Spin-Lattice Coupling Across The Magnetic Quantum-Phase Transition In Copper-Containing Coordination Polymers

We measured the infrared vibrational properties of two copper-containing coordination polymers, [Cu(pyz)(2)(2-HOpy)(2)]-(PF6)(2) and [Cu(pyz)(1.5)(4-HOpy)(2)](ClO4)(2), under different external stimuli in order to explore the microscopic aspects of spin-lattice coupling. While the temperature and pressure control hydrogen bonding, an applied field drives these materials from the antiferromagnetic -> fully saturated state. Analysis of the pyrazine (pyz)-related vibrational modes across the magnetic quantum-phase transition provides a superb local probe of magnetoelastic coupling because the pyz ligand functions as the primary exchange pathway and is present in both systems. Strikingly, the PF6- compound employs several pyz-related distortions in support of the magnetically driven transition, whereas the ClO4- system requires only a single out-of-plane pyz bending mode. Bringing these findings together with magnetoinfrared spectra from other copper complexes reveals spin-lattice coupling across the magnetic quantum-phase transition as a function of the structural and magnetic dimensionality. Coupling is maximized in [Cu(pyz)(1.5)(4-HOpy)(2)](ClO4)(2) because of its ladderlike character. Although spin-lattice interactions can also be explored under compression, differences in the local structure and dimensionality drive these materials to unique high-pressure phases. Symmetry analysis suggests that the high-pressure phase of the ClO4- compound may be ferroelectric.