| Summary: | We have studied, both experimentally and theoretically, the unusual temperature dependence of the phonon spectra in NdCoO<sub>3</sub>, SmCoO<sub>3</sub> and GdCoO<sub>3</sub>, where the Co<sup>3+</sup> ion is in the low-spin (LS) ground state, and at the finite temperature, the high-spin (HS) term has a nonzero concentration <inline-formula><math display="inline"><semantics><mrow><msub><mi>n</mi><mrow><mi>H</mi><mi>S</mi></mrow></msub></mrow></semantics></math></inline-formula> due to multiplicity fluctuations. We measured the absorption spectra in polycrystalline and nanostructured samples in the temperature range 3–550 K and found a quite strong breathing mode softening that cannot be explained by standard lattice anharmonicity. We showed that the anharmonicity in the electron–phonon interaction is responsible for this red shift proportional to the <inline-formula><math display="inline"><semantics><mrow><msub><mi>n</mi><mrow><mi>H</mi><mi>S</mi></mrow></msub></mrow></semantics></math></inline-formula> concentration.
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