In a recent letter,¹ Tang et al. have reported the synthesis and giant magnetoresistance properties of the substituted manganate, La_1–xAg_xMnO₃. It has been shown that single perovskite phase is obtained for x0.25 and two phases containing a magnetic perovskite phase and a nonmagnetic phase which is Ag metal are observed for x>0.25. This comment is to show that the reported Ag substituted compositions with x<0.25 are self-doped manganates La_1–xMnO_3–. The higher T_cs observed for these compositions are due to oxidation of Mn^{3 +} to Mn^{4 +} because of the presence of Ag during synthesis, as reported for thin film samples of La_1–xMnO_3–.^2,3 Even for x = 0.2, we have observed the presence of Ag metal in the powder x-ray diffraction (XRD) pattern and a self-doped composition La_0.8MnO_3– shows enhanced T_c after annealing with a smaller amount of Ag₂O at low temperatures.

Powder XRD patterns of a self-doped composition La_0.8MnO_3–(LMO) and another composition with x = 0.2 of Ag (LAMO), synthesized under identical conditions as reported by Tang et al.,¹ are compared in Fig. 1. Identical XRD patterns corresponding to a rhombohedral structure are ob-tained for both LMO and LAMO, with a slightly smaller lattice parameter for LAMO. The most intense reflection of Ag is expected at 38.1° using Cu K radiation and a weak reflection from the (113) set of planes from the rhombohedral lattice of La_1–xMnO_3– is also observed close to the same 2 value. Therefore, it is difficult to detect the presence of Ag in the low-Ag containing compositions reported in Ref. 1, as the peak at 2= 38.1° may be indexed to the reflection from the rhombohedral lattice. The inset of Fig. 1 shows the expanded XRD patterns of LMO and LAMO in this 2 range. The relative intensities of all other reflections are comparable for both compounds and therefore the enhanced intensity of the peak at 38.1° for LAMO is due to the overlapping reflection from Ag. Similarly, a very weak peak corresponding to the second most intense reflection of Ag is also observed at 44.3° in the XRD pattern of LAMO.

Figure 1.

A pellet of La_0.8MnO_3– was annealed at 850 °C after adding Ag₂O corresponding to x = 0.1 (Ag–LMO). Zero-field cooled (ZFC) magnetization curves of LMO and LAMO are shown in Fig. 2 along with the magnetization curve of Ag–LMO. It may be seen that Ag–LMO also shows T_c almost identical to that of LAMO. Earlier studies on thin film samples of La_1–xMnO_3– have indicated that T_c is considerably increased after annealing with Ag (Ref. 2) or in oxygen³ and this is due to the decrease in the Mn^{3 +} /Mn^{4 +} ratio due to incorporation of additional oxygen in the structure. A similar situation happens in the present case also. Ag₂O releases oxygen which oxygenates La_1–xMnO_3– thereby increasing T_c due to increase in the Mn^{4 +} content. The slightly lower value of the lattice parameter obtained for LAMO is consistent with the increase in the Mn^{4 +} content as reported.³ Therefore, the conclusion that Ag in LaMnO₃ forms a single perovskite phase is questionable.

Figure 2.