First order transition in trigonal structure ${\textbf{Ca}}{\textbf{Mn}}_{2}{\textbf{P}}_{2}$

TL;DR

This study investigates the structural, electronic, and magnetic properties of CaMn2P2, revealing a first-order phase transition at 69.5 K associated with symmetry change, unusual for similar compounds, and pressure-dependent transition behavior.

Contribution

It reports the first observation of a first-order structural transition in CaMn2P2 with detailed characterization, highlighting its unique properties among similar Mn-based compounds.

Findings

CaMn2P2 adopts a trigonal CaAl2Si2-type structure.

It exhibits an insulating ground state with two activation energies.

A first-order transition occurs at 69.5 K, shifting with pressure.

Abstract

We report structural and physical properties of the single crystalline ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$. The X-ray diffraction(XRD) results show that ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$ adopts the trigonal ${\mathrm{Ca}}{\mathrm{Al}}_{2}{\mathrm{Si}}_{2}$-type structure. Temperature dependent electrical resistivity $\rho(T)$ measurements indicate an insulating ground state for ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$ with activation energies of 40 meV and 0.64 meV for two distinct regions, respectively. Magnetization measurements show no apparent magnetic phase transition under 400 K. Different from other ${\mathrm{A}}{\mathrm{Mn}}_{2}{\mathrm{Pn}}_{2}$ (A = Ca, Sr, and Ba, and Pn = P, As, and Sb) compounds with the same structure, heat capacity $C_{\mathrm{p}}(T)$ and $\rho(T)$ reveal that ${\mathrm{Ca}}{\mathrm{Mn}}_{2}{\mathrm{P}}_{2}$ has a first-order transition at $T$ = 69.5 K and the transition temperature shifts to high temperature upon increasing pressure. The emergence of plenty of new Raman modes below the transition, clearly suggests a change in symmetry accompanying the transition. The combination of the structural, transport, thermal and magnetic measurements, points to an unusual origin of the transition.