Structural and magnetic phase transitions in (Chloroanilinium)2CuX4 (X = Cl, Br)

Successive structural phase transitions of (4-ClC₆H₄NH₃)₂CuCl₄, which occur in a very narrow temperature range were reinvestigated by Fourier transform nuclear quadrupole resonance (FT NQR) measurements. The phase transitions at 275.5 and 277.0 K were confirmed. The effect of the deuteration of the ammonium end on these transitions was studied. The³⁵Cl NQR frequencies of organic cation were observed to decrease by about 4 kHz and the phase transition temperatures to decrease by about 2 K by the deuteration, suggesting that the ?NH₃ ⁺ … Cl hydrogen bond is weakened by the deuteration. The magnetic phase transition temperature of 8.6 K showed no remarkable change within experimental error by the deuteration. It was found that the magnetically ordered state is broken by the radio-frequency magnetic field of about 15–35 Oe usually employed in pulsed NQR. However, in the deuterated compound (4-ClC₆H₄ND₃)₂CuCl₄, the ordered state was found to be stabler for the usual radiofrequency power. By combining with the NQR data of (4-ClC₆H₄NH₃)₂CuBr₄ and (3,5-Cl₂C₆H₃NH₃)₂CuCl₄, the possibility is discussed of tuning the interlayer interaction between the organic cation layer and the inorganic complex anion layer by the halogen substitution in the organic cation as well as by the halogen replacement in the inorganic complex anion.