Important information on the lattice dynamics can be obtained from the sound velocity vs(T) measurement. For La1-XSrXMnO3 (LSMO) and La1-XCaXMnO3 (LCMO), we reported the sound velocity anomalies associated with charge ordering. For the lower hole concentrations, the distribution of the temperature T*, at which vs anomaly occurs, was centered at X=1/8, which was consistent with the polaron-ordered phase reported by the neutron diffraction analyses. Fig.1 shows the temperature dependence of the sound velocity for the LSMO system (X=0.15). The vs anomalies were observed at the FM transition temperature Tc=170K and at the characteristic temperature T*=140K. Fig.2 shows the phase diagram of the LSMO system. T* was centered at X=1/8 and the similar behavior was also observed in the LCMO system.
left:Fig,1:The sound velocity for the LSMO system (X=0.15) right:Fig.2: Determined phase diaagram of the LSMO system by the vs(T) measurements
Fig. 3 shows the normalized sound velocity vs(T)/vs(300K) for the LCMO (X=0.30~0.80) samples. For the X=0.30 sample, vs(T) showed an abrupt upturn below the FM transition temperature Tc. However, the vs(T) upturn becomes moderate for X=0.40 and 0.45. For the X=0.48, 0.50, 0.55 samples, where the charge order (CO) phase is observed, the anomalous vs(T) upturn was observable below the charge ordered temperature TCO with a slight vs(T) softening just above TCO. The vs softening becomes deeper with increasing the Ca content X (=0.60, 0.67). In this way, the vs behavior in the CO phase shows the characteristic temperature dependence at X=0.50, 0.67 and 0.75, which may results from the charge stripe structure observed by the electron diffraction.
Fig,3:The normalized sound velocity vs(T)/vs(300K) for the LCMO system (X=0.30~0.95)
As recent revived studies on perovskite-based manganese have confirmed, it has been widely recognized that the local lattice distortions of the MnO6 octahedra play an important role in determining the transport properties of the doped holes and the complex behaviors of magnetic and structural transitions. The thermal conductivity k is a valuable tool to investigate the effect of the lattice dynamics near the phase transitions of the manganite system. Fig.4(a) shows the temperature dependence of the thermal conductivity k(T) for the LCMO system (X=0~0.35). For the ferromagnetic X<0.15 sample, whose electrical resistivity r(T) behaves nonmetallic, k(T) is low and no anomaly is observed around Tc. k(T) of La0.85Ca0.15MnO3 suggests the existence of very strong phonon scattering over the entire temperature range. On the other hand, for the X>0.20 samples which show the FM metallic behavior, we can see that the k(T) shows an enhancement below Tc, which increases and becomes step-like just below Tc for X=0.25 and 0.30. The k(T) enhancement should not be linked only to the ferromagnetism but rather to the electronic transition to metallic state which suppresses significantly the amplitude of Jahn-Teller (J-T) distortion.
Fig.4(b) shows the thermal conductivity k(T) for the X=0.40~0.60 samples. For the X=0.40 and 0.45 samples which are FM and metallic, k(T) shows the upturn below Tc, though the step-like jump below Tc almost disappears. Because of reduction in J-T active Mn3+ ion concentration with increasing X, the effect of relaxation of the local lattice distortion may be somewhat weakened in these samples. For the X=0.50 sample, in which the FM state vanishes and the CO state appears, the magnitude of k(T) drastically decreases and the k(T) reduction is observable around the CO transition temperature TCO (=230K). Thus, the k(T) behaviors of X=0.45 and 0.50 samples show a clear contrast reflecting the difference in the ordered phase. The X=0.60 sample shows the similar k(T) behavior as X=0.50. The thermal dilatation dL(T)/L for X=0.45 decreases below Tc with decreasing temperature, while for the CO samples (X>0.50), the thermal dilatation shows quite a opposite character; dL(T)/L abruptly increases below TCO. The increase of dL(T)/L below TCO in contrast to the decrease below the Curie temperature Tc of the metallic samples suggests that the lattice distortion rather increases below TCO. It should also be noticed that r(T) increases below TCO, which may inhibit the relaxation of the J-T distortion due to mobile carriers. Therefore the k(T) reduction below TCO may come from the increase of the lattice distortion.
Fig,4:Thermal conductivity k(T) for the LCMO system
The phase diagram was proposed for the LSMO and LCMO systems using the temperatures at which the anomalies were observed by the electrical resistivity r(T), thermal conductivity k(T), thermal dilatation dL(T)/L and the sound velocity vs(T).
Fig.5:Phase diagram of the LSMO system
Fig.6:Phase diagram of the LCMO system