Exchange Striction in NiO

Exchange-striction effects have been studied in both the antiferromagnetic and paramagnetic states of NiO. Near-neighbor spin correlation functions as functions of temperature for various choices of J₂{J₁} have been calculated using Green's-function techniques and have been compared with our experimental x-ray data measured between 6 and 698^. The temperature dependence of the calculated trigonal distortion using J₂{J₁}=1. 5 yields a good fit with the measurements which vary from 0 at T₍ to 4. 5' at 0^. The isotropic volume contraction (strain) due to the exchange striction is small and never exceeds 310^-4 throughout the temperature range; hence, these latter measurements do not provide detailed information on the correlation functions as was the case for MnO.