Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

1B, 4C, 16A2/2004G244

Orbital Ordering and the dilute effect in copper fluoride
Youichi MURAKAMI*1,2, Naka TATAMI1, Hidekazu TAMURA1, Hironori NAKAO1,Takeshi MATSUMURA1, Yusuke WAKABAYASHI3, Hiroshi SAWA3, Shinichi ITO4
1
Department of Physics, Tohoku University, Sendai 980-8578, Japan
2
Synchrotron Radiation Research Center, JAERI, Sayo, 679-5148, Japan
3
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
4
KEK-KENS, Tsukuba, Ibaraki 305-0801, JapanIntroduction
It is well kown that charge-, spin-, and orbital orderings
play important roles in strongly correlated electronsystems. The long-range orderings have successfullyobserved by resonant x-ray scattering (RXS) and neutron
scattering and so on. Impurity effect in the ordered states
has raised interesting results; for example, in high Tc
cuprates a small amount of impurity destroys thesuperconductivity and revive the magnetic order. The
impurity effect in localied spin systems has also been
intensively studied in the past 30 years. On the other
hand, few studies on the effect are reported in orbitallyodered systems. We have investigated the effect on atypical orbital ordering system KCuF3 by substituting Znfor Cu. In this case we call the effect as the dilute effect10because Zn ion has a closed shell (3d ).
Experimental Results
Powder Diffraction and Resonant X-ray Scattering
High quality single crystals were grown using
Bridgman method.
Powder samples with uniform
concentraiton were prepared by crushing the single
crystals. Powder diffraction experiments were carried outat BL-1B using the imaging plate. The RXS experimentsat Cu K absorption edge energy have been done at BL-4C
and BL-16A2 with four circle diffractometer.
Figure 1 shows the lattice constants a, c, measured byFigure 1. Lattice constants a, c at room temperature as afunction of Cu concentration. Inset: Concentration
dependence of RXS intensities.

powder diffraction experiments, as a function of Cu
concentration at room temperature. Increasing Zn ions
we found the difference between a and c become smalland zero around x = 0.6. This result indicates that a
structural phase transition from tetragnal to cuic occurs
near this concentration. It is supposed Jahn-Teller
distortion due to the orbital ordering in high Cu
concentration releases with decreasing Cu concentration.
To confirm this we have applied the RXS technique tothis system. The RXS intensity corresponds to the orderparameter of the orbital ordering through Jahn-Teller
effect. The RXS intensities rapidly decrease withdecreasing Cu ions and disappear around x = 0.6 at room
temperature as shown in the inset of Fig. 1. Therefore, itis elucidated that the tetragonal and cubic phases are an
orbital ordered and disordered phases, respectively.
We have made concentration-temparature phase
diagram from the powder diffraction data measured inhigh and low temperature regions using furnace andcryostat. The phase diagram is shown in Fig. 2. The
ordering temperature decreases with decreasing Cu ionsand disappears at x = 0.55 in the extrapolation to T = 0 K.
This concentration is much larger than that of theconventional dilute mangets: the arrow in Fig. 2 shows
the concentration of order-disorder threshold in a 3dimensional magnet of simple cubic. This result isattributed to the orbital modulation around the Zn site; themodulation may expand to several unit cells for one Znion.

Figure 2. Cu concentration vs. temprature phase
diagram.
* murakami@iiyo.phys.tohoku.ac.jp

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