Experimental determination of ionicity in MnO nanoparticles

Phonons are exquisitely sensitive to finite-length scale effects in complex materials because they are intimately connected to charge, polarizability, and structure, and a quantitative analysis of their behavior can reveal microscopic aspects of chemical bonding. To investigate these effects in a model correlated oxide, we measured the infrared vibrational properties of 8-nm particles of MnO, compared the results with the analogous bulk material, and quantified the phonon confinement with a calculation of the Born effective charge. Our analysis reveals that the Born effective charge decreases by ∼20%, compared to the bulk material. Moreover, this change impacts both ionicity and polarizability. Specifically, we find that MnO nanoparticles are ∼12% less ionic than the corresponding bulk. This discovery is important for understanding finite-length scale effects in this simple binary oxide and the more complicated functional oxides that emanate from this parent compound.