Raman scattering spectroscopic observation of a ferroelastic crossover in bond-frustrated PrCd$_3$P$_3$

Abstract

2D magnetism in triangular lattices has already shown potential for hosting exotic magnetic states. Control of these magnetic states, both in terms of magnetic properties and in terms of charge doping would be the next step. This makes materials which combine triangular lattice magnetic layers with layers hosting interesting structural or electronic properties particularly useful. PrCd$_3$P$_3$, studied in this work, is one of a family of materials where triangular lattice layers of magnetic rare earth ions alternate with semiconducting hexagonal CdP layers. Using Raman scattering spectroscopy we uncover a structural instability in the CdP layers, associated with a soft mode behavior of a phonon in these layers. Raman scattering detects crystal electric field excitations, and confirms a singlet ground state for Pr$^{3+}$ and splitting of the doublet levels as a result of the structural instability in CdP layers. While Pr$^{3+}$ is non-magnetic in PrCd$_3$P$_3$ we speculate that this family of materials can realize control of the magnetic layer through the CdP layer which can become ferroelectric under strain that would relieve frustration.

Figures (6)

• Figure 1: Structure of PrCd$_3$P$_3$, viewed along (a) skewed crystallographic a-axis and (b) [001] direction. (c) Cd$_{trig}$/P$_{trig}$ layer viewed along the [001] direction.
• Figure 2: Raman scattering spectra of PrCd$_3$P$_3$ with frequencies of Raman active phonons and crystal field excitations marked by vertical lines. (a) spectra in RL scattering channel between 7 and 240 K, (b) spectra in RR scattering channel between 7 and 180 K. (c) Raman scattering spectra of NdCd$_3$P$_3$ in XX, YY, and XY scattering channels at 215 K. Black, red, and blue vertical dashed lines show observed frequencies of phonons of PrCd$_3$P$_3$, calculated frequencies are marked in the bottom panel, see Table \ref{['tab:phonon_table']} for exact frequencies. Green vertical dashed lines show observed positions of crystal field excitations, with the dashed lines with shorter dashes marking observed interband crystal field excitations (see Table \ref{['tab:CEFcalc_table']} for exact frequencies). The comparison of PrCd$_3$P$_3$ and NdCd$_3$P$_3$ spectra allows one to identify phonons vs crystal field excitations of Pr$^{3+}$ in the Raman scattering spectra of PrCd$_3$P$_3$.
• Figure 3: $\Gamma$-point atomic displacements for six DFT-calculated phonons of PrCd$_3$P$_3$. Phonons are labeled by Ph1-Ph6, with the dominant atomic motion of each mode noted in brackets. The phonon labels are color-coded dependent on which layer the atoms belong to. Top: atomic displacements viewed along [120] direction. Bottom: atomic displacements viewed along [001] direction for single layers of displaced atomic sites.
• Figure 4: Temperature dependence of the lowest frequency $E_{2g}$ phonon (Ph1). (a) Raman spectra in the spectral region around the phonon, the intensity is normalized to spectral weight of A$_{1g}$ phonon at 30.8 meV. (b) Temperature dependence of the squared soft mode energy (black squares), showing softening and subsequent hardening in the low temperature regime. A fit to a linear model in the high-temperature regime is shown by red dashed line and yields T=70 K for the structural transition temperature. (c) Temperature dependence of the full-width at half maximum (FWHM) of the Lorenztian phonon lineshape.
• Figure 5: Temperature dependence of (a) phonon frequencies and (b) width (FWHM); The phonon labels are color-coded dependent on which layer the atoms belong to, as marked in the PrCd$_3$P$_3$ crystal structure shown in panel (c). Top: phonons involving motion of the interstitial layer. Middle: phonons involving motion of the P$_{oct}$ layer. Bottom: phonons involving motion of the Cd$_{tet}$ layer. The structural instability temperature obtained from the soft mode (70 K) is marked with a dashed vertical line.