Title: Collapsed Tetragonal Phase Transition in LaRu$_2$P$_2$,
Abstract: The structural properties of LaRu$_2$P$_2$ under external pressure have been
studied up to 14 GPa, employing high-energy x-ray diffraction in a
diamond-anvil pressure cell. At ambient conditions, LaRu$_2$P$_2$ (I4/mmm) has
a tetragonal structure with a bulk modulus of $B=105(2)$ GPa and exhibits
superconductivity at $T_c= 4.1$ K. With the application of pressure,
LaRu$_2$P$_2$ undergoes a phase transition to a collapsed tetragonal (cT) state
with a bulk modulus of $B=175(5)$ GPa. At the transition, the c-lattice
parameter exhibits a sharp decrease with a concurrent increase of the a-lattice
parameter. The cT phase transition in LaRu$_2$P$_2$ is consistent with a second
order transition, and was found to be temperature dependent, increasing from
$P=3.9(3)$ GPa at 160 K to $P=4.6(3)$ GPa at 300 K. In total, our data are
consistent with the cT transition being near, but slightly above 2 GPa at 5 K.
Finally, we compare the effect of physical and chemical pressure in the
RRu$_2$P$_2$ (R = Y, La-Er, Yb) isostructural series of compounds and find them
to be analogous.