rails 3 render xml irrespective of request.format

I am trying to create an API and i need to return xml irrespective of
the request format. now i have the following in my controller

def index
@posts = Post.all
respond_to do |format|

i have a index.xml.builder

'/posts.xml' works for me but not '/posts'

i tried request.format = :xml which gave me a SystemStackError (stack
level too deep):. why is it happening.

How can i force the controller to render xml for all types of
request ? so that i dont need to specify the format in the url which
looks clean and tidy