  Unusual phases and phase transitions are seen at the magnetic-nonmagnetic
boundary in Ce, Eu and Yb-based compounds. EuNi$_2$P$_{2}$ is a very unusual
valence fluctuating Eu system, because at low temperatures the Eu valence stays
close to 2.5 instead of approaching an integer value. Eu valence and thus the
magnetic property in this system can be tuned by Ge substitution in P site as
EuNi$_2$Ge$_{2}$ is known to exhibit antiferromagnetc (AFM) ordering of
divalent Eu moments with $T_N$ = 30 K. We have grown
EuNi$_2$(P$_{1-x}$Ge$_x$)$_2$ (0.0 $\leq$ $x$ $\leq$ 0.5) single crystals and
studied their magnetic, thermodynamic and transport properties. Increasing Ge
doping to $x >$ 0.4 results in a well-defined AFM ordered state with $T_N$ = 12
K for $x$ = 0.5. Moreover, the reduced value of magnetic entropy for $x$ = 0.5
at $T_N$ suggests the presence of valance fluctuation/ Kondo effect in this
compound. Interestingly, the specific heat exhibits an enhanced Sommerfeld
coefficient upon Ge doping. Subsequently, electronic structure calculations
lead to a non-integral valence in EuNi$_2$P$_{2}$ but a stable divalent Eu
state in EuNi$_2$Ge$_{2}$ which is in good agreement with experimental results.
