CONFORMITY TO NATURAL LAWS OF STRUCTURE CHANGES IN (Hg,Ce,Cu)(Sr,R)₂(R,Sr)Cu₂O_6+d (R=Pr, Nd, Sm, Dy, Ho, Er, Yb, Lu), (Hg,Ce,Cu)(Sr,R)₂(R,Ca)Cu₂O_6+d (R=Ce, Nd, Eu) AND (Hg,Ce,Cu)(Sr,R)₂(R,Ñe)₂Cu₂O_8+d (R=Ho, Lu, Y) SERIES.

V. Luparev, G.Kuz'micheva

M.V.Lomonosov State Academy of Fine Chemical Technology, Moscow 117571,Russia
E-mail: kuz7micheva@glasnet.ru

Keywords: superconductivity, new mercury Sr-based cuprates, crystal structure, correlation.

Recently we have successfully prepared a new superconducting and non-superconducting phases with perovskite-like structures: (Hg,Ce,Cu)(Sr,R)₂(R,Sr)Cu₂O_6+d (R,Sr-1212) for R=Pr, Nd (Tc=50K), Sm, Dy, Ho (Tc=70K), Er, Yb, Lu (Tc=75K), (Hg,Ce,Cu)(Sr,R)₂(R,Ca)Cu₂O_6+d (R,Ca-1212) for Ce (Tc=108K), Nd, Eu and (Hg,Ce,Cu)(Sr,R)₂(R,Ce)₂Cu₂O_8+d (R,Ce-1222) for R=Ho, Lu, Y series.

They were synthesized in two stages: solid-phase sintering of the components without mercury oxide under normal conditions (t=890-930^oC, =10-16h, air) followed by calcination a precursor with mercury oxide under high pressure (p=3.5-5.0GPa, t=750-850^oC, =5-15min). The 1212 and 1222 phases are tetragonal with a space group (sp.gr) P4/mmm for R,Sr -1212 (a=3.82-3.87A c=11.97-12.20A), R,Ca-1212 (a=3.84-3.86Å c=11.98-12.19Å) structures and I4/mmm for R,Ce-1222 (a=3.802-3.839Å c=27.814-28.134Å) structure. Refinement of the structures (DBWS 9411 program) and crystallochemical analysis of results have been carried out.

The content of Ce in Hg-sites, Ln in Sr-sites decreases and Cu in Hg-sites, Sr in Ln-sites decreases from Pr to Ho(Y) in R,Sr-1212 series but then increases from Ho to Lu in the same series. The change of (Hg,Ce,Cu)-O bond direction from <110> to <hk0> with the motion from Pr to Lu has been observed. The content of Cu in Hg-sites decreases from Ho to Lu in R,Ce-1222 series (M-O(1) bond directed along <hk0>).

The relation between the formal charge of Cu cation (FC Cu), the Cu-O(2) and Cu-O(3) distances (O2-the apical oxygen atom, O3-the basal atoms forming the pyramids) (d_Cu-O(2),A and d_Cu-O(3),A), the Cu-O(3)-Cu angle (j), the (Hg,Ce,Cu)-O(1) and (Hg,Ce,Cu)-O(2) (O2-the apical oxygen atom, O1-the basal atoms) (d_M-O(1) and d_M-O(2) ), (Sr,R)-O(1) (d_A-O(1)), Cu-Cu (d_Cu-Cu) distances and the critical temperature (T_c,K) of all phases has been established. From the depedences obtained it might be concluded that the highest critical temperatures is accounted for optimum combination of composition and structural characteristics, namely

FC Cu=2.11-2.27,j =170-178, d_Cu-O(2)=2.17-2.35Å, d_Cu-O(3)=1.910-1.939Å, d_M-O(1)=2.13-2.35Å, d_M-O(2)=2.14-2.16Å, d_A-O(1)=2.53-2.77Å d_Cu-Cu/c=0.270-0.281Å , (Hg,Ce,Cu)-O(1) bond directed along <hk0> or <110> for R,Sr-1212.

FC Cu=2.22-2.26, j=171-174, d_Cu-O(2)=2.12-2.18Å, d_Cu-O(3)=1.912-1.920Å, d_M-O(2)~2.16Å, d_A-O(1)=2.50-2.57Å, d_Cu-Cu/c~0.204, M-O(1) bond directed along <110> for R,Ce-1222.

FC Cu=2.20-2.26,j =175-179, d_Cu-O(2)=2.18-2.25Å, d_Cu-O(3)=1.919-1.926Å, d_M-O(2)=2.05-2.20Å, d_A-O(1)=2.56-2.74Å, d_Cu-Cu/c=0.201-0.212, M-O(1) bond directed along <hk0> for R,Ce-1222.

FC Cu=2.18-2.25, j=173-175, d_Cu-O(2)=2.04-2.12Å, d_Cu-O(3)=1.925-1.932Å, d_M-O(2)=2.00-2.10Å, d_A-O(1)~3.04Å, d_Cu-Cu/c=0.212-0.215, M-O(1) bond directed along <100> for R,Ce-1222.

Structural characteristics of superconductivity for investigated phases were offered.