Local Structure in Pyrochlore Niobates
S. Torigoe1, N. Hanasaki2, Y. Nogami1, D. Matsumura3, K. Yoshii3,Y. Yoneda3 and Y. Nishihata3
1
Department of Physics, Okayama University, 2Department of Physics, Osaka University,
3
Synchrotron Radiation Research Center, Japan Atomic Energy Agency

Fourier Transform of k3-weighted χ( k)The spin frustration in the pyrochlore oxides, which have the chemical formula of A2B2O7, provides
interesting phenomena such as the spin ice state, where the two-in-two-out spin configuration is
produced in the tetrahedra of the A site. If it is possible that the occupation of the electron (hole) in theB site in the pyrochlore structure corresponds to the out (in) state of the spin, one can expect the“charge ice” state as the analogy of the spin ice. If this local charge order is also coupled with thelattice distortion, the local dipole moment is anticipated to be formed in the Nb tetrahedra. For therealization of the charge ice state, the average valence of the transition-metal ion has to be a half
integer in the pyrochlore oxides. Istomin et al. synthesized the pyrochlore niobates YCaNb2O7 andNdCaNb2O7 [1], in which the Nb valence is +4.5. Recently, Cava et al. also reported the neutron
diffraction in YCaNb2O7 and performed its structure analysis [2].
The pyrochlore niobates YCaNb2O7 and NdCaNb2O7 are synthesized by use of the floating zone
furnace. In these compounds, we find the Debye relaxation of the dielectric constants suggesting theferroelectric state. It is essential to investigate the Nb charge and its position, in order to investigate themechanism of the dielectric properties. We measured the X-ray absorption spectra (XAS) in
SPring-8(BL14B1). The Nb K edge energy depends on the average charge of the Nb ion. The Nbcharge in YCaNb2O7 and NdCaNb2O7 is estimated to be about 4.5 by the energy position of the K edge.
This XAS result is in agreement with the Nb charge estimated by the thermobalance method. Figure 1
shows the Fourier transform of the EXAFS (extended X-ray absorption fine structure) function. Thepeak around r=1.5Åcorresponds to the nearest oxygen from the Nb ion, while the peak around r=3.3Å to the nearest neighbor Nb ion and the Y, Nd and Ca ions in the A-site. In the pyrochlore oxides,
the B-site ion is located in the center of the oxygen octahedron. The oxygen sites around the Nb ion
are equivalent in ideal pyrochlore structure. The height of the peak around r=1.5Åis reduced inYCaNb2O7 and NdCaNb2O7, as compared to Cd2Nb2O7, which has the ideal pyrochlore structure at theroom temperature. Thus, the Fourier transform of the EXAFS function reveals that the nearest oxygensites are not equivalent in YCaNb2O7 and NdCaNb2O7. Cava et al. reported that the Nb ion isdisplaced either towards or away from the centers of the B-site tetrahedra [2]. Assuming this type of
the Nb displacement, we calculated the EXAFS function in the cluster containing the Nd, Y, Ca, Nb
and O ions around the Nb ion. Here, we consider the case of the two Nb displacements (the two-in ortwo-out configuration) or the four Nb
displacements (the two-in-two-out configuration)of the B-site tetrahedra. In the two-in-two-out
Cd2Nb2O7
configuration, it is difficult to reproduce theY2Nb2O7
structure of the two peaks around r=3Å, since
10 Nb-O Nb-A,Nb YCaNb2O7
the Nb-Nb distance is not sensitive to the Nb
NdCaNb2O7
displacement. This analysis around the peaks near
r=1.5 Å and r=3.3 Å indicate the Nb
5displacements of about 0.041Å~0.103Å, whichis in agreement with the result of the neutronRTdiffraction [2]. I will discuss the origin of theferroelectric state and the Nb displacement in the0pyrochlore niobates.
0123456[1] S.Y.Istomin et al., Mater. Res. Bull. 32, 421 (1997).
distance r(Å)[2] T.M.McQueen et al., J. Phys: Condens. Matter 20,235210 (2008).

Fig.1 : The Fourier transform of the EXAFS functionin the pyrochlore Niobates