  We present the growth and basic magnetic and transport properties of
Cr$_{1+x}$Pt$_{5-x}$P. We show that single crystals can readily be grown from a
high-temperature solution created by adding dilute quantities of Cr to Pt-P
based melts. Like other 1-5-1 compounds, Cr$_{1+x}$Pt$_{5-x}$P adopts a
tetragonal P4/mmm structure composed face-sharing CrPt$_3$ like slabs that are
broken up along the c-axis by sheets of P atoms. EDS and X-ray diffraction
measurements both suggest Cr$_{1+x}$Pt$_{5-x}$P has mixed occupancy between Cr
and Pt atoms, similar to what is found in the closely related compound
CrPt$_3$, giving real compositions of Cr$_{1.5}$Pt$_{4.5}$P (x = 0.5). We
report that Cr$_{1.5}$Pt$_{4.5}$P orders ferromagnetically at T$_C$ = 464.5 K
with a saturated moment of $\approx$ 2.1 $\mu_{\textit{B}}$/Cr at 1.8 K. Likely
owing to the strong spin-orbit coupling associated with the large quantity of
high Z Pt atoms, Cr$_{1.5}$Pt$_{4.5}$P has exceptionally strong planar
anisotropy with estimated anisotropy fields of 345 kOe and 220 kOe at 1.8 K and
300 K respectively. The resistance of Cr$_{1.5}$Pt$_{4.5}$P has a metallic
temperature dependence with relatively weak magnetoresistance. Electronic band
structure calculations show that CrPt$_5$P has a large peak in the density of
states near the Fermi level which is split into spin majority and minority
bands in the ferromagnetic state. Furthermore, the calculations suggest
substantial hybridization between Cr-3d and Pt-5d states near the Fermi level,
in agreement with the experimentally measured anisotropy.
