4) 0.7353 (3) 0.0148 (2) 0 0.9993 (5) 0.7551 (2) 0.0120 (1)O1 (1) 2a 0 0.952 (1) 0 0.0003 (24) 0 0.932 (2) 0 0.030 (3)O1 (2) 2b 0 0.070 (1) 0.5 0.037 (4) 0 0.060 (2) 0.5 0.033 (3)O2 (1) 2a 0 0.623 (1) 0 0.0017 (22) 0 0.616 (1) 0 0.012 (2)O2 (2) 2b 0 0.348 (3) 0.5 0.029 (4) 0 0.340 (1) 0.5 0.017 (3)O3 (1) 2a 0 0.377 (1) 0 0.024 (3) 0 0.379 (1) 0 0.004 (2)O3 (2) 2b 0 0.634 (2) 0.5 0.038 (4) 0 0.6387 (9) 0.5 0.004 (2)O4 (1) 8f 0.2144 (8) 0.8754 (8) 0.7304 (8) 0.011 (1) 0.203 (1) 0.878 (1) 0.7285 (9) 0.018 (1)O4 (2) 8f 0.669 (1) 0.6168 (8) 0.2132 (9) 0.032 (2) 0.680 (1) 0.6184 (9) 0.2103 (9) 0.024 (1)

878 Journal of the American Ceramic Society—Cai et al. Vol. 93, No. 3

occupy an off-center site with mirror symmetry similar to theNb51 ions (see Figs. 7(g) and (h)). Edge-sharing Gd1O8 poly-hedra align along the [001] direction in both the HT and the LTphases. The distance along [001] between neighboring Gd131

ions is constant and equal to c/2 at HT. However, at LT, thedistance is alternatively c/21d0 or c/2 �d0 (d0/2 is the projectedshifting distance of Gd31 along [001]). Gd131 ions are displacedby 0.07 A at 295 K and by 0.26 A at 100 K from the geometricalcenter of the GdO8 polyhedra.

It is therefore clear that the phase transition in Gd3NbO7 ischiefly mediated by the off-center shift of both Nb51 and Gd131

ions in their corresponding polyhedra.Finally, it is quite interesting that in the LT phase, the off-

center distance of both Nb51 and Gd131 ions increases withdecreasing temperature, and that the relative displacement of theNb51 and Gd131 ions is antiparallel (see Fig. 7(e)). This suggeststhe possibility of an antiferrielectric behavior, and while ourpreliminary tests (piezoelectric activity and polarization hyster-esis) have not yielded a significant insight in this respect, furtherinvestigation is warranted.

IV. Conclusion

Gd3NbO7 was successfully synthesized using the conventionalsolid-state process. SHG and heat capacity measurements indi-cated a phase transition in Gd3NbO7 at about 340 K. High-

resolution XRDwas conducted at 100, 295, 345, and 400 K. Theappearance of (201) reflection upon cooling confirmed the phasetransition. Rietveld refinement was performed on the XRD pat-terns at the four above-mentioned temperatures and resolvedthe controversy regarding the space group of the LT (Cm2m)and HT (Cmcm) phases. It was also concluded that the phasetransition upon cooling is mainly due to the off-center shifts ofNb51 and one-third of the Gd31 ions within their correspondingpolyhedra.

Acknowledgments

The authors would like to thank all the scientists, especially Brian Toby, at11-BM Advanced Photon Source, Argonne National Lab.

References

1J. G. Allpress and H. J. Rossell, ‘‘Fluorite-Related Phases Ln3MO7,Ln5Rare-Earth, Y, or Sc, M5Nb, Sb, or Ta .1. Crystal-Chemistry,’’ J. SolidState Chem., 27 [1] 105–14 (1979).

2N. Barrier, P. Gall, and P. Gougeon, ‘‘Rare-Earth Site Splitting in Sm3MoO7,’’Acta Crystallogr. Sect. C: Cryst. Struct. Commun., 63, I102–4 (2007).

3N. Barrier and P. Gougeon, ‘‘Pr3MoO7,’’ Acta Crystallogr. Sect. E: Struct.Rep. Online, 59, I22–4 (2003).

4L. Cai and J. C. Nino, ‘‘Structure and Dielectric Properties of Ln3NbO7

(Ln5Nd, Gd, Dy, Er, Yb and Y),’’ J. Eur. Ceram. Soc., 27 [13–15] 3971–6 (2007).5T. Fennell, S. T. Bramwell, and M. A. Green, ‘‘Structural and Magnetic Char-

acterization of Ho3SbO7 and Dy3SbO7,’’ Can. J. Phys., 79 [11–12] 1415–9 (2001).

