For your info: it looks like the stable phase at room temperature is not the monoclinic: http://dx.doi.org/10.1016/j.jssc.2004.11.012
Thank you for your information. The Materials Project calculated both rhombohedral and monoclinic structures of Ba2BiTaO6 and the energy difference between these two phases is less than 1meV, within the accuracy of our calculations. The band gaps for these two structures are also quite close. Please check the link below.
The monoclinic phase is only stable at low temperature, while at room temperature the stable phase is rhombohedral based on the paper you referred. This information is actually already noted in the tags on the webpages above the band structure plot. for these two phases. Please let us know if you have more questions. Thanks.