Structures and Optical Properties of Pt and Pd Compounds with Charge-Density-Waves - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 11

3.6

δ

3.4

-

3+ ρ

3- ρ

M---X-----M

l 2

l 1 l 2

3.2

L

3.0

2.8

X = I

7

15 14

8

2.6

11

6

10 4

5

16

X = Br

13

2.4 3

9 2 1

X = Cl

12

2.2

l 1

2.0

4.8 5.0 5.2 5.4 5.6 5.8 6.0

(M-M) distance L (Å)

Fig. 2.3 Correlation between the M-M distance ( L ) and the M-X distance ( l 1 and l 2 ). The material
for each number is listed in Table 2.1 . Data of the homometal compounds are represented by
circles for M ¼ Pt, triangles for M ¼ Pd, and squares for M ¼ Ni. Data of the heterometal

compounds with M ¼ Pt and Pd are represented by diamonds . Open and filled marks indicate the

compounds with Y ¼ ClO 4 and Y ¼ halogen, respectively. (Reprinted figure with permission

from [ 5 ])

2.2.1 Control of CDW Amplitudes

By replacing metals (M Cl, Br, or I), the

Coulomb repulsion energy ( U ) on the metal site and the transfer energy ( t ) between
the nearest-neighbor two metal sites can be considerably changed. It is because the
expanses of d z 2 orbitals of M and p z orbitals of X are different among the respective
metal ions (M) and bridging halogen ions (X) [ 13 , 32 ]. As a result, we can obtain a
variety of compounds having different bridging halogen displacements

¼ Pt or Pd) and bridging halogens (X ¼

from the
midpoint between the neighboring two M ions (see the inset of Fig. 2.3 ). The systematic
structural studies revealed that

d

is closely correlated to the metal to metal (M-M)
distance L , which is strongly dependent on the choice of the ligand (A) and the counter
anion (Y) and also on the strength of the H-bond between A and Y [ 32 ].
In Table 2.1 , the M-M distance L , the metal to halogen (M-X) distance l 1 and l 2
( l 1 < l 2 ), the halogen displacement

d

d ( l 2 l 1 ¼ 2 d ), and the distortion parameter d

( d ¼ / L ) are listed for various MX-chain compounds. In Fig. 2.3 , the values of l 1

and l 2 are plotted as a function of L . The dotted line indicates l 1 ¼ l 2 ¼ L /2, which
is a hypothetical line for no bridging halogen displacements (

2 d

0). The Ni
compounds having the MH states are located on this line. The deviation of l 1 and
l 2 from the dotted line corresponds to a magnitude of

d ¼

. The most significant feature
in Fig. 2.3 is that the data points fall on almost the same lines according to the
choice of bridging halogen (X) ions (Cl ,Br

d

or I ), while M, A, and Y are
replaced. It indicates the strong correlation between