Vibrational and electronic properties of MP15 polyphosphides: Crystalline RbP15, KP15, and NaP15.

Abstract

First- and second-order Raman scattering and photoluminescence experiments were carried out to investigate the vibrational and electronic properties of crystalline M${\mathrm{P}}_{15}$ polyphosphides (M=Rb,K,Na). The M${\mathrm{P}}_{15}$ crystalline structures are based on infinite P tubes held together by either the metal atoms M or by van der Waals forces. The properties of the vibrational and electronic states can be discussed in terms of intertube and intratube correlations. Two different sets of modes are observed in the Raman spectra: low-frequency modes in the frequency range up to 140 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and high-frequency modes between 200 and 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The low-frequency modes correspond to intertube vibrations, while the origin of the high-frequency ones is attributed to intratube vibrations. The electronic states that define the energy gap ${E}_{g}$ are identified as being mainly of intratube nature. The behavior of the Raman modes and photoluminescence bands was studied as a function of changes in volume and temperature, with the ``explicit'' contributions to the temperature coefficients separated from the volume-related effects. The mode-Gr\"uneisen parameters of the intertube and intratube phonons differ by one order of magnitude, indicating a breakdown of the Gr\"uneisen approximation for M${\mathrm{P}}_{15}$ crystals. Strong anharmonic interactions determine the behavior of the real and imaginary parts of the self-energy of the high-frequency intratube phonons. The temperature dependence of the band gap is mostly due to the explicit term, indicating a rather strong electron-phonon coupling. The covalent contribution to the energy gap is the dominant one and can explain the different energies of the gaps of M${\mathrm{P}}_{15}$ polyphosphides and Hittorf's phosphorus.