Systematics in the thermopower of electron-doped layered manganites

Abstract

We report on a comparative study of the transport properties of lightly La-doped layered CaO (CaMnO 3 ) m (mϭ1, 2, and ϱ͒ and similar Mn 3ϩ /Mn 4ϩ ratios. In contrast to the large and rather nonsystematic variations of the resistivity (T) for samples with the same doping and various m, the absolute thermopower ͑S͒ exhibits remarkable systematics. In the paramagnetic ͑PM͒ state, S varies linearly with temperature and its extrapolation to Tϭ0 has a finite intercept. The intercept and the slope of these lines vary systematically with m. In particular, within the experimental error, the inverse slopes of the straight lines increase linearly with 1/m, which in its turn varies linearly with the average number of Mn-Mn near neighbors. The implications of this simple systematics are discussed using a model developed by Cutler and Mott ͓Phys. Rev. 181, 1336 ͑1968͔͒ for transport by activated hopping of carriers obeying nondegenerate statistics. This model ͑or a possible extension into the semidegenerate regime͒ leads also to a simple, qualitative interpretation of S(T) of these materials on crossing the Néel temperature (T N ) from the PM state and below T N .

Non-adiabatic small polaron hopping in then= 3 Ruddlesden–Popper compound Ca4Mn3O10

“…A similar explanation has been proposed by Fisher for the change in ∂S/∂T they observe at T N for the n = 1, 2, ∞ members of (CaMnO 3 ) n (CaO) 82 and the antiferromagnet BaCoS 2 83 .…”

Synthesis of SrMnO3 powders with different morphologies by hydrothermal method

“…Furthermore, each rod has a cusp at one side, with the median diameter of 3 µm and length of 30 µm, respectively. It is evident that the morphology of as-prepared SrMnO 3 is quite different from that with KMnO 4 , Mn(CH 3 COO) 2 and Sr(CH 3 COO) 2 as the starting materials.…”

“…Moreover, it is clear that the intensity of the diffraction peaks increased and the halfpeak breadth of the diffraction peaks reduced with prolonging the reaction time, which indicated that the crystallinity of the samples increases gradually on prolonging the reaction time. Chemical analyses of the products indicate that the molar ratio of Sr:Mn is close to the stoichiometry of 1:1, and the average valence of A typical SEM image of SrMnO 3 sample synthesized with KMnO 4 , Mn(CH 3 COO) 2 and Sr(CH 3 COO) 2 as the starting materials is given in figure 3. From figure 3, it is observed that there are a large amount of microsized crystal grains free from any other particles in the sample.…”

“…3 Results and discussion 3.1 Synthesis of SrMnO 3 with KMnO 4 , Mn(CH 3 COO) 2 and Sr(CH 3 COO) 2 as the starting materials Figure 1 shows the XRD patterns for the samples synthesized with KMnO 4 , Mn(CH 3 COO) 2 and Sr(CH 3 COO) 2 as the starting materials at 200°C for the selected time between 6h and 72h. All peaks could be indexed to the hexagonal perovskite phase of SrMnO 3 , matching the JCPDS data (Card No: 72-0197) perfectly with a lattice parameter of a =5.4497Å; c=9.0976Å.…”

“…The two end members RMnO 3 and AMnO 3 are A-and G-type antiferromagnetic (AFM) insulators, respectively. Up to now, most of the studies have concentrated on the R-rich side and very little work has been reported on the A-rich side [2][3][4]. And if we focus on the prototypical La 1-x Sr x MnO 3 family, we may find that a strong effort has been devoted in studying LaMnO 3 , whereas the other parent compound, SrMnO 3 , has been mostly disregarded.…”