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Abstract
The effective ionic charges in lithium niobate that displays the largest spontaneous polarisation among known ferroelectrics materials, significantly vary with temperature, contrary to an approximation commonly made in the calculations of polarisation. The temperature dependence of the ionic effective charges is deduced from that of the TO-LO splittings of infrared-active phonons measured by infrared reflectivity spectroscopy at room and medium temperatures, and by infrared emissivity at high temperature up to 1550 K.