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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 thetrigonal ${\mathrm{Ca}}{\mathrm{Al}}_{2}{\mathrm{Si}}_{2}$-type structure.Temperature dependent electrical resistivity $\rho(T)$ measurements indicate aninsulating 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 phasetransition 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 transitionat $T$ = 69.5 K and the transition temperature shifts to high temperature uponincreasing pressure. The emergence of plenty of new Raman modes below thetransition, clearly suggests a change in symmetry accompanying the transition.The combination of the structural, transport, thermal and magneticmeasurements, points to an unusual origin of the transition.

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