  We present a catalogue of X-ray selected galaxy clusters and groups as a
first release of the 2XMMi/SDSS Galaxy Cluster Survey. The survey is a search
for galaxy clusters detected serendipitously in observations with XMM-Newton in
the footprint of the Sloan Digital Sky Survey (SDSS). The main aims of the
survey are to identify new X-ray galaxy clusters, investigate their X-ray
scaling relations, identify distant cluster candidates and study the
correlation of the X-ray and optical properties. In this paper we describe the
basic strategy to identify and characterize the X-ray cluster candidates that
currently comprise 1180 objects selected from the second XMM-Newton
serendipitous source catalogue (2XMMi-DR3). Cross-correlation of the initial
catalogue with recently published optically selected SDSS galaxy cluster
catalogues yields photometric redshifts for 275 objects. Of these, 182 clusters
have at least one member with a spectroscopic redshift from existing public
data (SDSS-DR8). Here we present the X-ray properties of the first cluster
sample which comprises 175 clusters, among which 139 objects are new X-ray
discoveries while the others were previously known as X-ray sources. The first
cluster sample from the survey covers a wide range of redshifts from 0.09 to
0.61, bolometric luminosities L_500 = 1.9 x 10^42 - 1.2 x 10^45 erg/s, and
masses M_500 = 2.3 x 10^13 - 4.9 x 10^14 Msun. We extend the relation between
the X-ray bolometric luminosity L_500 and the X-ray temperature towards
significantly lower T and L and still find that the slope of the linear L-T
relation is consistent with values published for high luminosities.
