  Massive galaxy clusters are interesting astrophysical and cosmological study
objects, but are relatively rare. In the redshift range z = 0.25 to 0.5 which
is, for example, a favourable region for gravitational lensing studies, about
100 such systems are known. Most of them have been studied in X-rays. In this
paper we study the six remaining massive clusters in this redshift interval in
the highly complete CLASSIX survey which have so far not been observed with
sufficiently deep exposures in X-rays. With data from our new XMM-Newton
observations we characterise their structures, derive X-ray properties such as
the X-ray luminosity and intra-cluster medium temperature and estimate their
gas and total masses. We find that one cluster, RXCJ1230.7+3439, is dynamically
young with three distinct substructures in the cluster outskirts and
RXCJ1310.9+2157/RXCJ1310.4+2151 is a double cluster system. Mass determination
is difficult in the systems with substructure. We therefore discuss several
methods of mass estimation including scaling relations. In summary we find that
five of the six study targets are indeed massive clusters as expected, while
the last cluster RXCJ2116.2-0309 is a close projection of a distant and a
nearby cluster which has led to a previous overestimation of its mass. In the
XMM-Newton observation fields we also find three low redshift clusters close to
the targets which are also analysed and described here. In the field of
RXCJ2116.2-0309 we discover serendipitously a highly variable X-ray source
which has decreased its flux within a year by more than a factor of eight. This
source is most probably an AGN.
