The ability to control material properties using light is a significant challenge with potential for numerous applications. Ultrashort laser pulses offer a promising approach for contactless, ultrafast, and selective control, particularly for optically driven photonic devices, memories, and actuators. Ideal photo-responses require room-temperature switching, a wide bistability thermal regime, persistent photoinduced states, threshold switching with single laser shots, and ultrafast dynamics. Molecular materials, especially cyanide-bridged bimetallic assemblies, are attractive candidates exhibiting inter-metallic charge transfer (CT) leading to switchable functionalities (conductivity, ferroelectricity, color, etc.) under photoexcitation. This research focuses on Rb_0.94Mn_0.94Co_0.06[Fe(CN)₆]_0.98 (RbMn_0.94Co_0.06Fe), a cyanide-bridged bimetallic assembly designed to exhibit a wide thermal hysteresis centered at room temperature between Mn^IIFe^III tetragonal and Mn^IIIFe^II cubic phases. The study aims to understand the ultrafast and permanent photoinduced phase transition (PIPT) in this material using a newly developed streaming powder diffraction technique coupled with time-resolved X-ray diffraction (TR-XRD). This technique is essential for probing the out-of-equilibrium structural dynamics of this irreversible transformation. Understanding the underlying mechanisms of this PIPT can pave the way for developing new, efficient optical switching devices operating at room temperature.

Numerous studies have explored photoinduced phase transitions (PIPTs) in various materials. Cyanide-bridged bimetallic assemblies have shown particular promise due to their switchable magnetic, optical, and electrical properties resulting from photoinduced charge transfer. Previous research has demonstrated laser-induced reversible spin transitions and other PIPTs in related materials, but often at low temperatures or requiring multiple laser shots. The challenge lies in achieving persistent, ultrafast PIPTs at room temperature, utilizing a single laser pulse. Existing techniques have limitations in monitoring the ultrafast dynamics of non-reversible transformations. This work builds upon previous studies by developing a novel streaming powder diffraction technique enabling real-time observation of the structural changes during the PIPT process.

The RbMn_0.94Co_0.06Fe material was synthesized and characterized using magnetic measurements, X-ray diffraction (XRD), and infrared spectroscopy. The material exhibits a clear charge-transfer based phase transition, with a wide thermal hysteresis (75 K) centered at room temperature, monitored through the thermal dependence of χ_mT (molar magnetic susceptibility χ_m and temperature T). Powder XRD revealed significant changes in lattice parameters between the high-temperature (HT) cubic (F43m) and low-temperature (LT) tetragonal (F42m) phases. A novel streaming powder diffraction technique was developed and employed to monitor ultrafast structural dynamics using TR-XRD at the ID09 beamline of the ESRF. This technique involved dispersing microcrystals of RbMn_0.94Co_0.06Fe in ethanol and streaming them through a liquid jet. This jet was positioned in the path of synchronized optical pump (1 ps pulses at 650 nm) and X-ray probe (~35 ps pulses) beams. The closed-loop circulation system included a cooling device to ensure each diffraction pattern was measured from a fresh, unexcited batch of crystals. The time-resolved diffraction patterns were analyzed using Rietveld refinement to track the time evolution of lattice parameters, phase fractions, and bond lengths. Data was obtained for various laser fluences (0-150 mJ cm⁻²) and pump-probe delays (−3 ns to +10 μs). The analysis focused on two time-dependent phases: a photoexcited tetragonal (PT) phase and a photoinduced cubic (PIC) phase.

Time-resolved XRD revealed that at low fluences, the initial LT phase undergoes lattice distortions within 100 ps, exhibiting increased volume and reduced ferroelastic distortion. These changes are attributed to the formation of photoinduced Mn^IIIFe^II CT polarons and subsequent reverse Jahn-Teller distortions. At higher fluences (above a threshold of ~10 mJ cm⁻²), a complete and ultrafast (within 100 ps) tetragonal-to-cubic phase transition occurs. This transition is driven by an increase in the relative volume expansion of the PT lattice. The photoinduced cubic (PIC) phase is structurally similar to the thermodynamically stable HT phase and persists long after the laser pulse. The Rietveld analysis showed that approximately 10% of the MnFe units are photoexcited at the threshold, and almost complete conversion to the PIC phase is achieved above 100 mJ cm⁻². The dynamics exhibits a clear threshold behavior, characteristic of a cooperative PIPT. The Mn-N bond lengths were observed to change within 100 ps, further supporting the ultrafast nature of the structural reorganization. The timescale of the phase transition matches the acoustic propagation time across the crystals, indicating an elastically driven process. The observed structural changes and timescale strongly suggest that the PIPT mechanism is governed by the interplay between the elastic energy of the lattice and the charge-transfer polarons.

The findings demonstrate that a single laser shot can induce a persistent, ultrafast PIPT at room temperature. This contrasts with previous observations of PIPTs that often required multiple laser shots or occurred at lower temperatures. The observed threshold behavior and fluence dependence strongly support a cooperative mechanism, where a critical density of photoinduced polarons initiates a macroscopic phase transition. The Landau theory framework, accounting for the coupling between charge transfer and lattice distortion, successfully explains the observed behavior. The competition between the lattice pressure pushing the polarons back to their ground state and the pulling forces of the expanded lattice favoring the higher-volume state determines the threshold and cooperativity. The development and successful application of the streaming powder diffraction technique provide a powerful method for studying ultrafast, irreversible phase transitions in materials science.

This study demonstrates the feasibility of inducing an ultrafast and persistent photoinduced phase transition at room temperature using a single laser pulse. The observed phenomenon is explained by a cooperative mechanism driven by the elastic interactions within the crystal lattice. The new streaming powder diffraction technique allows for detailed study of the microscopic and macroscopic structural dynamics of ultrafast non-reversible phase transitions. This approach opens new avenues for developing advanced optical switching devices with improved performance and operational characteristics. Future research could explore other materials with similar bistability characteristics and investigate the potential for further enhancing the speed and efficiency of the photoinduced phase transition.

The study primarily focuses on the structural dynamics and does not directly probe the electronic transitions. While the timescale of the phase transition is established, the precise mechanism of polaron formation and their interaction with the lattice could benefit from further investigation using complementary techniques. The limited number of crystals in the X-ray focus may influence the precise measurements of the phase transition, and future work could increase the density of crystals in the jet to improve the signal. The current system is limited to 10 µs time window, and future adaptation to the X-ray free-electron laser beamlines could enable sub-picosecond time resolution.