The Linear Thermal Expansion of Sodium Tungstate Between 20° and 600°

(1) The linear thermal expansion of anhydrous sodium tungstate has been determined between 20° and 600°C by means of an interferometer, with results given by the following equations: $$\begin{array}{c}{\mathrm{v=v}}_{20}{\text{[1+53.194(t−20)10}}^{\text{−6}}{\text{+41.5(t−20)}}^2{10}^{\text{−9}}\mathrm{],}\\ {\mathrm{\Delta l/l}}_{20}{\text{=17.73(t−20)10}}^{\text{−6}}{\text{+13.83(t−20)}}^2{10}^{\text{−9}},\\ {\alpha }_m{\text{=17.73×10}}^{\text{−6}}{\text{+13.83(t−20)10}}^{\text{−9}},\\ {\alpha }_t{\text{=17.73×10}}^{\text{−6}}{\text{+27.66(t−20)10}}^{\text{−9}}.\end{array}$$ (2) The results show that at atmospheric pressure the salt is trimorphous. Modification III is stable on heating up to 585° at which temperature it transforms to II with an increase in volume of 17.4 percent. Form II is stable over a very short temperature interval, whose exact extent could not be determined, and then transforms to Na₂WO₄ I with a decrease in volume of 0.12 percent.