  We report synthesis, structural and magnetic (DC and AC) properties of Boron
substituted MgCNi3 superconductor. A series of polycrystalline bulk samples
Mg1.2C1.6-xBxNi3 (x = 0.0, 0.08 and 0.16) is synthesized through standard
solid-state reaction route, which are found to crystallize in cubic perovskite
structure with space group Pm3m. Rietveld analysis of observed XRD data show
that lattice parameters expand from a = 3.8106 (4) {\AA} for pure, to 3.8164
(2) {\AA} and 3.8173 (5) {\AA} for 5% and 10% Boron substituted samples
respectively. DC magnetization exhibited superconducting transition (Tc) at
around 7.3 K for pure sample, and the same decreases slightly with Boron
substitution. The lower critical field (Hc1) at 2 K is around 150 Oe for pure
sample, which increases slightly with Boron substitution. For pure sample the
upper critical field (Hc2) being determined from AC susceptibility measurements
is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively,
which decreases to 5.57 kOe and 42.5 kOe respectively for 10% Boron substituted
sample. 10% Boron substitution at Carbon site has decreased both the Hc2 and
Tc. On the other hand lower critical field (Hc1) at 2 K is slightly increased
from around 150 Oe for pure sample, to 200 Oe for 10% Boron substituted sample.
Seemingly, the Carbon site Boron substitution induced disorder though has
increased slightly the Hc1 but with simultaneous decrease in superconducting
transition temperature (Tc) and upper critical field (Hc2). The high relative
proportion of Ni in studied MgCNi3 suggests that magnetic interactions are
important and non-oxide perovskite structure make it interesting.
