Is it possible to do post-doc in a subject different from our PhD research topic?
The answer is a definite YES. There are two ways to look at the postdoc experience. You can either continue to work in the same or related area or expand the scope of your experience by choosing a different area. While each has its advantages and disadvantages, there is no one rule of thumb. It is of course convenient to stay on the same area and perhaps more opportunities exist. However, the future of research is on interdisciplinary skills. This means go outside the box and try to learn as much as possible and broaden your skill sets. This way, you have an opportunity to expand on what you have already learned into some other area. Hence, you would increase your marketability when you are done with your post doc.