Dilatometric Study of LiNH₄SO₄ Crystals with Manganese Impurity

A crystal of lithium-ammonium sulfate with a manganese impurity of 5% has been synthesized and its structural parameters (the atomic coordinates and the unit cell parameters) have been specified. The introduction of the impurity was found to change the absolute values of the thermal expansion coefficient Δl/l₀, but not its behavior. Furthermore, a negative thermal expansion along the specimen Z direction is revealed near the phase transition point. It is shown that the introduction of the impurity shifts the phase transition point toward lower temperatures from 461 K (for the pure crystal) to 455.7 K (for the impurity-doped crystal), reduces the linear expansion coefficient αi in the interval of positive thermal expansion, increases it in the interval of negative thermal expansion, and extends the temperature interval, where the thermal expansion coefficient is negative. Indicative surfaces of the thermal expansion coefficient in the ferroelectric and paraelectric phases have been plotted. The independent study and the verification of the phase transition have been performed with the use of the method of differential thermal analysis. (author)