Extended Moment Formation and Second Neighbor Coupling in Li₂CuO₂

Comprised of ferromagnetic edge-sharing CuO$_2$ chains that order in antialigned fashion at T$_N$=9 K, Li$_2$CuO$_2$ is found from local spin density calculations to display several surprising characteristics: (1) the ordered moment/f.u. of 0.92 $\mu_B$ is the largest for any low dimensional cuprate system, in agreement with experiment, (2) 40% of this moment lies on the neighboring O ions, making it the largest oxygen moment yet reported, and (3) the second neighbors couplings are larger than nearest neighbors couplings. All of these phenomena arise naturally due to a well defined effective $d_{yz}$ type orbital that includes very strong O $p_{\sigma}$ character. We interpret the large moment as surviving reduction by quantum fluctuations due to extension caused by the $d-p$ hybridization.