Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science4C, 9C/2005G134

Orbital ordering in RVO3 controlled by hydrostatic pressure
Hironori NAKAO1, *, Daisuke BIZEN1, Keisuke NAKATSUKA1, Youichi MURAKAMI1,22, 3
Shigeki MIYASAKA , Yoshinori TOKURA
1
Department of Physics, Tohoku University, Sendai 980-8578, Japan2
Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
3
CERC, AIST, Tsukuba 305-8562, Japan
Introduction
In addition to the charge and spin degrees of freedom,
the orbital degree of freedom plays an important role indetermining the electric and magnetic properties of theperovskite transition metal oxides. The orbital ordering isstrongly coupled with the lattice distortion, i.e. JahnTeller distortion. Therefore, the purpose of the present
study is to demonstrate the control of the orbital ordering
phases in terms of the lattice distortion by applying
pressure.
RVO3 (R: rear earth or Y) shows various physicalproperties coupled with the spin and orbital states
depending on R-ions [1]. RVO3 (Tb-La) with large Rionic radius undergoes a phase transition from an orbital
disordered state to a G-type orbital ordering (G-OO) at
TOO1. The crystal structure also changes fromorthorhombic (Pbnm) to monoclinic structure (P21/a) at
TOO1 reflecting the orbital state. Moreover, the ground stateof the magnetic structure becomes C-type spin ordering
influenced by the orbital ordering. RVO3 (Lu-Y) withsmaller R-ionic radius has the G-OO phase at TOO2< T<
TOO1, and a C-type orbital ordering (C-OO) with Pbnm
below TOO2. In addition, the ground state of the magneticstructure becomes the G-type spin ordering. Consequently,the ground state of the orbital ordering in RVO3 changes
from G-OO to C-OO by R-ions substitution. In order toclarify the pressure effect on the orbital states, thepressure-temperature phase diagrams of the orbital
ordering in RVO3 (R = Y, Tb), which locates near thephase boundary between G-OO and C-OO, have beenstudied by x-ray scattering technique.
Experiments
The high-quality single crystals of RVO3 were grownby a floating-zone method. X-ray scattering experiments
were carried out at beam line 4C and 9C using a fourcircle diffractometer. The x-ray energy was tuned to 18.0
keV with a Si(111) double-crystal monochromator, andthe beam was focused by a bent cylindrical mirror. For thelow-temperature and high-pressure experiments, a heliumgas driven diamond-anvil cell (DAC) was mounted on aclosed-cycle He cryostat, and the gas pressure and thetemperature were controlled by a computer. Pressure wasgenerated in a DAC using a 1:1 mixture of pentane andisopentane pressure medium, and was calibrated from alattice constant of NaCl enclosed with the sample in the3DAC. A typical sample size is 0.08x0.08x0.02 mm .
Results and discussions

The pressure-temperature phase diagram of the orbital
state was studied in YVO3, which has the C-OO phase as
the ground state. The intensity of the forbidden reflection(401) of Pbnm and the temperature dependences of lattice
constants were measured. The forbidden reflection is only
observed in the G-OO phase with the space group P21/a,and the intensity should be proportional to the square of
the order parameter.

The figure shows the temperature dependences of the(401) intensity under high pressure. The intensities at ~0.67 GPa are observed below TOO1 ~ 200 K and disappears
at TOO2 ~ 80 K as shown by closed circles. Namely, thespace group becomes P21/a of the G-OO at TOO2< T< TOO1,
and the C-OO appears below TOO2. The results are
consistent with the previous experiment in ambientpressure [2]. With increasing pressure, TOO2 clearly
increases and TOO1 decreases. As a result, applying
pressure stabilizes the C-OO phase, and the intermediatephase, G-OO, is expected to disappear above 6 GPa.
Namely, the phase transition from the orbital disorderedstate to the C-OO can be observed, and such a phasetransition has never been observed in the study of R-ions
substitution.
TbVO3 has the G-type as the ground state, and is
expected to locate near phase boundary between C-OO
and G-OO. We also try to switch the ground state oforbital ordering by applying pressure in TbVO3.
References[1] S. Miyasaka et al., Phys. Rev. B 68, 100406 (2003).
[2] G. R. Blake et al., Phys. Rev. B 65, 174112 (2002).
* nakao@iiyo.phys.tohoku.ac.jpUsers' Report133