Zener polarons

Fourth, Jaime and Salamon (1999) have pointed out that a(T) increases more sharply than exponentially on cooling to rmax in fig. 30 and that the additional entropy transported increases on crossing the () -() phase boundary at tc. This observation is consistent with a progressive transformation from Zener to small polarons in the hole-poor phase as the hole concentration x = 0.30 in this phase is diluted by the trapping of Zener polarons in the hole-rich phase. Such a transformation would double the number of sites available to a polaron and would therefore increase the a of eq. (26) by reducing c = (1 — r)2x toward c = (1 — r)x, where r is the ratio of trapped to free polarons. In the O phase, most of the polarons appear to be small polarons at 7 N. 

Fig. 50. Type-CE antiferromagnetic order in a-b planes antiferromagnetic coupling above c-axis in space group Pbnm (a) ordering of localized occupied e orbitals after Goodenough (1955), and (b) ordering of Zener polarons.

Fig. 57. Inverse magnetic susceptibility l/x(T) of Y0.5Cao.5Mn03 and schematic picture of the magnetic state (a) PM above Tqo with formation of Zener polarons setting in below 450 K (b) PM Zener polarons order between Tqo ant-l 7n (c) CE AFI structure below 7n, after Daoud-Aladine et al. (2002).

Above Tc, the conductivity of the O phase fits the adiabatic small-polaron model a - (A/T) exp(-Ea/kT) over the entire doping range [162]. As already discussed, an electron tunnels from an Mn(III) to either an Mn(IV) ion or a Zener Mn(III)—O—Mn(IV) pair in a time Zh > where is the period of the locally cooperative oxygen displacements that, for x < 0.5, dress the mobile holes as Mn(IV) ions or as Mn(III)—O—Mn(IV) pairs. Fast electron transfer (zh < occurs within a Zener polaron. 

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