Elastic coupling and anelastic relaxation associated with multiple phase transitions in para-chloroanilinium tetrachlorocuprate, [p-ClC6H4NH3]2CuCl4

The perovskite-like salt [p-CIC6H4NH3](2)CuCl4 1 exhibits a wealth of magnetic and structural phase transitions which have been probed by variable temperature single crystal X-ray diffraction, SQUID magnetometry, resonant ultrasound spectroscopy (RUS), EPR spectroscopy, DSC measurements and DFT calculations. Single crystal X-ray diffraction studies between room temperature and 3 K reveal a rich tapestry of structural changes; at 298 K the structure conforms to a monoclinic setting but undergoes a first order phase transition upon cooling below similar to 275 K to a higher symmetry orthorhombic cell. This is facilitated by a transition to an intermediate phase at similar to 277 K. Whilst the intermediate phase has a limited stability window (similar to 2 K) and has not been structurally determined, the two discrete phase transitions at 275.5 K and 277 K have been clearly detected by differential scanning calorimetry, EPR spectroscopy and RUS studies. On further cooling a dynamic relaxation process is observed in RUS measurements, evidenced by a Debye-like peak in the dissipation at similar to 140 K. Residual electron density maps from single crystal X-ray diffraction studies reveal that this may be associated with a freezing out of the NH3 center dot center dot center dot Cl hydrogen-bonding between cation and anion frameworks upon cooling. The activation barrier for this order/disorder process was estimated to be at least 27 kJ mol(-1) from the RUS data. Variable temperature dc SQUID data reveal that 1 is a 2D ferromagnet with antiferromagnetic interactions between layers below 9 K. Analysis of the temperature dependence of the magnetic susceptibility for T > 40 K reveals that 1 exhibits Curie Weiss behaviour with theta = +22.8 K indicative of dominant ferromagnetic interactions. Good agreement is observed between the strength of the ferromagnetic interaction extracted from the Weiss constant (J/k = +22.8 K) and that calculated by DFT (J/k = +25 to +28 K) and from EPR studies (J/k= +17 K). The presence of short range ferromagnetic interactions is reflected in a marked temperature dependence of the g-factors determined from EPR spectroscopy below 20 K and possibly a small elastic anomaly in the RUS data. RUS studies indicate a very small elastic anomaly associated with the transition to long range order, implying weak or no magnetoelastic effect. At fields above similar to 15 G a spin flip transition is induced and 1 displays metamagnetic behaviour, with a saturation magnetization of 0.96 mu(B). (C) 2012 Elsevier B.V. All rights reserved.