Fig. 7. (a) Polyhedral view of the Gd3NbO7 unit cell. (b) [0.105, �1, 0.132] view of the NbO6 octahedron at the high- and low-temperature phases. Atthe high-temperature phase, the position of the centered Nb (black circle) has both twofold (line and ellipse) and mirror (translucent plane) symmetry. Atthe low-temperature phase, Nb displaces away from the twofold axes but still in the mirror plane. (c) [1, 0.02,�0.19] view of the NbO6 octahedron at thehigh- and low-temperature phases. (d) [100] view of Gd3NbO7, rendered spheres indicate atomic positions at the high-temperature phase; black circlesshow atomic positions at the low-temperature phase. (e) [100] view of spacing filling Nb and Gd-1 chains parallel to the [001] direction; black circles showthe center positions at the high-temperature phase, whose spacing is c/2. The black arrows above the atoms indicate the displacement orientation fromthe center positions. (f) Gd1O8 polyhedron at high- and low-temperature phases. At the low-temperature phase, Gd1 moves away from the twofold axeswithin the mirror plane. (g) [�.171, �0.06, 1] view of the Gd1O8 polyhedron at high- and low-temperature phases.

March 2010 Phase Transition in Weberite-Type Gd3NbO7 879

6W. R. Gemmill, M. D. Smith, and H. C. Z. Loye, ‘‘Synthesis and CrystalStructure of the Fluorite-Related Ruthenate, Trilanthanum Ruthenium Septaox-ide La3RuO7,’’ J. Chem. Crystallogr., 37 [12] 793–5 (2007).

7W. R. Gemmill, M. D. Smith, Y. A. Mozharivsky, G. J. Miller, and H. C. zurLoye, ‘‘Crystal Growth, Structural Transitions, and Magnetic Properties of theFluorite-Related Osmates: Sm3OsO7, Eu3OsO7, and Gd3OsO7,’’ Inorg. Chem., 44[20] 7047–55 (2005).

8W. R. Gemmill, M. D. Smith, and H. C. zur Loye, ‘‘Crystal Growth, Obser-vation, and Characterization of the Low-Temperature Structure of the Fluorite-Related Ruthenates: Sm3RuO7 and Eu3RuO7,’’ Inorg. Chem., 43 [14] 4254–61(2004).

9J. E. Greedan, N. P. Raju, A. Wegner, P. Gougeon, and J. Padiou, ‘‘A Study ofthe Structure and Electronic and Thermal Properties of Quasi-One-DimensionalLa3MoO7,’’ J. Solid State Chem., 129 [2] 320–7 (1997).

10D. Harada and Y. Hinatsu, ‘‘A Study of theMagnetic and Thermal Propertiesof Ln3RuO7 (Ln5Sm, Eu),’’ J. Solid State Chem., 158 [2] 245–53 (2001).

11D. Harada and Y. Hinatsu, ‘‘Magnetic and Calorimetric Studies on One-Dimensional Ln3RuO7 (Ln5Pr, Gd),’’ J. Solid State Chem., 164 [1] 163–8(2002).

12D. Harada, Y. Hinatsu, and Y. Ishii, ‘‘Studies on the Magnetic and StructuralPhase Transitions of Nd3RuO7,’’ J. Phys. Condens. Mat., 13 [48] 10825–36(2001).

13Y. Hinatsu, M. Wakeshima, N. Kawabuchi, and N. Taira, ‘‘Structures andMagnetic Properties of Rare Earth Rhenium Oxides Ln3ReO7 (Ln5Gd, Tb, andDy),’’ J. Alloys Compd., 374 [1–2] 79–83 (2004).

14N. Ishizawa, K. Hiraga, D.du Boulay, H. Hibino, T. Ida, and S. Oishi,‘‘A Non-Centrosymmetric Polymorph of Gd3RuO7,’’ Acta Crystallogr. Sect.E: Struct. Rep. Online, 62, I13–6 (2006).

15N. Ishizawa, T. Suwa, and K. Tateishi, ‘‘Dy3RuO7 with Partial StructuralDisorder,’’ Acta Crystallogr. Sect. E: Struct. Rep. Online, 63, I163–U97 (2007).

16N. Ishizawa, T. Suwa, K. Tateishi, and L. R. Hester, ‘‘Synchrotron X-RayStudy of Non-Centrosymmetric Tb3RuO7 with Partial Structural Disorder,’’ ActaCrystallogr. Sect. C: Cryst. Struct. Commun., 63, I43–6 (2007).

17N. Ishizawa, K. Tateishi, S. Kondo, and T. Suwa, ‘‘Structural Phase Transi-tion of Gd3RuO7,’’ Inorg. Chem., 47 [2] 560–6 (2008).

18A. Kahnharari, L. Mazerolles, D. Michel, and F. Robert, ‘‘StructuralDescription of La3NbO7,’’ J. Solid State Chem., 116 [1] 103–6 (1995).

19P. Khalifah, R. W. Erwin, J. W. Lynn, Q. Huang, B. Batlogg, and R. J. Cava,‘‘Magnetic and Electronic Characterization of Quasi-One-DimensionalLa3RuO7,’’ Phys. Rev. B, 60 [13] 9573–8 (1999).

20P. Khalifah, Q. Huang, J. W. Lynn, R. W. Erwin, and R. J. Cava, ‘‘Synthesisand Crystal Structure of La3RuO7,’’ Mater. Res. Bull., 35 [1] 1–7 (2000).

