Pressure induced phase transformation in BaWO4

A new dense form of BaWO₄(BaWO₄-II) was prepared under high pressure. The phase boundary between the normal pressure form (BaWO₄-I, scheelite structure) and BaWO₄-II was determined as P(kb) = 26.₇+0.26₅T(°C), (T=600–1000 °(C). Crystallographic data were obtained from the single crystal and powder X-ray analyses. BaWO₄-II is monoclinic with 8 formula units in the unit cell. The possible space group is $\text{P}\text{2}{}_{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}$ and the cell parameters are; $\text{a}\text{= 13.159}\text{A}\text{?}\text{,}\text{b}\text{= 7.161}\text{A}\text{?}\text{,}\text{c}\text{= 7.499}\text{A}\text{?}\text{, β = 93.76°}$ and the cell $\text{volume}\text{= 705}\text{A}\text{?}{}^3$. The volume decrease upon transformation is estimated to be 12.1%.