chloride in a saturated solution of thiourea [@23]. The crystals grow after about two weeks in a solution maintained at a temperature of 35 degrees centigrade. Yellow-brown single crystals are tetragonal prisms along the $c$-axis (Fig. 3a). The space group of DTN is $I4$, with two molecules in the unit cell, whose dimensions are $a=b=9.558$ Å$ $ and $c=8.981$ Å. The $S = 1$ Ni atoms form two interpenetrating tetragonal lattices. The thioureas are bounded to the nickel by means of the sulfur atoms in a square array. The spin must be in an environment of approximate uniaxial symmetry (Fig. 3b). The metal is in a noncentrosymmetric environment of fourfold symmetry. All molecules are oriented in the same way and held together by Cl-N hydrogen bonds. The Cl-Ni-Cl axes all lie parallel to the crystal $c$-axis. The samples were always chemically analyzed for their main constituents. Polarized light allowed us to verify the excellent quality of the samples and to check the orientation of the crystallographic axes for the measurements, as determined by $X$-ray. The sample masses in our experiments ranged from a few milligrams to 2 g.

![\[fig3\]a) Sample of DTN grown in a water solution. b) Schematic crystal 