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 J₂J₁ have been calculated using Green's-function techniques and have been compared with our experimental x-ray data measured between 6 and 698^∘{}K. The temperature dependence of the calculated trigonal distortion using J₂J₁=1.5 yields a good fit with the measurements which vary from 0 at TN to 4.5{'} at 0^∘{}K. The isotropic volume contraction (strain) due to the exchange striction is small and never exceeds {~} 3×{}10^-4 throughout the temperature range; hence, these latter measurements do not provide detailed information on the correlation functions as was the case for MnO.