APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010;
Portland, Oregon
Session X36: Focus Session: Bulk Properties of Complex Oxides -- Novel Systems
2:30 PM–5:30 PM,
Thursday, March 18, 2010
Room: E146
Sponsoring
Units:
DMP GMAG
Chair: Ram Seshadri, University of California, Santa Barbara
Abstract ID: BAPS.2010.MAR.X36.1
Abstract: X36.00001 : Hollandites: a novel class of oxides with unusual properties
2:30 PM–3:06 PM
Preview Abstract
Abstract
Author:
Yutaka Ueda
(Institute for Solid State Physics, University of Tokyo)
Hollandite with a general chemical formula $A_{x}M_{8}$O$_{16}$
($x\le $2) is
one of minerals. Its crystal structure consists of the
$M_{8}$O$_{16}$-framework and $A$-cations at tunnel sites in the
$M_{8}$O$_{16}$-framework. The most popular crystal symmetry is a
tetragonal
$I$4/$m$ in which the crystallographycally independent $M$ site
is unique, hence,
$A^{+}_{2}M_{8}$O$_{16}$ is a mixed valent oxide with
$M^{3+}$/$M^{4+}$ = 1/3.
We have synthesized K$_{2}$V$_{8}$O$_{16}$ [1] and
K$_{2}$Cr$_{8}$O$_{16}$
[2] in both powder and single crystal form under high pressure
and found
metal-insulator (MI) transitions. The manner of MI transitions,
however, is
very different between both compounds. K$_{2}$V$_{8}$O$_{16}$
exhibits a
first order MI transition at around 170 K. The magnetic
susceptibility is
reduced to a small value at the transition, suggesting the
formation of
V$^{4+}$-V$^{4+}$ singlet pairs and V$^{3+}$-V$^{3+ }$pairs in
the low
temperature insulator phase. The transition is accompanied by the
structural
change from a tetragonal to a monoclinic structure. The low
temperature
phase has a superlattice of $\surd $2 x $\surd $2 x 2, suggesting
a charge
ordering of V$^{4+}$ and V$^{3+}$. On the other hand,
K$_{2}$Cr$_{8}$O$_{16}$ is a ferromagnetic metal (or half-metal)
with
$T_{C}$ = 180 K and shows a transition to an insulator at 95 K
without any
apparent structural change but retaining ferromagnetism.
K$_{2}$Cr$_{8}$O$_{16}$ is quite unique in three aspects: It has
a rare
mixed valence of Cr$^{3+}$ and Cr$^{4+}$; it has a metal (or
half-metal) to
insulator transition in a ferromagnetic state; and the resulting low
temperature phase is a rare case of a ferromagnetic insulator. This
discovery could open a new frontier on the relation of magnetism and
conducting properties in strongly correlated electron systems.
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References:\\[0pt]
[1] M. Isobe, S. Koishi, N. Kouno, J. Yamaura, T. Yamauchi, H.
Ueda, H. Gotou, T. Yagi, and Y. Ueda, J. Phys. Soc. Jpn.
\textbf{75} (2006) 073801.
\\[0pt]
[2] K. Hasegawa, M. Isobe, T. Yamauchi, H. Ueda, J-I. Yamaura, H.
Gotou, T. Yagi, H. Sato, and Y. Ueda, Phys. Rev. Lett.
\textbf{103 }(2009) 146403.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2010.MAR.X36.1