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The physics underlying the complex glassy phenomena, which accompany theformation of polarons in optimally doped manganites (ODM) is a cumbersome issuewith many unexplained aspects. In this article we present $^{139}$La and$^{55}$Mn NMR in the temperature range 80K - 900K of ODMLa$_{0.67}$Ca$_{0.33}$MnO$_3$. We show that local lattice distortions,established in the Paramagnetic (PM) phase for $T<700$K, induce a genuinespin-glass state, which for $T<T_c$ consolidates with the Ferromagnetic (FM)state into a single thermodynamic phase. Comparative NMR experiments onLa$_{0.77}$Ca$_{0.23}$MnO$_3$, La$_{0.59}$Ca$_{0.41}$MnO$_3$, andLa$_{0.70}$Sr$_{0.30}$MnO$_3$ demonstrate the dominant role of latticedistortions, which appear to control (i) the stability of the spin glass phasecomponent and (ii) the kind (1st or 2nd order) of the PM-FM phase transition.The experimental results are in agreement with the predictions of thecompressible random bond - random field Ising model, where consideration of astrain field induced by lattice distortions, is shown to invoke at $T_c$ adiscontinuous (1st order like) change of both the FM and the "glassy"Edwards-Anderson (EA) order parameters.

Spin order and lattice frustration in optimally doped manganites. A high temperature NMR study.