# Collapsed Tetragonal Phase Transition in LaRu$_2$P$_2$

## TL;DR

This study investigates the pressure-induced phase transition in LaRu₂P₂, revealing a collapsed tetragonal phase at high pressure, with implications for understanding structural and superconducting properties under external pressure.

## Contribution

The paper provides the first detailed high-pressure x-ray diffraction analysis of LaRu₂P₂, identifying a second-order collapsed tetragonal phase transition and comparing effects of physical and chemical pressure.

## Key findings

- LaRu₂P₂ transitions to a collapsed tetragonal phase above ~2 GPa.
- The cT phase exhibits a higher bulk modulus of 175 GPa.
- The phase transition is temperature-dependent, occurring at slightly higher pressures at room temperature.

## 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.