Spin-spin interaction of Ni2+ ions in ZnSiF6 center dot 6H(2)O

The angular dependences of the electron spin resonance spectrum of 1% Ni2ions in a ZnSiF6. 6H(2)O matrix are investigated experimentally at 36 GHz a nd 4.2 K. Besides the main spectrum of the isolated ion, we observed a spec trum due to interacting pairs of Ni2+ ions, located in the first (nn) and s econd (2n) coordination spheres and coupled by, besides the magnetic dipole -dipole interaction, isotropic exchange: J(nn)=(-197 +/- 1)x10(-4), J(2n al pha)=(-5 +/- 1)x10(-4), and J(2n beta)=(3 +/- 2)x10(-4) cm(-1). Lines due t o other isolated Ni2+ ions, which have a different initial splitting D, are also present in the spectrum with intensity comparable to the pair spectru m. Low-symmetry distortions of the crystal field are observed, caused by a pair of impurity ions located close to one another. It is shown that the pr eviously proposed interpretation is incorrect. (C) 1999 American Institute of Physics. [S1063-7834(99)01409-4].