Existing data placement algorithms for wireless data broadcast generally make assumptions that the clients' queries are already known and the distribution of access frequencies of their queries can be obtained a priori. Unfortunately, these assumptions are not realistic in most real life applications because new mobile clients may join in anytime and clients may be reluctant to disclose their queries (due to privacy concerns). In this paper, we study the data placement problem of periodic XML data broadcast in mobile wireless environments. This is an important issue, particularly when XML becomes prevalent in today's ubiquitous Web and mobile computing devices. Taking advantage of the structured characteristics of XML data, we are able to generate effective broadcast programs based purely on XML data on the server without any knowledge of the clients' access patterns. This not only makes our work distinguished from previous studies, but also enables it to have broader applicability. We discuss structural sharing in XML data which forms the basis of our novel data placement algorithm. The proposed placement algorithm is validated through a set of experiments and the results show that our algorithm can effectively place XML data on air and significantly improve the overall access efficiency.