  Ca2-xNaxCuO2Cl2 is a structural analogue to the La2-xSrxCuO4 superconductor.
By substituting Na+ for Ca2+ mobile holes are introduced into the CuO2 planes.
A series of experiments shows that the Na content x depends not only on the
pressure during synthesis but also on the synthesis temperature and time.
Ca2-xNaxCuO2Cl2 crystals (x=0.10 and Tc,on=13.5 K) up to 1x1x0.05 mm3 have been
grown by a flux method at 35 kbar and the temperature was slowly ramped from
1250 C to 1050 C within 8 h. Applying higher synthesis pressure (up to 55 kbar)
and temperature (up to 1700 C) results in polycrystalline samples with highest
Tc,on=28.0 K at x=0.20. Further experiments are under way to optimize
conditions to grow crystals with extended concentration of Na and Tc
characteristic for optomally doped and overdoped regions.
