The high-pressure behavior of both $\beta$- and $\alpha -\mathrm{Ba}(\mathrm{OH}{)}_2$ was investigated by in situ powder synchrotron x-ray diffraction. Pressures up to 13 GPa were generated using a diamond-anvil cell. At least one phase transition was detected in $\beta -\mathrm{Ba}(\mathrm{OH}{)}_2$ $[{P2\mathrm{}}_1/n;a=9.3396\mathrm{}(2),b=7.8550\mathrm{}(2),c=6.7267\mathrm{}(2)\mathrm{}\mathrm{\AA },\beta =95.607\mathrm{}(2)\mathrm{}°$ at 0.95 GPa], indicated by the appearance of an additional low-angle peak. The high-pressure ${\beta }_2$ phase $[{P2\mathrm{}}_1/c;a=11.6632\mathrm{}(8),b=7.6684\mathrm{}(5),c=10.7785\mathrm{}(7)\mathrm{}\mathrm{\AA },\beta =108.881\mathrm{}(5)\mathrm{}°$ at 5.40 GPa] is characterized by a doubling of the unit-cell volume. Bulk moduli were determined for $\beta -\mathrm{Ba}(\mathrm{OH}{)}_2$ with $K_0=40\mathrm{}\left(1\right)\mathrm{}\mathrm{GPa}$ ${(K}^{\prime }$ fixed to 6) and for ${\beta }_2-\mathrm{Ba}(\mathrm{OH}{)}_2$ with $K_0=60\mathrm{}\left(4\right)\mathrm{}\mathrm{GPa}$ ${(K}^{\prime }$ fixed to 6). There are indications for two other phase transitions between 8 and 10 GPa. Compressing $\alpha -\mathrm{Ba}(\mathrm{OH}{)}_2$ produces new diffraction patterns, which cannot be interpreted unambiguously. Upon pressure release down to 0.3 GPa, the ${\beta }_2$ phase is recovered.

• Received 7 February 2002

DOI:https://doi.org/10.1103/PhysRevB.66.214103