Magnetic energy investigations on Ni2(PO4) (OH) and NaNiPO4

The Ni-Ni magnetic exchange parameters for the one-dimensional structures Ni2(PO4) (OH) and NaNiPO4 were calculated using the combined SIESTA_TB2J programs. This approach facilitates the generation of a Hamiltonian in terms of localized atomic orbitals and, in turn, calculates the exchange coupling parameters with minimum user input. In the case of Ni2(PO4) (OH), SIESTA_DFT was used to calculate the exchange parameters for a very complex crystal structure containing a basis set of twelve Ni atoms. The calculations generated exchange parameters for 354 Ni pairs for Ni2(PO4) (OH) and 124 Ni pairs for NaNiPO4. For both Ni2(PO4) (OH) and NaNi(PO4), the results not only provided exchange coupling parameters for links identified by simple examination of the crystal structure but also identified additional exchange couplings that account for the correlated magnetic behavior of 1D chains with their magnetic environment. A surprising discovery is that the Ni-O-P-O-Ni bonding yielded exchange parameters equal to -1.13 meV for large Ni-Ni separations at 6.248 Å.