Efficient launch of shear phonons in photostrictive halide perovskites

TL;DR

The study demonstrates an efficient, nonthermal route to launch shear (TA) phonons in a lead-free halide double perovskite (Cs2AgBiBr6) by exploiting anisotropic photostriction in the tetragonal phase. Time-domain Brillouin spectroscopy reveals coherent LA and TA phonons in the tetragonal phase, with TA polarization dictated by the c-axis projection on the surface, while only LA phonons appear in the cubic phase; the TA_y mode exhibits strong dispersion changes near the tetragonal–cubic transition at $T_c \approx 122\ \mathrm{K}$. The mechanism relies on anisotropic lattice expansion under photoexcitation, leading to an in-plane driving force that preferentially excites TA_y, and is supported by DFT-calculated mode mixing and polarization analysis. These findings open avenues for tunable hypersonic control and phononic transducers in perovskites by leveraging phase transitions and electron–phonon coupling, with a photostriction coefficient on the order of $|\alpha_i| \sim 10^{-23}$ cm$^3$.

Abstract

Optical generation of transverse coherent phonons by femtosecond light pulses is appealing for high-speed sub-THz active control of material properties. Lead-free double perovskite semiconductors, such as Cs2AgBiBr6, attract particular interest due to their cubic to tetragonal phase transition below room temperature and strong polaron effects from carrier-lattice coupling. Here, we reveal that the anisotropic photostriction in halide perovskites with tetragonal crystal structure represents an efficient non-thermal tool for generating transverse coherent phonons. In particular, we demonstrate that along with compressive strain, optical generation of photoexcited carriers leads to strong shear strain in Cs2AgBiBr6 below the phase transition temperature of 122 K. Using time-domain Brillouin spectroscopy, we observe coherent transverse and longitudinal acoustic phonons with comparable amplitudes in the tetragonal phase, while in the cubic phase only longitudinal phonons are generated. The polarization of the photoinduced transverse phonons is dictated by the projection of the c-axis on the surface plane, which leads to a prominent anisotropic polarization response in the detection. The generated strain pulses correspond to transverse acoustic soft eigenmodes with a strong temperature dependence of dispersion, which provides an additional degree of freedom for active hypersonic control.

Figures (7)

• Figure 1: Ultrafast generation of strain pulses. (a) The single Cs$_2$AgBiBr$_6$ crystal under study. Arrows point along the main crystallographic directions of the available facets. (b) Reflectivity (blue), transmission (green), and photoluminescence (PL, red) measured at $T=2$ K. The reflectivity spectrum shows the exciton resonance at 2.82 eV. The transmission spectrum is measured in the vicinity of the absorption edge. Vertical arrows indicate the photon energies in the time-resolved pump-probe and continuous wave ($cw$) Brillouin light scattering (BLS) experiments. (c) Schematic presentation of the excitation of a strain pulses by a fs optical pump pulse along the $\mathbf{z}'$ direction. The color gradient indicates the surface layer containing the photoexcited carriers. In the cubic phase, the photogenerated stress driving force $\mathbf{\Phi}\|\mathbf{z}'$ results in compressive strain. Only a LA phonon pulse is launched in this case, as shown by the green wiggly arrow. In the tetragonal phase, the expansion coefficient along the [001] direction (c-axis) is different from that along the [100] and [010] directions. Then the driving force $\mathbf{\Phi}$ acquires an in-plane component (along $y'$) and leads to shear strain. If the expansion coefficients have different signs (see green and magenta arrows), the shear strain is significant, giving rise to TA and LA pulses with comparable amplitudes (red and green wiggly arrows).
• Figure 2: Dynamics of pump-induced reflectivity modulation. (a) and (b) Examples of pump-probe transients of differential reflectivity $\Delta R/R$ from the Cs$_{2}$AgBiBr$_{6}$ crystal at $T=5$ K, i.e. in the tetragonal phase, measured along the crystal direction [011] ([111] in the cubic phase) in HF and LF experiments with repetition frequencies of 80 MHz and 30 kHz, respectively. In (a) the photon energies of the pump $h\nu_{\rm pump}$ and probe $h\nu_{\rm probe}$ pulses are set to 2.638 and 2.339 eV, respectively. The insets show fast oscillatory signals below and above the structural phase transitions, taken at $T=5$ K and 150 K. In (b) $h\nu_{\rm pump}=2.818$ eV and $h\nu_{\rm probe}=2$ eV. The inset shows the polar plot of the amplitudes of the fast oscillatory components with frequencies $f_{\rm LA}=18.6$ GHz and $f_{\rm TA_{\rm y}}=9$ GHz (blue and red diamonds, respectively) as function of the direction $\theta$ of the probe pulse polarization relative to the $y'$-axis. The solid black lines show possible projections of the directions of the c-axis on the crystal surface. The solid red and blue lines are fits using the function $A-B\sin^2(\theta)$. (c) The top and middle curves correspond to fast Fourier transform (FFT) spectra of the pump-probe signals in (a), measured at 5 K (red) showing two peaks in the tetragonal phase, and measured at 150 K (green) with only one peak in the cubic phase. The lower spectrum corresponds to cw Brillouin light scattering (BLS) measured in the back reflection geometry at the photon energy $h\nu_{\rm BLS}=2.287$ eV for $T=5$ K. (d) Frequencies of the FFT peaks of the coherent longitudinal LA (diamonds), transverse acoustic TA$_{\rm x}$ (circles) and TA$_{\rm y}$ (squares) phonons as function of the probe photon energy. The $cw$ BLS data are shown with the blue symbols, the LF data with the green symbols, and the HF data with the yellow symbols. Linear fits through zero frequency are presented by the solid blue and yellow lines.
• Figure 3: Shear strain generation. Sketch illustrating the occurrence of a transverse component of the driving force in the tetragonal phase. The colored spheres show the positions of the atoms, the red arrows indicate the direction of the force acting on the individual atoms and the blue arrows show the average stress force acting in the surface plane. The red arrows with shorter lengths result from the projection of the stress vector into the $y'z'$ plane. They have to be taken into account twice due to the two atoms in the cell along the $\mathbf{x}'$ axis.
• Figure 4: Temperature dependence of the phonon frequencies. The data are shown by orange symbols for the HF pump-probe, by the green symbols for the LF pump-probe, and by blue symbols for the $cw$ BLS spectroscopy (recalculated for the photon energy of $h\nu=2.34$ eV). (a) The diamond symbols correspond to the LA phonons, (b) squares to the TA$_{\rm y}$ phonons, circles to the TA$_{\rm x}$ phonons. The vertical dashed line corresponds to the phase transition temperature $T_c=122$ K.
• Figure S1: (a) XRD powder pattern of the grained Cs$_{2}$AgBiBr$_{6}$ crystals measured at room temperature. The peaks can be assigned to a simple cubic crystal structure Adam-2016. Peaks expected, but not resolved are marked with asterisks. (b) Simulated XRD powder pattern of the Cs$_{2}$AgBiBr$_{6}$ perovskite. The simulated data were generated with the Mercury CCDC software using the crystallographic data from doi:10.1021/jacs.7b01629.