Physica B 284}288 (2000) 1007}1008

Heat transport in TH-phase (NdCe Sr ) CuO
\V\W V W superconductors
Manabu Ikebe*, Hiroyuki Fujishiro
Faculty of Engineering, Department of Materials Science, Iwate University, 4-3-5 Ueda, Morioka 020-8551, JapanAbstract
The thermal conductivity i of (NdCe Sr ) CuO (NCSCO) with TH-structure have been measured and\V\W V W 
compared with those of La
Sr CuO (LSCO) with T-structure and of Nd
Ce CuO (NCCO) with T-structure.
\V V
\V V
The phonon scattering mechanisms e!ective in NCSCO have been analyzed.  2000 Elsevier Science B.V. All rights
reserved.
Keywords: Apical oxygen; NdSr CuO ; Phonon scattering; Thermal conductivity; TH-phase superconductor
\V V

Among the so-called (214) oxide superconductors,
there are three closely related tetragonal structures which
are named as T-phase with Cu}O octahedra(LaSr CuO (LSCO)), T-phase with Cu}O squares
\V V
(NdCe CuO (NCCO)) and TH-phase with Cu}O\V V
pyramids ((NdCe Sr ) CuO (NCSCO)). The\V\W V W magnitude of the thermal conductivity i and its temperature dependence are quite di!erent between T- andT-phase [1,2]. The existence of two apical oxygen atoms
above and below the unit Cu}O square may explain thereduce of i in the T-phase system. Although there aremany investigations about the structure and superconductivity for TH-phase system, there have been no reported data for thermal transport properties. We fabricated the single-phase sintered TH-phase superconductors, (NdCe Sr ) CuO (x"0.0875, y"0.2), and\V\W V W 
measured the thermal conductivity. TH-phase materials
have an intermediate structure between T- and T-phasewith one apical oxygen above the Cu}O square.
(NdCe Sr ) CuO (x"0.0875, y"0.2) poly\V\W V W crystals were prepared from stoichiometric mixtures of
Nd O , CeO , SrCO and CuO raw powders. The mix tures were calcined twice at 9003C for 12 h in air. They
were pressed into pellets and sintered at 11503C for 18 hin air and furnace-cooled. Finally, the samples were* Corresponding author. Fax: #81-19-621-6373.
E-mail address: ikebe@iwate-u.ac.jp (M. Ikebe)heat-treated at various temperatures ¹ from 10003C to&2
12103C for 24 h in #owing oxygen and furnace-cooled.
The thermal conductivity was measured between 10 and
150 K by a continuous heat-#ow method using an automated measuring system [1]. The electrical resistivityo(¹) of the samples decreased with increasing ¹ . The&2¹ "12103C sample with single-phase TH structure has&2
a small and metallic o(¹), and exhibits the highest¹ (¹"23 K, ¹"16 K).
 
Fig. 1 shows the temperature dependence of the thermal conductivity i(¹) of (NdCe Sr ) CuO (TH\V\W V W 
phase,¹"16 K), LaSr CuO(T-phase:
\V V
x"0.15: ¹ "38 K) and NdCe CuO (T-phase:

\V V

x"0.15: ¹ "20 K). In contrast to T-phase, the mag
nitude of i is comparatively small for T- and TH-phasesamples. There is some di!erence in i(¹) behavior between T- and TH-phase. In T-phase, i(¹) is reduced atlower temperatures more heavily than that in TH-phase.
We assume that electron component i (¹) follows theWiedemann}Franz law and calculate i (¹) on the basis
of the relaxation time approximation [3,4], where thetotal phonon scattering rate q\ is given byq\"q\#Ux exp(!H /a¹)#S(¹x)


"#P(¹x)#E(¹x)g(x, y)#K(¹x).

(1)

Here, x is the reduced phonon frequency, q is thephonon relaxation time due to grain boundaries and P, S,
E and Ux exp(!H /a¹) (H "400 K, a"1.8) refer to""0921-4526/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 3 5 1 - 0

1008

M. Ikebe, H. Fujishiro / Physica B 284}288 (2000) 1007}1008

Fig. 1. Temperature dependence of the thermal conductivity
i(¹) of the La
Sr CuO (x"0.15: T-phase), NdCe CuO\V V
\V V
(x"0.15: T-phase) and (NdCe Sr ) CuO (x"0.0875,\V\W V W 
y"0.2: TH-phase) samples. The "tting curves of i for the T-,
TH- and T-phase are also shown.
Table 1Fitting parameters determined by i analyses for T-, TH- andT-phaseq\(s\)U(s\)S(K\s\)
P(K\s\)
E(K\s\)
K(K\s\)determined parameters are summarized in Table 1. The
phonon scattering strengths by point defects (P), sheetlike faults (S) and umklapp process (U) for TH-phase arelarger than those for T-phase. However, a steeper reduction in i(¹) of LSCO (T-phase) at low temperatures canbe reproduced only by a larger K¹x term than that forTH-phase. We have proposed that a larger K¹x term ofT-phase comes from the tunneling-like motion of theapical oxygens related to the structural instability between LTO phase and HTT phase, because the structuraltransformation is provoked most directly by the changeof apical oxygen sites [4,5]. For TH-phase, K term isabout a half of that for T-phase, which might re#ect therelative number of apical oxygens in both structures. The
phonon scattering by two-level-like tunneling of apicaloxygen may reduce the phonon transport also in THphase NCSCO, though not so strongly as in T-phase
LSCO.

T-phase [1]TH-phaseT-phase [2]References

6.4;10
05.5;10
1.5;10
2.5;10
2.5;10

6.7;10
3.7;10
1.4;10
3.0;10
01.3;10

3.6;10
6.9;10
04.3;10
00[1] H. Fujishiro, M. Ikebe, M. Yagi, K. Nakasato, Y. Shibazaki,T. Fukase, J. Low Temp. Phys. 105 (1996) 981.
[2] M. Ikebe, H. Fujishiro, M. Yagi, T. Fukase, Superlatt. andMicrostruc. 21 (1997) 357.
[3] R. Berman, Thermal conduction in Solids, Clarendon Press,Oxford, New York, 1976.
[4] L. Tewordt, T. Wokhausen, Solid State Commun. 75 (1990)
515.
[5] H. Fujishiro, M. Ikebe, Physica B 263 (1999) 691.

the strength of the scattering by point defects, sheet-likefaults, electrons and other phonons, respectively. The"tting curves for the thermal conductivity i of T-, Tand TH-phase samples are also shown in Fig. 1. The