21H. Nishimine, Y. Doi, Y. Hinatsu, andM. Sato, ‘‘Phase Transition of Ln3IrO7

(Ln5Pr, Nd, Sm, Eu) and its Low-Temperature Structure,’’ J. Ceram. Soc. Jpn.,115 [1346] 577–81 (2007).

22H. P. Rooksby and E. A. D. White, ‘‘Rare-Earth Niobates and Tantalates ofDefect Fluorite-Type and Weberite-Type Structures,’’ J. Am. Ceram. Soc., 47 [2]94–6 (1964).

23H. J. Rossell, ‘‘Fluorite-Related Phases Ln3MO7, Ln5Rare-Earth, Y or Sc,M5Nb, Sb, or Ta .2. Structure Determination,’’ J. Solid State Chem., 27 [1] 115–22 (1979).

24F. P. F. Vanberkel and D. J. W. Ijdo, ‘‘The Orthorhombic Fluorite Related-Compounds Ln3RuO7, Ln5Nd, Sm and Eu,’’ Mater. Res. Bull., 21 [9] 1103–6(1986).

25J. F. Vente, R. B. Helmholdt, and D. J. W. Ijdo, ‘‘The Structure and Mag-netic-Properties of Pr3MO7 with M5Nb, Ta, and Sb,’’ J. Solid State Chem., 108[1] 18–23 (1994).

26M. Wakeshima and Y. Hinatsu, ‘‘Magnetic Properties of Lanthanide Rhe-nium Oxides Ln3ReO7 (Ln5Sm, Eu, Ho),’’ J. Solid State Chem., 179 [11] 3575–81(2006).

27M. Wakeshima, H. Nishimine, and Y. Hinatsu, ‘‘Crystal Structures and Mag-netic Properties of Rare Earth Tantalates RE3TaO7 (RE5Rare Earths),’’ J. Phys.Condens. Mater., 16 [23] 4103–20 (2004).

28G. Wltschek, H. Paulus, I. Svoboda, H. Ehrenberg, and H. Fuess, ‘‘CrystalStructure and Magnetic Properties of Sm3ReO7,’’ J. Solid State Chem., 125 [1] 1–4(1996).

29A. V. Astafyev, V. P. Sirotinkin, and S. Y. Stefanovich, ‘‘Phase-Transitions inthe Compounds Sm3NbO7 and Gd3NbO7 with a Fluorite-Like Structure,’’Kristallografiya, 30 [3] 603–4 (1985).

30A. N. Klimenko, V. M. Ionov, N. A. Tomilin, V. S. Sergeev, V. P. Sirotinkin,A. E. Prozorovskii, V. B. Rybakov, and S. G. Zhukov, ‘‘Phase-Transformations inRare-Earth-Metals of R3NbO7 Niobates at High-Temperatures,’’ Zh NeorgKhim1, 35 [3] 599–603 (1990).

31S. A. Kovyazina, L. A. Perelyaeva, I. A. Leonidov, and Y. A. Bakhteeva,‘‘High-Temperature Structural Disorder in R3NbO7,’’ J. Struct. Chem., 44 [6] 975–9 (2003).

32H. Nishimine, M. Wakeshima, and Y. Hinatsu, ‘‘Structures, Magnetic, andThermal Properties of Ln3MoO7 (Ln5La, Pr, Nd, Sm, and Eu),’’ J. Solid StateChem., 178 [4] 1221–9 (2005).

33V. P. Sirotinkin, A. A. Evdokimov, and V. K. Trunov, ‘‘Parameter Improve-ment of Nucleus of Ln3NbO7 and Ln3TaO7 Compounds,’’ Zh Neorg Khim1, 27[7] 1648–51 (1982).

34ASTM International. ASTM E1269-05, Standard Test Method for Determin-ing Specific Heat Capacity by Differential Scanning Calorimetry. ASTM Interna-tional, West Conshohocken, PA, 2005.

35R. Shirley, The CRYSFIRE System for Automatic Powder Indexing. (2004).36L. Cai and J. C. Nino, ‘‘Complex Ceramic Structures. 1.Weberites,’’ Acta

Crystallogr. Sect. B: Struct. Sci., B65, 269–90 (2009).37G. Frenzen, W. Massa, D. Babel, N. Ruchaud, J. Grannec, A. Tressaud, and

P. Hagenmuller, ‘‘Structure and Magnetic-Behavior of the Na2NiInF7 Weberite,’’J. Solid State Chem., 98 [1] 121–7 (1992).

38W. Kraus and G. Nolze, Powdercell for Windows Version 2.4. (2000).39B. H. Toby, ‘‘EXPGUI, A Graphical User Interface for GSAS,’’ J. Appl.

Crystallogr., 34, 210–3 (2001).40A. C. Larson and R. B. V. Dreele, ‘‘General Structure Analysis System

(GSAS) Los Alamos National Laboratory Report, LAUR 86-748, 2004. &

880 Journal of the American Ceramic Society—Cai et al. Vol. 93, No. 3