Thl.: Ri elwc ld mc thod ofstructurl.: refinement
from powder dilTr<:lction patterns is widely used with neutron
data and int:reasingly so with x* ray data . [n this p<:lper the
Ril.:tveld method has hccn cxplainl.:d. As an ex.ample the
t: rystal :-.truclure of LazCu04 at rollm It:mperaturc has been
refined using x*ray powder dirfr<:lctiol1 data. The crystal
,lruc lure is llflhorombi c wilh a = 5.154S(4)A .
h = 5.411116(1)A . c = 13.1529 (9)A. and space gfllUp em,"'
A sample of La2Cu04 has been oxidized to La2Cu04+0 and
its magnetic susceptibily as a function or temperature has
heen studied. The new m(ltcrial is supcrconductor with
Tc == 30K. Further investigation is in progress to determine
the va lue or () and the position of the extra oxygen atOl11s in
tht..: cell.